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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
d1be9408 72A named array in scalar context is quite different from what would at
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73first glance appear to be a list in scalar context. You can't get a list
74like C<(1,2,3)> into being in scalar context, because the compiler knows
75the context at compile time. It would generate the scalar comma operator
76there, not the list construction version of the comma. That means it
77was never a list to start with.
78
79In general, functions in Perl that serve as wrappers for system calls
f86cebdf 80of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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81true when they succeed and C<undef> otherwise, as is usually mentioned
82in the descriptions below. This is different from the C interfaces,
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83which return C<-1> on failure. Exceptions to this rule are C<wait>,
84C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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85variable on failure. Other functions do not, except accidentally.
86
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87=head2 Perl Functions by Category
88
89Here are Perl's functions (including things that look like
5a964f20 90functions, like some keywords and named operators)
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91arranged by category. Some functions appear in more
92than one place.
93
13a2d996 94=over 4
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95
96=item Functions for SCALARs or strings
97
22fae026 98C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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99C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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101
102=item Regular expressions and pattern matching
103
ab4f32c2 104C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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105
106=item Numeric functions
107
22fae026
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108C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109C<sin>, C<sqrt>, C<srand>
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110
111=item Functions for real @ARRAYs
112
22fae026 113C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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114
115=item Functions for list data
116
ab4f32c2 117C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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118
119=item Functions for real %HASHes
120
22fae026 121C<delete>, C<each>, C<exists>, C<keys>, C<values>
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122
123=item Input and output functions
124
22fae026
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125C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
129C<warn>, C<write>
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130
131=item Functions for fixed length data or records
132
22fae026 133C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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134
135=item Functions for filehandles, files, or directories
136
22fae026 137C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
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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
22fae026
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168C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
169C<untie>, C<use>
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170
171=item Low-level socket functions
172
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
121910a4 294 -T File is an ASCII text file (heuristic guess).
2cdbc966 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>
9124316e 342or C<-B> is used on a filehandle, the current IO buffer is examined
19799a22 343rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 344file, or a file at EOF when testing a filehandle. Because you have to
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345read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 347
19799a22 348If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 349the special filehandle consisting of a solitary underline, then the stat
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350structure of the previous file test (or stat operator) is used, saving
351a system call. (This doesn't work with C<-t>, and you need to remember
352that lstat() and C<-l> will leave values in the stat structure for the
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
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389specified number of wallclock seconds have elapsed. If SECONDS is not
390specified, the value stored in C<$_> is used. (On some machines,
391unfortunately, the elapsed time may be up to one second less or more
392than you specified because of how seconds are counted, and process
393scheduling may delay the delivery of the signal even further.)
394
395Only one timer may be counting at once. Each call disables the
396previous timer, and an argument of C<0> may be supplied to cancel the
397previous timer without starting a new one. The returned value is the
398amount of time remaining on the previous timer.
a0d0e21e 399
4633a7c4 400For delays of finer granularity than one second, you may use Perl's
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401four-argument version of select() leaving the first three arguments
402undefined, or you might be able to use the C<syscall> interface to
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403access setitimer(2) if your system supports it. The Time::HiRes
404module (from CPAN, and starting from Perl 5.8 part of the standard
405distribution) may also prove useful.
2b5ab1e7 406
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407It is usually a mistake to intermix C<alarm> and C<sleep> calls.
408(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 409
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410If you want to use C<alarm> to time out a system call you need to use an
411C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 412fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 413restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 414modulo the caveats given in L<perlipc/"Signals">.
ff68c719 415
416 eval {
f86cebdf 417 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 418 alarm $timeout;
ff68c719 419 $nread = sysread SOCKET, $buffer, $size;
36477c24 420 alarm 0;
ff68c719 421 };
ff68c719 422 if ($@) {
f86cebdf 423 die unless $@ eq "alarm\n"; # propagate unexpected errors
ff68c719 424 # timed out
425 }
426 else {
427 # didn't
428 }
429
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430=item atan2 Y,X
431
432Returns the arctangent of Y/X in the range -PI to PI.
433
ca6e1c26 434For the tangent operation, you may use the C<Math::Trig::tan>
28757baa 435function, or use the familiar relation:
436
437 sub tan { sin($_[0]) / cos($_[0]) }
438
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439=item bind SOCKET,NAME
440
441Binds a network address to a socket, just as the bind system call
19799a22 442does. Returns true if it succeeded, false otherwise. NAME should be a
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443packed address of the appropriate type for the socket. See the examples in
444L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 445
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446=item binmode FILEHANDLE, DISCIPLINE
447
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448=item binmode FILEHANDLE
449
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450Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
451on systems where the run-time libraries distinguish between binary and
30168b04 452text files. If FILEHANDLE is an expression, the value is taken as the
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453name of the filehandle. DISCIPLINE can be either of C<:raw> for
454binary mode or C<:crlf> for "text" mode. If the DISCIPLINE is
455omitted, it defaults to C<:raw>. Returns true on success, C<undef> on
456failure. The C<:raw> are C<:clrf>, and any other directives of the
457form C<:...>, are called I/O I<disciplines>.
30168b04 458
ed53a2bb
JH
459The C<open> pragma can be used to establish default I/O disciplines.
460See L<open>.
461
462In general, binmode() should be called after open() but before any I/O
463is done on the filehandle. Calling binmode() will flush any possibly
464pending buffered input or output data on the handle. The only
465exception to this is the C<:encoding> discipline that changes
466the default character encoding of the handle, see L<open>.
467The C<:encoding> discipline sometimes needs to be called in
468mid-stream, and it doesn't flush the stream.
16fe6d59 469
16fe6d59
GS
470On some systems binmode() is necessary when you're not working with a
471text file. For the sake of portability it is a good idea to always use
472it when appropriate, and to never use it when it isn't appropriate.
30168b04
GS
473
474In other words: Regardless of platform, use binmode() on binary
475files, and do not use binmode() on text files.
19799a22
GS
476
477The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
478system all work together to let the programmer treat a single
479character (C<\n>) as the line terminator, irrespective of the external
480representation. On many operating systems, the native text file
481representation matches the internal representation, but on some
482platforms the external representation of C<\n> is made up of more than
483one character.
484
68bd7414
NIS
485Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
486character to end each line in the external representation of text (even
5e12dbfa 487though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
68bd7414
NIS
488on Unix and most VMS files). Consequently binmode() has no effect on
489these operating systems. In other systems like OS/2, DOS and the various
490flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>, but
491what's stored in text files are the two characters C<\cM\cJ>. That means
492that, if you don't use binmode() on these systems, C<\cM\cJ> sequences on
493disk will be converted to C<\n> on input, and any C<\n> in your program
5e12dbfa
PP
494will be converted back to C<\cM\cJ> on output. This is what you want for
495text files, but it can be disastrous for binary files.
30168b04
GS
496
497Another consequence of using binmode() (on some systems) is that
498special end-of-file markers will be seen as part of the data stream.
499For systems from the Microsoft family this means that if your binary
4375e838 500data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
501the file, unless you use binmode().
502
503binmode() is not only important for readline() and print() operations,
504but also when using read(), seek(), sysread(), syswrite() and tell()
505(see L<perlport> for more details). See the C<$/> and C<$\> variables
506in L<perlvar> for how to manually set your input and output
507line-termination sequences.
a0d0e21e 508
4633a7c4 509=item bless REF,CLASSNAME
a0d0e21e
LW
510
511=item bless REF
512
2b5ab1e7
TC
513This function tells the thingy referenced by REF that it is now an object
514in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 515is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7
TC
516it returns the reference for convenience. Always use the two-argument
517version if the function doing the blessing might be inherited by a
518derived class. See L<perltoot> and L<perlobj> for more about the blessing
519(and blessings) of objects.
a0d0e21e 520
57668c4d 521Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7
TC
522Namespaces with all lowercase names are considered reserved for
523Perl pragmata. Builtin types have all uppercase names, so to prevent
524confusion, you may wish to avoid such package names as well. Make sure
525that CLASSNAME is a true value.
60ad88b8
GS
526
527See L<perlmod/"Perl Modules">.
528
a0d0e21e
LW
529=item caller EXPR
530
531=item caller
532
5a964f20 533Returns the context of the current subroutine call. In scalar context,
28757baa 534returns the caller's package name if there is a caller, that is, if
19799a22 535we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 536otherwise. In list context, returns
a0d0e21e 537
748a9306 538 ($package, $filename, $line) = caller;
a0d0e21e
LW
539
540With EXPR, it returns some extra information that the debugger uses to
541print a stack trace. The value of EXPR indicates how many call frames
542to go back before the current one.
543
f3aa04c2 544 ($package, $filename, $line, $subroutine, $hasargs,
e476b1b5 545 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 546
951ba7fe 547Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 548call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 549C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 550C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 551C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 552$filename is C<(eval)>, but $evaltext is undefined. (Note also that
dc848c6f 553each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
277ddfaf 554frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
e476b1b5
GS
555frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
556was compiled with. The C<$hints> and C<$bitmask> values are subject to
557change between versions of Perl, and are not meant for external use.
748a9306
LW
558
559Furthermore, when called from within the DB package, caller returns more
7660c0ab 560detailed information: it sets the list variable C<@DB::args> to be the
54310121 561arguments with which the subroutine was invoked.
748a9306 562
7660c0ab 563Be aware that the optimizer might have optimized call frames away before
19799a22 564C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 565might not return information about the call frame you expect it do, for
b76cc8ba 566C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 567previous time C<caller> was called.
7660c0ab 568
a0d0e21e
LW
569=item chdir EXPR
570
ffce7b87 571Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 572changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 573changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
574variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
575neither is set, C<chdir> does nothing. It returns true upon success,
576false otherwise. See the example under C<die>.
a0d0e21e
LW
577
578=item chmod LIST
579
580Changes the permissions of a list of files. The first element of the
4633a7c4 581list must be the numerical mode, which should probably be an octal
2f9daede
TP
582number, and which definitely should I<not> a string of octal digits:
583C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 584successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
585
586 $cnt = chmod 0755, 'foo', 'bar';
587 chmod 0755, @executables;
f86cebdf
GS
588 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
589 # --w----r-T
2f9daede
TP
590 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
591 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 592
ca6e1c26
JH
593You can also import the symbolic C<S_I*> constants from the Fcntl
594module:
595
596 use Fcntl ':mode';
597
598 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
599 # This is identical to the chmod 0755 of the above example.
600
a0d0e21e
LW
601=item chomp VARIABLE
602
313c9f5c 603=item chomp( LIST )
a0d0e21e
LW
604
605=item chomp
606
2b5ab1e7
TC
607This safer version of L</chop> removes any trailing string
608that corresponds to the current value of C<$/> (also known as
28757baa 609$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
610number of characters removed from all its arguments. It's often used to
611remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
612that the final record may be missing its newline. When in paragraph
613mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
614When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
615a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 616remove anything.
19799a22 617If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
618
619 while (<>) {
620 chomp; # avoid \n on last field
621 @array = split(/:/);
5a964f20 622 # ...
a0d0e21e
LW
623 }
624
4bf21a6d
RD
625If VARIABLE is a hash, it chomps the hash's values, but not its keys.
626
a0d0e21e
LW
627You can actually chomp anything that's an lvalue, including an assignment:
628
629 chomp($cwd = `pwd`);
630 chomp($answer = <STDIN>);
631
632If you chomp a list, each element is chomped, and the total number of
633characters removed is returned.
634
635=item chop VARIABLE
636
313c9f5c 637=item chop( LIST )
a0d0e21e
LW
638
639=item chop
640
641Chops off the last character of a string and returns the character
5b3eff12 642chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 643scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
644If VARIABLE is a hash, it chops the hash's values, but not its keys.
645
5b3eff12 646You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
647
648If you chop a list, each element is chopped. Only the value of the
19799a22 649last C<chop> is returned.
a0d0e21e 650
19799a22 651Note that C<chop> returns the last character. To return all but the last
748a9306
LW
652character, use C<substr($string, 0, -1)>.
653
a0d0e21e
LW
654=item chown LIST
655
656Changes the owner (and group) of a list of files. The first two
19799a22
GS
657elements of the list must be the I<numeric> uid and gid, in that
658order. A value of -1 in either position is interpreted by most
659systems to leave that value unchanged. Returns the number of files
660successfully changed.
a0d0e21e
LW
661
662 $cnt = chown $uid, $gid, 'foo', 'bar';
663 chown $uid, $gid, @filenames;
664
54310121 665Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
666
667 print "User: ";
19799a22 668 chomp($user = <STDIN>);
5a964f20 669 print "Files: ";
19799a22 670 chomp($pattern = <STDIN>);
a0d0e21e
LW
671
672 ($login,$pass,$uid,$gid) = getpwnam($user)
673 or die "$user not in passwd file";
674
5a964f20 675 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
676 chown $uid, $gid, @ary;
677
54310121 678On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
679file unless you're the superuser, although you should be able to change
680the group to any of your secondary groups. On insecure systems, these
681restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
682On POSIX systems, you can detect this condition this way:
683
684 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
685 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 686
a0d0e21e
LW
687=item chr NUMBER
688
54310121 689=item chr
bbce6d69 690
a0d0e21e 691Returns the character represented by that NUMBER in the character set.
a0ed51b3 692For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
121910a4
JH
693chr(0x263a) is a Unicode smiley face. Note that characters from 127
694to 255 (inclusive) are by default not encoded in Unicode for backward
695compatibility reasons (but see L<encoding>).
aaa68c4a 696
b76cc8ba 697For the reverse, use L</ord>.
121910a4 698See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 699
7660c0ab 700If NUMBER is omitted, uses C<$_>.
bbce6d69 701
a0d0e21e
LW
702=item chroot FILENAME
703
54310121 704=item chroot
bbce6d69 705
5a964f20 706This function works like the system call by the same name: it makes the
4633a7c4 707named directory the new root directory for all further pathnames that
951ba7fe 708begin with a C</> by your process and all its children. (It doesn't
28757baa 709change your current working directory, which is unaffected.) For security
4633a7c4 710reasons, this call is restricted to the superuser. If FILENAME is
19799a22 711omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
712
713=item close FILEHANDLE
714
6a518fbc
TP
715=item close
716
9124316e
JH
717Closes the file or pipe associated with the file handle, returning
718true only if IO buffers are successfully flushed and closes the system
719file descriptor. Closes the currently selected filehandle if the
720argument is omitted.
fb73857a 721
722You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
723another C<open> on it, because C<open> will close it for you. (See
724C<open>.) However, an explicit C<close> on an input file resets the line
725counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 726
19799a22
GS
727If the file handle came from a piped open C<close> will additionally
728return false if one of the other system calls involved fails or if the
fb73857a 729program exits with non-zero status. (If the only problem was that the
b76cc8ba 730program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 731also waits for the process executing on the pipe to complete, in case you
b76cc8ba 732want to look at the output of the pipe afterwards, and
2b5ab1e7 733implicitly puts the exit status value of that command into C<$?>.
5a964f20 734
73689b13
GS
735Prematurely closing the read end of a pipe (i.e. before the process
736writing to it at the other end has closed it) will result in a
737SIGPIPE being delivered to the writer. If the other end can't
738handle that, be sure to read all the data before closing the pipe.
739
fb73857a 740Example:
a0d0e21e 741
fb73857a 742 open(OUTPUT, '|sort >foo') # pipe to sort
743 or die "Can't start sort: $!";
5a964f20 744 #... # print stuff to output
fb73857a 745 close OUTPUT # wait for sort to finish
746 or warn $! ? "Error closing sort pipe: $!"
747 : "Exit status $? from sort";
748 open(INPUT, 'foo') # get sort's results
749 or die "Can't open 'foo' for input: $!";
a0d0e21e 750
5a964f20
TC
751FILEHANDLE may be an expression whose value can be used as an indirect
752filehandle, usually the real filehandle name.
a0d0e21e
LW
753
754=item closedir DIRHANDLE
755
19799a22 756Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
757system call.
758
759DIRHANDLE may be an expression whose value can be used as an indirect
760dirhandle, usually the real dirhandle name.
a0d0e21e
LW
761
762=item connect SOCKET,NAME
763
764Attempts to connect to a remote socket, just as the connect system call
19799a22 765does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
766packed address of the appropriate type for the socket. See the examples in
767L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 768
cb1a09d0
AD
769=item continue BLOCK
770
771Actually a flow control statement rather than a function. If there is a
98293880
JH
772C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
773C<foreach>), it is always executed just before the conditional is about to
774be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
775it can be used to increment a loop variable, even when the loop has been
776continued via the C<next> statement (which is similar to the C C<continue>
777statement).
778
98293880 779C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
780block. C<last> and C<redo> will behave as if they had been executed within
781the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
782block, it may be more entertaining.
783
784 while (EXPR) {
785 ### redo always comes here
786 do_something;
787 } continue {
788 ### next always comes here
789 do_something_else;
790 # then back the top to re-check EXPR
791 }
792 ### last always comes here
793
794Omitting the C<continue> section is semantically equivalent to using an
19799a22 795empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
796to check the condition at the top of the loop.
797
a0d0e21e
LW
798=item cos EXPR
799
d6217f1e
GS
800=item cos
801
5a964f20 802Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 803takes cosine of C<$_>.
a0d0e21e 804
ca6e1c26 805For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa 806function, or use this relation:
807
808 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
809
a0d0e21e
LW
810=item crypt PLAINTEXT,SALT
811
f86cebdf 812Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
813(assuming that you actually have a version there that has not been
814extirpated as a potential munition). This can prove useful for checking
815the password file for lousy passwords, amongst other things. Only the
816guys wearing white hats should do this.
a0d0e21e 817
85c16d83
JH
818Note that C<crypt> is intended to be a one-way function, much like
819breaking eggs to make an omelette. There is no (known) corresponding
820decrypt function (in other words, the crypt() is a one-way hash
821function). As a result, this function isn't all that useful for
11155c91 822cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 823
85c16d83
JH
824When verifying an existing encrypted string you should use the
825encrypted text as the salt (like C<crypt($plain, $crypted) eq
826$crypted>). This allows your code to work with the standard C<crypt>
827and with more exotic implementations. In other words, do not assume
828anything about the returned string itself, or how many bytes in
829the encrypted string matter.
830
831Traditionally the result is a string of 13 bytes: two first bytes of
832the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
833the first eight bytes of the encrypted string mattered, but
834alternative hashing schemes (like MD5), higher level security schemes
835(like C2), and implementations on non-UNIX platforms may produce
836different strings.
837
838When choosing a new salt create a random two character string whose
839characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
840'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
e71965be 841
a0d0e21e
LW
842Here's an example that makes sure that whoever runs this program knows
843their own password:
844
845 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
846
847 system "stty -echo";
848 print "Password: ";
e71965be 849 chomp($word = <STDIN>);
a0d0e21e
LW
850 print "\n";
851 system "stty echo";
852
e71965be 853 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
854 die "Sorry...\n";
855 } else {
856 print "ok\n";
54310121 857 }
a0d0e21e 858
9f8f0c9d 859Of course, typing in your own password to whoever asks you
748a9306 860for it is unwise.
a0d0e21e 861
19799a22
GS
862The L<crypt> function is unsuitable for encrypting large quantities
863of data, not least of all because you can't get the information
864back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
865on your favorite CPAN mirror for a slew of potentially useful
866modules.
867
d1be9408 868If using crypt() on a Unicode string (which potentially has
85c16d83
JH
869characters with codepoints above 255), Perl tries to make sense of
870the situation by using only the low eight bits of the characters when
871calling crypt().
872
aa689395 873=item dbmclose HASH
a0d0e21e 874
19799a22 875[This function has been largely superseded by the C<untie> function.]
a0d0e21e 876
aa689395 877Breaks the binding between a DBM file and a hash.
a0d0e21e 878
19799a22 879=item dbmopen HASH,DBNAME,MASK
a0d0e21e 880
19799a22 881[This function has been largely superseded by the C<tie> function.]
a0d0e21e 882
7b8d334a 883This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
884hash. HASH is the name of the hash. (Unlike normal C<open>, the first
885argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395 886is the name of the database (without the F<.dir> or F<.pag> extension if
887any). If the database does not exist, it is created with protection
19799a22
GS
888specified by MASK (as modified by the C<umask>). If your system supports
889only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 890program. In older versions of Perl, if your system had neither DBM nor
19799a22 891ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395 892sdbm(3).
893
894If you don't have write access to the DBM file, you can only read hash
895variables, not set them. If you want to test whether you can write,
19799a22 896either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 897which will trap the error.
a0d0e21e 898
19799a22
GS
899Note that functions such as C<keys> and C<values> may return huge lists
900when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
901function to iterate over large DBM files. Example:
902
903 # print out history file offsets
904 dbmopen(%HIST,'/usr/lib/news/history',0666);
905 while (($key,$val) = each %HIST) {
906 print $key, ' = ', unpack('L',$val), "\n";
907 }
908 dbmclose(%HIST);
909
cb1a09d0 910See also L<AnyDBM_File> for a more general description of the pros and
184e9718 911cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 912rich implementation.
4633a7c4 913
2b5ab1e7
TC
914You can control which DBM library you use by loading that library
915before you call dbmopen():
916
917 use DB_File;
918 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
919 or die "Can't open netscape history file: $!";
920
a0d0e21e
LW
921=item defined EXPR
922
54310121 923=item defined
bbce6d69 924
2f9daede
TP
925Returns a Boolean value telling whether EXPR has a value other than
926the undefined value C<undef>. If EXPR is not present, C<$_> will be
927checked.
928
929Many operations return C<undef> to indicate failure, end of file,
930system error, uninitialized variable, and other exceptional
931conditions. This function allows you to distinguish C<undef> from
932other values. (A simple Boolean test will not distinguish among
7660c0ab 933C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 934false.) Note that since C<undef> is a valid scalar, its presence
19799a22 935doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TP
936returns C<undef> when its argument is an empty array, I<or> when the
937element to return happens to be C<undef>.
938
f10b0346
GS
939You may also use C<defined(&func)> to check whether subroutine C<&func>
940has ever been defined. The return value is unaffected by any forward
847c7ebe
DD
941declarations of C<&foo>. Note that a subroutine which is not defined
942may still be callable: its package may have an C<AUTOLOAD> method that
943makes it spring into existence the first time that it is called -- see
944L<perlsub>.
f10b0346
GS
945
946Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
947used to report whether memory for that aggregate has ever been
948allocated. This behavior may disappear in future versions of Perl.
949You should instead use a simple test for size:
950
951 if (@an_array) { print "has array elements\n" }
952 if (%a_hash) { print "has hash members\n" }
2f9daede
TP
953
954When used on a hash element, it tells you whether the value is defined,
dc848c6f 955not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 956purpose.
a0d0e21e
LW
957
958Examples:
959
960 print if defined $switch{'D'};
961 print "$val\n" while defined($val = pop(@ary));
962 die "Can't readlink $sym: $!"
963 unless defined($value = readlink $sym);
a0d0e21e 964 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 965 $debugging = 0 unless defined $debugging;
a0d0e21e 966
19799a22 967Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 968discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 969defined values. For example, if you say
a5f75d66
AD
970
971 "ab" =~ /a(.*)b/;
972
7660c0ab 973The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 974matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 975matched something that happened to be zero characters long. This is all
a5f75d66 976very above-board and honest. When a function returns an undefined value,
2f9daede 977it's an admission that it couldn't give you an honest answer. So you
19799a22 978should use C<defined> only when you're questioning the integrity of what
7660c0ab 979you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TP
980what you want.
981
dc848c6f 982See also L</undef>, L</exists>, L</ref>.
2f9daede 983
a0d0e21e
LW
984=item delete EXPR
985
01020589
GS
986Given an expression that specifies a hash element, array element, hash slice,
987or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 988In the case of an array, if the array elements happen to be at the end,
b76cc8ba 989the size of the array will shrink to the highest element that tests
8216c1fd 990true for exists() (or 0 if no such element exists).
a0d0e21e 991
01020589
GS
992Returns each element so deleted or the undefined value if there was no such
993element. Deleting from C<$ENV{}> modifies the environment. Deleting from
994a hash tied to a DBM file deletes the entry from the DBM file. Deleting
995from a C<tie>d hash or array may not necessarily return anything.
996
8ea97a1e
GS
997Deleting an array element effectively returns that position of the array
998to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
999element with exists() will return false. Note that deleting array
1000elements in the middle of an array will not shift the index of the ones
1001after them down--use splice() for that. See L</exists>.
8ea97a1e 1002
01020589 1003The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1004
5f05dabc 1005 foreach $key (keys %HASH) {
1006 delete $HASH{$key};
a0d0e21e
LW
1007 }
1008
01020589
GS
1009 foreach $index (0 .. $#ARRAY) {
1010 delete $ARRAY[$index];
1011 }
1012
1013And so do these:
5f05dabc 1014
01020589
GS
1015 delete @HASH{keys %HASH};
1016
9740c838 1017 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1018
2b5ab1e7 1019But both of these are slower than just assigning the empty list
01020589
GS
1020or undefining %HASH or @ARRAY:
1021
1022 %HASH = (); # completely empty %HASH
1023 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1024
01020589
GS
1025 @ARRAY = (); # completely empty @ARRAY
1026 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1027
1028Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1029operation is a hash element, array element, hash slice, or array slice
1030lookup:
a0d0e21e
LW
1031
1032 delete $ref->[$x][$y]{$key};
5f05dabc 1033 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1034
01020589
GS
1035 delete $ref->[$x][$y][$index];
1036 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1037
a0d0e21e
LW
1038=item die LIST
1039
19799a22
GS
1040Outside an C<eval>, prints the value of LIST to C<STDERR> and
1041exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1042exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1043status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1044an C<eval(),> the error message is stuffed into C<$@> and the
1045C<eval> is terminated with the undefined value. This makes
1046C<die> the way to raise an exception.
a0d0e21e
LW
1047
1048Equivalent examples:
1049
1050 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1051 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
1052
1053If the value of EXPR does not end in a newline, the current script line
1054number and input line number (if any) are also printed, and a newline
883faa13
GS
1055is supplied. Note that the "input line number" (also known as "chunk")
1056is subject to whatever notion of "line" happens to be currently in
1057effect, and is also available as the special variable C<$.>.
1058See L<perlvar/"$/"> and L<perlvar/"$.">.
1059
1060Hint: sometimes appending C<", stopped"> to your message
7660c0ab 1061will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
1062appended. Suppose you are running script "canasta".
1063
1064 die "/etc/games is no good";
1065 die "/etc/games is no good, stopped";
1066
1067produce, respectively
1068
1069 /etc/games is no good at canasta line 123.
1070 /etc/games is no good, stopped at canasta line 123.
1071
2b5ab1e7 1072See also exit(), warn(), and the Carp module.
a0d0e21e 1073
7660c0ab
A
1074If LIST is empty and C<$@> already contains a value (typically from a
1075previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a 1076This is useful for propagating exceptions:
1077
1078 eval { ... };
1079 die unless $@ =~ /Expected exception/;
1080
ad216e65
JH
1081If LIST is empty and C<$@> contains an object reference that has a
1082C<PROPAGATE> method, that method will be called with additional file
1083and line number parameters. The return value replaces the value in
1084C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
1085were called.
1086
7660c0ab 1087If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1088
52531d10
GS
1089die() can also be called with a reference argument. If this happens to be
1090trapped within an eval(), $@ contains the reference. This behavior permits
1091a more elaborate exception handling implementation using objects that
4375e838 1092maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1093is sometimes preferable to matching particular string values of $@ using
1094regular expressions. Here's an example:
1095
1096 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1097 if ($@) {
1098 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1099 # handle Some::Module::Exception
1100 }
1101 else {
1102 # handle all other possible exceptions
1103 }
1104 }
1105
19799a22 1106Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1107them, you may want to overload stringification operations on such custom
1108exception objects. See L<overload> for details about that.
1109
19799a22
GS
1110You can arrange for a callback to be run just before the C<die>
1111does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1112handler will be called with the error text and can change the error
1113message, if it sees fit, by calling C<die> again. See
1114L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1115L<"eval BLOCK"> for some examples. Although this feature was meant
1116to be run only right before your program was to exit, this is not
1117currently the case--the C<$SIG{__DIE__}> hook is currently called
1118even inside eval()ed blocks/strings! If one wants the hook to do
1119nothing in such situations, put
fb73857a 1120
1121 die @_ if $^S;
1122
19799a22
GS
1123as the first line of the handler (see L<perlvar/$^S>). Because
1124this promotes strange action at a distance, this counterintuitive
b76cc8ba 1125behavior may be fixed in a future release.
774d564b 1126
a0d0e21e
LW
1127=item do BLOCK
1128
1129Not really a function. Returns the value of the last command in the
1130sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1131modifier, executes the BLOCK once before testing the loop condition.
1132(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1133
4968c1e4 1134C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1135C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1136See L<perlsyn> for alternative strategies.
4968c1e4 1137
a0d0e21e
LW
1138=item do SUBROUTINE(LIST)
1139
1140A deprecated form of subroutine call. See L<perlsub>.
1141
1142=item do EXPR
1143
1144Uses the value of EXPR as a filename and executes the contents of the
1145file as a Perl script. Its primary use is to include subroutines
1146from a Perl subroutine library.
1147
1148 do 'stat.pl';
1149
1150is just like
1151
986b19de 1152 eval `cat stat.pl`;
a0d0e21e 1153
2b5ab1e7
TC
1154except that it's more efficient and concise, keeps track of the current
1155filename for error messages, searches the @INC libraries, and updates
1156C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1157variables. It also differs in that code evaluated with C<do FILENAME>
1158cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1159same, however, in that it does reparse the file every time you call it,
1160so you probably don't want to do this inside a loop.
a0d0e21e 1161
8e30cc93 1162If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1163error. If C<do> can read the file but cannot compile it, it
8e30cc93
MG
1164returns undef and sets an error message in C<$@>. If the file is
1165successfully compiled, C<do> returns the value of the last expression
1166evaluated.
1167
a0d0e21e 1168Note that inclusion of library modules is better done with the
19799a22 1169C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1170and raise an exception if there's a problem.
a0d0e21e 1171
5a964f20
TC
1172You might like to use C<do> to read in a program configuration
1173file. Manual error checking can be done this way:
1174
b76cc8ba 1175 # read in config files: system first, then user
f86cebdf 1176 for $file ("/share/prog/defaults.rc",
b76cc8ba 1177 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1178 {
5a964f20 1179 unless ($return = do $file) {
f86cebdf
GS
1180 warn "couldn't parse $file: $@" if $@;
1181 warn "couldn't do $file: $!" unless defined $return;
1182 warn "couldn't run $file" unless $return;
5a964f20
TC
1183 }
1184 }
1185
a0d0e21e
LW
1186=item dump LABEL
1187
1614b0e3
JD
1188=item dump
1189
19799a22
GS
1190This function causes an immediate core dump. See also the B<-u>
1191command-line switch in L<perlrun>, which does the same thing.
1192Primarily this is so that you can use the B<undump> program (not
1193supplied) to turn your core dump into an executable binary after
1194having initialized all your variables at the beginning of the
1195program. When the new binary is executed it will begin by executing
1196a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1197Think of it as a goto with an intervening core dump and reincarnation.
1198If C<LABEL> is omitted, restarts the program from the top.
1199
1200B<WARNING>: Any files opened at the time of the dump will I<not>
1201be open any more when the program is reincarnated, with possible
b76cc8ba 1202resulting confusion on the part of Perl.
19799a22
GS
1203
1204This function is now largely obsolete, partly because it's very
1205hard to convert a core file into an executable, and because the
1206real compiler backends for generating portable bytecode and compilable
1207C code have superseded it.
1208
1209If you're looking to use L<dump> to speed up your program, consider
1210generating bytecode or native C code as described in L<perlcc>. If
1211you're just trying to accelerate a CGI script, consider using the
210b36aa 1212C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1213You might also consider autoloading or selfloading, which at least
b76cc8ba 1214make your program I<appear> to run faster.
5a964f20 1215
aa689395 1216=item each HASH
1217
5a964f20 1218When called in list context, returns a 2-element list consisting of the
aa689395 1219key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1220it. When called in scalar context, returns only the key for the next
e902a979 1221element in the hash.
2f9daede 1222
ab192400
GS
1223Entries are returned in an apparently random order. The actual random
1224order is subject to change in future versions of perl, but it is guaranteed
19799a22 1225to be in the same order as either the C<keys> or C<values> function
ab192400
GS
1226would produce on the same (unmodified) hash.
1227
1228When the hash is entirely read, a null array is returned in list context
19799a22
GS
1229(which when assigned produces a false (C<0>) value), and C<undef> in
1230scalar context. The next call to C<each> after that will start iterating
1231again. There is a single iterator for each hash, shared by all C<each>,
1232C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TP
1233reading all the elements from the hash, or by evaluating C<keys HASH> or
1234C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1235iterating over it, you may get entries skipped or duplicated, so
1236don't. Exception: It is always safe to delete the item most recently
1237returned by C<each()>, which means that the following code will work:
1238
1239 while (($key, $value) = each %hash) {
1240 print $key, "\n";
1241 delete $hash{$key}; # This is safe
1242 }
aa689395 1243
f86cebdf 1244The following prints out your environment like the printenv(1) program,
aa689395 1245only in a different order:
a0d0e21e
LW
1246
1247 while (($key,$value) = each %ENV) {
1248 print "$key=$value\n";
1249 }
1250
19799a22 1251See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1252
1253=item eof FILEHANDLE
1254
4633a7c4
LW
1255=item eof ()
1256
a0d0e21e
LW
1257=item eof
1258
1259Returns 1 if the next read on FILEHANDLE will return end of file, or if
1260FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1261gives the real filehandle. (Note that this function actually
19799a22 1262reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1263interactive context.) Do not read from a terminal file (or call
19799a22 1264C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1265as terminals may lose the end-of-file condition if you do.
1266
820475bd
GS
1267An C<eof> without an argument uses the last file read. Using C<eof()>
1268with empty parentheses is very different. It refers to the pseudo file
1269formed from the files listed on the command line and accessed via the
61eff3bc
JH
1270C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1271as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd
GS
1272used will cause C<@ARGV> to be examined to determine if input is
1273available.
1274
61eff3bc 1275In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1276detect the end of each file, C<eof()> will only detect the end of the
1277last file. Examples:
a0d0e21e 1278
748a9306
LW
1279 # reset line numbering on each input file
1280 while (<>) {
b76cc8ba 1281 next if /^\s*#/; # skip comments
748a9306 1282 print "$.\t$_";
5a964f20
TC
1283 } continue {
1284 close ARGV if eof; # Not eof()!
748a9306
LW
1285 }
1286
a0d0e21e
LW
1287 # insert dashes just before last line of last file
1288 while (<>) {
5a964f20 1289 if (eof()) { # check for end of current file
a0d0e21e 1290 print "--------------\n";
2b5ab1e7 1291 close(ARGV); # close or last; is needed if we
748a9306 1292 # are reading from the terminal
a0d0e21e
LW
1293 }
1294 print;
1295 }
1296
a0d0e21e 1297Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1298input operators typically return C<undef> when they run out of data, or if
1299there was an error.
a0d0e21e
LW
1300
1301=item eval EXPR
1302
1303=item eval BLOCK
1304
c7cc6f1c
GS
1305In the first form, the return value of EXPR is parsed and executed as if it
1306were a little Perl program. The value of the expression (which is itself
5a964f20 1307determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1308errors, executed in the lexical context of the current Perl program, so
1309that any variable settings or subroutine and format definitions remain
1310afterwards. Note that the value is parsed every time the eval executes.
1311If EXPR is omitted, evaluates C<$_>. This form is typically used to
1312delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1313
1314In the second form, the code within the BLOCK is parsed only once--at the
1315same time the code surrounding the eval itself was parsed--and executed
1316within the context of the current Perl program. This form is typically
1317used to trap exceptions more efficiently than the first (see below), while
1318also providing the benefit of checking the code within BLOCK at compile
1319time.
1320
1321The final semicolon, if any, may be omitted from the value of EXPR or within
1322the BLOCK.
1323
1324In both forms, the value returned is the value of the last expression
5a964f20 1325evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1326as with subroutines. The expression providing the return value is evaluated
5a964f20 1327in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1328See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1329
19799a22
GS
1330If there is a syntax error or runtime error, or a C<die> statement is
1331executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1332error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1333string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c
GS
1334warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1335To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1336L</warn> and L<perlvar>.
a0d0e21e 1337
19799a22
GS
1338Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1339determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1340is implemented. It is also Perl's exception trapping mechanism, where
1341the die operator is used to raise exceptions.
1342
1343If the code to be executed doesn't vary, you may use the eval-BLOCK
1344form to trap run-time errors without incurring the penalty of
1345recompiling each time. The error, if any, is still returned in C<$@>.
1346Examples:
1347
54310121 1348 # make divide-by-zero nonfatal
a0d0e21e
LW
1349 eval { $answer = $a / $b; }; warn $@ if $@;
1350
1351 # same thing, but less efficient
1352 eval '$answer = $a / $b'; warn $@ if $@;
1353
1354 # a compile-time error
5a964f20 1355 eval { $answer = }; # WRONG
a0d0e21e
LW
1356
1357 # a run-time error
1358 eval '$answer ='; # sets $@
1359
2b5ab1e7
TC
1360Due to the current arguably broken state of C<__DIE__> hooks, when using
1361the C<eval{}> form as an exception trap in libraries, you may wish not
1362to trigger any C<__DIE__> hooks that user code may have installed.
1363You can use the C<local $SIG{__DIE__}> construct for this purpose,
1364as shown in this example:
774d564b 1365
1366 # a very private exception trap for divide-by-zero
f86cebdf
GS
1367 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1368 warn $@ if $@;
774d564b 1369
1370This is especially significant, given that C<__DIE__> hooks can call
19799a22 1371C<die> again, which has the effect of changing their error messages:
774d564b 1372
1373 # __DIE__ hooks may modify error messages
1374 {
f86cebdf
GS
1375 local $SIG{'__DIE__'} =
1376 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1377 eval { die "foo lives here" };
1378 print $@ if $@; # prints "bar lives here"
774d564b 1379 }
1380
19799a22 1381Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1382may be fixed in a future release.
1383
19799a22 1384With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1385being looked at when:
1386
1387 eval $x; # CASE 1
1388 eval "$x"; # CASE 2
1389
1390 eval '$x'; # CASE 3
1391 eval { $x }; # CASE 4
1392
5a964f20 1393 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1394 $$x++; # CASE 6
1395
2f9daede 1396Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1397the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1398the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1399and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1400does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TP
1401purely visual reasons, but it also has the advantage of compiling at
1402compile-time instead of at run-time.) Case 5 is a place where
19799a22 1403normally you I<would> like to use double quotes, except that in this
2f9daede
TP
1404particular situation, you can just use symbolic references instead, as
1405in case 6.
a0d0e21e 1406
4968c1e4 1407C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1408C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1409
a0d0e21e
LW
1410=item exec LIST
1411
8bf3b016
GS
1412=item exec PROGRAM LIST
1413
19799a22
GS
1414The C<exec> function executes a system command I<and never returns>--
1415use C<system> instead of C<exec> if you want it to return. It fails and
1416returns false only if the command does not exist I<and> it is executed
fb73857a 1417directly instead of via your system's command shell (see below).
a0d0e21e 1418
19799a22
GS
1419Since it's a common mistake to use C<exec> instead of C<system>, Perl
1420warns you if there is a following statement which isn't C<die>, C<warn>,
1421or C<exit> (if C<-w> is set - but you always do that). If you
1422I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1423can use one of these styles to avoid the warning:
1424
5a964f20
TC
1425 exec ('foo') or print STDERR "couldn't exec foo: $!";
1426 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1427
5a964f20 1428If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1429with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1430If there is only one scalar argument or an array with one element in it,
1431the argument is checked for shell metacharacters, and if there are any,
1432the entire argument is passed to the system's command shell for parsing
1433(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1434If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1435words and passed directly to C<execvp>, which is more efficient.
19799a22 1436Examples:
a0d0e21e 1437
19799a22
GS
1438 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1439 exec "sort $outfile | uniq";
a0d0e21e
LW
1440
1441If you don't really want to execute the first argument, but want to lie
1442to the program you are executing about its own name, you can specify
1443the program you actually want to run as an "indirect object" (without a
1444comma) in front of the LIST. (This always forces interpretation of the
54310121 1445LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1446the list.) Example:
1447
1448 $shell = '/bin/csh';
1449 exec $shell '-sh'; # pretend it's a login shell
1450
1451or, more directly,
1452
1453 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1454
bb32b41a
GS
1455When the arguments get executed via the system shell, results will
1456be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1457for details.
1458
19799a22
GS
1459Using an indirect object with C<exec> or C<system> is also more
1460secure. This usage (which also works fine with system()) forces
1461interpretation of the arguments as a multivalued list, even if the
1462list had just one argument. That way you're safe from the shell
1463expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1464
1465 @args = ( "echo surprise" );
1466
2b5ab1e7 1467 exec @args; # subject to shell escapes
f86cebdf 1468 # if @args == 1
2b5ab1e7 1469 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1470
1471The first version, the one without the indirect object, ran the I<echo>
1472program, passing it C<"surprise"> an argument. The second version
1473didn't--it tried to run a program literally called I<"echo surprise">,
1474didn't find it, and set C<$?> to a non-zero value indicating failure.
1475
0f897271
GS
1476Beginning with v5.6.0, Perl will attempt to flush all files opened for
1477output before the exec, but this may not be supported on some platforms
1478(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1479in English) or call the C<autoflush()> method of C<IO::Handle> on any
1480open handles in order to avoid lost output.
1481
19799a22 1482Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1483any C<DESTROY> methods in your objects.
1484
a0d0e21e
LW
1485=item exists EXPR
1486
01020589 1487Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1488returns true if the specified element in the hash or array has ever
1489been initialized, even if the corresponding value is undefined. The
1490element is not autovivified if it doesn't exist.
a0d0e21e 1491
01020589
GS
1492 print "Exists\n" if exists $hash{$key};
1493 print "Defined\n" if defined $hash{$key};
1494 print "True\n" if $hash{$key};
1495
1496 print "Exists\n" if exists $array[$index];
1497 print "Defined\n" if defined $array[$index];
1498 print "True\n" if $array[$index];
a0d0e21e 1499
8ea97a1e 1500A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1501it exists, but the reverse doesn't necessarily hold true.
1502
afebc493
GS
1503Given an expression that specifies the name of a subroutine,
1504returns true if the specified subroutine has ever been declared, even
1505if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1506does not count as declaring it. Note that a subroutine which does not
1507exist may still be callable: its package may have an C<AUTOLOAD>
1508method that makes it spring into existence the first time that it is
1509called -- see L<perlsub>.
afebc493
GS
1510
1511 print "Exists\n" if exists &subroutine;
1512 print "Defined\n" if defined &subroutine;
1513
a0d0e21e 1514Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1515operation is a hash or array key lookup or subroutine name:
a0d0e21e 1516
2b5ab1e7
TC
1517 if (exists $ref->{A}->{B}->{$key}) { }
1518 if (exists $hash{A}{B}{$key}) { }
1519
01020589
GS
1520 if (exists $ref->{A}->{B}->[$ix]) { }
1521 if (exists $hash{A}{B}[$ix]) { }
1522
afebc493
GS
1523 if (exists &{$ref->{A}{B}{$key}}) { }
1524
01020589
GS
1525Although the deepest nested array or hash will not spring into existence
1526just because its existence was tested, any intervening ones will.
61eff3bc 1527Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1528into existence due to the existence test for the $key element above.
1529This happens anywhere the arrow operator is used, including even:
5a964f20 1530
2b5ab1e7
TC
1531 undef $ref;
1532 if (exists $ref->{"Some key"}) { }
1533 print $ref; # prints HASH(0x80d3d5c)
1534
1535This surprising autovivification in what does not at first--or even
1536second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1537release.
a0d0e21e 1538
479ba383
GS
1539See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1540on how exists() acts when used on a pseudo-hash.
e0478e5a 1541
afebc493
GS
1542Use of a subroutine call, rather than a subroutine name, as an argument
1543to exists() is an error.
1544
1545 exists &sub; # OK
1546 exists &sub(); # Error
1547
a0d0e21e
LW
1548=item exit EXPR
1549
2b5ab1e7 1550Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1551
1552 $ans = <STDIN>;
1553 exit 0 if $ans =~ /^[Xx]/;
1554
19799a22 1555See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1556universally recognized values for EXPR are C<0> for success and C<1>
1557for error; other values are subject to interpretation depending on the
1558environment in which the Perl program is running. For example, exiting
155969 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1560the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1561
19799a22
GS
1562Don't use C<exit> to abort a subroutine if there's any chance that
1563someone might want to trap whatever error happened. Use C<die> instead,
1564which can be trapped by an C<eval>.
28757baa 1565
19799a22 1566The exit() function does not always exit immediately. It calls any
2b5ab1e7 1567defined C<END> routines first, but these C<END> routines may not
19799a22 1568themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1569be called are called before the real exit. If this is a problem, you
1570can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1571See L<perlmod> for details.
5a964f20 1572
a0d0e21e
LW
1573=item exp EXPR
1574
54310121 1575=item exp
bbce6d69 1576
b76cc8ba 1577Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1578If EXPR is omitted, gives C<exp($_)>.
1579
1580=item fcntl FILEHANDLE,FUNCTION,SCALAR
1581
f86cebdf 1582Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1583
1584 use Fcntl;
1585
0ade1984 1586first to get the correct constant definitions. Argument processing and
b76cc8ba 1587value return works just like C<ioctl> below.
a0d0e21e
LW
1588For example:
1589
1590 use Fcntl;
5a964f20
TC
1591 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1592 or die "can't fcntl F_GETFL: $!";
1593
19799a22 1594You don't have to check for C<defined> on the return from C<fnctl>.
951ba7fe
GS
1595Like C<ioctl>, it maps a C<0> return from the system call into
1596C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1597in numeric context. It is also exempt from the normal B<-w> warnings
1598on improper numeric conversions.
5a964f20 1599
19799a22 1600Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1601doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1602manpage to learn what functions are available on your system.
a0d0e21e
LW
1603
1604=item fileno FILEHANDLE
1605
2b5ab1e7
TC
1606Returns the file descriptor for a filehandle, or undefined if the
1607filehandle is not open. This is mainly useful for constructing
19799a22 1608bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1609If FILEHANDLE is an expression, the value is taken as an indirect
1610filehandle, generally its name.
5a964f20 1611
b76cc8ba 1612You can use this to find out whether two handles refer to the
5a964f20
TC
1613same underlying descriptor:
1614
1615 if (fileno(THIS) == fileno(THAT)) {
1616 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1617 }
1618
1619(Filehandles connected to memory objects via new features of C<open> may
1620return undefined even though they are open.)
1621
a0d0e21e
LW
1622
1623=item flock FILEHANDLE,OPERATION
1624
19799a22
GS
1625Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1626for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1627machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1628C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1629only entire files, not records.
1630
1631Two potentially non-obvious but traditional C<flock> semantics are
1632that it waits indefinitely until the lock is granted, and that its locks
1633B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1634fewer guarantees. This means that files locked with C<flock> may be
1635modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1636your port's specific documentation, or your system-specific local manpages
1637for details. It's best to assume traditional behavior if you're writing
1638portable programs. (But if you're not, you should as always feel perfectly
1639free to write for your own system's idiosyncrasies (sometimes called
1640"features"). Slavish adherence to portability concerns shouldn't get
1641in the way of your getting your job done.)
a3cb178b 1642
8ebc5c01 1643OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1644LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1645you can use the symbolic names if you import them from the Fcntl module,
68dc0745 1646either individually, or as a group using the ':flock' tag. LOCK_SH
1647requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1648releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1649LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745 1650waiting for the lock (check the return status to see if you got it).
1651
2b5ab1e7
TC
1652To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1653before locking or unlocking it.
8ebc5c01 1654
f86cebdf 1655Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1656locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1657are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1658implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01 1659differing semantics shouldn't bite too many people.
1660
19799a22
GS
1661Note also that some versions of C<flock> cannot lock things over the
1662network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1663that. If you like you can force Perl to ignore your system's flock(2)
1664function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 1665the switch C<-Ud_flock> to the F<Configure> program when you configure
1666perl.
4633a7c4
LW
1667
1668Here's a mailbox appender for BSD systems.
a0d0e21e 1669
7e1af8bc 1670 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1671
1672 sub lock {
7e1af8bc 1673 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1674 # and, in case someone appended
1675 # while we were waiting...
1676 seek(MBOX, 0, 2);
1677 }
1678
1679 sub unlock {
7e1af8bc 1680 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1681 }
1682
1683 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1684 or die "Can't open mailbox: $!";
1685
1686 lock();
1687 print MBOX $msg,"\n\n";
1688 unlock();
1689
2b5ab1e7
TC
1690On systems that support a real flock(), locks are inherited across fork()
1691calls, whereas those that must resort to the more capricious fcntl()
1692function lose the locks, making it harder to write servers.
1693
cb1a09d0 1694See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1695
1696=item fork
1697
2b5ab1e7
TC
1698Does a fork(2) system call to create a new process running the
1699same program at the same point. It returns the child pid to the
1700parent process, C<0> to the child process, or C<undef> if the fork is
1701unsuccessful. File descriptors (and sometimes locks on those descriptors)
1702are shared, while everything else is copied. On most systems supporting
1703fork(), great care has gone into making it extremely efficient (for
1704example, using copy-on-write technology on data pages), making it the
1705dominant paradigm for multitasking over the last few decades.
5a964f20 1706
0f897271
GS
1707Beginning with v5.6.0, Perl will attempt to flush all files opened for
1708output before forking the child process, but this may not be supported
1709on some platforms (see L<perlport>). To be safe, you may need to set
1710C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1711C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1712
19799a22 1713If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1714accumulate zombies. On some systems, you can avoid this by setting
1715C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1716forking and reaping moribund children.
cb1a09d0 1717
28757baa 1718Note that if your forked child inherits system file descriptors like
1719STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1720if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1721backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1722You should reopen those to F</dev/null> if it's any issue.
28757baa 1723
cb1a09d0
AD
1724=item format
1725
19799a22 1726Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1727example:
1728
54310121 1729 format Something =
cb1a09d0
AD
1730 Test: @<<<<<<<< @||||| @>>>>>
1731 $str, $%, '$' . int($num)
1732 .
1733
1734 $str = "widget";
184e9718 1735 $num = $cost/$quantity;
cb1a09d0
AD
1736 $~ = 'Something';
1737 write;
1738
1739See L<perlform> for many details and examples.
1740
8903cb82 1741=item formline PICTURE,LIST
a0d0e21e 1742
5a964f20 1743This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1744too. It formats (see L<perlform>) a list of values according to the
1745contents of PICTURE, placing the output into the format output
7660c0ab 1746accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1747Eventually, when a C<write> is done, the contents of
a0d0e21e 1748C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1749yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1750does one C<formline> per line of form, but the C<formline> function itself
748a9306 1751doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1752that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1753You may therefore need to use multiple formlines to implement a single
1754record format, just like the format compiler.
1755
19799a22 1756Be careful if you put double quotes around the picture, because an C<@>
748a9306 1757character may be taken to mean the beginning of an array name.
19799a22 1758C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1759
1760=item getc FILEHANDLE
1761
1762=item getc
1763
1764Returns the next character from the input file attached to FILEHANDLE,
2b5ab1e7
TC
1765or the undefined value at end of file, or if there was an error.
1766If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1767efficient. However, it cannot be used by itself to fetch single
1768characters without waiting for the user to hit enter. For that, try
1769something more like:
4633a7c4
LW
1770
1771 if ($BSD_STYLE) {
1772 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1773 }
1774 else {
54310121 1775 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1776 }
1777
1778 $key = getc(STDIN);
1779
1780 if ($BSD_STYLE) {
1781 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1782 }
1783 else {
5f05dabc 1784 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1785 }
1786 print "\n";
1787
54310121 1788Determination of whether $BSD_STYLE should be set
1789is left as an exercise to the reader.
cb1a09d0 1790
19799a22 1791The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1792systems purporting POSIX compliance. See also the C<Term::ReadKey>
1793module from your nearest CPAN site; details on CPAN can be found on
1794L<perlmodlib/CPAN>.
a0d0e21e
LW
1795
1796=item getlogin
1797
5a964f20
TC
1798Implements the C library function of the same name, which on most
1799systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1800use C<getpwuid>.
a0d0e21e 1801
f86702cc 1802 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1803
19799a22
GS
1804Do not consider C<getlogin> for authentication: it is not as
1805secure as C<getpwuid>.
4633a7c4 1806
a0d0e21e
LW
1807=item getpeername SOCKET
1808
1809Returns the packed sockaddr address of other end of the SOCKET connection.
1810
4633a7c4
LW
1811 use Socket;
1812 $hersockaddr = getpeername(SOCK);
19799a22 1813 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1814 $herhostname = gethostbyaddr($iaddr, AF_INET);
1815 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1816
1817=item getpgrp PID
1818
47e29363 1819Returns the current process group for the specified PID. Use
7660c0ab 1820a PID of C<0> to get the current process group for the
4633a7c4 1821current process. Will raise an exception if used on a machine that
f86cebdf 1822doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1823group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1824does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1825
1826=item getppid
1827
1828Returns the process id of the parent process.
1829
1830=item getpriority WHICH,WHO
1831
4633a7c4
LW
1832Returns the current priority for a process, a process group, or a user.
1833(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1834machine that doesn't implement getpriority(2).
a0d0e21e
LW
1835
1836=item getpwnam NAME
1837
1838=item getgrnam NAME
1839
1840=item gethostbyname NAME
1841
1842=item getnetbyname NAME
1843
1844=item getprotobyname NAME
1845
1846=item getpwuid UID
1847
1848=item getgrgid GID
1849
1850=item getservbyname NAME,PROTO
1851
1852=item gethostbyaddr ADDR,ADDRTYPE
1853
1854=item getnetbyaddr ADDR,ADDRTYPE
1855
1856=item getprotobynumber NUMBER
1857
1858=item getservbyport PORT,PROTO
1859
1860=item getpwent
1861
1862=item getgrent
1863
1864=item gethostent
1865
1866=item getnetent
1867
1868=item getprotoent
1869
1870=item getservent
1871
1872=item setpwent
1873
1874=item setgrent
1875
1876=item sethostent STAYOPEN
1877
1878=item setnetent STAYOPEN
1879
1880=item setprotoent STAYOPEN
1881
1882=item setservent STAYOPEN
1883
1884=item endpwent
1885
1886=item endgrent
1887
1888=item endhostent
1889
1890=item endnetent
1891
1892=item endprotoent
1893
1894=item endservent
1895
1896These routines perform the same functions as their counterparts in the
5a964f20 1897system library. In list context, the return values from the
a0d0e21e
LW
1898various get routines are as follows:
1899
1900 ($name,$passwd,$uid,$gid,
6ee623d5 1901 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1902 ($name,$passwd,$gid,$members) = getgr*
1903 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1904 ($name,$aliases,$addrtype,$net) = getnet*
1905 ($name,$aliases,$proto) = getproto*
1906 ($name,$aliases,$port,$proto) = getserv*
1907
1908(If the entry doesn't exist you get a null list.)
1909
4602f195
JH
1910The exact meaning of the $gcos field varies but it usually contains
1911the real name of the user (as opposed to the login name) and other
1912information pertaining to the user. Beware, however, that in many
1913system users are able to change this information and therefore it
106325ad 1914cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
1915L<perlsec>). The $passwd and $shell, user's encrypted password and
1916login shell, are also tainted, because of the same reason.
4602f195 1917
5a964f20 1918In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1919lookup by name, in which case you get the other thing, whatever it is.
1920(If the entry doesn't exist you get the undefined value.) For example:
1921
5a964f20
TC
1922 $uid = getpwnam($name);
1923 $name = getpwuid($num);
1924 $name = getpwent();
1925 $gid = getgrnam($name);
1926 $name = getgrgid($num;
1927 $name = getgrent();
1928 #etc.
a0d0e21e 1929
4602f195
JH
1930In I<getpw*()> the fields $quota, $comment, and $expire are special
1931cases in the sense that in many systems they are unsupported. If the
1932$quota is unsupported, it is an empty scalar. If it is supported, it
1933usually encodes the disk quota. If the $comment field is unsupported,
1934it is an empty scalar. If it is supported it usually encodes some
1935administrative comment about the user. In some systems the $quota
1936field may be $change or $age, fields that have to do with password
1937aging. In some systems the $comment field may be $class. The $expire
1938field, if present, encodes the expiration period of the account or the
1939password. For the availability and the exact meaning of these fields
1940in your system, please consult your getpwnam(3) documentation and your
1941F<pwd.h> file. You can also find out from within Perl what your
1942$quota and $comment fields mean and whether you have the $expire field
1943by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1944C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1945files are only supported if your vendor has implemented them in the
1946intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
1947shadow versions if you're running under privilege or if there exists
1948the shadow(3) functions as found in System V ( this includes Solaris
1949and Linux.) Those systems which implement a proprietary shadow password
1950facility are unlikely to be supported.
6ee623d5 1951
19799a22 1952The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1953the login names of the members of the group.
1954
1955For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1956C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1957C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1958addresses returned by the corresponding system library call. In the
1959Internet domain, each address is four bytes long and you can unpack it
1960by saying something like:
1961
1962 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1963
2b5ab1e7
TC
1964The Socket library makes this slightly easier:
1965
1966 use Socket;
1967 $iaddr = inet_aton("127.1"); # or whatever address
1968 $name = gethostbyaddr($iaddr, AF_INET);
1969
1970 # or going the other way
19799a22 1971 $straddr = inet_ntoa($iaddr);
2b5ab1e7 1972
19799a22
GS
1973If you get tired of remembering which element of the return list
1974contains which return value, by-name interfaces are provided
1975in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1976C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1977and C<User::grent>. These override the normal built-ins, supplying
1978versions that return objects with the appropriate names
1979for each field. For example:
5a964f20
TC
1980
1981 use File::stat;
1982 use User::pwent;
1983 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1984
b76cc8ba
NIS
1985Even though it looks like they're the same method calls (uid),
1986they aren't, because a C<File::stat> object is different from
19799a22 1987a C<User::pwent> object.
5a964f20 1988
a0d0e21e
LW
1989=item getsockname SOCKET
1990
19799a22
GS
1991Returns the packed sockaddr address of this end of the SOCKET connection,
1992in case you don't know the address because you have several different
1993IPs that the connection might have come in on.
a0d0e21e 1994
4633a7c4
LW
1995 use Socket;
1996 $mysockaddr = getsockname(SOCK);
19799a22 1997 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 1998 printf "Connect to %s [%s]\n",
19799a22
GS
1999 scalar gethostbyaddr($myaddr, AF_INET),
2000 inet_ntoa($myaddr);
a0d0e21e
LW
2001
2002=item getsockopt SOCKET,LEVEL,OPTNAME
2003
5a964f20 2004Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
2005
2006=item glob EXPR
2007
0a753a76 2008=item glob
2009
2b5ab1e7
TC
2010Returns the value of EXPR with filename expansions such as the
2011standard Unix shell F</bin/csh> would do. This is the internal function
61eff3bc
JH
2012implementing the C<< <*.c> >> operator, but you can use it directly.
2013If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2b5ab1e7 2014discussed in more detail in L<perlop/"I/O Operators">.
a0d0e21e 2015
3a4b19e4
GS
2016Beginning with v5.6.0, this operator is implemented using the standard
2017C<File::Glob> extension. See L<File::Glob> for details.
2018
a0d0e21e
LW
2019=item gmtime EXPR
2020
d1be9408 2021Converts a time as returned by the time function to an 8-element list
54310121 2022with the time localized for the standard Greenwich time zone.
4633a7c4 2023Typically used as follows:
a0d0e21e 2024
b76cc8ba 2025 # 0 1 2 3 4 5 6 7
48a26b3a 2026 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
2027 gmtime(time);
2028
48a26b3a
GS
2029All list elements are numeric, and come straight out of the C `struct
2030tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2031specified time. $mday is the day of the month, and $mon is the month
2032itself, in the range C<0..11> with 0 indicating January and 11
2033indicating December. $year is the number of years since 1900. That
2034is, $year is C<123> in year 2023. $wday is the day of the week, with
20350 indicating Sunday and 3 indicating Wednesday. $yday is the day of
b76cc8ba 2036the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
2037
2038Note that the $year element is I<not> simply the last two digits of
2039the year. If you assume it is, then you create non-Y2K-compliant
2040programs--and you wouldn't want to do that, would you?
2f9daede 2041
abd75f24
GS
2042The proper way to get a complete 4-digit year is simply:
2043
2044 $year += 1900;
2045
2046And to get the last two digits of the year (e.g., '01' in 2001) do:
2047
2048 $year = sprintf("%02d", $year % 100);
2049
48a26b3a 2050If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2051
48a26b3a 2052In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76 2053
2054 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2055
19799a22 2056Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 2057and the strftime(3) function available via the POSIX module.
7660c0ab 2058
2b5ab1e7
TC
2059This scalar value is B<not> locale dependent (see L<perllocale>), but
2060is instead a Perl builtin. Also see the C<Time::Local> module, and the
2061strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
2062get somewhat similar but locale dependent date strings, set up your
2063locale environment variables appropriately (please see L<perllocale>)
2064and try for example:
2065
2066 use POSIX qw(strftime);
2b5ab1e7 2067 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 2068
2b5ab1e7
TC
2069Note that the C<%a> and C<%b> escapes, which represent the short forms
2070of the day of the week and the month of the year, may not necessarily
2071be three characters wide in all locales.
0a753a76 2072
a0d0e21e
LW
2073=item goto LABEL
2074
748a9306
LW
2075=item goto EXPR
2076
a0d0e21e
LW
2077=item goto &NAME
2078
7660c0ab 2079The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2080execution there. It may not be used to go into any construct that
7660c0ab 2081requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2082also can't be used to go into a construct that is optimized away,
19799a22 2083or to get out of a block or subroutine given to C<sort>.
0a753a76 2084It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2085including out of subroutines, but it's usually better to use some other
19799a22 2086construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2087need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2088(The difference being that C does not offer named loops combined with
2089loop control. Perl does, and this replaces most structured uses of C<goto>
2090in other languages.)
a0d0e21e 2091
7660c0ab
A
2092The C<goto-EXPR> form expects a label name, whose scope will be resolved
2093dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2094necessarily recommended if you're optimizing for maintainability:
2095
2096 goto ("FOO", "BAR", "GLARCH")[$i];
2097
1b6921cb
BT
2098The C<goto-&NAME> form is quite different from the other forms of
2099C<goto>. In fact, it isn't a goto in the normal sense at all, and
2100doesn't have the stigma associated with other gotos. Instead, it
2101exits the current subroutine (losing any changes set by local()) and
2102immediately calls in its place the named subroutine using the current
2103value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2104load another subroutine and then pretend that the other subroutine had
2105been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2106in the current subroutine are propagated to the other subroutine.)
2107After the C<goto>, not even C<caller> will be able to tell that this
2108routine was called first.
2109
2110NAME needn't be the name of a subroutine; it can be a scalar variable
2111containing a code reference, or a block which evaluates to a code
2112reference.
a0d0e21e
LW
2113
2114=item grep BLOCK LIST
2115
2116=item grep EXPR,LIST
2117
2b5ab1e7
TC
2118This is similar in spirit to, but not the same as, grep(1) and its
2119relatives. In particular, it is not limited to using regular expressions.
2f9daede 2120
a0d0e21e 2121Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2122C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2123elements for which the expression evaluated to true. In scalar
2124context, returns the number of times the expression was true.
a0d0e21e
LW
2125
2126 @foo = grep(!/^#/, @bar); # weed out comments
2127
2128or equivalently,
2129
2130 @foo = grep {!/^#/} @bar; # weed out comments
2131
be3174d2
GS
2132Note that C<$_> is an alias to the list value, so it can be used to
2133modify the elements of the LIST. While this is useful and supported,
2134it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2135Similarly, grep returns aliases into the original list, much as a for
2136loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2137element of a list returned by grep (for example, in a C<foreach>, C<map>
2138or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2139This is usually something to be avoided when writing clear code.
a0d0e21e 2140
19799a22 2141See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2142
a0d0e21e
LW
2143=item hex EXPR
2144
54310121 2145=item hex
bbce6d69 2146
2b5ab1e7
TC
2147Interprets EXPR as a hex string and returns the corresponding value.
2148(To convert strings that might start with either 0, 0x, or 0b, see
2149L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TP
2150
2151 print hex '0xAf'; # prints '175'
2152 print hex 'aF'; # same
a0d0e21e 2153
19799a22 2154Hex strings may only represent integers. Strings that would cause
53305cf1
NC
2155integer overflow trigger a warning. Leading whitespace is not stripped,
2156unlike oct().
19799a22 2157
a0d0e21e
LW
2158=item import
2159
19799a22 2160There is no builtin C<import> function. It is just an ordinary
4633a7c4 2161method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2162names to another module. The C<use> function calls the C<import> method
cea6626f 2163for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2164
2165=item index STR,SUBSTR,POSITION
2166
2167=item index STR,SUBSTR
2168
2b5ab1e7
TC
2169The index function searches for one string within another, but without
2170the wildcard-like behavior of a full regular-expression pattern match.
2171It returns the position of the first occurrence of SUBSTR in STR at
2172or after POSITION. If POSITION is omitted, starts searching from the
2173beginning of the string. The return value is based at C<0> (or whatever
2174you've set the C<$[> variable to--but don't do that). If the substring
2175is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2176
2177=item int EXPR
2178
54310121 2179=item int
bbce6d69 2180
7660c0ab 2181Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2182You should not use this function for rounding: one because it truncates
2183towards C<0>, and two because machine representations of floating point
2184numbers can sometimes produce counterintuitive results. For example,
2185C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2186because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2187the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2188functions will serve you better than will int().
a0d0e21e
LW
2189
2190=item ioctl FILEHANDLE,FUNCTION,SCALAR
2191
2b5ab1e7 2192Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2193
4633a7c4 2194 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2195
2b5ab1e7 2196to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2197exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2198own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2199(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2200may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2201written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2202will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2203has no string value but does have a numeric value, that value will be
2204passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2205true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2206functions may be needed to manipulate the values of structures used by
b76cc8ba 2207C<ioctl>.
a0d0e21e 2208
19799a22 2209The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2210
2211 if OS returns: then Perl returns:
2212 -1 undefined value
2213 0 string "0 but true"
2214 anything else that number
2215
19799a22 2216Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2217still easily determine the actual value returned by the operating
2218system:
2219
2b5ab1e7 2220 $retval = ioctl(...) || -1;
a0d0e21e
LW
2221 printf "System returned %d\n", $retval;
2222
c2611fb3 2223The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2224about improper numeric conversions.
2225
19799a22
GS
2226Here's an example of setting a filehandle named C<REMOTE> to be
2227non-blocking at the system level. You'll have to negotiate C<$|>
2228on your own, though.
2229
2230 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2231
2232 $flags = fcntl(REMOTE, F_GETFL, 0)
2233 or die "Can't get flags for the socket: $!\n";
2234
2235 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2236 or die "Can't set flags for the socket: $!\n";
2237
a0d0e21e
LW
2238=item join EXPR,LIST
2239
2b5ab1e7
TC
2240Joins the separate strings of LIST into a single string with fields
2241separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2242
2b5ab1e7 2243 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2244
eb6e2d6f
GS
2245Beware that unlike C<split>, C<join> doesn't take a pattern as its
2246first argument. Compare L</split>.
a0d0e21e 2247
aa689395 2248=item keys HASH
2249
19799a22 2250Returns a list consisting of all the keys of the named hash. (In
1d2dff63 2251scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2252an apparently random order. The actual random order is subject to
2253change in future versions of perl, but it is guaranteed to be the same
19799a22 2254order as either the C<values> or C<each> function produces (given
ab192400
GS
2255that the hash has not been modified). As a side effect, it resets
2256HASH's iterator.
a0d0e21e 2257
aa689395 2258Here is yet another way to print your environment:
a0d0e21e
LW
2259
2260 @keys = keys %ENV;
2261 @values = values %ENV;
b76cc8ba 2262 while (@keys) {
a0d0e21e
LW
2263 print pop(@keys), '=', pop(@values), "\n";
2264 }
2265
2266or how about sorted by key:
2267
2268 foreach $key (sort(keys %ENV)) {
2269 print $key, '=', $ENV{$key}, "\n";
2270 }
2271
8ea1e5d4
GS
2272The returned values are copies of the original keys in the hash, so
2273modifying them will not affect the original hash. Compare L</values>.
2274
19799a22 2275To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2276Here's a descending numeric sort of a hash by its values:
4633a7c4 2277
5a964f20 2278 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2279 printf "%4d %s\n", $hash{$key}, $key;
2280 }
2281
19799a22 2282As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395 2283allocated for the given hash. This can gain you a measure of efficiency if
2284you know the hash is going to get big. (This is similar to pre-extending
2285an array by assigning a larger number to $#array.) If you say
55497cff 2286
2287 keys %hash = 200;
2288
ab192400
GS
2289then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2290in fact, since it rounds up to the next power of two. These
55497cff 2291buckets will be retained even if you do C<%hash = ()>, use C<undef
2292%hash> if you want to free the storage while C<%hash> is still in scope.
2293You can't shrink the number of buckets allocated for the hash using
19799a22 2294C<keys> in this way (but you needn't worry about doing this by accident,
55497cff 2295as trying has no effect).
2296
19799a22 2297See also C<each>, C<values> and C<sort>.
ab192400 2298
b350dd2f 2299=item kill SIGNAL, LIST
a0d0e21e 2300
b350dd2f 2301Sends a signal to a list of processes. Returns the number of
517db077
GS
2302processes successfully signaled (which is not necessarily the
2303same as the number actually killed).
a0d0e21e
LW
2304
2305 $cnt = kill 1, $child1, $child2;
2306 kill 9, @goners;
2307
b350dd2f
GS
2308If SIGNAL is zero, no signal is sent to the process. This is a
2309useful way to check that the process is alive and hasn't changed
2310its UID. See L<perlport> for notes on the portability of this
2311construct.
2312
2313Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2314process groups instead of processes. (On System V, a negative I<PROCESS>
2315number will also kill process groups, but that's not portable.) That
2316means you usually want to use positive not negative signals. You may also
da0045b7 2317use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2318
2319=item last LABEL
2320
2321=item last
2322
2323The C<last> command is like the C<break> statement in C (as used in
2324loops); it immediately exits the loop in question. If the LABEL is
2325omitted, the command refers to the innermost enclosing loop. The
2326C<continue> block, if any, is not executed:
2327
4633a7c4
LW
2328 LINE: while (<STDIN>) {
2329 last LINE if /^$/; # exit when done with header
5a964f20 2330 #...
a0d0e21e
LW
2331 }
2332
4968c1e4 2333C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2334C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2335a grep() or map() operation.
4968c1e4 2336
6c1372ed
GS
2337Note that a block by itself is semantically identical to a loop
2338that executes once. Thus C<last> can be used to effect an early
2339exit out of such a block.
2340
98293880
JH
2341See also L</continue> for an illustration of how C<last>, C<next>, and
2342C<redo> work.
1d2dff63 2343
a0d0e21e
LW
2344=item lc EXPR
2345
54310121 2346=item lc
bbce6d69 2347
d1be9408 2348Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2349implementing the C<\L> escape in double-quoted strings. Respects
2350current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2351and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2352
7660c0ab 2353If EXPR is omitted, uses C<$_>.
bbce6d69 2354
a0d0e21e
LW
2355=item lcfirst EXPR
2356
54310121 2357=item lcfirst
bbce6d69 2358
ad0029c4
JH
2359Returns the value of EXPR with the first character lowercased. This
2360is the internal function implementing the C<\l> escape in
2361double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2362locale> in force. See L<perllocale> and L<perlunicode> for more
2363details about locale and Unicode support.
a0d0e21e 2364
7660c0ab 2365If EXPR is omitted, uses C<$_>.
bbce6d69 2366
a0d0e21e
LW
2367=item length EXPR
2368
54310121 2369=item length
bbce6d69 2370
a0ed51b3 2371Returns the length in characters of the value of EXPR. If EXPR is
b76cc8ba 2372omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2373an entire array or hash to find out how many elements these have.
2374For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e
LW
2375
2376=item link OLDFILE,NEWFILE
2377
19799a22 2378Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2379success, false otherwise.
a0d0e21e
LW
2380
2381=item listen SOCKET,QUEUESIZE
2382
19799a22 2383Does the same thing that the listen system call does. Returns true if
b76cc8ba 2384it succeeded, false otherwise. See the example in
cea6626f 2385L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2386
2387=item local EXPR
2388
19799a22 2389You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2390what most people think of as "local". See
13a2d996 2391L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2392
5a964f20
TC
2393A local modifies the listed variables to be local to the enclosing
2394block, file, or eval. If more than one value is listed, the list must
2395be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2396for details, including issues with tied arrays and hashes.
a0d0e21e 2397
a0d0e21e
LW
2398=item localtime EXPR
2399
19799a22 2400Converts a time as returned by the time function to a 9-element list
5f05dabc 2401with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2402follows:
2403
54310121 2404 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2405 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2406 localtime(time);
2407
48a26b3a
GS
2408All list elements are numeric, and come straight out of the C `struct
2409tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2410specified time. $mday is the day of the month, and $mon is the month
2411itself, in the range C<0..11> with 0 indicating January and 11
2412indicating December. $year is the number of years since 1900. That
2413is, $year is C<123> in year 2023. $wday is the day of the week, with
24140 indicating Sunday and 3 indicating Wednesday. $yday is the day of
874b1813 2415the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
48a26b3a
GS
2416is true if the specified time occurs during daylight savings time,
2417false otherwise.
2418
2419Note that the $year element is I<not> simply the last two digits of
2420the year. If you assume it is, then you create non-Y2K-compliant
2421programs--and you wouldn't want to do that, would you?
54310121 2422
abd75f24
GS
2423The proper way to get a complete 4-digit year is simply:
2424
2425 $year += 1900;
2426
2427And to get the last two digits of the year (e.g., '01' in 2001) do:
2428
2429 $year = sprintf("%02d", $year % 100);
2430
48a26b3a 2431If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2432
48a26b3a 2433In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2434
5f05dabc 2435 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2436
a3cb178b 2437This scalar value is B<not> locale dependent, see L<perllocale>, but
68f8bed4
JH
2438instead a Perl builtin. Also see the C<Time::Local> module
2439(to convert the second, minutes, hours, ... back to seconds since the
2440stroke of midnight the 1st of January 1970, the value returned by
ca6e1c26 2441time()), and the strftime(3) and mktime(3) functions available via the
68f8bed4
JH
2442POSIX module. To get somewhat similar but locale dependent date
2443strings, set up your locale environment variables appropriately
2444(please see L<perllocale>) and try for example:
a3cb178b 2445
5a964f20 2446 use POSIX qw(strftime);
2b5ab1e7 2447 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2448
2449Note that the C<%a> and C<%b>, the short forms of the day of the week
2450and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2451
07698885 2452=item lock THING
19799a22
GS
2453
2454This function places an advisory lock on a variable, subroutine,
2455or referenced object contained in I<THING> until the lock goes out
2456of scope. This is a built-in function only if your version of Perl
43d3ddbe
JH
2457was built with threading enabled, and if you've said C<use Thread>.
2458Otherwise a user-defined function by this name will be called.
2459See L<Thread>.
19799a22 2460
a0d0e21e
LW
2461=item log EXPR
2462
54310121 2463=item log
bbce6d69 2464
2b5ab1e7
TC
2465Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2466returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2467The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2468divided by the natural log of N. For example:
2469
2470 sub log10 {
2471 my $n = shift;
2472 return log($n)/log(10);
b76cc8ba 2473 }
2b5ab1e7
TC
2474
2475See also L</exp> for the inverse operation.
a0d0e21e 2476
a0d0e21e
LW
2477=item lstat EXPR
2478
54310121 2479=item lstat
bbce6d69 2480
19799a22 2481Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2482special C<_> filehandle) but stats a symbolic link instead of the file
2483the symbolic link points to. If symbolic links are unimplemented on
19799a22 2484your system, a normal C<stat> is done.
a0d0e21e 2485
7660c0ab 2486If EXPR is omitted, stats C<$_>.
bbce6d69 2487
a0d0e21e
LW
2488=item m//
2489
2490The match operator. See L<perlop>.
2491
2492=item map BLOCK LIST
2493
2494=item map EXPR,LIST
2495
19799a22
GS
2496Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2497C<$_> to each element) and returns the list value composed of the
2498results of each such evaluation. In scalar context, returns the
2499total number of elements so generated. Evaluates BLOCK or EXPR in
2500list context, so each element of LIST may produce zero, one, or
2501more elements in the returned value.
dd99ebda 2502
a0d0e21e
LW
2503 @chars = map(chr, @nums);
2504
2505translates a list of numbers to the corresponding characters. And
2506
4633a7c4 2507 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2508
2509is just a funny way to write
2510
2511 %hash = ();
2512 foreach $_ (@array) {
4633a7c4 2513 $hash{getkey($_)} = $_;
a0d0e21e
LW
2514 }
2515
be3174d2
GS
2516Note that C<$_> is an alias to the list value, so it can be used to
2517modify the elements of the LIST. While this is useful and supported,
2518it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2519Using a regular C<foreach> loop for this purpose would be clearer in
2520most cases. See also L</grep> for an array composed of those items of
2521the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2522
205fdb4d
NC
2523C<{> starts both hash references and blocks, so C<map { ...> could be either
2524the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2525ahead for the closing C<}> it has to take a guess at which its dealing with
2526based what it finds just after the C<{>. Usually it gets it right, but if it
2527doesn't it won't realize something is wrong until it gets to the C<}> and
2528encounters the missing (or unexpected) comma. The syntax error will be
2529reported close to the C<}> but you'll need to change something near the C<{>
2530such as using a unary C<+> to give perl some help:
2531
2532 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2533 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2534 %hash = map { ("\L$_", 1) } @array # this also works
2535 %hash = map { lc($_), 1 } @array # as does this.
2536 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2537
205fdb4d
NC
2538 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2539
2540or to force an anon hash constructor use C<+{>
2541
2542 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2543
2544and you get list of anonymous hashes each with only 1 entry.
2545
19799a22 2546=item mkdir FILENAME,MASK
a0d0e21e 2547
5a211162
GS
2548=item mkdir FILENAME
2549
0591cd52 2550Creates the directory specified by FILENAME, with permissions
19799a22
GS
2551specified by MASK (as modified by C<umask>). If it succeeds it
2552returns true, otherwise it returns false and sets C<$!> (errno).
5a211162 2553If omitted, MASK defaults to 0777.
0591cd52 2554
19799a22 2555In general, it is better to create directories with permissive MASK,
0591cd52 2556and let the user modify that with their C<umask>, than it is to supply
19799a22 2557a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2558The exceptions to this rule are when the file or directory should be
2559kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2560C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2561
cc1852e8
JH
2562Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2563number of trailing slashes. Some operating and filesystems do not get
2564this right, so Perl automatically removes all trailing slashes to keep
2565everyone happy.
2566
a0d0e21e
LW
2567=item msgctl ID,CMD,ARG
2568
f86cebdf 2569Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2570
2571 use IPC::SysV;
2572
7660c0ab
A
2573first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2574then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2575structure. Returns like C<ioctl>: the undefined value for error,
2576C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2577L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2578
2579=item msgget KEY,FLAGS
2580
f86cebdf 2581Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2582id, or the undefined value if there is an error. See also
2583L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2584
a0d0e21e
LW
2585=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2586
2587Calls the System V IPC function msgrcv to receive a message from
2588message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2589SIZE. Note that when a message is received, the message type as a
2590native long integer will be the first thing in VAR, followed by the
2591actual message. This packing may be opened with C<unpack("l! a*")>.
2592Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2593an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2594C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2595
2596=item msgsnd ID,MSG,FLAGS
2597
2598Calls the System V IPC function msgsnd to send the message MSG to the
2599message queue ID. MSG must begin with the native long integer message
2600type, and be followed by the length of the actual message, and finally
2601the message itself. This kind of packing can be achieved with
2602C<pack("l! a*", $type, $message)>. Returns true if successful,
2603or false if there is an error. See also C<IPC::SysV>
2604and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2605
2606=item my EXPR
2607
09bef843
SB
2608=item my EXPR : ATTRIBUTES
2609
19799a22
GS
2610A C<my> declares the listed variables to be local (lexically) to the
2611enclosing block, file, or C<eval>. If
5f05dabc 2612more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2613L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2614
a0d0e21e
LW
2615=item next LABEL
2616
2617=item next
2618
2619The C<next> command is like the C<continue> statement in C; it starts
2620the next iteration of the loop:
2621
4633a7c4
LW
2622 LINE: while (<STDIN>) {
2623 next LINE if /^#/; # discard comments
5a964f20 2624 #...
a0d0e21e
LW
2625 }
2626
2627Note that if there were a C<continue> block on the above, it would get
2628executed even on discarded lines. If the LABEL is omitted, the command
2629refers to the innermost enclosing loop.
2630
4968c1e4 2631C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2632C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2633a grep() or map() operation.
4968c1e4 2634
6c1372ed
GS
2635Note that a block by itself is semantically identical to a loop
2636that executes once. Thus C<next> will exit such a block early.
2637
98293880
JH
2638See also L</continue> for an illustration of how C<last>, C<next>, and
2639C<redo> work.
1d2dff63 2640
a0d0e21e
LW
2641=item no Module LIST
2642
7660c0ab 2643See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2644
2645=item oct EXPR
2646
54310121 2647=item oct
bbce6d69 2648
4633a7c4 2649Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2650value. (If EXPR happens to start off with C<0x>, interprets it as a
2651hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
2652binary string. Leading whitespace is ignored in all three cases.)
2653The following will handle decimal, binary, octal, and hex in the standard
2654Perl or C notation:
a0d0e21e
LW
2655
2656 $val = oct($val) if $val =~ /^0/;
2657
19799a22
GS
2658If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2659in octal), use sprintf() or printf():
2660
2661 $perms = (stat("filename"))[2] & 07777;
2662 $oct_perms = sprintf "%lo", $perms;
2663
2664The oct() function is commonly used when a string such as C<644> needs
2665to be converted into a file mode, for example. (Although perl will
2666automatically convert strings into numbers as needed, this automatic
2667conversion assumes base 10.)
a0d0e21e
LW
2668
2669=item open FILEHANDLE,EXPR
2670
68bd7414
NIS
2671=item open FILEHANDLE,MODE,EXPR
2672
2673=item open FILEHANDLE,MODE,EXPR,LIST
2674
ba964c95
T
2675=item open FILEHANDLE,MODE,REFERENCE
2676
a0d0e21e
LW
2677=item open FILEHANDLE
2678
2679Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
2680FILEHANDLE.
2681
2682(The following is a comprehensive reference to open(): for a gentler
2683introduction you may consider L<perlopentut>.)
2684
2685If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2686assigned a reference to a new anonymous filehandle, otherwise if
2687FILEHANDLE is an expression, its value is used as the name of the real
2688filehandle wanted. (This is considered a symbolic reference, so C<use
2689strict 'refs'> should I<not> be in effect.)
2690
2691If EXPR is omitted, the scalar variable of the same name as the
2692FILEHANDLE contains the filename. (Note that lexical variables--those
2693declared with C<my>--will not work for this purpose; so if you're
2694using C<my>, specify EXPR in your call to open.)
2695
2696If three or more arguments are specified then the mode of opening and
2697the file name are separate. If MODE is C<< '<' >> or nothing, the file
2698is opened for input. If MODE is C<< '>' >>, the file is truncated and
2699opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 2700the file is opened for appending, again being created if necessary.
5a964f20 2701
ed53a2bb
JH
2702You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2703indicate that you want both read and write access to the file; thus
2704C<< '+<' >> is almost always preferred for read/write updates--the C<<
2705'+>' >> mode would clobber the file first. You can't usually use
2706either read-write mode for updating textfiles, since they have
2707variable length records. See the B<-i> switch in L<perlrun> for a
2708better approach. The file is created with permissions of C<0666>
2709modified by the process' C<umask> value.
2710
2711These various prefixes correspond to the fopen(3) modes of C<'r'>,
2712C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2713
6170680b
IZ
2714In the 2-arguments (and 1-argument) form of the call the mode and
2715filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
2716spaces. It is possible to omit the mode in these forms if the mode is
2717C<< '<' >>.
6170680b 2718
7660c0ab 2719If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2720command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2721C<'|'>, the filename is interpreted as a command which pipes output to
2722us. See L<perlipc/"Using open() for IPC">
19799a22 2723for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2724that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2725and L<perlipc/"Bidirectional Communication with Another Process">
2726for alternatives.)
cb1a09d0 2727
ed53a2bb
JH
2728For three or more arguments if MODE is C<'|-'>, the filename is
2729interpreted as a command to which output is to be piped, and if MODE
2730is C<'-|'>, the filename is interpreted as a command which pipes
2731output to us. In the 2-arguments (and 1-argument) form one should
2732replace dash (C<'-'>) with the command.
2733See L<perlipc/"Using open() for IPC"> for more examples of this.
2734(You are not allowed to C<open> to a command that pipes both in I<and>
2735out, but see L<IPC::Open2>, L<IPC::Open3>, and
2736L<perlipc/"Bidirectional Communication"> for alternatives.)
2737
2738In the three-or-more argument form of pipe opens, if LIST is specified
2739(extra arguments after the command name) then LIST becomes arguments
2740to the command invoked if the platform supports it. The meaning of
2741C<open> with more than three arguments for non-pipe modes is not yet
2742specified. Experimental "layers" may give extra LIST arguments
2743meaning.
6170680b
IZ
2744
2745In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 2746and opening C<< '>-' >> opens STDOUT.
6170680b 2747
ed53a2bb
JH
2748You may use the three-argument form of open to specify
2749I<I/O disciplines> that affect how the input and output
9124316e
JH
2750are processed: see L</binmode> and L<open>. For example
2751
2752 open(FH, "<:utf8", "file")
2753
2754will open the UTF-8 encoded file containing Unicode characters,
2755see L<perluniintro>.
ed53a2bb
JH
2756
2757Open returns nonzero upon success, the undefined value otherwise. If
2758the C<open> involved a pipe, the return value happens to be the pid of
2759the subprocess.
cb1a09d0 2760
ed53a2bb
JH
2761If you're running Perl on a system that distinguishes between text
2762files and binary files, then you should check out L</binmode> for tips
2763for dealing with this. The key distinction between systems that need
2764C<binmode> and those that don't is their text file formats. Systems
2765like Unix, MacOS, and Plan9, which delimit lines with a single
2766character, and which encode that character in C as C<"\n">, do not
2767need C<binmode>. The rest need it.
cb1a09d0 2768
ed53a2bb
JH
2769In the three argument form MODE may also contain a list of IO "layers"
2770(see L<open> and L<PerlIO> for more details) to be applied to the
2771handle. This can be used to achieve the effect of C<binmode> as well
2772as more complex behaviours.
68bd7414 2773
fb73857a 2774When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2775if the request failed, so C<open> is frequently used in connection with
2776C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2777where you want to make a nicely formatted error message (but there are
5a964f20 2778modules that can help with that problem)) you should always check
19799a22 2779the return value from opening a file. The infrequent exception is when
fb73857a 2780working with an unopened filehandle is actually what you want to do.
2781
ed53a2bb
JH
2782As a special case the 3 arg form with a read/write mode and the third
2783argument being C<undef>:
b76cc8ba
NIS
2784
2785 open(TMP, "+>", undef) or die ...
2786
2787opens a filehandle to an anonymous temporary file.
2788
ba964c95
T
2789File handles can be opened to "in memory" files held in Perl scalars via:
2790
2791 open($fh,'>', \$variable) || ..
2792
cb1a09d0 2793Examples:
a0d0e21e
LW
2794
2795 $ARTICLE = 100;
2796 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2797 while (<ARTICLE>) {...
2798
6170680b 2799 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2800 # if the open fails, output is discarded
a0d0e21e 2801
6170680b 2802 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2803 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2804
6170680b
IZ
2805 open(DBASE, '+<dbase.mine') # ditto
2806 or die "Can't open 'dbase.mine' for update: $!";
2807
2808 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2809 or die "Can't start caesar: $!";
a0d0e21e 2810
6170680b
IZ
2811 open(ARTICLE, "caesar <$article |") # ditto
2812 or die "Can't start caesar: $!";
2813
2814 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
fb73857a 2815 or die "Can't start sort: $!";
a0d0e21e 2816
ba964c95
T
2817 # in memory files
2818 open(MEMORY,'>', \$var)
2819 or die "Can't open memory file: $!";
2820 print MEMORY "foo!\n"; # output will end up in $var
2821
a0d0e21e
LW
2822 # process argument list of files along with any includes
2823
2824 foreach $file (@ARGV) {
2825 process($file, 'fh00');
2826 }
2827
2828 sub process {
5a964f20 2829 my($filename, $input) = @_;
a0d0e21e
LW
2830 $input++; # this is a string increment
2831 unless (open($input, $filename)) {
2832 print STDERR "Can't open $filename: $!\n";
2833 return;
2834 }
2835
5a964f20 2836 local $_;
a0d0e21e
LW
2837 while (<$input>) { # note use of indirection
2838 if (/^#include "(.*)"/) {
2839 process($1, $input);
2840 next;
2841 }
5a964f20 2842 #... # whatever
a0d0e21e
LW
2843 }
2844 }
2845
2846You may also, in the Bourne shell tradition, specify an EXPR beginning
61eff3bc 2847with C<< '>&' >>, in which case the rest of the string is interpreted as the
5a964f20 2848name of a filehandle (or file descriptor, if numeric) to be
61eff3bc
JH
2849duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2850C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
a0d0e21e 2851mode you specify should match the mode of the original filehandle.
184e9718 2852(Duping a filehandle does not take into account any existing contents of
9124316e 2853IO buffers.) If you use the 3 arg form then you can pass either a number,
b76cc8ba 2854the name of a filehandle or the normal "reference to a glob".
6170680b 2855
a0d0e21e
LW
2856Here is a script that saves, redirects, and restores STDOUT and
2857STDERR:
2858
2859 #!/usr/bin/perl
b76cc8ba 2860 open(my $oldout, ">&", \*STDOUT);
5a964f20 2861 open(OLDERR, ">&STDERR");
a0d0e21e 2862
6170680b
IZ
2863 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2864 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
a0d0e21e
LW
2865
2866 select(STDERR); $| = 1; # make unbuffered
2867 select(STDOUT); $| = 1; # make unbuffered
2868
2869 print STDOUT "stdout 1\n"; # this works for
2870 print STDERR "stderr 1\n"; # subprocesses too
2871
2872 close(STDOUT);
2873 close(STDERR);
2874
5a964f20
TC
2875 open(STDOUT, ">&OLDOUT");
2876 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2877
2878 print STDOUT "stdout 2\n";
2879 print STDERR "stderr 2\n";
2880
df632fdf
JH
2881If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2882do an equivalent of C's C<fdopen> of that file descriptor; this is
2883more parsimonious of file descriptors. For example:
a0d0e21e
LW
2884
2885 open(FILEHANDLE, "<&=$fd")
df632fdf 2886
b76cc8ba 2887or
df632fdf 2888
b76cc8ba 2889 open(FILEHANDLE, "<&=", $fd)
a0d0e21e 2890
df632fdf
JH
2891Note that if Perl is using the standard C libraries' fdopen() then on
2892many UNIX systems, fdopen() is known to fail when file descriptors
4af147f6 2893exceed a certain value, typically 255. If you need more file
b76cc8ba 2894descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
4af147f6 2895
df632fdf
JH
2896You can see whether Perl has been compiled with PerlIO or not by
2897running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2898is C<define>, you have PerlIO, otherwise you don't.
2899
6170680b
IZ
2900If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2901with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 2902there is an implicit fork done, and the return value of open is the pid
7660c0ab 2903of the child within the parent process, and C<0> within the child
184e9718 2904process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2905The filehandle behaves normally for the parent, but i/o to that
2906filehandle is piped from/to the STDOUT/STDIN of the child process.
2907In the child process the filehandle isn't opened--i/o happens from/to
2908the new STDOUT or STDIN. Typically this is used like the normal
2909piped open when you want to exercise more control over just how the
2910pipe command gets executed, such as when you are running setuid, and
54310121 2911don't want to have to scan shell commands for metacharacters.
6170680b 2912The following triples are more or less equivalent:
a0d0e21e
LW
2913
2914 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
2915 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2916 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
b76cc8ba 2917 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
a0d0e21e
LW
2918
2919 open(FOO, "cat -n '$file'|");
6170680b
IZ
2920 open(FOO, '-|', "cat -n '$file'");
2921 open(FOO, '-|') || exec 'cat', '-n', $file;
b76cc8ba
NIS
2922 open(FOO, '-|', "cat", '-n', $file);
2923
2924The last example in each block shows the pipe as "list form", which is
2925not yet supported on all platforms.
a0d0e21e 2926
4633a7c4
LW
2927See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2928
0f897271
GS
2929Beginning with v5.6.0, Perl will attempt to flush all files opened for
2930output before any operation that may do a fork, but this may not be
2931supported on some platforms (see L<perlport>). To be safe, you may need
2932to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2933of C<IO::Handle> on any open handles.
2934
ed53a2bb
JH
2935On systems that support a close-on-exec flag on files, the flag will
2936be set for the newly opened file descriptor as determined by the value
2937of $^F. See L<perlvar/$^F>.
a0d0e21e 2938
0dccf244
CS
2939Closing any piped filehandle causes the parent process to wait for the
2940child to finish, and returns the status value in C<$?>.
2941
ed53a2bb
JH
2942The filename passed to 2-argument (or 1-argument) form of open() will
2943have leading and trailing whitespace deleted, and the normal
2944redirection characters honored. This property, known as "magic open",
5a964f20 2945can often be used to good effect. A user could specify a filename of
7660c0ab 2946F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2947
2948 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2949 open(FH, $filename) or die "Can't open $filename: $!";
2950
6170680b
IZ
2951Use 3-argument form to open a file with arbitrary weird characters in it,
2952
2953 open(FOO, '<', $file);
2954
2955otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
2956
2957 $file =~ s#^(\s)#./$1#;
2958 open(FOO, "< $file\0");
2959
a31a806a 2960(this may not work on some bizarre filesystems). One should
106325ad 2961conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
2962of open():
2963
2964 open IN, $ARGV[0];
2965
2966will allow the user to specify an argument of the form C<"rsh cat file |">,
2967but will not work on a filename which happens to have a trailing space, while
2968
2969 open IN, '<', $ARGV[0];
2970
2971will have exactly the opposite restrictions.
2972
19799a22 2973If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
2974should use the C<sysopen> function, which involves no such magic (but
2975may use subtly different filemodes than Perl open(), which is mapped
2976to C fopen()). This is
5a964f20
TC
2977another way to protect your filenames from interpretation. For example:
2978
2979 use IO::Handle;
2980 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2981 or die "sysopen $path: $!";
2982 $oldfh = select(HANDLE); $| = 1; select($oldfh);
38762f02 2983 print HANDLE "stuff $$\n";
5a964f20
TC
2984 seek(HANDLE, 0, 0);
2985 print "File contains: ", <HANDLE>;
2986
7660c0ab
A
2987Using the constructor from the C<IO::Handle> package (or one of its
2988subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2989filehandles that have the scope of whatever variables hold references to
2990them, and automatically close whenever and however you leave that scope:
c07a80fd 2991
5f05dabc 2992 use IO::File;
5a964f20 2993 #...
c07a80fd 2994 sub read_myfile_munged {
2995 my $ALL = shift;
5f05dabc 2996 my $handle = new IO::File;
c07a80fd 2997 open($handle, "myfile") or die "myfile: $!";
2998 $first = <$handle>
2999 or return (); # Automatically closed here.
3000 mung $first or die "mung failed"; # Or here.
3001 return $first, <$handle> if $ALL; # Or here.
3002 $first; # Or here.
3003 }
3004
b687b08b 3005See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
3006
3007=item opendir DIRHANDLE,EXPR
3008
19799a22
GS
3009Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3010C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a0d0e21e
LW
3011DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3012
3013=item ord EXPR
3014
54310121 3015=item ord
bbce6d69 3016
121910a4
JH
3017Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3018or Unicode) value of the first character of EXPR. If EXPR is omitted,
3019uses C<$_>.
3020
3021For the reverse, see L</chr>.
3022See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 3023
77ca0c92
LW
3024=item our EXPR
3025
9969eac4
BS
3026=item our EXPR : ATTRIBUTES
3027
77ca0c92
LW
3028An C<our> declares the listed variables to be valid globals within
3029the enclosing block, file, or C<eval>. That is, it has the same
3030scoping rules as a "my" declaration, but does not create a local
3031variable. If more than one value is listed, the list must be placed
3032in parentheses. The C<our> declaration has no semantic effect unless
3033"use strict vars" is in effect, in which case it lets you use the
3034declared global variable without qualifying it with a package name.
3035(But only within the lexical scope of the C<our> declaration. In this
3036it differs from "use vars", which is package scoped.)
3037
f472eb5c
GS
3038An C<our> declaration declares a global variable that will be visible
3039across its entire lexical scope, even across package boundaries. The
3040package in which the variable is entered is determined at the point
3041of the declaration, not at the point of use. This means the following
3042behavior holds:
3043
3044 package Foo;
3045 our $bar; # declares $Foo::bar for rest of lexical scope
3046 $bar = 20;
3047
3048 package Bar;
3049 print $bar; # prints 20
3050
3051Multiple C<our> declarations in the same lexical scope are allowed
3052if they are in different packages. If they happened to be in the same
3053package, Perl will emit warnings if you have asked for them.
3054
3055 use warnings;
3056 package Foo;
3057 our $bar; # declares $Foo::bar for rest of lexical scope
3058 $bar = 20;
3059
3060 package Bar;
3061 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3062 print $bar; # prints 30
3063
3064 our $bar; # emits warning
3065
9969eac4
BS
3066An C<our> declaration may also have a list of attributes associated
3067with it. B<WARNING>: This is an experimental feature that may be
3068changed or removed in future releases of Perl. It should not be
3069relied upon.
3070
51d2bbcc 3071The only currently recognized attribute is C<unique> which indicates
9969eac4
BS
3072that a single copy of the global is to be used by all interpreters
3073should the program happen to be running in a multi-interpreter
3074environment. (The default behaviour would be for each interpreter to
3075have its own copy of the global.) In such an environment, this
3076attribute also has the effect of making the global readonly.
3077Examples:
3078
51d2bbcc
JH
3079 our @EXPORT : unique = qw(foo);
3080 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3081 our $VERSION : unique = "1.00";
9969eac4
BS
3082
3083Multi-interpreter environments can come to being either through the
3084fork() emulation on Windows platforms, or by embedding perl in a
51d2bbcc 3085multi-threaded application. The C<unique> attribute does nothing in
9969eac4
BS
3086all other environments.
3087
a0d0e21e
LW
3088=item pack TEMPLATE,LIST
3089
2b6c5635
GS
3090Takes a LIST of values and converts it into a string using the rules
3091given by the TEMPLATE. The resulting string is the concatenation of
3092the converted values. Typically, each converted value looks
3093like its machine-level representation. For example, on 32-bit machines
3094a converted integer may be represented by a sequence of 4 bytes.
3095
3096The TEMPLATE is a
a0d0e21e
LW
3097sequence of characters that give the order and type of values, as
3098follows:
3099
5a929a98 3100 a A string with arbitrary binary data, will be null padded.
121910a4
JH
3101 A A text (ASCII) string, will be space padded.
3102 Z A null terminated (ASCIZ) string, will be null padded.
5a929a98 3103
2b6c5635
GS
3104 b A bit string (ascending bit order inside each byte, like vec()).
3105 B A bit string (descending bit order inside each byte).
a0d0e21e
LW
3106 h A hex string (low nybble first).
3107 H A hex string (high nybble first).
3108
3109 c A signed char value.
a0ed51b3 3110 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 3111
a0d0e21e
LW
3112 s A signed short value.
3113 S An unsigned short value.
96e4d5b1 3114 (This 'short' is _exactly_ 16 bits, which may differ from
851646ae
JH
3115 what a local C compiler calls 'short'. If you want
3116 native-length shorts, use the '!' suffix.)
96e4d5b1 3117
a0d0e21e
LW
3118 i A signed integer value.
3119 I An unsigned integer value.
19799a22 3120 (This 'integer' is _at_least_ 32 bits wide. Its exact
f86cebdf
GS
3121 size depends on what a local C compiler calls 'int',
3122 and may even be larger than the 'long' described in
3123 the next item.)
96e4d5b1 3124
a0d0e21e
LW
3125 l A signed long value.
3126 L An unsigned long value.
96e4d5b1 3127 (This 'long' is _exactly_ 32 bits, which may differ from
851646ae
JH
3128 what a local C compiler calls 'long'. If you want
3129 native-length longs, use the '!' suffix.)
a0d0e21e 3130
5d11dd56
MG
3131 n An unsigned short in "network" (big-endian) order.
3132 N An unsigned long in "network" (big-endian) order.
3133 v An unsigned short in "VAX" (little-endian) order.
3134 V An unsigned long in "VAX" (little-endian) order.
96e4d5b1 3135 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3136 _exactly_ 32 bits, respectively.)
a0d0e21e 3137
dae0da7a
JH
3138 q A signed quad (64-bit) value.
3139 Q An unsigned quad value.
851646ae
JH
3140 (Quads are available only if your system supports 64-bit
3141 integer values _and_ if Perl has been compiled to support those.
dae0da7a
JH
3142 Causes a fatal error otherwise.)
3143
a0d0e21e
LW
3144 f A single-precision float in the native format.
3145 d A double-precision float in the native format.
3146
3147 p A pointer to a null-terminated string.
3148 P A pointer to a structure (fixed-length string).
3149
3150 u A uuencoded string.
ad0029c4
JH
3151 U A Unicode character number. Encodes to UTF-8 internally
3152 (or UTF-EBCDIC in EBCDIC platforms).
a0d0e21e 3153
96e4d5b1 3154 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
3155 integer in base 128, most significant digit first, with as
3156 few digits as possible. Bit eight (the high bit) is set
3157 on each byte except the last.
def98dd4 3158
a0d0e21e
LW
3159 x A null byte.
3160 X Back up a byte.
3161 @ Null fill to absolute position.
3162
5a929a98
VU
3163The following rules apply:
3164
3165=over 8
3166
3167=item *
3168
5a964f20 3169Each letter may optionally be followed by a number giving a repeat
951ba7fe
GS
3170count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3171C<H>, and C<P> the pack function will gobble up that many values from
5a929a98 3172the LIST. A C<*> for the repeat count means to use however many items are
951ba7fe
GS
3173left, except for C<@>, C<x>, C<X>, where it is equivalent
3174to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2b6c5635
GS
3175same).
3176
951ba7fe 3177When used with C<Z>, C<*> results in the addition of a trailing null
2b6c5635
GS
3178byte (so the packed result will be one longer than the byte C<length>
3179of the item).
3180
951ba7fe 3181The repeat count for C<u> is interpreted as the maximal number of bytes
2b6c5635 3182to encode per line of output, with 0 and 1 replaced by 45.
5a929a98
VU
3183
3184=item *
3185
951ba7fe 3186The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5a929a98 3187string of length count, padding with nulls or spaces as necessary. When
951ba7fe
GS
3188unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3189after the first null, and C<a> returns data verbatim. When packing,
3190C<a>, and C<Z> are equivalent.
2b6c5635
GS
3191
3192If the value-to-pack is too long, it is truncated. If too long and an
951ba7fe
GS
3193explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3194by a null byte. Thus C<Z> always packs a trailing null byte under
2b6c5635 3195all circumstances.
5a929a98
VU
3196
3197=item *
3198
951ba7fe 3199Likewise, the C<b> and C<B> fields pack a string that many bits long.
c73032f5
IZ
3200Each byte of the input field of pack() generates 1 bit of the result.
3201Each result bit is based on the least-significant bit of the corresponding
3202input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3203C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3204
3205Starting from the beginning of the input string of pack(), each 8-tuple
951ba7fe 3206of bytes is converted to 1 byte of output. With format C<b>
c73032f5 3207the first byte of the 8-tuple determines the least-significant bit of a
951ba7fe 3208byte, and with format C<B> it determines the most-significant bit of
c73032f5
IZ
3209a byte.
3210
3211If the length of the input string is not exactly divisible by 8, the
3212remainder is packed as if the input string were padded by null bytes
3213at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3214
3215If the input string of pack() is longer than needed, extra bytes are ignored.
2b6c5635
GS
3216A C<*> for the repeat count of pack() means to use all the bytes of
3217the input field. On unpack()ing the bits are converted to a string
3218of C<"0">s and C<"1">s.
5a929a98
VU
3219
3220=item *
3221
951ba7fe 3222The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
851646ae 3223representable as hexadecimal digits, 0-9a-f) long.
5a929a98 3224
c73032f5
IZ
3225Each byte of the input field of pack() generates 4 bits of the result.
3226For non-alphabetical bytes the result is based on the 4 least-significant
3227bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3228bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3229C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3230is compatible with the usual hexadecimal digits, so that C<"a"> and
3231C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3232C<"g".."z"> and C<"G".."Z"> is not well-defined.
3233
3234Starting from the beginning of the input string of pack(), each pair
951ba7fe 3235of bytes is converted to 1 byte of output. With format C<h> the
c73032f5 3236first byte of the pair determines the least-significant nybble of the
951ba7fe 3237output byte, and with format C<H> it determines the most-significant
c73032f5
IZ
3238nybble.
3239
3240If the length of the input string is not even, it behaves as if padded
3241by a null byte at the end. Similarly, during unpack()ing the "extra"
3242nybbles are ignored.
3243
3244If the input string of pack() is longer than needed, extra bytes are ignored.
3245A C<*> for the repeat count of pack() means to use all the bytes of
3246the input field. On unpack()ing the bits are converted to a string
3247of hexadecimal digits.
3248
5a929a98
VU
3249=item *
3250
951ba7fe 3251The C<p> type packs a pointer to a null-terminated string. You are
5a929a98
VU
3252responsible for ensuring the string is not a temporary value (which can
3253potentially get deallocated before you get around to using the packed result).
951ba7fe
GS
3254The C<P> type packs a pointer to a structure of the size indicated by the
3255length. A NULL pointer is created if the corresponding value for C<p> or
3256C<P> is C<undef>, similarly for unpack().
5a929a98
VU
3257
3258=item *
3259
951ba7fe
GS
3260The C</> template character allows packing and unpacking of strings where
3261the packed structure contains a byte count followed by the string itself.
17f4a12d 3262You write I<length-item>C</>I<string-item>.
43192e07
IP
3263
3264The I<length-item> can be any C<pack> template letter,
3265and describes how the length value is packed.
3266The ones likely to be of most use are integer-packing ones like
951ba7fe
GS
3267C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3268and C<N> (for Sun XDR).
43192e07
IP
3269
3270The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3271For C<unpack> the length of the string is obtained from the I<length-item>,
3272but if you put in the '*' it will be ignored.
3273
17f4a12d
IZ
3274 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3275 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3276 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
43192e07
IP
3277
3278The I<length-item> is not returned explicitly from C<unpack>.
3279
951ba7fe
GS
3280Adding a count to the I<length-item> letter is unlikely to do anything
3281useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3282I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
43192e07
IP
3283which Perl does not regard as legal in numeric strings.
3284
3285=item *
3286