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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
3 | perlfunc - Perl builtin functions | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | The functions in this section can serve as terms in an expression. | |
8 | They fall into two major categories: list operators and named unary | |
9 | operators. These differ in their precedence relationship with a | |
10 | following comma. (See the precedence table in L<perlop>.) List | |
11 | operators take more than one argument, while unary operators can never | |
12 | take more than one argument. Thus, a comma terminates the argument of | |
13 | a unary operator, but merely separates the arguments of a list | |
14 | operator. A unary operator generally provides a scalar context to its | |
2b5ab1e7 | 15 | argument, while a list operator may provide either scalar or list |
a0d0e21e | 16 | contexts for its arguments. If it does both, the scalar arguments will |
5f05dabc | 17 | be first, and the list argument will follow. (Note that there can ever |
0f31cffe | 18 | be only one such list argument.) For instance, splice() has three scalar |
2b5ab1e7 TC |
19 | arguments followed by a list, whereas gethostbyname() has four scalar |
20 | arguments. | |
a0d0e21e LW |
21 | |
22 | In the syntax descriptions that follow, list operators that expect a | |
23 | list (and provide list context for the elements of the list) are shown | |
24 | with LIST as an argument. Such a list may consist of any combination | |
25 | of scalar arguments or list values; the list values will be included | |
26 | in the list as if each individual element were interpolated at that | |
27 | point in the list, forming a longer single-dimensional list value. | |
28 | Elements of the LIST should be separated by commas. | |
29 | ||
30 | Any function in the list below may be used either with or without | |
31 | parentheses around its arguments. (The syntax descriptions omit the | |
5f05dabc | 32 | parentheses.) If you use the parentheses, the simple (but occasionally |
19799a22 | 33 | surprising) rule is this: It I<looks> like a function, therefore it I<is> a |
a0d0e21e LW |
34 | function, and precedence doesn't matter. Otherwise it's a list |
35 | operator or unary operator, and precedence does matter. And whitespace | |
36 | between the function and left parenthesis doesn't count--so you need to | |
37 | be 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. | |
a0d0e21e LW |
44 | |
45 | If you run Perl with the B<-w> switch it can warn you about this. For | |
46 | example, 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 | ||
2b5ab1e7 TC |
51 | A few functions take no arguments at all, and therefore work as neither |
52 | unary nor list operators. These include such functions as C<time> | |
53 | and C<endpwent>. For example, C<time+86_400> always means | |
54 | C<time() + 86_400>. | |
55 | ||
a0d0e21e | 56 | For functions that can be used in either a scalar or list context, |
54310121 | 57 | nonabortive failure is generally indicated in a scalar context by |
a0d0e21e LW |
58 | returning the undefined value, and in a list context by returning the |
59 | null list. | |
60 | ||
5a964f20 TC |
61 | Remember the following important rule: There is B<no rule> that relates |
62 | the behavior of an expression in list context to its behavior in scalar | |
63 | context, or vice versa. It might do two totally different things. | |
a0d0e21e | 64 | Each operator and function decides which sort of value it would be most |
2b5ab1e7 | 65 | appropriate to return in scalar context. Some operators return the |
5a964f20 | 66 | length of the list that would have been returned in list context. Some |
a0d0e21e LW |
67 | operators return the first value in the list. Some operators return the |
68 | last value in the list. Some operators return a count of successful | |
69 | operations. In general, they do what you want, unless you want | |
70 | consistency. | |
71 | ||
5a964f20 TC |
72 | An named array in scalar context is quite different from what would at |
73 | first glance appear to be a list in scalar context. You can't get a list | |
74 | like C<(1,2,3)> into being in scalar context, because the compiler knows | |
75 | the context at compile time. It would generate the scalar comma operator | |
76 | there, not the list construction version of the comma. That means it | |
77 | was never a list to start with. | |
78 | ||
79 | In general, functions in Perl that serve as wrappers for system calls | |
f86cebdf | 80 | of the same name (like chown(2), fork(2), closedir(2), etc.) all return |
5a964f20 TC |
81 | true when they succeed and C<undef> otherwise, as is usually mentioned |
82 | in the descriptions below. This is different from the C interfaces, | |
19799a22 GS |
83 | which return C<-1> on failure. Exceptions to this rule are C<wait>, |
84 | C<waitpid>, and C<syscall>. System calls also set the special C<$!> | |
5a964f20 TC |
85 | variable on failure. Other functions do not, except accidentally. |
86 | ||
cb1a09d0 AD |
87 | =head2 Perl Functions by Category |
88 | ||
89 | Here are Perl's functions (including things that look like | |
5a964f20 | 90 | functions, like some keywords and named operators) |
cb1a09d0 AD |
91 | arranged by category. Some functions appear in more |
92 | than one place. | |
93 | ||
94 | =over | |
95 | ||
96 | =item Functions for SCALARs or strings | |
97 | ||
22fae026 | 98 | C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>, |
ab4f32c2 A |
99 | C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>, |
100 | C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///> | |
cb1a09d0 AD |
101 | |
102 | =item Regular expressions and pattern matching | |
103 | ||
ab4f32c2 | 104 | C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//> |
cb1a09d0 AD |
105 | |
106 | =item Numeric functions | |
107 | ||
22fae026 TM |
108 | C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>, |
109 | C<sin>, C<sqrt>, C<srand> | |
cb1a09d0 AD |
110 | |
111 | =item Functions for real @ARRAYs | |
112 | ||
22fae026 | 113 | C<pop>, C<push>, C<shift>, C<splice>, C<unshift> |
cb1a09d0 AD |
114 | |
115 | =item Functions for list data | |
116 | ||
ab4f32c2 | 117 | C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack> |
cb1a09d0 AD |
118 | |
119 | =item Functions for real %HASHes | |
120 | ||
22fae026 | 121 | C<delete>, C<each>, C<exists>, C<keys>, C<values> |
cb1a09d0 AD |
122 | |
123 | =item Input and output functions | |
124 | ||
22fae026 TM |
125 | C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>, |
126 | C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>, | |
127 | C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>, | |
128 | C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>, | |
129 | C<warn>, C<write> | |
cb1a09d0 AD |
130 | |
131 | =item Functions for fixed length data or records | |
132 | ||
22fae026 | 133 | C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec> |
cb1a09d0 AD |
134 | |
135 | =item Functions for filehandles, files, or directories | |
136 | ||
22fae026 | 137 | C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>, |
5ff3f7a4 GS |
138 | C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, |
139 | C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>, | |
140 | C<unlink>, C<utime> | |
cb1a09d0 AD |
141 | |
142 | =item Keywords related to the control flow of your perl program | |
143 | ||
98293880 JH |
144 | C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>, |
145 | C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray> | |
cb1a09d0 | 146 | |
54310121 | 147 | =item Keywords related to scoping |
cb1a09d0 | 148 | |
22fae026 | 149 | C<caller>, C<import>, C<local>, C<my>, C<package>, C<use> |
cb1a09d0 AD |
150 | |
151 | =item Miscellaneous functions | |
152 | ||
22fae026 TM |
153 | C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>, |
154 | C<scalar>, C<undef>, C<wantarray> | |
cb1a09d0 AD |
155 | |
156 | =item Functions for processes and process groups | |
157 | ||
22fae026 | 158 | C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>, |
ab4f32c2 | 159 | C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>, |
22fae026 | 160 | C<times>, C<wait>, C<waitpid> |
cb1a09d0 AD |
161 | |
162 | =item Keywords related to perl modules | |
163 | ||
22fae026 | 164 | C<do>, C<import>, C<no>, C<package>, C<require>, C<use> |
cb1a09d0 AD |
165 | |
166 | =item Keywords related to classes and object-orientedness | |
167 | ||
22fae026 TM |
168 | C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>, |
169 | C<untie>, C<use> | |
cb1a09d0 AD |
170 | |
171 | =item Low-level socket functions | |
172 | ||
22fae026 TM |
173 | C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>, |
174 | C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>, | |
175 | C<socket>, C<socketpair> | |
cb1a09d0 AD |
176 | |
177 | =item System V interprocess communication functions | |
178 | ||
22fae026 TM |
179 | C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>, |
180 | C<shmctl>, C<shmget>, C<shmread>, C<shmwrite> | |
cb1a09d0 AD |
181 | |
182 | =item Fetching user and group info | |
183 | ||
22fae026 TM |
184 | C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>, |
185 | C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>, | |
186 | C<getpwuid>, C<setgrent>, C<setpwent> | |
cb1a09d0 AD |
187 | |
188 | =item Fetching network info | |
189 | ||
22fae026 TM |
190 | C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>, |
191 | C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, | |
192 | C<getprotobyname>, C<getprotobynumber>, C<getprotoent>, | |
193 | C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>, | |
194 | C<setnetent>, C<setprotoent>, C<setservent> | |
cb1a09d0 AD |
195 | |
196 | =item Time-related functions | |
197 | ||
22fae026 | 198 | C<gmtime>, C<localtime>, C<time>, C<times> |
cb1a09d0 | 199 | |
37798a01 | 200 | =item Functions new in perl5 |
201 | ||
22fae026 TM |
202 | C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>, |
203 | C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>, | |
204 | C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>, | |
205 | C<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 | 208 | operator, which can be used in expressions. |
37798a01 | 209 | |
210 | =item Functions obsoleted in perl5 | |
211 | ||
22fae026 | 212 | C<dbmclose>, C<dbmopen> |
37798a01 | 213 | |
cb1a09d0 AD |
214 | =back |
215 | ||
60f9f73c JH |
216 | =head2 Portability |
217 | ||
2b5ab1e7 TC |
218 | Perl was born in Unix and can therefore access all common Unix |
219 | system calls. In non-Unix environments, the functionality of some | |
220 | Unix system calls may not be available, or details of the available | |
221 | functionality may differ slightly. The Perl functions affected | |
60f9f73c JH |
222 | by this are: |
223 | ||
224 | C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>, | |
225 | C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>, | |
226 | C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>, | |
227 | C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>, | |
228 | C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, | |
229 | C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>, | |
230 | C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>, | |
231 | C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>, | |
232 | C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>, | |
2b5ab1e7 | 233 | C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>, |
60f9f73c JH |
234 | C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>, |
235 | C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>, | |
236 | C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>, | |
2b5ab1e7 TC |
237 | C<shmwrite>, C<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>, |
238 | C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>, | |
239 | C<utime>, C<wait>, C<waitpid> | |
60f9f73c JH |
240 | |
241 | For more information about the portability of these functions, see | |
242 | L<perlport> and other available platform-specific documentation. | |
243 | ||
cb1a09d0 AD |
244 | =head2 Alphabetical Listing of Perl Functions |
245 | ||
a0d0e21e LW |
246 | =over 8 |
247 | ||
22fae026 | 248 | =item I<-X> FILEHANDLE |
a0d0e21e | 249 | |
22fae026 | 250 | =item I<-X> EXPR |
a0d0e21e | 251 | |
22fae026 | 252 | =item I<-X> |
a0d0e21e LW |
253 | |
254 | A file test, where X is one of the letters listed below. This unary | |
255 | operator takes one argument, either a filename or a filehandle, and | |
256 | tests the associated file to see if something is true about it. If the | |
7660c0ab | 257 | argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN. |
19799a22 | 258 | Unless otherwise documented, it returns C<1> for true and C<''> for false, or |
a0d0e21e LW |
259 | the undefined value if the file doesn't exist. Despite the funny |
260 | names, precedence is the same as any other named unary operator, and | |
261 | the argument may be parenthesized like any other unary operator. The | |
262 | operator may be any of: | |
7e778d91 IZ |
263 | X<-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> |
264 | X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C> | |
a0d0e21e LW |
265 | |
266 | -r File is readable by effective uid/gid. | |
267 | -w File is writable by effective uid/gid. | |
268 | -x File is executable by effective uid/gid. | |
269 | -o File is owned by effective uid. | |
270 | ||
271 | -R File is readable by real uid/gid. | |
272 | -W File is writable by real uid/gid. | |
273 | -X File is executable by real uid/gid. | |
274 | -O File is owned by real uid. | |
275 | ||
276 | -e File exists. | |
277 | -z File has zero size. | |
54310121 | 278 | -s File has nonzero size (returns size). |
a0d0e21e LW |
279 | |
280 | -f File is a plain file. | |
281 | -d File is a directory. | |
282 | -l File is a symbolic link. | |
9c4d0f16 | 283 | -p File is a named pipe (FIFO), or Filehandle is a pipe. |
a0d0e21e LW |
284 | -S File is a socket. |
285 | -b File is a block special file. | |
286 | -c File is a character special file. | |
287 | -t Filehandle is opened to a tty. | |
288 | ||
289 | -u File has setuid bit set. | |
290 | -g File has setgid bit set. | |
291 | -k File has sticky bit set. | |
292 | ||
2cdbc966 JD |
293 | -T File is an ASCII text file. |
294 | -B File is a "binary" file (opposite of -T). | |
a0d0e21e LW |
295 | |
296 | -M Age of file in days when script started. | |
297 | -A Same for access time. | |
298 | -C Same for inode change time. | |
299 | ||
a0d0e21e LW |
300 | Example: |
301 | ||
302 | while (<>) { | |
303 | chop; | |
304 | next unless -f $_; # ignore specials | |
5a964f20 | 305 | #... |
a0d0e21e LW |
306 | } |
307 | ||
5ff3f7a4 GS |
308 | The interpretation of the file permission operators C<-r>, C<-R>, |
309 | C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode | |
310 | of the file and the uids and gids of the user. There may be other | |
311 | reasons you can't actually read, write, or execute the file. Such | |
312 | reasons may be for example network filesystem access controls, ACLs | |
313 | (access control lists), read-only filesystems, and unrecognized | |
314 | executable formats. | |
315 | ||
2b5ab1e7 TC |
316 | Also note that, for the superuser on the local filesystems, the C<-r>, |
317 | C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1 | |
5ff3f7a4 GS |
318 | if any execute bit is set in the mode. Scripts run by the superuser |
319 | may thus need to do a stat() to determine the actual mode of the file, | |
2b5ab1e7 | 320 | or temporarily set their effective uid to something else. |
5ff3f7a4 GS |
321 | |
322 | If you are using ACLs, there is a pragma called C<filetest> that may | |
323 | produce more accurate results than the bare stat() mode bits. | |
5ff3f7a4 GS |
324 | When under the C<use filetest 'access'> the above-mentioned filetests |
325 | will test whether the permission can (not) be granted using the | |
468541a8 | 326 | access() family of system calls. Also note that the C<-x> and C<-X> may |
5ff3f7a4 GS |
327 | under this pragma return true even if there are no execute permission |
328 | bits set (nor any extra execute permission ACLs). This strangeness is | |
329 | due to the underlying system calls' definitions. Read the | |
330 | documentation for the C<filetest> pragma for more information. | |
331 | ||
a0d0e21e LW |
332 | Note that C<-s/a/b/> does not do a negated substitution. Saying |
333 | C<-exp($foo)> still works as expected, however--only single letters | |
334 | following a minus are interpreted as file tests. | |
335 | ||
336 | The C<-T> and C<-B> switches work as follows. The first block or so of the | |
337 | file is examined for odd characters such as strange control codes or | |
5a964f20 | 338 | characters with the high bit set. If too many strange characters (E<gt>30%) |
a0d0e21e LW |
339 | are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file |
340 | containing null in the first block is considered a binary file. If C<-T> | |
341 | or C<-B> is used on a filehandle, the current stdio buffer is examined | |
19799a22 | 342 | rather than the first block. Both C<-T> and C<-B> return true on a null |
54310121 | 343 | file, or a file at EOF when testing a filehandle. Because you have to |
4633a7c4 LW |
344 | read a file to do the C<-T> test, on most occasions you want to use a C<-f> |
345 | against the file first, as in C<next unless -f $file && -T $file>. | |
a0d0e21e | 346 | |
19799a22 | 347 | If any of the file tests (or either the C<stat> or C<lstat> operators) are given |
28757baa | 348 | the special filehandle consisting of a solitary underline, then the stat |
a0d0e21e LW |
349 | structure of the previous file test (or stat operator) is used, saving |
350 | a system call. (This doesn't work with C<-t>, and you need to remember | |
351 | that lstat() and C<-l> will leave values in the stat structure for the | |
352 | symbolic link, not the real file.) Example: | |
353 | ||
354 | print "Can do.\n" if -r $a || -w _ || -x _; | |
355 | ||
356 | stat($filename); | |
357 | print "Readable\n" if -r _; | |
358 | print "Writable\n" if -w _; | |
359 | print "Executable\n" if -x _; | |
360 | print "Setuid\n" if -u _; | |
361 | print "Setgid\n" if -g _; | |
362 | print "Sticky\n" if -k _; | |
363 | print "Text\n" if -T _; | |
364 | print "Binary\n" if -B _; | |
365 | ||
366 | =item abs VALUE | |
367 | ||
54310121 | 368 | =item abs |
bbce6d69 | 369 | |
a0d0e21e | 370 | Returns the absolute value of its argument. |
7660c0ab | 371 | If VALUE is omitted, uses C<$_>. |
a0d0e21e LW |
372 | |
373 | =item accept NEWSOCKET,GENERICSOCKET | |
374 | ||
f86cebdf | 375 | Accepts an incoming socket connect, just as the accept(2) system call |
19799a22 | 376 | does. Returns the packed address if it succeeded, false otherwise. |
2b5ab1e7 | 377 | See the example in L<perlipc/"Sockets: Client/Server Communication">. |
a0d0e21e | 378 | |
8d2a6795 GS |
379 | On systems that support a close-on-exec flag on files, the flag will |
380 | be set for the newly opened file descriptor, as determined by the | |
381 | value of $^F. See L<perlvar/$^F>. | |
382 | ||
a0d0e21e LW |
383 | =item alarm SECONDS |
384 | ||
54310121 | 385 | =item alarm |
bbce6d69 | 386 | |
a0d0e21e | 387 | Arranges to have a SIGALRM delivered to this process after the |
bbce6d69 | 388 | specified number of seconds have elapsed. If SECONDS is not specified, |
7660c0ab | 389 | the value stored in C<$_> is used. (On some machines, |
a0d0e21e LW |
390 | unfortunately, the elapsed time may be up to one second less than you |
391 | specified because of how seconds are counted.) Only one timer may be | |
392 | counting at once. Each call disables the previous timer, and an | |
7660c0ab | 393 | argument of C<0> may be supplied to cancel the previous timer without |
a0d0e21e LW |
394 | starting a new one. The returned value is the amount of time remaining |
395 | on the previous timer. | |
396 | ||
4633a7c4 | 397 | For delays of finer granularity than one second, you may use Perl's |
19799a22 GS |
398 | four-argument version of select() leaving the first three arguments |
399 | undefined, or you might be able to use the C<syscall> interface to | |
2b5ab1e7 TC |
400 | access setitimer(2) if your system supports it. The Time::HiRes module |
401 | from CPAN may also prove useful. | |
402 | ||
68f8bed4 JH |
403 | It is usually a mistake to intermix C<alarm> and C<sleep> calls. |
404 | (C<sleep> may be internally implemented in your system with C<alarm>) | |
a0d0e21e | 405 | |
19799a22 GS |
406 | If you want to use C<alarm> to time out a system call you need to use an |
407 | C<eval>/C<die> pair. You can't rely on the alarm causing the system call to | |
f86cebdf | 408 | fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to |
19799a22 | 409 | restart system calls on some systems. Using C<eval>/C<die> always works, |
5a964f20 | 410 | modulo the caveats given in L<perlipc/"Signals">. |
ff68c719 | 411 | |
412 | eval { | |
f86cebdf | 413 | local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required |
36477c24 | 414 | alarm $timeout; |
ff68c719 | 415 | $nread = sysread SOCKET, $buffer, $size; |
36477c24 | 416 | alarm 0; |
ff68c719 | 417 | }; |
ff68c719 | 418 | if ($@) { |
f86cebdf | 419 | die unless $@ eq "alarm\n"; # propagate unexpected errors |
ff68c719 | 420 | # timed out |
421 | } | |
422 | else { | |
423 | # didn't | |
424 | } | |
425 | ||
a0d0e21e LW |
426 | =item atan2 Y,X |
427 | ||
428 | Returns the arctangent of Y/X in the range -PI to PI. | |
429 | ||
ca6e1c26 | 430 | For the tangent operation, you may use the C<Math::Trig::tan> |
28757baa | 431 | function, or use the familiar relation: |
432 | ||
433 | sub tan { sin($_[0]) / cos($_[0]) } | |
434 | ||
a0d0e21e LW |
435 | =item bind SOCKET,NAME |
436 | ||
437 | Binds a network address to a socket, just as the bind system call | |
19799a22 | 438 | does. Returns true if it succeeded, false otherwise. NAME should be a |
4633a7c4 LW |
439 | packed address of the appropriate type for the socket. See the examples in |
440 | L<perlipc/"Sockets: Client/Server Communication">. | |
a0d0e21e LW |
441 | |
442 | =item binmode FILEHANDLE | |
443 | ||
19799a22 GS |
444 | Arranges for FILEHANDLE to be read or written in "binary" mode on |
445 | systems whose run-time libraries force the programmer to guess | |
446 | between binary and text files. If FILEHANDLE is an expression, the | |
447 | value is taken as the name of the filehandle. binmode() should be | |
448 | called after the C<open> but before any I/O is done on the filehandle. | |
449 | The only way to reset binary mode on a filehandle is to reopen the | |
450 | file. | |
451 | ||
452 | The operating system, device drivers, C libraries, and Perl run-time | |
453 | system all conspire to let the programmer conveniently treat a | |
454 | simple, one-byte C<\n> as the line terminator, irrespective of its | |
455 | external representation. On Unix and its brethren, the native file | |
456 | representation exactly matches the internal representation, making | |
457 | everyone's lives unbelievably simpler. Consequently, L<binmode> | |
458 | has no effect under Unix, Plan9, or Mac OS, all of which use C<\n> | |
459 | to end each line. (Unix and Plan9 think C<\n> means C<\cJ> and | |
460 | C<\r> means C<\cM>, whereas the Mac goes the other way--it uses | |
461 | C<\cM> for c<\n> and C<\cJ> to mean C<\r>. But that's ok, because | |
462 | it's only one byte, and the internal and external representations | |
463 | match.) | |
464 | ||
465 | In legacy systems like MS-DOS and its embellishments, your program | |
466 | sees a C<\n> as a simple C<\cJ> (just as in Unix), but oddly enough, | |
467 | that's not what's physically stored on disk. What's worse, these | |
468 | systems refuse to help you with this; it's up to you to remember | |
469 | what to do. And you mustn't go applying binmode() with wild abandon, | |
470 | either, because if your system does care about binmode(), then using | |
471 | it when you shouldn't is just as perilous as failing to use it when | |
472 | you should. | |
473 | ||
474 | That means that on any version of Microsoft WinXX that you might | |
475 | care to name (or not), binmode() causes C<\cM\cJ> sequences on disk | |
476 | to be converted to C<\n> when read into your program, and causes | |
477 | any C<\n> in your program to be converted back to C<\cM\cJ> on | |
478 | output to disk. This sad discrepancy leads to no end of | |
479 | problems in not just the readline operator, but also when using | |
480 | seek(), tell(), and read() calls. See L<perlport> for other painful | |
481 | details. See the C<$/> and C<$\> variables in L<perlvar> for how | |
482 | to manually set your input and output line-termination sequences. | |
a0d0e21e | 483 | |
4633a7c4 | 484 | =item bless REF,CLASSNAME |
a0d0e21e LW |
485 | |
486 | =item bless REF | |
487 | ||
2b5ab1e7 TC |
488 | This function tells the thingy referenced by REF that it is now an object |
489 | in the CLASSNAME package. If CLASSNAME is omitted, the current package | |
19799a22 | 490 | is used. Because a C<bless> is often the last thing in a constructor, |
2b5ab1e7 TC |
491 | it returns the reference for convenience. Always use the two-argument |
492 | version if the function doing the blessing might be inherited by a | |
493 | derived class. See L<perltoot> and L<perlobj> for more about the blessing | |
494 | (and blessings) of objects. | |
a0d0e21e | 495 | |
57668c4d | 496 | Consider always blessing objects in CLASSNAMEs that are mixed case. |
2b5ab1e7 TC |
497 | Namespaces with all lowercase names are considered reserved for |
498 | Perl pragmata. Builtin types have all uppercase names, so to prevent | |
499 | confusion, you may wish to avoid such package names as well. Make sure | |
500 | that CLASSNAME is a true value. | |
60ad88b8 GS |
501 | |
502 | See L<perlmod/"Perl Modules">. | |
503 | ||
a0d0e21e LW |
504 | =item caller EXPR |
505 | ||
506 | =item caller | |
507 | ||
5a964f20 | 508 | Returns the context of the current subroutine call. In scalar context, |
28757baa | 509 | returns the caller's package name if there is a caller, that is, if |
19799a22 | 510 | we're in a subroutine or C<eval> or C<require>, and the undefined value |
5a964f20 | 511 | otherwise. In list context, returns |
a0d0e21e | 512 | |
748a9306 | 513 | ($package, $filename, $line) = caller; |
a0d0e21e LW |
514 | |
515 | With EXPR, it returns some extra information that the debugger uses to | |
516 | print a stack trace. The value of EXPR indicates how many call frames | |
517 | to go back before the current one. | |
518 | ||
f3aa04c2 | 519 | ($package, $filename, $line, $subroutine, $hasargs, |
e476b1b5 | 520 | $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i); |
e7ea3e70 | 521 | |
951ba7fe | 522 | Here $subroutine may be C<(eval)> if the frame is not a subroutine |
19799a22 | 523 | call, but an C<eval>. In such a case additional elements $evaltext and |
7660c0ab | 524 | C<$is_require> are set: C<$is_require> is true if the frame is created by a |
19799a22 | 525 | C<require> or C<use> statement, $evaltext contains the text of the |
dc848c6f | 526 | C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement, |
951ba7fe | 527 | $filename is C<(eval)>, but $evaltext is undefined. (Note also that |
dc848c6f | 528 | each C<use> statement creates a C<require> frame inside an C<eval EXPR>) |
e476b1b5 GS |
529 | frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller |
530 | was compiled with. The C<$hints> and C<$bitmask> values are subject to | |
531 | change between versions of Perl, and are not meant for external use. | |
748a9306 LW |
532 | |
533 | Furthermore, when called from within the DB package, caller returns more | |
7660c0ab | 534 | detailed information: it sets the list variable C<@DB::args> to be the |
54310121 | 535 | arguments with which the subroutine was invoked. |
748a9306 | 536 | |
7660c0ab | 537 | Be aware that the optimizer might have optimized call frames away before |
19799a22 | 538 | C<caller> had a chance to get the information. That means that C<caller(N)> |
7660c0ab | 539 | might not return information about the call frame you expect it do, for |
19799a22 GS |
540 | C<N E<gt> 1>. In particular, C<@DB::args> might have information from the |
541 | previous time C<caller> was called. | |
7660c0ab | 542 | |
a0d0e21e LW |
543 | =item chdir EXPR |
544 | ||
2b5ab1e7 | 545 | Changes the working directory to EXPR, if possible. If EXPR is omitted, |
0bfc1ec4 GS |
546 | changes to the directory specified by C<$ENV{HOME}>, if set; if not, |
547 | changes to the directory specified by C<$ENV{LOGDIR}>. If neither is | |
548 | set, C<chdir> does nothing. It returns true upon success, false | |
549 | otherwise. See the example under C<die>. | |
a0d0e21e LW |
550 | |
551 | =item chmod LIST | |
552 | ||
553 | Changes the permissions of a list of files. The first element of the | |
4633a7c4 | 554 | list must be the numerical mode, which should probably be an octal |
2f9daede TP |
555 | number, and which definitely should I<not> a string of octal digits: |
556 | C<0644> is okay, C<'0644'> is not. Returns the number of files | |
dc848c6f | 557 | successfully changed. See also L</oct>, if all you have is a string. |
a0d0e21e LW |
558 | |
559 | $cnt = chmod 0755, 'foo', 'bar'; | |
560 | chmod 0755, @executables; | |
f86cebdf GS |
561 | $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to |
562 | # --w----r-T | |
2f9daede TP |
563 | $mode = '0644'; chmod oct($mode), 'foo'; # this is better |
564 | $mode = 0644; chmod $mode, 'foo'; # this is best | |
a0d0e21e | 565 | |
ca6e1c26 JH |
566 | You can also import the symbolic C<S_I*> constants from the Fcntl |
567 | module: | |
568 | ||
569 | use Fcntl ':mode'; | |
570 | ||
571 | chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables; | |
572 | # This is identical to the chmod 0755 of the above example. | |
573 | ||
a0d0e21e LW |
574 | =item chomp VARIABLE |
575 | ||
576 | =item chomp LIST | |
577 | ||
578 | =item chomp | |
579 | ||
2b5ab1e7 TC |
580 | This safer version of L</chop> removes any trailing string |
581 | that corresponds to the current value of C<$/> (also known as | |
28757baa | 582 | $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total |
583 | number of characters removed from all its arguments. It's often used to | |
584 | remove the newline from the end of an input record when you're worried | |
2b5ab1e7 TC |
585 | that the final record may be missing its newline. When in paragraph |
586 | mode (C<$/ = "">), it removes all trailing newlines from the string. | |
4c5a6083 GS |
587 | When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is |
588 | a reference to an integer or the like, see L<perlvar>) chomp() won't | |
19799a22 GS |
589 | remove anything. |
590 | If VARIABLE is omitted, it chomps C<$_>. Example: | |
a0d0e21e LW |
591 | |
592 | while (<>) { | |
593 | chomp; # avoid \n on last field | |
594 | @array = split(/:/); | |
5a964f20 | 595 | # ... |
a0d0e21e LW |
596 | } |
597 | ||
598 | You can actually chomp anything that's an lvalue, including an assignment: | |
599 | ||
600 | chomp($cwd = `pwd`); | |
601 | chomp($answer = <STDIN>); | |
602 | ||
603 | If you chomp a list, each element is chomped, and the total number of | |
604 | characters removed is returned. | |
605 | ||
606 | =item chop VARIABLE | |
607 | ||
608 | =item chop LIST | |
609 | ||
610 | =item chop | |
611 | ||
612 | Chops off the last character of a string and returns the character | |
613 | chopped. It's used primarily to remove the newline from the end of an | |
614 | input record, but is much more efficient than C<s/\n//> because it neither | |
7660c0ab | 615 | scans nor copies the string. If VARIABLE is omitted, chops C<$_>. |
a0d0e21e LW |
616 | Example: |
617 | ||
618 | while (<>) { | |
619 | chop; # avoid \n on last field | |
620 | @array = split(/:/); | |
5a964f20 | 621 | #... |
a0d0e21e LW |
622 | } |
623 | ||
624 | You can actually chop anything that's an lvalue, including an assignment: | |
625 | ||
626 | chop($cwd = `pwd`); | |
627 | chop($answer = <STDIN>); | |
628 | ||
629 | If you chop a list, each element is chopped. Only the value of the | |
19799a22 | 630 | last C<chop> is returned. |
a0d0e21e | 631 | |
19799a22 | 632 | Note that C<chop> returns the last character. To return all but the last |
748a9306 LW |
633 | character, use C<substr($string, 0, -1)>. |
634 | ||
a0d0e21e LW |
635 | =item chown LIST |
636 | ||
637 | Changes the owner (and group) of a list of files. The first two | |
19799a22 GS |
638 | elements of the list must be the I<numeric> uid and gid, in that |
639 | order. A value of -1 in either position is interpreted by most | |
640 | systems to leave that value unchanged. Returns the number of files | |
641 | successfully changed. | |
a0d0e21e LW |
642 | |
643 | $cnt = chown $uid, $gid, 'foo', 'bar'; | |
644 | chown $uid, $gid, @filenames; | |
645 | ||
54310121 | 646 | Here's an example that looks up nonnumeric uids in the passwd file: |
a0d0e21e LW |
647 | |
648 | print "User: "; | |
19799a22 | 649 | chomp($user = <STDIN>); |
5a964f20 | 650 | print "Files: "; |
19799a22 | 651 | chomp($pattern = <STDIN>); |
a0d0e21e LW |
652 | |
653 | ($login,$pass,$uid,$gid) = getpwnam($user) | |
654 | or die "$user not in passwd file"; | |
655 | ||
5a964f20 | 656 | @ary = glob($pattern); # expand filenames |
a0d0e21e LW |
657 | chown $uid, $gid, @ary; |
658 | ||
54310121 | 659 | On most systems, you are not allowed to change the ownership of the |
4633a7c4 LW |
660 | file unless you're the superuser, although you should be able to change |
661 | the group to any of your secondary groups. On insecure systems, these | |
662 | restrictions may be relaxed, but this is not a portable assumption. | |
19799a22 GS |
663 | On POSIX systems, you can detect this condition this way: |
664 | ||
665 | use POSIX qw(sysconf _PC_CHOWN_RESTRICTED); | |
666 | $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED); | |
4633a7c4 | 667 | |
a0d0e21e LW |
668 | =item chr NUMBER |
669 | ||
54310121 | 670 | =item chr |
bbce6d69 | 671 | |
a0d0e21e | 672 | Returns the character represented by that NUMBER in the character set. |
a0ed51b3 | 673 | For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and |
2b5ab1e7 TC |
674 | chr(0x263a) is a Unicode smiley face (but only within the scope of |
675 | a C<use utf8>). For the reverse, use L</ord>. | |
676 | See L<utf8> for more about Unicode. | |
a0d0e21e | 677 | |
7660c0ab | 678 | If NUMBER is omitted, uses C<$_>. |
bbce6d69 | 679 | |
a0d0e21e LW |
680 | =item chroot FILENAME |
681 | ||
54310121 | 682 | =item chroot |
bbce6d69 | 683 | |
5a964f20 | 684 | This function works like the system call by the same name: it makes the |
4633a7c4 | 685 | named directory the new root directory for all further pathnames that |
951ba7fe | 686 | begin with a C</> by your process and all its children. (It doesn't |
28757baa | 687 | change your current working directory, which is unaffected.) For security |
4633a7c4 | 688 | reasons, this call is restricted to the superuser. If FILENAME is |
19799a22 | 689 | omitted, does a C<chroot> to C<$_>. |
a0d0e21e LW |
690 | |
691 | =item close FILEHANDLE | |
692 | ||
6a518fbc TP |
693 | =item close |
694 | ||
19799a22 | 695 | Closes the file or pipe associated with the file handle, returning true |
a0d0e21e | 696 | only if stdio successfully flushes buffers and closes the system file |
19799a22 | 697 | descriptor. Closes the currently selected filehandle if the argument |
6a518fbc | 698 | is omitted. |
fb73857a | 699 | |
700 | You don't have to close FILEHANDLE if you are immediately going to do | |
19799a22 GS |
701 | another C<open> on it, because C<open> will close it for you. (See |
702 | C<open>.) However, an explicit C<close> on an input file resets the line | |
703 | counter (C<$.>), while the implicit close done by C<open> does not. | |
fb73857a | 704 | |
19799a22 GS |
705 | If the file handle came from a piped open C<close> will additionally |
706 | return false if one of the other system calls involved fails or if the | |
fb73857a | 707 | program exits with non-zero status. (If the only problem was that the |
2b5ab1e7 TC |
708 | program exited non-zero C<$!> will be set to C<0>.) Closing a pipe |
709 | also waits for the process executing on the pipe to complete, in case you | |
710 | want to look at the output of the pipe afterwards, and | |
711 | implicitly puts the exit status value of that command into C<$?>. | |
5a964f20 | 712 | |
73689b13 GS |
713 | Prematurely closing the read end of a pipe (i.e. before the process |
714 | writing to it at the other end has closed it) will result in a | |
715 | SIGPIPE being delivered to the writer. If the other end can't | |
716 | handle that, be sure to read all the data before closing the pipe. | |
717 | ||
fb73857a | 718 | Example: |
a0d0e21e | 719 | |
fb73857a | 720 | open(OUTPUT, '|sort >foo') # pipe to sort |
721 | or die "Can't start sort: $!"; | |
5a964f20 | 722 | #... # print stuff to output |
fb73857a | 723 | close OUTPUT # wait for sort to finish |
724 | or warn $! ? "Error closing sort pipe: $!" | |
725 | : "Exit status $? from sort"; | |
726 | open(INPUT, 'foo') # get sort's results | |
727 | or die "Can't open 'foo' for input: $!"; | |
a0d0e21e | 728 | |
5a964f20 TC |
729 | FILEHANDLE may be an expression whose value can be used as an indirect |
730 | filehandle, usually the real filehandle name. | |
a0d0e21e LW |
731 | |
732 | =item closedir DIRHANDLE | |
733 | ||
19799a22 | 734 | Closes a directory opened by C<opendir> and returns the success of that |
5a964f20 TC |
735 | system call. |
736 | ||
737 | DIRHANDLE may be an expression whose value can be used as an indirect | |
738 | dirhandle, usually the real dirhandle name. | |
a0d0e21e LW |
739 | |
740 | =item connect SOCKET,NAME | |
741 | ||
742 | Attempts to connect to a remote socket, just as the connect system call | |
19799a22 | 743 | does. Returns true if it succeeded, false otherwise. NAME should be a |
4633a7c4 LW |
744 | packed address of the appropriate type for the socket. See the examples in |
745 | L<perlipc/"Sockets: Client/Server Communication">. | |
a0d0e21e | 746 | |
cb1a09d0 AD |
747 | =item continue BLOCK |
748 | ||
749 | Actually a flow control statement rather than a function. If there is a | |
98293880 JH |
750 | C<continue> BLOCK attached to a BLOCK (typically in a C<while> or |
751 | C<foreach>), it is always executed just before the conditional is about to | |
752 | be evaluated again, just like the third part of a C<for> loop in C. Thus | |
cb1a09d0 AD |
753 | it can be used to increment a loop variable, even when the loop has been |
754 | continued via the C<next> statement (which is similar to the C C<continue> | |
755 | statement). | |
756 | ||
98293880 | 757 | C<last>, C<next>, or C<redo> may appear within a C<continue> |
19799a22 GS |
758 | block. C<last> and C<redo> will behave as if they had been executed within |
759 | the main block. So will C<next>, but since it will execute a C<continue> | |
1d2dff63 GS |
760 | block, it may be more entertaining. |
761 | ||
762 | while (EXPR) { | |
763 | ### redo always comes here | |
764 | do_something; | |
765 | } continue { | |
766 | ### next always comes here | |
767 | do_something_else; | |
768 | # then back the top to re-check EXPR | |
769 | } | |
770 | ### last always comes here | |
771 | ||
772 | Omitting the C<continue> section is semantically equivalent to using an | |
19799a22 | 773 | empty one, logically enough. In that case, C<next> goes directly back |
1d2dff63 GS |
774 | to check the condition at the top of the loop. |
775 | ||
a0d0e21e LW |
776 | =item cos EXPR |
777 | ||
5a964f20 | 778 | Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, |
7660c0ab | 779 | takes cosine of C<$_>. |
a0d0e21e | 780 | |
ca6e1c26 | 781 | For the inverse cosine operation, you may use the C<Math::Trig::acos()> |
28757baa | 782 | function, or use this relation: |
783 | ||
784 | sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) } | |
785 | ||
a0d0e21e LW |
786 | =item crypt PLAINTEXT,SALT |
787 | ||
f86cebdf | 788 | Encrypts a string exactly like the crypt(3) function in the C library |
4633a7c4 LW |
789 | (assuming that you actually have a version there that has not been |
790 | extirpated as a potential munition). This can prove useful for checking | |
791 | the password file for lousy passwords, amongst other things. Only the | |
792 | guys wearing white hats should do this. | |
a0d0e21e | 793 | |
19799a22 | 794 | Note that C<crypt> is intended to be a one-way function, much like breaking |
11155c91 CS |
795 | eggs to make an omelette. There is no (known) corresponding decrypt |
796 | function. As a result, this function isn't all that useful for | |
797 | cryptography. (For that, see your nearby CPAN mirror.) | |
2f9daede | 798 | |
e71965be RS |
799 | When verifying an existing encrypted string you should use the encrypted |
800 | text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This | |
19799a22 | 801 | allows your code to work with the standard C<crypt> and with more |
e71965be RS |
802 | exotic implementations. When choosing a new salt create a random two |
803 | character string whose characters come from the set C<[./0-9A-Za-z]> | |
804 | (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). | |
805 | ||
a0d0e21e LW |
806 | Here's an example that makes sure that whoever runs this program knows |
807 | their own password: | |
808 | ||
809 | $pwd = (getpwuid($<))[1]; | |
a0d0e21e LW |
810 | |
811 | system "stty -echo"; | |
812 | print "Password: "; | |
e71965be | 813 | chomp($word = <STDIN>); |
a0d0e21e LW |
814 | print "\n"; |
815 | system "stty echo"; | |
816 | ||
e71965be | 817 | if (crypt($word, $pwd) ne $pwd) { |
a0d0e21e LW |
818 | die "Sorry...\n"; |
819 | } else { | |
820 | print "ok\n"; | |
54310121 | 821 | } |
a0d0e21e | 822 | |
9f8f0c9d | 823 | Of course, typing in your own password to whoever asks you |
748a9306 | 824 | for it is unwise. |
a0d0e21e | 825 | |
19799a22 GS |
826 | The L<crypt> function is unsuitable for encrypting large quantities |
827 | of data, not least of all because you can't get the information | |
828 | back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories | |
829 | on your favorite CPAN mirror for a slew of potentially useful | |
830 | modules. | |
831 | ||
aa689395 | 832 | =item dbmclose HASH |
a0d0e21e | 833 | |
19799a22 | 834 | [This function has been largely superseded by the C<untie> function.] |
a0d0e21e | 835 | |
aa689395 | 836 | Breaks the binding between a DBM file and a hash. |
a0d0e21e | 837 | |
19799a22 | 838 | =item dbmopen HASH,DBNAME,MASK |
a0d0e21e | 839 | |
19799a22 | 840 | [This function has been largely superseded by the C<tie> function.] |
a0d0e21e | 841 | |
7b8d334a | 842 | This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a |
19799a22 GS |
843 | hash. HASH is the name of the hash. (Unlike normal C<open>, the first |
844 | argument is I<not> a filehandle, even though it looks like one). DBNAME | |
aa689395 | 845 | is the name of the database (without the F<.dir> or F<.pag> extension if |
846 | any). If the database does not exist, it is created with protection | |
19799a22 GS |
847 | specified by MASK (as modified by the C<umask>). If your system supports |
848 | only the older DBM functions, you may perform only one C<dbmopen> in your | |
aa689395 | 849 | program. In older versions of Perl, if your system had neither DBM nor |
19799a22 | 850 | ndbm, calling C<dbmopen> produced a fatal error; it now falls back to |
aa689395 | 851 | sdbm(3). |
852 | ||
853 | If you don't have write access to the DBM file, you can only read hash | |
854 | variables, not set them. If you want to test whether you can write, | |
19799a22 | 855 | either use file tests or try setting a dummy hash entry inside an C<eval>, |
aa689395 | 856 | which will trap the error. |
a0d0e21e | 857 | |
19799a22 GS |
858 | Note that functions such as C<keys> and C<values> may return huge lists |
859 | when used on large DBM files. You may prefer to use the C<each> | |
a0d0e21e LW |
860 | function to iterate over large DBM files. Example: |
861 | ||
862 | # print out history file offsets | |
863 | dbmopen(%HIST,'/usr/lib/news/history',0666); | |
864 | while (($key,$val) = each %HIST) { | |
865 | print $key, ' = ', unpack('L',$val), "\n"; | |
866 | } | |
867 | dbmclose(%HIST); | |
868 | ||
cb1a09d0 | 869 | See also L<AnyDBM_File> for a more general description of the pros and |
184e9718 | 870 | cons of the various dbm approaches, as well as L<DB_File> for a particularly |
cb1a09d0 | 871 | rich implementation. |
4633a7c4 | 872 | |
2b5ab1e7 TC |
873 | You can control which DBM library you use by loading that library |
874 | before you call dbmopen(): | |
875 | ||
876 | use DB_File; | |
877 | dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db") | |
878 | or die "Can't open netscape history file: $!"; | |
879 | ||
a0d0e21e LW |
880 | =item defined EXPR |
881 | ||
54310121 | 882 | =item defined |
bbce6d69 | 883 | |
2f9daede TP |
884 | Returns a Boolean value telling whether EXPR has a value other than |
885 | the undefined value C<undef>. If EXPR is not present, C<$_> will be | |
886 | checked. | |
887 | ||
888 | Many operations return C<undef> to indicate failure, end of file, | |
889 | system error, uninitialized variable, and other exceptional | |
890 | conditions. This function allows you to distinguish C<undef> from | |
891 | other values. (A simple Boolean test will not distinguish among | |
7660c0ab | 892 | C<undef>, zero, the empty string, and C<"0">, which are all equally |
2f9daede | 893 | false.) Note that since C<undef> is a valid scalar, its presence |
19799a22 | 894 | doesn't I<necessarily> indicate an exceptional condition: C<pop> |
2f9daede TP |
895 | returns C<undef> when its argument is an empty array, I<or> when the |
896 | element to return happens to be C<undef>. | |
897 | ||
f10b0346 GS |
898 | You may also use C<defined(&func)> to check whether subroutine C<&func> |
899 | has ever been defined. The return value is unaffected by any forward | |
900 | declarations of C<&foo>. | |
901 | ||
902 | Use of C<defined> on aggregates (hashes and arrays) is deprecated. It | |
903 | used to report whether memory for that aggregate has ever been | |
904 | allocated. This behavior may disappear in future versions of Perl. | |
905 | You should instead use a simple test for size: | |
906 | ||
907 | if (@an_array) { print "has array elements\n" } | |
908 | if (%a_hash) { print "has hash members\n" } | |
2f9daede TP |
909 | |
910 | When used on a hash element, it tells you whether the value is defined, | |
dc848c6f | 911 | not whether the key exists in the hash. Use L</exists> for the latter |
2f9daede | 912 | purpose. |
a0d0e21e LW |
913 | |
914 | Examples: | |
915 | ||
916 | print if defined $switch{'D'}; | |
917 | print "$val\n" while defined($val = pop(@ary)); | |
918 | die "Can't readlink $sym: $!" | |
919 | unless defined($value = readlink $sym); | |
a0d0e21e | 920 | sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } |
2f9daede | 921 | $debugging = 0 unless defined $debugging; |
a0d0e21e | 922 | |
19799a22 | 923 | Note: Many folks tend to overuse C<defined>, and then are surprised to |
7660c0ab | 924 | discover that the number C<0> and C<""> (the zero-length string) are, in fact, |
2f9daede | 925 | defined values. For example, if you say |
a5f75d66 AD |
926 | |
927 | "ab" =~ /a(.*)b/; | |
928 | ||
7660c0ab | 929 | The pattern match succeeds, and C<$1> is defined, despite the fact that it |
a5f75d66 | 930 | matched "nothing". But it didn't really match nothing--rather, it |
2b5ab1e7 | 931 | matched something that happened to be zero characters long. This is all |
a5f75d66 | 932 | very above-board and honest. When a function returns an undefined value, |
2f9daede | 933 | it's an admission that it couldn't give you an honest answer. So you |
19799a22 | 934 | should use C<defined> only when you're questioning the integrity of what |
7660c0ab | 935 | you're trying to do. At other times, a simple comparison to C<0> or C<""> is |
2f9daede TP |
936 | what you want. |
937 | ||
dc848c6f | 938 | See also L</undef>, L</exists>, L</ref>. |
2f9daede | 939 | |
a0d0e21e LW |
940 | =item delete EXPR |
941 | ||
01020589 GS |
942 | Given an expression that specifies a hash element, array element, hash slice, |
943 | or array slice, deletes the specified element(s) from the hash or array. | |
8216c1fd GS |
944 | In the case of an array, if the array elements happen to be at the end, |
945 | the size of the array will shrink to the highest element that tests | |
946 | true for exists() (or 0 if no such element exists). | |
a0d0e21e | 947 | |
01020589 GS |
948 | Returns each element so deleted or the undefined value if there was no such |
949 | element. Deleting from C<$ENV{}> modifies the environment. Deleting from | |
950 | a hash tied to a DBM file deletes the entry from the DBM file. Deleting | |
951 | from a C<tie>d hash or array may not necessarily return anything. | |
952 | ||
8ea97a1e GS |
953 | Deleting an array element effectively returns that position of the array |
954 | to its initial, uninitialized state. Subsequently testing for the same | |
8216c1fd GS |
955 | element with exists() will return false. Note that deleting array |
956 | elements in the middle of an array will not shift the index of the ones | |
957 | after them down--use splice() for that. See L</exists>. | |
8ea97a1e | 958 | |
01020589 | 959 | The following (inefficiently) deletes all the values of %HASH and @ARRAY: |
a0d0e21e | 960 | |
5f05dabc | 961 | foreach $key (keys %HASH) { |
962 | delete $HASH{$key}; | |
a0d0e21e LW |
963 | } |
964 | ||
01020589 GS |
965 | foreach $index (0 .. $#ARRAY) { |
966 | delete $ARRAY[$index]; | |
967 | } | |
968 | ||
969 | And so do these: | |
5f05dabc | 970 | |
01020589 GS |
971 | delete @HASH{keys %HASH}; |
972 | ||
9740c838 | 973 | delete @ARRAY[0 .. $#ARRAY]; |
5f05dabc | 974 | |
2b5ab1e7 | 975 | But both of these are slower than just assigning the empty list |
01020589 GS |
976 | or undefining %HASH or @ARRAY: |
977 | ||
978 | %HASH = (); # completely empty %HASH | |
979 | undef %HASH; # forget %HASH ever existed | |
2b5ab1e7 | 980 | |
01020589 GS |
981 | @ARRAY = (); # completely empty @ARRAY |
982 | undef @ARRAY; # forget @ARRAY ever existed | |
2b5ab1e7 TC |
983 | |
984 | Note that the EXPR can be arbitrarily complicated as long as the final | |
01020589 GS |
985 | operation is a hash element, array element, hash slice, or array slice |
986 | lookup: | |
a0d0e21e LW |
987 | |
988 | delete $ref->[$x][$y]{$key}; | |
5f05dabc | 989 | delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys}; |
a0d0e21e | 990 | |
01020589 GS |
991 | delete $ref->[$x][$y][$index]; |
992 | delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices]; | |
993 | ||
a0d0e21e LW |
994 | =item die LIST |
995 | ||
19799a22 GS |
996 | Outside an C<eval>, prints the value of LIST to C<STDERR> and |
997 | exits with the current value of C<$!> (errno). If C<$!> is C<0>, | |
998 | exits with the value of C<($? E<gt>E<gt> 8)> (backtick `command` | |
999 | status). If C<($? E<gt>E<gt> 8)> is C<0>, exits with C<255>. Inside | |
1000 | an C<eval(),> the error message is stuffed into C<$@> and the | |
1001 | C<eval> is terminated with the undefined value. This makes | |
1002 | C<die> the way to raise an exception. | |
a0d0e21e LW |
1003 | |
1004 | Equivalent examples: | |
1005 | ||
1006 | die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; | |
54310121 | 1007 | chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" |
a0d0e21e LW |
1008 | |
1009 | If the value of EXPR does not end in a newline, the current script line | |
1010 | number and input line number (if any) are also printed, and a newline | |
883faa13 GS |
1011 | is supplied. Note that the "input line number" (also known as "chunk") |
1012 | is subject to whatever notion of "line" happens to be currently in | |
1013 | effect, and is also available as the special variable C<$.>. | |
1014 | See L<perlvar/"$/"> and L<perlvar/"$.">. | |
1015 | ||
1016 | Hint: sometimes appending C<", stopped"> to your message | |
7660c0ab | 1017 | will cause it to make better sense when the string C<"at foo line 123"> is |
a0d0e21e LW |
1018 | appended. Suppose you are running script "canasta". |
1019 | ||
1020 | die "/etc/games is no good"; | |
1021 | die "/etc/games is no good, stopped"; | |
1022 | ||
1023 | produce, respectively | |
1024 | ||
1025 | /etc/games is no good at canasta line 123. | |
1026 | /etc/games is no good, stopped at canasta line 123. | |
1027 | ||
2b5ab1e7 | 1028 | See also exit(), warn(), and the Carp module. |
a0d0e21e | 1029 | |
7660c0ab A |
1030 | If LIST is empty and C<$@> already contains a value (typically from a |
1031 | previous eval) that value is reused after appending C<"\t...propagated">. | |
fb73857a | 1032 | This is useful for propagating exceptions: |
1033 | ||
1034 | eval { ... }; | |
1035 | die unless $@ =~ /Expected exception/; | |
1036 | ||
7660c0ab | 1037 | If C<$@> is empty then the string C<"Died"> is used. |
fb73857a | 1038 | |
52531d10 GS |
1039 | die() can also be called with a reference argument. If this happens to be |
1040 | trapped within an eval(), $@ contains the reference. This behavior permits | |
1041 | a more elaborate exception handling implementation using objects that | |
1042 | maintain arbitary state about the nature of the exception. Such a scheme | |
1043 | is sometimes preferable to matching particular string values of $@ using | |
1044 | regular expressions. Here's an example: | |
1045 | ||
1046 | eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) }; | |
1047 | if ($@) { | |
1048 | if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) { | |
1049 | # handle Some::Module::Exception | |
1050 | } | |
1051 | else { | |
1052 | # handle all other possible exceptions | |
1053 | } | |
1054 | } | |
1055 | ||
19799a22 | 1056 | Because perl will stringify uncaught exception messages before displaying |
52531d10 GS |
1057 | them, you may want to overload stringification operations on such custom |
1058 | exception objects. See L<overload> for details about that. | |
1059 | ||
19799a22 GS |
1060 | You can arrange for a callback to be run just before the C<die> |
1061 | does its deed, by setting the C<$SIG{__DIE__}> hook. The associated | |
1062 | handler will be called with the error text and can change the error | |
1063 | message, if it sees fit, by calling C<die> again. See | |
1064 | L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and | |
1065 | L<"eval BLOCK"> for some examples. Although this feature was meant | |
1066 | to be run only right before your program was to exit, this is not | |
1067 | currently the case--the C<$SIG{__DIE__}> hook is currently called | |
1068 | even inside eval()ed blocks/strings! If one wants the hook to do | |
1069 | nothing in such situations, put | |
fb73857a | 1070 | |
1071 | die @_ if $^S; | |
1072 | ||
19799a22 GS |
1073 | as the first line of the handler (see L<perlvar/$^S>). Because |
1074 | this promotes strange action at a distance, this counterintuitive | |
1075 | behavior may be fixed in a future release. | |
774d564b | 1076 | |
a0d0e21e LW |
1077 | =item do BLOCK |
1078 | ||
1079 | Not really a function. Returns the value of the last command in the | |
1080 | sequence of commands indicated by BLOCK. When modified by a loop | |
98293880 JH |
1081 | modifier, executes the BLOCK once before testing the loop condition. |
1082 | (On other statements the loop modifiers test the conditional first.) | |
a0d0e21e | 1083 | |
4968c1e4 | 1084 | C<do BLOCK> does I<not> count as a loop, so the loop control statements |
2b5ab1e7 TC |
1085 | C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. |
1086 | See L<perlsyn> for alternative strategies. | |
4968c1e4 | 1087 | |
a0d0e21e LW |
1088 | =item do SUBROUTINE(LIST) |
1089 | ||
1090 | A deprecated form of subroutine call. See L<perlsub>. | |
1091 | ||
1092 | =item do EXPR | |
1093 | ||
1094 | Uses the value of EXPR as a filename and executes the contents of the | |
1095 | file as a Perl script. Its primary use is to include subroutines | |
1096 | from a Perl subroutine library. | |
1097 | ||
1098 | do 'stat.pl'; | |
1099 | ||
1100 | is just like | |
1101 | ||
fb73857a | 1102 | scalar eval `cat stat.pl`; |
a0d0e21e | 1103 | |
2b5ab1e7 TC |
1104 | except that it's more efficient and concise, keeps track of the current |
1105 | filename for error messages, searches the @INC libraries, and updates | |
1106 | C<%INC> if the file is found. See L<perlvar/Predefined Names> for these | |
1107 | variables. It also differs in that code evaluated with C<do FILENAME> | |
1108 | cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the | |
1109 | same, however, in that it does reparse the file every time you call it, | |
1110 | so you probably don't want to do this inside a loop. | |
a0d0e21e | 1111 | |
8e30cc93 | 1112 | If C<do> cannot read the file, it returns undef and sets C<$!> to the |
2b5ab1e7 | 1113 | error. If C<do> can read the file but cannot compile it, it |
8e30cc93 MG |
1114 | returns undef and sets an error message in C<$@>. If the file is |
1115 | successfully compiled, C<do> returns the value of the last expression | |
1116 | evaluated. | |
1117 | ||
a0d0e21e | 1118 | Note that inclusion of library modules is better done with the |
19799a22 | 1119 | C<use> and C<require> operators, which also do automatic error checking |
4633a7c4 | 1120 | and raise an exception if there's a problem. |
a0d0e21e | 1121 | |
5a964f20 TC |
1122 | You might like to use C<do> to read in a program configuration |
1123 | file. Manual error checking can be done this way: | |
1124 | ||
1125 | # read in config files: system first, then user | |
f86cebdf | 1126 | for $file ("/share/prog/defaults.rc", |
2b5ab1e7 TC |
1127 | "$ENV{HOME}/.someprogrc") |
1128 | { | |
5a964f20 | 1129 | unless ($return = do $file) { |
f86cebdf GS |
1130 | warn "couldn't parse $file: $@" if $@; |
1131 | warn "couldn't do $file: $!" unless defined $return; | |
1132 | warn "couldn't run $file" unless $return; | |
5a964f20 TC |
1133 | } |
1134 | } | |
1135 | ||
a0d0e21e LW |
1136 | =item dump LABEL |
1137 | ||
1614b0e3 JD |
1138 | =item dump |
1139 | ||
19799a22 GS |
1140 | This function causes an immediate core dump. See also the B<-u> |
1141 | command-line switch in L<perlrun>, which does the same thing. | |
1142 | Primarily this is so that you can use the B<undump> program (not | |
1143 | supplied) to turn your core dump into an executable binary after | |
1144 | having initialized all your variables at the beginning of the | |
1145 | program. When the new binary is executed it will begin by executing | |
1146 | a C<goto LABEL> (with all the restrictions that C<goto> suffers). | |
1147 | Think of it as a goto with an intervening core dump and reincarnation. | |
1148 | If C<LABEL> is omitted, restarts the program from the top. | |
1149 | ||
1150 | B<WARNING>: Any files opened at the time of the dump will I<not> | |
1151 | be open any more when the program is reincarnated, with possible | |
1152 | resulting confusion on the part of Perl. | |
1153 | ||
1154 | This function is now largely obsolete, partly because it's very | |
1155 | hard to convert a core file into an executable, and because the | |
1156 | real compiler backends for generating portable bytecode and compilable | |
1157 | C code have superseded it. | |
1158 | ||
1159 | If you're looking to use L<dump> to speed up your program, consider | |
1160 | generating bytecode or native C code as described in L<perlcc>. If | |
1161 | you're just trying to accelerate a CGI script, consider using the | |
1162 | C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI. | |
1163 | You might also consider autoloading or selfloading, which at least | |
1164 | make your program I<appear> to run faster. | |
5a964f20 | 1165 | |
aa689395 | 1166 | =item each HASH |
1167 | ||
5a964f20 | 1168 | When called in list context, returns a 2-element list consisting of the |
aa689395 | 1169 | key and value for the next element of a hash, so that you can iterate over |
5a964f20 | 1170 | it. When called in scalar context, returns the key for only the "next" |
e902a979 | 1171 | element in the hash. |
2f9daede | 1172 | |
ab192400 GS |
1173 | Entries are returned in an apparently random order. The actual random |
1174 | order is subject to change in future versions of perl, but it is guaranteed | |
19799a22 | 1175 | to be in the same order as either the C<keys> or C<values> function |
ab192400 GS |
1176 | would produce on the same (unmodified) hash. |
1177 | ||
1178 | When the hash is entirely read, a null array is returned in list context | |
19799a22 GS |
1179 | (which when assigned produces a false (C<0>) value), and C<undef> in |
1180 | scalar context. The next call to C<each> after that will start iterating | |
1181 | again. There is a single iterator for each hash, shared by all C<each>, | |
1182 | C<keys>, and C<values> function calls in the program; it can be reset by | |
2f9daede TP |
1183 | reading all the elements from the hash, or by evaluating C<keys HASH> or |
1184 | C<values HASH>. If you add or delete elements of a hash while you're | |
1185 | iterating over it, you may get entries skipped or duplicated, so don't. | |
aa689395 | 1186 | |
f86cebdf | 1187 | The following prints out your environment like the printenv(1) program, |
aa689395 | 1188 | only in a different order: |
a0d0e21e LW |
1189 | |
1190 | while (($key,$value) = each %ENV) { | |
1191 | print "$key=$value\n"; | |
1192 | } | |
1193 | ||
19799a22 | 1194 | See also C<keys>, C<values> and C<sort>. |
a0d0e21e LW |
1195 | |
1196 | =item eof FILEHANDLE | |
1197 | ||
4633a7c4 LW |
1198 | =item eof () |
1199 | ||
a0d0e21e LW |
1200 | =item eof |
1201 | ||
1202 | Returns 1 if the next read on FILEHANDLE will return end of file, or if | |
1203 | FILEHANDLE is not open. FILEHANDLE may be an expression whose value | |
5a964f20 | 1204 | gives the real filehandle. (Note that this function actually |
19799a22 | 1205 | reads a character and then C<ungetc>s it, so isn't very useful in an |
748a9306 | 1206 | interactive context.) Do not read from a terminal file (or call |
19799a22 | 1207 | C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such |
748a9306 LW |
1208 | as terminals may lose the end-of-file condition if you do. |
1209 | ||
820475bd GS |
1210 | An C<eof> without an argument uses the last file read. Using C<eof()> |
1211 | with empty parentheses is very different. It refers to the pseudo file | |
1212 | formed from the files listed on the command line and accessed via the | |
1213 | C<E<lt>E<gt>> operator. Since C<E<lt>E<gt>> isn't explicitly opened, | |
1214 | as a normal filehandle is, an C<eof()> before C<E<lt>E<gt>> has been | |
1215 | used will cause C<@ARGV> to be examined to determine if input is | |
1216 | available. | |
1217 | ||
1218 | In a C<while (E<lt>E<gt>)> loop, C<eof> or C<eof(ARGV)> can be used to | |
1219 | detect the end of each file, C<eof()> will only detect the end of the | |
1220 | last file. Examples: | |
a0d0e21e | 1221 | |
748a9306 LW |
1222 | # reset line numbering on each input file |
1223 | while (<>) { | |
5a964f20 | 1224 | next if /^\s*#/; # skip comments |
748a9306 | 1225 | print "$.\t$_"; |
5a964f20 TC |
1226 | } continue { |
1227 | close ARGV if eof; # Not eof()! | |
748a9306 LW |
1228 | } |
1229 | ||
a0d0e21e LW |
1230 | # insert dashes just before last line of last file |
1231 | while (<>) { | |
5a964f20 | 1232 | if (eof()) { # check for end of current file |
a0d0e21e | 1233 | print "--------------\n"; |
2b5ab1e7 | 1234 | close(ARGV); # close or last; is needed if we |
748a9306 | 1235 | # are reading from the terminal |
a0d0e21e LW |
1236 | } |
1237 | print; | |
1238 | } | |
1239 | ||
a0d0e21e | 1240 | Practical hint: you almost never need to use C<eof> in Perl, because the |
3ce0d271 GS |
1241 | input operators typically return C<undef> when they run out of data, or if |
1242 | there was an error. | |
a0d0e21e LW |
1243 | |
1244 | =item eval EXPR | |
1245 | ||
1246 | =item eval BLOCK | |
1247 | ||
c7cc6f1c GS |
1248 | In the first form, the return value of EXPR is parsed and executed as if it |
1249 | were a little Perl program. The value of the expression (which is itself | |
5a964f20 | 1250 | determined within scalar context) is first parsed, and if there weren't any |
c7cc6f1c | 1251 | errors, executed in the context of the current Perl program, so that any |
5f05dabc | 1252 | variable settings or subroutine and format definitions remain afterwards. |
c7cc6f1c GS |
1253 | Note that the value is parsed every time the eval executes. If EXPR is |
1254 | omitted, evaluates C<$_>. This form is typically used to delay parsing | |
1255 | and subsequent execution of the text of EXPR until run time. | |
1256 | ||
1257 | In the second form, the code within the BLOCK is parsed only once--at the | |
1258 | same time the code surrounding the eval itself was parsed--and executed | |
1259 | within the context of the current Perl program. This form is typically | |
1260 | used to trap exceptions more efficiently than the first (see below), while | |
1261 | also providing the benefit of checking the code within BLOCK at compile | |
1262 | time. | |
1263 | ||
1264 | The final semicolon, if any, may be omitted from the value of EXPR or within | |
1265 | the BLOCK. | |
1266 | ||
1267 | In both forms, the value returned is the value of the last expression | |
5a964f20 | 1268 | evaluated inside the mini-program; a return statement may be also used, just |
c7cc6f1c | 1269 | as with subroutines. The expression providing the return value is evaluated |
5a964f20 | 1270 | in void, scalar, or list context, depending on the context of the eval itself. |
c7cc6f1c | 1271 | See L</wantarray> for more on how the evaluation context can be determined. |
a0d0e21e | 1272 | |
19799a22 GS |
1273 | If there is a syntax error or runtime error, or a C<die> statement is |
1274 | executed, an undefined value is returned by C<eval>, and C<$@> is set to the | |
a0d0e21e | 1275 | error message. If there was no error, C<$@> is guaranteed to be a null |
19799a22 | 1276 | string. Beware that using C<eval> neither silences perl from printing |
c7cc6f1c GS |
1277 | warnings to STDERR, nor does it stuff the text of warning messages into C<$@>. |
1278 | To do either of those, you have to use the C<$SIG{__WARN__}> facility. See | |
1279 | L</warn> and L<perlvar>. | |
a0d0e21e | 1280 | |
19799a22 GS |
1281 | Note that, because C<eval> traps otherwise-fatal errors, it is useful for |
1282 | determining whether a particular feature (such as C<socket> or C<symlink>) | |
a0d0e21e LW |
1283 | is implemented. It is also Perl's exception trapping mechanism, where |
1284 | the die operator is used to raise exceptions. | |
1285 | ||
1286 | If the code to be executed doesn't vary, you may use the eval-BLOCK | |
1287 | form to trap run-time errors without incurring the penalty of | |
1288 | recompiling each time. The error, if any, is still returned in C<$@>. | |
1289 | Examples: | |
1290 | ||
54310121 | 1291 | # make divide-by-zero nonfatal |
a0d0e21e LW |
1292 | eval { $answer = $a / $b; }; warn $@ if $@; |
1293 | ||
1294 | # same thing, but less efficient | |
1295 | eval '$answer = $a / $b'; warn $@ if $@; | |
1296 | ||
1297 | # a compile-time error | |
5a964f20 | 1298 | eval { $answer = }; # WRONG |
a0d0e21e LW |
1299 | |
1300 | # a run-time error | |
1301 | eval '$answer ='; # sets $@ | |
1302 | ||
2b5ab1e7 TC |
1303 | Due to the current arguably broken state of C<__DIE__> hooks, when using |
1304 | the C<eval{}> form as an exception trap in libraries, you may wish not | |
1305 | to trigger any C<__DIE__> hooks that user code may have installed. | |
1306 | You can use the C<local $SIG{__DIE__}> construct for this purpose, | |
1307 | as shown in this example: | |
774d564b | 1308 | |
1309 | # a very private exception trap for divide-by-zero | |
f86cebdf GS |
1310 | eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; |
1311 | warn $@ if $@; | |
774d564b | 1312 | |
1313 | This is especially significant, given that C<__DIE__> hooks can call | |
19799a22 | 1314 | C<die> again, which has the effect of changing their error messages: |
774d564b | 1315 | |
1316 | # __DIE__ hooks may modify error messages | |
1317 | { | |
f86cebdf GS |
1318 | local $SIG{'__DIE__'} = |
1319 | sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; | |
c7cc6f1c GS |
1320 | eval { die "foo lives here" }; |
1321 | print $@ if $@; # prints "bar lives here" | |
774d564b | 1322 | } |
1323 | ||
19799a22 | 1324 | Because this promotes action at a distance, this counterintuitive behavior |
2b5ab1e7 TC |
1325 | may be fixed in a future release. |
1326 | ||
19799a22 | 1327 | With an C<eval>, you should be especially careful to remember what's |
a0d0e21e LW |
1328 | being looked at when: |
1329 | ||
1330 | eval $x; # CASE 1 | |
1331 | eval "$x"; # CASE 2 | |
1332 | ||
1333 | eval '$x'; # CASE 3 | |
1334 | eval { $x }; # CASE 4 | |
1335 | ||
5a964f20 | 1336 | eval "\$$x++"; # CASE 5 |
a0d0e21e LW |
1337 | $$x++; # CASE 6 |
1338 | ||
2f9daede | 1339 | Cases 1 and 2 above behave identically: they run the code contained in |
19799a22 | 1340 | the variable $x. (Although case 2 has misleading double quotes making |
2f9daede | 1341 | the reader wonder what else might be happening (nothing is).) Cases 3 |
7660c0ab | 1342 | and 4 likewise behave in the same way: they run the code C<'$x'>, which |
19799a22 | 1343 | does nothing but return the value of $x. (Case 4 is preferred for |
2f9daede TP |
1344 | purely visual reasons, but it also has the advantage of compiling at |
1345 | compile-time instead of at run-time.) Case 5 is a place where | |
19799a22 | 1346 | normally you I<would> like to use double quotes, except that in this |
2f9daede TP |
1347 | particular situation, you can just use symbolic references instead, as |
1348 | in case 6. | |
a0d0e21e | 1349 | |
4968c1e4 | 1350 | C<eval BLOCK> does I<not> count as a loop, so the loop control statements |
2b5ab1e7 | 1351 | C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. |
4968c1e4 | 1352 | |
a0d0e21e LW |
1353 | =item exec LIST |
1354 | ||
8bf3b016 GS |
1355 | =item exec PROGRAM LIST |
1356 | ||
19799a22 GS |
1357 | The C<exec> function executes a system command I<and never returns>-- |
1358 | use C<system> instead of C<exec> if you want it to return. It fails and | |
1359 | returns false only if the command does not exist I<and> it is executed | |
fb73857a | 1360 | directly instead of via your system's command shell (see below). |
a0d0e21e | 1361 | |
19799a22 GS |
1362 | Since it's a common mistake to use C<exec> instead of C<system>, Perl |
1363 | warns you if there is a following statement which isn't C<die>, C<warn>, | |
1364 | or C<exit> (if C<-w> is set - but you always do that). If you | |
1365 | I<really> want to follow an C<exec> with some other statement, you | |
55d729e4 GS |
1366 | can use one of these styles to avoid the warning: |
1367 | ||
5a964f20 TC |
1368 | exec ('foo') or print STDERR "couldn't exec foo: $!"; |
1369 | { exec ('foo') }; print STDERR "couldn't exec foo: $!"; | |
55d729e4 | 1370 | |
5a964f20 | 1371 | If there is more than one argument in LIST, or if LIST is an array |
f86cebdf | 1372 | with more than one value, calls execvp(3) with the arguments in LIST. |
5a964f20 TC |
1373 | If there is only one scalar argument or an array with one element in it, |
1374 | the argument is checked for shell metacharacters, and if there are any, | |
1375 | the entire argument is passed to the system's command shell for parsing | |
1376 | (this is C</bin/sh -c> on Unix platforms, but varies on other platforms). | |
1377 | If there are no shell metacharacters in the argument, it is split into | |
19799a22 GS |
1378 | words and passed directly to C<execvp>, which is more efficient. |
1379 | Examples: | |
a0d0e21e | 1380 | |
19799a22 GS |
1381 | exec '/bin/echo', 'Your arguments are: ', @ARGV; |
1382 | exec "sort $outfile | uniq"; | |
a0d0e21e LW |
1383 | |
1384 | If you don't really want to execute the first argument, but want to lie | |
1385 | to the program you are executing about its own name, you can specify | |
1386 | the program you actually want to run as an "indirect object" (without a | |
1387 | comma) in front of the LIST. (This always forces interpretation of the | |
54310121 | 1388 | LIST as a multivalued list, even if there is only a single scalar in |
a0d0e21e LW |
1389 | the list.) Example: |
1390 | ||
1391 | $shell = '/bin/csh'; | |
1392 | exec $shell '-sh'; # pretend it's a login shell | |
1393 | ||
1394 | or, more directly, | |
1395 | ||
1396 | exec {'/bin/csh'} '-sh'; # pretend it's a login shell | |
1397 | ||
bb32b41a GS |
1398 | When the arguments get executed via the system shell, results will |
1399 | be subject to its quirks and capabilities. See L<perlop/"`STRING`"> | |
1400 | for details. | |
1401 | ||
19799a22 GS |
1402 | Using an indirect object with C<exec> or C<system> is also more |
1403 | secure. This usage (which also works fine with system()) forces | |
1404 | interpretation of the arguments as a multivalued list, even if the | |
1405 | list had just one argument. That way you're safe from the shell | |
1406 | expanding wildcards or splitting up words with whitespace in them. | |
5a964f20 TC |
1407 | |
1408 | @args = ( "echo surprise" ); | |
1409 | ||
2b5ab1e7 | 1410 | exec @args; # subject to shell escapes |
f86cebdf | 1411 | # if @args == 1 |
2b5ab1e7 | 1412 | exec { $args[0] } @args; # safe even with one-arg list |
5a964f20 TC |
1413 | |
1414 | The first version, the one without the indirect object, ran the I<echo> | |
1415 | program, passing it C<"surprise"> an argument. The second version | |
1416 | didn't--it tried to run a program literally called I<"echo surprise">, | |
1417 | didn't find it, and set C<$?> to a non-zero value indicating failure. | |
1418 | ||
19799a22 | 1419 | Note that C<exec> will not call your C<END> blocks, nor will it call |
7660c0ab A |
1420 | any C<DESTROY> methods in your objects. |
1421 | ||
a0d0e21e LW |
1422 | =item exists EXPR |
1423 | ||
01020589 | 1424 | Given an expression that specifies a hash element or array element, |
8ea97a1e GS |
1425 | returns true if the specified element in the hash or array has ever |
1426 | been initialized, even if the corresponding value is undefined. The | |
1427 | element is not autovivified if it doesn't exist. | |
a0d0e21e | 1428 | |
01020589 GS |
1429 | print "Exists\n" if exists $hash{$key}; |
1430 | print "Defined\n" if defined $hash{$key}; | |
1431 | print "True\n" if $hash{$key}; | |
1432 | ||
1433 | print "Exists\n" if exists $array[$index]; | |
1434 | print "Defined\n" if defined $array[$index]; | |
1435 | print "True\n" if $array[$index]; | |
a0d0e21e | 1436 | |
8ea97a1e | 1437 | A hash or array element can be true only if it's defined, and defined if |
a0d0e21e LW |
1438 | it exists, but the reverse doesn't necessarily hold true. |
1439 | ||
afebc493 GS |
1440 | Given an expression that specifies the name of a subroutine, |
1441 | returns true if the specified subroutine has ever been declared, even | |
1442 | if it is undefined. Mentioning a subroutine name for exists or defined | |
1443 | does not count as declaring it. | |
1444 | ||
1445 | print "Exists\n" if exists &subroutine; | |
1446 | print "Defined\n" if defined &subroutine; | |
1447 | ||
a0d0e21e | 1448 | Note that the EXPR can be arbitrarily complicated as long as the final |
afebc493 | 1449 | operation is a hash or array key lookup or subroutine name: |
a0d0e21e | 1450 | |
2b5ab1e7 TC |
1451 | if (exists $ref->{A}->{B}->{$key}) { } |
1452 | if (exists $hash{A}{B}{$key}) { } | |
1453 | ||
01020589 GS |
1454 | if (exists $ref->{A}->{B}->[$ix]) { } |
1455 | if (exists $hash{A}{B}[$ix]) { } | |
1456 | ||
afebc493 GS |
1457 | if (exists &{$ref->{A}{B}{$key}}) { } |
1458 | ||
01020589 GS |
1459 | Although the deepest nested array or hash will not spring into existence |
1460 | just because its existence was tested, any intervening ones will. | |
1461 | Thus C<$ref-E<gt>{"A"}> and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring | |
1462 | into existence due to the existence test for the $key element above. | |
1463 | This happens anywhere the arrow operator is used, including even: | |
5a964f20 | 1464 | |
2b5ab1e7 TC |
1465 | undef $ref; |
1466 | if (exists $ref->{"Some key"}) { } | |
1467 | print $ref; # prints HASH(0x80d3d5c) | |
1468 | ||
1469 | This surprising autovivification in what does not at first--or even | |
1470 | second--glance appear to be an lvalue context may be fixed in a future | |
5a964f20 | 1471 | release. |
a0d0e21e | 1472 | |
e0478e5a MS |
1473 | See L<perlref/"Pseudo-hashes"> for specifics on how exists() acts when |
1474 | used on a pseudo-hash. | |
1475 | ||
afebc493 GS |
1476 | Use of a subroutine call, rather than a subroutine name, as an argument |
1477 | to exists() is an error. | |
1478 | ||
1479 | exists ⊂ # OK | |
1480 | exists &sub(); # Error | |
1481 | ||
a0d0e21e LW |
1482 | =item exit EXPR |
1483 | ||
2b5ab1e7 | 1484 | Evaluates EXPR and exits immediately with that value. Example: |
a0d0e21e LW |
1485 | |
1486 | $ans = <STDIN>; | |
1487 | exit 0 if $ans =~ /^[Xx]/; | |
1488 | ||
19799a22 | 1489 | See also C<die>. If EXPR is omitted, exits with C<0> status. The only |
2b5ab1e7 TC |
1490 | universally recognized values for EXPR are C<0> for success and C<1> |
1491 | for error; other values are subject to interpretation depending on the | |
1492 | environment in which the Perl program is running. For example, exiting | |
1493 | 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause | |
1494 | the mailer to return the item undelivered, but that's not true everywhere. | |
a0d0e21e | 1495 | |
19799a22 GS |
1496 | Don't use C<exit> to abort a subroutine if there's any chance that |
1497 | someone might want to trap whatever error happened. Use C<die> instead, | |
1498 | which can be trapped by an C<eval>. | |
28757baa | 1499 | |
19799a22 | 1500 | The exit() function does not always exit immediately. It calls any |
2b5ab1e7 | 1501 | defined C<END> routines first, but these C<END> routines may not |
19799a22 | 1502 | themselves abort the exit. Likewise any object destructors that need to |
2b5ab1e7 TC |
1503 | be called are called before the real exit. If this is a problem, you |
1504 | can call C<POSIX:_exit($status)> to avoid END and destructor processing. | |
87275199 | 1505 | See L<perlmod> for details. |
5a964f20 | 1506 | |
a0d0e21e LW |
1507 | =item exp EXPR |
1508 | ||
54310121 | 1509 | =item exp |
bbce6d69 | 1510 | |
2b5ab1e7 | 1511 | Returns I<e> (the natural logarithm base) to the power of EXPR. |
a0d0e21e LW |
1512 | If EXPR is omitted, gives C<exp($_)>. |
1513 | ||
1514 | =item fcntl FILEHANDLE,FUNCTION,SCALAR | |
1515 | ||
f86cebdf | 1516 | Implements the fcntl(2) function. You'll probably have to say |
a0d0e21e LW |
1517 | |
1518 | use Fcntl; | |
1519 | ||
0ade1984 | 1520 | first to get the correct constant definitions. Argument processing and |
19799a22 | 1521 | value return works just like C<ioctl> below. |
a0d0e21e LW |
1522 | For example: |
1523 | ||
1524 | use Fcntl; | |
5a964f20 TC |
1525 | fcntl($filehandle, F_GETFL, $packed_return_buffer) |
1526 | or die "can't fcntl F_GETFL: $!"; | |
1527 | ||
19799a22 | 1528 | You don't have to check for C<defined> on the return from C<fnctl>. |
951ba7fe GS |
1529 | Like C<ioctl>, it maps a C<0> return from the system call into |
1530 | C<"0 but true"> in Perl. This string is true in boolean context and C<0> | |
2b5ab1e7 TC |
1531 | in numeric context. It is also exempt from the normal B<-w> warnings |
1532 | on improper numeric conversions. | |
5a964f20 | 1533 | |
19799a22 | 1534 | Note that C<fcntl> will produce a fatal error if used on a machine that |
2b5ab1e7 TC |
1535 | doesn't implement fcntl(2). See the Fcntl module or your fcntl(2) |
1536 | manpage to learn what functions are available on your system. | |
a0d0e21e LW |
1537 | |
1538 | =item fileno FILEHANDLE | |
1539 | ||
2b5ab1e7 TC |
1540 | Returns the file descriptor for a filehandle, or undefined if the |
1541 | filehandle is not open. This is mainly useful for constructing | |
19799a22 | 1542 | bitmaps for C<select> and low-level POSIX tty-handling operations. |
2b5ab1e7 TC |
1543 | If FILEHANDLE is an expression, the value is taken as an indirect |
1544 | filehandle, generally its name. | |
5a964f20 TC |
1545 | |
1546 | You can use this to find out whether two handles refer to the | |
1547 | same underlying descriptor: | |
1548 | ||
1549 | if (fileno(THIS) == fileno(THAT)) { | |
1550 | print "THIS and THAT are dups\n"; | |
1551 | } | |
a0d0e21e LW |
1552 | |
1553 | =item flock FILEHANDLE,OPERATION | |
1554 | ||
19799a22 GS |
1555 | Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true |
1556 | for success, false on failure. Produces a fatal error if used on a | |
2b5ab1e7 | 1557 | machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3). |
19799a22 | 1558 | C<flock> is Perl's portable file locking interface, although it locks |
2b5ab1e7 TC |
1559 | only entire files, not records. |
1560 | ||
1561 | Two potentially non-obvious but traditional C<flock> semantics are | |
1562 | that it waits indefinitely until the lock is granted, and that its locks | |
1563 | B<merely advisory>. Such discretionary locks are more flexible, but offer | |
19799a22 GS |
1564 | fewer guarantees. This means that files locked with C<flock> may be |
1565 | modified by programs that do not also use C<flock>. See L<perlport>, | |
2b5ab1e7 TC |
1566 | your port's specific documentation, or your system-specific local manpages |
1567 | for details. It's best to assume traditional behavior if you're writing | |
1568 | portable programs. (But if you're not, you should as always feel perfectly | |
1569 | free to write for your own system's idiosyncrasies (sometimes called | |
1570 | "features"). Slavish adherence to portability concerns shouldn't get | |
1571 | in the way of your getting your job done.) | |
a3cb178b | 1572 | |
8ebc5c01 | 1573 | OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with |
1574 | LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but | |
ea3105be | 1575 | you can use the symbolic names if you import them from the Fcntl module, |
68dc0745 | 1576 | either individually, or as a group using the ':flock' tag. LOCK_SH |
1577 | requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN | |
ea3105be GS |
1578 | releases a previously requested lock. If LOCK_NB is bitwise-or'ed with |
1579 | LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking | |
68dc0745 | 1580 | waiting for the lock (check the return status to see if you got it). |
1581 | ||
2b5ab1e7 TC |
1582 | To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE |
1583 | before locking or unlocking it. | |
8ebc5c01 | 1584 | |
f86cebdf | 1585 | Note that the emulation built with lockf(3) doesn't provide shared |
8ebc5c01 | 1586 | locks, and it requires that FILEHANDLE be open with write intent. These |
2b5ab1e7 | 1587 | are the semantics that lockf(3) implements. Most if not all systems |
f86cebdf | 1588 | implement lockf(3) in terms of fcntl(2) locking, though, so the |
8ebc5c01 | 1589 | differing semantics shouldn't bite too many people. |
1590 | ||
19799a22 GS |
1591 | Note also that some versions of C<flock> cannot lock things over the |
1592 | network; you would need to use the more system-specific C<fcntl> for | |
f86cebdf GS |
1593 | that. If you like you can force Perl to ignore your system's flock(2) |
1594 | function, and so provide its own fcntl(2)-based emulation, by passing | |
8ebc5c01 | 1595 | the switch C<-Ud_flock> to the F<Configure> program when you configure |
1596 | perl. | |
4633a7c4 LW |
1597 | |
1598 | Here's a mailbox appender for BSD systems. | |
a0d0e21e | 1599 | |
7e1af8bc | 1600 | use Fcntl ':flock'; # import LOCK_* constants |
a0d0e21e LW |
1601 | |
1602 | sub lock { | |
7e1af8bc | 1603 | flock(MBOX,LOCK_EX); |
a0d0e21e LW |
1604 | # and, in case someone appended |
1605 | # while we were waiting... | |
1606 | seek(MBOX, 0, 2); | |
1607 | } | |
1608 | ||
1609 | sub unlock { | |
7e1af8bc | 1610 | flock(MBOX,LOCK_UN); |
a0d0e21e LW |
1611 | } |
1612 | ||
1613 | open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}") | |
1614 | or die "Can't open mailbox: $!"; | |
1615 | ||
1616 | lock(); | |
1617 | print MBOX $msg,"\n\n"; | |
1618 | unlock(); | |
1619 | ||
2b5ab1e7 TC |
1620 | On systems that support a real flock(), locks are inherited across fork() |
1621 | calls, whereas those that must resort to the more capricious fcntl() | |
1622 | function lose the locks, making it harder to write servers. | |
1623 | ||
cb1a09d0 | 1624 | See also L<DB_File> for other flock() examples. |
a0d0e21e LW |
1625 | |
1626 | =item fork | |
1627 | ||
2b5ab1e7 TC |
1628 | Does a fork(2) system call to create a new process running the |
1629 | same program at the same point. It returns the child pid to the | |
1630 | parent process, C<0> to the child process, or C<undef> if the fork is | |
1631 | unsuccessful. File descriptors (and sometimes locks on those descriptors) | |
1632 | are shared, while everything else is copied. On most systems supporting | |
1633 | fork(), great care has gone into making it extremely efficient (for | |
1634 | example, using copy-on-write technology on data pages), making it the | |
1635 | dominant paradigm for multitasking over the last few decades. | |
5a964f20 | 1636 | |
45bc9206 | 1637 | All files opened for output are flushed before forking the child process. |
a0d0e21e | 1638 | |
19799a22 | 1639 | If you C<fork> without ever waiting on your children, you will |
2b5ab1e7 TC |
1640 | accumulate zombies. On some systems, you can avoid this by setting |
1641 | C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of | |
1642 | forking and reaping moribund children. | |
cb1a09d0 | 1643 | |
28757baa | 1644 | Note that if your forked child inherits system file descriptors like |
1645 | STDIN and STDOUT that are actually connected by a pipe or socket, even | |
2b5ab1e7 | 1646 | if you exit, then the remote server (such as, say, a CGI script or a |
19799a22 | 1647 | backgrounded job launched from a remote shell) won't think you're done. |
2b5ab1e7 | 1648 | You should reopen those to F</dev/null> if it's any issue. |
28757baa | 1649 | |
cb1a09d0 AD |
1650 | =item format |
1651 | ||
19799a22 | 1652 | Declare a picture format for use by the C<write> function. For |
cb1a09d0 AD |
1653 | example: |
1654 | ||
54310121 | 1655 | format Something = |
cb1a09d0 AD |
1656 | Test: @<<<<<<<< @||||| @>>>>> |
1657 | $str, $%, '$' . int($num) | |
1658 | . | |
1659 | ||
1660 | $str = "widget"; | |
184e9718 | 1661 | $num = $cost/$quantity; |
cb1a09d0 AD |
1662 | $~ = 'Something'; |
1663 | write; | |
1664 | ||
1665 | See L<perlform> for many details and examples. | |
1666 | ||
8903cb82 | 1667 | =item formline PICTURE,LIST |
a0d0e21e | 1668 | |
5a964f20 | 1669 | This is an internal function used by C<format>s, though you may call it, |
a0d0e21e LW |
1670 | too. It formats (see L<perlform>) a list of values according to the |
1671 | contents of PICTURE, placing the output into the format output | |
7660c0ab | 1672 | accumulator, C<$^A> (or C<$ACCUMULATOR> in English). |
19799a22 | 1673 | Eventually, when a C<write> is done, the contents of |
a0d0e21e | 1674 | C<$^A> are written to some filehandle, but you could also read C<$^A> |
7660c0ab | 1675 | yourself and then set C<$^A> back to C<"">. Note that a format typically |
19799a22 | 1676 | does one C<formline> per line of form, but the C<formline> function itself |
748a9306 | 1677 | doesn't care how many newlines are embedded in the PICTURE. This means |
4633a7c4 | 1678 | that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line. |
748a9306 LW |
1679 | You may therefore need to use multiple formlines to implement a single |
1680 | record format, just like the format compiler. | |
1681 | ||
19799a22 | 1682 | Be careful if you put double quotes around the picture, because an C<@> |
748a9306 | 1683 | character may be taken to mean the beginning of an array name. |
19799a22 | 1684 | C<formline> always returns true. See L<perlform> for other examples. |
a0d0e21e LW |
1685 | |
1686 | =item getc FILEHANDLE | |
1687 | ||
1688 | =item getc | |
1689 | ||
1690 | Returns the next character from the input file attached to FILEHANDLE, | |
2b5ab1e7 TC |
1691 | or the undefined value at end of file, or if there was an error. |
1692 | If FILEHANDLE is omitted, reads from STDIN. This is not particularly | |
1693 | efficient. However, it cannot be used by itself to fetch single | |
1694 | characters without waiting for the user to hit enter. For that, try | |
1695 | something more like: | |
4633a7c4 LW |
1696 | |
1697 | if ($BSD_STYLE) { | |
1698 | system "stty cbreak </dev/tty >/dev/tty 2>&1"; | |
1699 | } | |
1700 | else { | |
54310121 | 1701 | system "stty", '-icanon', 'eol', "\001"; |
4633a7c4 LW |
1702 | } |
1703 | ||
1704 | $key = getc(STDIN); | |
1705 | ||
1706 | if ($BSD_STYLE) { | |
1707 | system "stty -cbreak </dev/tty >/dev/tty 2>&1"; | |
1708 | } | |
1709 | else { | |
5f05dabc | 1710 | system "stty", 'icanon', 'eol', '^@'; # ASCII null |
4633a7c4 LW |
1711 | } |
1712 | print "\n"; | |
1713 | ||
54310121 | 1714 | Determination of whether $BSD_STYLE should be set |
1715 | is left as an exercise to the reader. | |
cb1a09d0 | 1716 | |
19799a22 | 1717 | The C<POSIX::getattr> function can do this more portably on |
2b5ab1e7 TC |
1718 | systems purporting POSIX compliance. See also the C<Term::ReadKey> |
1719 | module from your nearest CPAN site; details on CPAN can be found on | |
1720 | L<perlmodlib/CPAN>. | |
a0d0e21e LW |
1721 | |
1722 | =item getlogin | |
1723 | ||
5a964f20 TC |
1724 | Implements the C library function of the same name, which on most |
1725 | systems returns the current login from F</etc/utmp>, if any. If null, | |
19799a22 | 1726 | use C<getpwuid>. |
a0d0e21e | 1727 | |
f86702cc | 1728 | $login = getlogin || getpwuid($<) || "Kilroy"; |
a0d0e21e | 1729 | |
19799a22 GS |
1730 | Do not consider C<getlogin> for authentication: it is not as |
1731 | secure as C<getpwuid>. | |
4633a7c4 | 1732 | |
a0d0e21e LW |
1733 | =item getpeername SOCKET |
1734 | ||
1735 | Returns the packed sockaddr address of other end of the SOCKET connection. | |
1736 | ||
4633a7c4 LW |
1737 | use Socket; |
1738 | $hersockaddr = getpeername(SOCK); | |
19799a22 | 1739 | ($port, $iaddr) = sockaddr_in($hersockaddr); |
4633a7c4 LW |
1740 | $herhostname = gethostbyaddr($iaddr, AF_INET); |
1741 | $herstraddr = inet_ntoa($iaddr); | |
a0d0e21e LW |
1742 | |
1743 | =item getpgrp PID | |
1744 | ||
47e29363 | 1745 | Returns the current process group for the specified PID. Use |
7660c0ab | 1746 | a PID of C<0> to get the current process group for the |
4633a7c4 | 1747 | current process. Will raise an exception if used on a machine that |
f86cebdf | 1748 | doesn't implement getpgrp(2). If PID is omitted, returns process |
19799a22 | 1749 | group of current process. Note that the POSIX version of C<getpgrp> |
7660c0ab | 1750 | does not accept a PID argument, so only C<PID==0> is truly portable. |
a0d0e21e LW |
1751 | |
1752 | =item getppid | |
1753 | ||
1754 | Returns the process id of the parent process. | |
1755 | ||
1756 | =item getpriority WHICH,WHO | |
1757 | ||
4633a7c4 LW |
1758 | Returns the current priority for a process, a process group, or a user. |
1759 | (See L<getpriority(2)>.) Will raise a fatal exception if used on a | |
f86cebdf | 1760 | machine that doesn't implement getpriority(2). |
a0d0e21e LW |
1761 | |
1762 | =item getpwnam NAME | |
1763 | ||
1764 | =item getgrnam NAME | |
1765 | ||
1766 | =item gethostbyname NAME | |
1767 | ||
1768 | =item getnetbyname NAME | |
1769 | ||
1770 | =item getprotobyname NAME | |
1771 | ||
1772 | =item getpwuid UID | |
1773 | ||
1774 | =item getgrgid GID | |
1775 | ||
1776 | =item getservbyname NAME,PROTO | |
1777 | ||
1778 | =item gethostbyaddr ADDR,ADDRTYPE | |
1779 | ||
1780 | =item getnetbyaddr ADDR,ADDRTYPE | |
1781 | ||
1782 | =item getprotobynumber NUMBER | |
1783 | ||
1784 | =item getservbyport PORT,PROTO | |
1785 | ||
1786 | =item getpwent | |
1787 | ||
1788 | =item getgrent | |
1789 | ||
1790 | =item gethostent | |
1791 | ||
1792 | =item getnetent | |
1793 | ||
1794 | =item getprotoent | |
1795 | ||
1796 | =item getservent | |
1797 | ||
1798 | =item setpwent | |
1799 | ||
1800 | =item setgrent | |
1801 | ||
1802 | =item sethostent STAYOPEN | |
1803 | ||
1804 | =item setnetent STAYOPEN | |
1805 | ||
1806 | =item setprotoent STAYOPEN | |
1807 | ||
1808 | =item setservent STAYOPEN | |
1809 | ||
1810 | =item endpwent | |
1811 | ||
1812 | =item endgrent | |
1813 | ||
1814 | =item endhostent | |
1815 | ||
1816 | =item endnetent | |
1817 | ||
1818 | =item endprotoent | |
1819 | ||
1820 | =item endservent | |
1821 | ||
1822 | These routines perform the same functions as their counterparts in the | |
5a964f20 | 1823 | system library. In list context, the return values from the |
a0d0e21e LW |
1824 | various get routines are as follows: |
1825 | ||
1826 | ($name,$passwd,$uid,$gid, | |
6ee623d5 | 1827 | $quota,$comment,$gcos,$dir,$shell,$expire) = getpw* |
a0d0e21e LW |
1828 | ($name,$passwd,$gid,$members) = getgr* |
1829 | ($name,$aliases,$addrtype,$length,@addrs) = gethost* | |
1830 | ($name,$aliases,$addrtype,$net) = getnet* | |
1831 | ($name,$aliases,$proto) = getproto* | |
1832 | ($name,$aliases,$port,$proto) = getserv* | |
1833 | ||
1834 | (If the entry doesn't exist you get a null list.) | |
1835 | ||
5a964f20 | 1836 | In scalar context, you get the name, unless the function was a |
a0d0e21e LW |
1837 | lookup by name, in which case you get the other thing, whatever it is. |
1838 | (If the entry doesn't exist you get the undefined value.) For example: | |
1839 | ||
5a964f20 TC |
1840 | $uid = getpwnam($name); |
1841 | $name = getpwuid($num); | |
1842 | $name = getpwent(); | |
1843 | $gid = getgrnam($name); | |
1844 | $name = getgrgid($num; | |
1845 | $name = getgrent(); | |
1846 | #etc. | |
a0d0e21e | 1847 | |
19799a22 | 1848 | In I<getpw*()> the fields $quota, $comment, and $expire are |
2b5ab1e7 | 1849 | special cases in the sense that in many systems they are unsupported. |
19799a22 GS |
1850 | If the $quota is unsupported, it is an empty scalar. If it is |
1851 | supported, it usually encodes the disk quota. If the $comment | |
2b5ab1e7 TC |
1852 | field is unsupported, it is an empty scalar. If it is supported it |
1853 | usually encodes some administrative comment about the user. In some | |
19799a22 GS |
1854 | systems the $quota field may be $change or $age, fields that have |
1855 | to do with password aging. In some systems the $comment field may | |
1856 | be $class. The $expire field, if present, encodes the expiration | |
2b5ab1e7 TC |
1857 | period of the account or the password. For the availability and the |
1858 | exact meaning of these fields in your system, please consult your | |
1859 | getpwnam(3) documentation and your F<pwd.h> file. You can also find | |
19799a22 GS |
1860 | out from within Perl what your $quota and $comment fields mean |
1861 | and whether you have the $expire field by using the C<Config> module | |
2b5ab1e7 TC |
1862 | and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>, |
1863 | and C<d_pwexpire>. Shadow password files are only supported if your | |
1864 | vendor has implemented them in the intuitive fashion that calling the | |
1865 | regular C library routines gets the shadow versions if you're running | |
1866 | under privilege. Those that incorrectly implement a separate library | |
1867 | call are not supported. | |
6ee623d5 | 1868 | |
19799a22 | 1869 | The $members value returned by I<getgr*()> is a space separated list of |
a0d0e21e LW |
1870 | the login names of the members of the group. |
1871 | ||
1872 | For the I<gethost*()> functions, if the C<h_errno> variable is supported in | |
1873 | C, it will be returned to you via C<$?> if the function call fails. The | |
7660c0ab | 1874 | C<@addrs> value returned by a successful call is a list of the raw |
a0d0e21e LW |
1875 | addresses returned by the corresponding system library call. In the |
1876 | Internet domain, each address is four bytes long and you can unpack it | |
1877 | by saying something like: | |
1878 | ||
1879 | ($a,$b,$c,$d) = unpack('C4',$addr[0]); | |
1880 | ||
2b5ab1e7 TC |
1881 | The Socket library makes this slightly easier: |
1882 | ||
1883 | use Socket; | |
1884 | $iaddr = inet_aton("127.1"); # or whatever address | |
1885 | $name = gethostbyaddr($iaddr, AF_INET); | |
1886 | ||
1887 | # or going the other way | |
19799a22 | 1888 | $straddr = inet_ntoa($iaddr); |
2b5ab1e7 | 1889 | |
19799a22 GS |
1890 | If you get tired of remembering which element of the return list |
1891 | contains which return value, by-name interfaces are provided | |
1892 | in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>, | |
1893 | C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>, | |
1894 | and C<User::grent>. These override the normal built-ins, supplying | |
1895 | versions that return objects with the appropriate names | |
1896 | for each field. For example: | |
5a964f20 TC |
1897 | |
1898 | use File::stat; | |
1899 | use User::pwent; | |
1900 | $is_his = (stat($filename)->uid == pwent($whoever)->uid); | |
1901 | ||
1902 | Even though it looks like they're the same method calls (uid), | |
19799a22 GS |
1903 | they aren't, because a C<File::stat> object is different from |
1904 | a C<User::pwent> object. | |
5a964f20 | 1905 | |
a0d0e21e LW |
1906 | =item getsockname SOCKET |
1907 | ||
19799a22 GS |
1908 | Returns the packed sockaddr address of this end of the SOCKET connection, |
1909 | in case you don't know the address because you have several different | |
1910 | IPs that the connection might have come in on. | |
a0d0e21e | 1911 | |
4633a7c4 LW |
1912 | use Socket; |
1913 | $mysockaddr = getsockname(SOCK); | |
19799a22 GS |
1914 | ($port, $myaddr) = sockaddr_in($mysockaddr); |
1915 | printf "Connect to %s [%s]\n", | |
1916 | scalar gethostbyaddr($myaddr, AF_INET), | |
1917 | inet_ntoa($myaddr); | |
a0d0e21e LW |
1918 | |
1919 | =item getsockopt SOCKET,LEVEL,OPTNAME | |
1920 | ||
5a964f20 | 1921 | Returns the socket option requested, or undef if there is an error. |
a0d0e21e LW |
1922 | |
1923 | =item glob EXPR | |
1924 | ||
0a753a76 | 1925 | =item glob |
1926 | ||
2b5ab1e7 TC |
1927 | Returns the value of EXPR with filename expansions such as the |
1928 | standard Unix shell F</bin/csh> would do. This is the internal function | |
1929 | implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly. | |
1930 | If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is | |
1931 | discussed in more detail in L<perlop/"I/O Operators">. | |
a0d0e21e LW |
1932 | |
1933 | =item gmtime EXPR | |
1934 | ||
19799a22 | 1935 | Converts a time as returned by the time function to a 9-element list |
54310121 | 1936 | with the time localized for the standard Greenwich time zone. |
4633a7c4 | 1937 | Typically used as follows: |
a0d0e21e | 1938 | |
54310121 | 1939 | # 0 1 2 3 4 5 6 7 8 |
a0d0e21e LW |
1940 | ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = |
1941 | gmtime(time); | |
1942 | ||
19799a22 GS |
1943 | All list elements are numeric, and come straight out of a struct tm. |
1944 | In particular this means that $mon has the range C<0..11> and $wday | |
1945 | has the range C<0..6> with sunday as day C<0>. Also, $year is the | |
1946 | number of years since 1900, that is, $year is C<123> in year 2023, | |
2b5ab1e7 TC |
1947 | I<not> simply the last two digits of the year. If you assume it is, |
1948 | then you create non-Y2K-compliant programs--and you wouldn't want to do | |
1949 | that, would you? | |
2f9daede | 1950 | |
abd75f24 GS |
1951 | The proper way to get a complete 4-digit year is simply: |
1952 | ||
1953 | $year += 1900; | |
1954 | ||
1955 | And to get the last two digits of the year (e.g., '01' in 2001) do: | |
1956 | ||
1957 | $year = sprintf("%02d", $year % 100); | |
1958 | ||
2f9daede | 1959 | If EXPR is omitted, does C<gmtime(time())>. |
a0d0e21e | 1960 | |
f86cebdf | 1961 | In scalar context, returns the ctime(3) value: |
0a753a76 | 1962 | |
1963 | $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994" | |
1964 | ||
19799a22 | 1965 | Also see the C<timegm> function provided by the C<Time::Local> module, |
f86cebdf | 1966 | and the strftime(3) function available via the POSIX module. |
7660c0ab | 1967 | |
2b5ab1e7 TC |
1968 | This scalar value is B<not> locale dependent (see L<perllocale>), but |
1969 | is instead a Perl builtin. Also see the C<Time::Local> module, and the | |
1970 | strftime(3) and mktime(3) functions available via the POSIX module. To | |
7660c0ab A |
1971 | get somewhat similar but locale dependent date strings, set up your |
1972 | locale environment variables appropriately (please see L<perllocale>) | |
1973 | and try for example: | |
1974 | ||
1975 | use POSIX qw(strftime); | |
2b5ab1e7 | 1976 | $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime; |
7660c0ab | 1977 | |
2b5ab1e7 TC |
1978 | Note that the C<%a> and C<%b> escapes, which represent the short forms |
1979 | of the day of the week and the month of the year, may not necessarily | |
1980 | be three characters wide in all locales. | |
0a753a76 | 1981 | |
a0d0e21e LW |
1982 | =item goto LABEL |
1983 | ||
748a9306 LW |
1984 | =item goto EXPR |
1985 | ||
a0d0e21e LW |
1986 | =item goto &NAME |
1987 | ||
7660c0ab | 1988 | The C<goto-LABEL> form finds the statement labeled with LABEL and resumes |
a0d0e21e | 1989 | execution there. It may not be used to go into any construct that |
7660c0ab | 1990 | requires initialization, such as a subroutine or a C<foreach> loop. It |
0a753a76 | 1991 | also can't be used to go into a construct that is optimized away, |
19799a22 | 1992 | or to get out of a block or subroutine given to C<sort>. |
0a753a76 | 1993 | It can be used to go almost anywhere else within the dynamic scope, |
a0d0e21e | 1994 | including out of subroutines, but it's usually better to use some other |
19799a22 | 1995 | construct such as C<last> or C<die>. The author of Perl has never felt the |
7660c0ab | 1996 | need to use this form of C<goto> (in Perl, that is--C is another matter). |
a0d0e21e | 1997 | |
7660c0ab A |
1998 | The C<goto-EXPR> form expects a label name, whose scope will be resolved |
1999 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
748a9306 LW |
2000 | necessarily recommended if you're optimizing for maintainability: |
2001 | ||
2002 | goto ("FOO", "BAR", "GLARCH")[$i]; | |
2003 | ||
6cb9131c GS |
2004 | The C<goto-&NAME> form is quite different from the other forms of C<goto>. |
2005 | In fact, it isn't a goto in the normal sense at all, and doesn't have | |
2006 | the stigma associated with other gotos. Instead, it | |
2007 | substitutes a call to the named subroutine for the currently running | |
2008 | subroutine. This is used by C<AUTOLOAD> subroutines that wish to load | |
2009 | another subroutine and then pretend that the other subroutine had been | |
2010 | called in the first place (except that any modifications to C<@_> | |
2011 | in the current subroutine are propagated to the other subroutine.) | |
2012 | After the C<goto>, not even C<caller> will be able to tell that this | |
2013 | routine was called first. | |
2014 | ||
2015 | NAME needn't be the name of a subroutine; it can be a scalar variable | |
2016 | containing a code reference, or a block which evaluates to a code | |
2017 | reference. | |
a0d0e21e LW |
2018 | |
2019 | =item grep BLOCK LIST | |
2020 | ||
2021 | =item grep EXPR,LIST | |
2022 | ||
2b5ab1e7 TC |
2023 | This is similar in spirit to, but not the same as, grep(1) and its |
2024 | relatives. In particular, it is not limited to using regular expressions. | |
2f9daede | 2025 | |
a0d0e21e | 2026 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting |
7660c0ab | 2027 | C<$_> to each element) and returns the list value consisting of those |
19799a22 GS |
2028 | elements for which the expression evaluated to true. In scalar |
2029 | context, returns the number of times the expression was true. | |
a0d0e21e LW |
2030 | |
2031 | @foo = grep(!/^#/, @bar); # weed out comments | |
2032 | ||
2033 | or equivalently, | |
2034 | ||
2035 | @foo = grep {!/^#/} @bar; # weed out comments | |
2036 | ||
2b5ab1e7 TC |
2037 | Note that, because C<$_> is a reference into the list value, it can |
2038 | be used to modify the elements of the array. While this is useful and | |
2039 | supported, it can cause bizarre results if the LIST is not a named array. | |
2040 | Similarly, grep returns aliases into the original list, much as a for | |
2041 | loop's index variable aliases the list elements. That is, modifying an | |
19799a22 GS |
2042 | element of a list returned by grep (for example, in a C<foreach>, C<map> |
2043 | or another C<grep>) actually modifies the element in the original list. | |
2b5ab1e7 | 2044 | This is usually something to be avoided when writing clear code. |
a0d0e21e | 2045 | |
19799a22 | 2046 | See also L</map> for a list composed of the results of the BLOCK or EXPR. |
38325410 | 2047 | |
a0d0e21e LW |
2048 | =item hex EXPR |
2049 | ||
54310121 | 2050 | =item hex |
bbce6d69 | 2051 | |
2b5ab1e7 TC |
2052 | Interprets EXPR as a hex string and returns the corresponding value. |
2053 | (To convert strings that might start with either 0, 0x, or 0b, see | |
2054 | L</oct>.) If EXPR is omitted, uses C<$_>. | |
2f9daede TP |
2055 | |
2056 | print hex '0xAf'; # prints '175' | |
2057 | print hex 'aF'; # same | |
a0d0e21e | 2058 | |
19799a22 | 2059 | Hex strings may only represent integers. Strings that would cause |
c6edd1b7 | 2060 | integer overflow trigger a warning. |
19799a22 | 2061 | |
a0d0e21e LW |
2062 | =item import |
2063 | ||
19799a22 | 2064 | There is no builtin C<import> function. It is just an ordinary |
4633a7c4 | 2065 | method (subroutine) defined (or inherited) by modules that wish to export |
19799a22 | 2066 | names to another module. The C<use> function calls the C<import> method |
54310121 | 2067 | for the package used. See also L</use()>, L<perlmod>, and L<Exporter>. |
a0d0e21e LW |
2068 | |
2069 | =item index STR,SUBSTR,POSITION | |
2070 | ||
2071 | =item index STR,SUBSTR | |
2072 | ||
2b5ab1e7 TC |
2073 | The index function searches for one string within another, but without |
2074 | the wildcard-like behavior of a full regular-expression pattern match. | |
2075 | It returns the position of the first occurrence of SUBSTR in STR at | |
2076 | or after POSITION. If POSITION is omitted, starts searching from the | |
2077 | beginning of the string. The return value is based at C<0> (or whatever | |
2078 | you've set the C<$[> variable to--but don't do that). If the substring | |
2079 | is not found, returns one less than the base, ordinarily C<-1>. | |
a0d0e21e LW |
2080 | |
2081 | =item int EXPR | |
2082 | ||
54310121 | 2083 | =item int |
bbce6d69 | 2084 | |
7660c0ab | 2085 | Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>. |
2b5ab1e7 TC |
2086 | You should not use this function for rounding: one because it truncates |
2087 | towards C<0>, and two because machine representations of floating point | |
2088 | numbers can sometimes produce counterintuitive results. For example, | |
2089 | C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's | |
2090 | because it's really more like -268.99999999999994315658 instead. Usually, | |
19799a22 | 2091 | the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil> |
2b5ab1e7 | 2092 | functions will serve you better than will int(). |
a0d0e21e LW |
2093 | |
2094 | =item ioctl FILEHANDLE,FUNCTION,SCALAR | |
2095 | ||
2b5ab1e7 | 2096 | Implements the ioctl(2) function. You'll probably first have to say |
a0d0e21e | 2097 | |
4633a7c4 | 2098 | require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph |
a0d0e21e | 2099 | |
2b5ab1e7 | 2100 | to get the correct function definitions. If F<ioctl.ph> doesn't |
a0d0e21e | 2101 | exist or doesn't have the correct definitions you'll have to roll your |
4633a7c4 | 2102 | own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>. |
5a964f20 | 2103 | (There is a Perl script called B<h2ph> that comes with the Perl kit that |
54310121 | 2104 | may help you in this, but it's nontrivial.) SCALAR will be read and/or |
4633a7c4 | 2105 | written depending on the FUNCTION--a pointer to the string value of SCALAR |
19799a22 | 2106 | will be passed as the third argument of the actual C<ioctl> call. (If SCALAR |
4633a7c4 LW |
2107 | has no string value but does have a numeric value, that value will be |
2108 | passed rather than a pointer to the string value. To guarantee this to be | |
19799a22 GS |
2109 | true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack> |
2110 | functions may be needed to manipulate the values of structures used by | |
2111 | C<ioctl>. | |
a0d0e21e | 2112 | |
19799a22 | 2113 | The return value of C<ioctl> (and C<fcntl>) is as follows: |
a0d0e21e LW |
2114 | |
2115 | if OS returns: then Perl returns: | |
2116 | -1 undefined value | |
2117 | 0 string "0 but true" | |
2118 | anything else that number | |
2119 | ||
19799a22 | 2120 | Thus Perl returns true on success and false on failure, yet you can |
a0d0e21e LW |
2121 | still easily determine the actual value returned by the operating |
2122 | system: | |
2123 | ||
2b5ab1e7 | 2124 | $retval = ioctl(...) || -1; |
a0d0e21e LW |
2125 | printf "System returned %d\n", $retval; |
2126 | ||
c2611fb3 | 2127 | The special string "C<0> but true" is exempt from B<-w> complaints |
5a964f20 TC |
2128 | about improper numeric conversions. |
2129 | ||
19799a22 GS |
2130 | Here's an example of setting a filehandle named C<REMOTE> to be |
2131 | non-blocking at the system level. You'll have to negotiate C<$|> | |
2132 | on your own, though. | |
2133 | ||
2134 | use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK); | |
2135 | ||
2136 | $flags = fcntl(REMOTE, F_GETFL, 0) | |
2137 | or die "Can't get flags for the socket: $!\n"; | |
2138 | ||
2139 | $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK) | |
2140 | or die "Can't set flags for the socket: $!\n"; | |
2141 | ||
a0d0e21e LW |
2142 | =item join EXPR,LIST |
2143 | ||
2b5ab1e7 TC |
2144 | Joins the separate strings of LIST into a single string with fields |
2145 | separated by the value of EXPR, and returns that new string. Example: | |
a0d0e21e | 2146 | |
2b5ab1e7 | 2147 | $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); |
a0d0e21e | 2148 | |
eb6e2d6f GS |
2149 | Beware that unlike C<split>, C<join> doesn't take a pattern as its |
2150 | first argument. Compare L</split>. | |
a0d0e21e | 2151 | |
aa689395 | 2152 | =item keys HASH |
2153 | ||
19799a22 | 2154 | Returns a list consisting of all the keys of the named hash. (In |
1d2dff63 | 2155 | scalar context, returns the number of keys.) The keys are returned in |
ab192400 GS |
2156 | an apparently random order. The actual random order is subject to |
2157 | change in future versions of perl, but it is guaranteed to be the same | |
19799a22 | 2158 | order as either the C<values> or C<each> function produces (given |
ab192400 GS |
2159 | that the hash has not been modified). As a side effect, it resets |
2160 | HASH's iterator. | |
a0d0e21e | 2161 | |
aa689395 | 2162 | Here is yet another way to print your environment: |
a0d0e21e LW |
2163 | |
2164 | @keys = keys %ENV; | |
2165 | @values = values %ENV; | |
19799a22 | 2166 | while (@keys) { |
a0d0e21e LW |
2167 | print pop(@keys), '=', pop(@values), "\n"; |
2168 | } | |
2169 | ||
2170 | or how about sorted by key: | |
2171 | ||
2172 | foreach $key (sort(keys %ENV)) { | |
2173 | print $key, '=', $ENV{$key}, "\n"; | |
2174 | } | |
2175 | ||
19799a22 | 2176 | To sort a hash by value, you'll need to use a C<sort> function. |
aa689395 | 2177 | Here's a descending numeric sort of a hash by its values: |
4633a7c4 | 2178 | |
5a964f20 | 2179 | foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { |
4633a7c4 LW |
2180 | printf "%4d %s\n", $hash{$key}, $key; |
2181 | } | |
2182 | ||
19799a22 | 2183 | As an lvalue C<keys> allows you to increase the number of hash buckets |
aa689395 | 2184 | allocated for the given hash. This can gain you a measure of efficiency if |
2185 | you know the hash is going to get big. (This is similar to pre-extending | |
2186 | an array by assigning a larger number to $#array.) If you say | |
55497cff | 2187 | |
2188 | keys %hash = 200; | |
2189 | ||
ab192400 GS |
2190 | then C<%hash> will have at least 200 buckets allocated for it--256 of them, |
2191 | in fact, since it rounds up to the next power of two. These | |
55497cff | 2192 | buckets will be retained even if you do C<%hash = ()>, use C<undef |
2193 | %hash> if you want to free the storage while C<%hash> is still in scope. | |
2194 | You can't shrink the number of buckets allocated for the hash using | |
19799a22 | 2195 | C<keys> in this way (but you needn't worry about doing this by accident, |
55497cff | 2196 | as trying has no effect). |
2197 | ||
19799a22 | 2198 | See also C<each>, C<values> and C<sort>. |
ab192400 | 2199 | |
b350dd2f | 2200 | =item kill SIGNAL, LIST |
a0d0e21e | 2201 | |
b350dd2f | 2202 | Sends a signal to a list of processes. Returns the number of |
517db077 GS |
2203 | processes successfully signaled (which is not necessarily the |
2204 | same as the number actually killed). | |
a0d0e21e LW |
2205 | |
2206 | $cnt = kill 1, $child1, $child2; | |
2207 | kill 9, @goners; | |
2208 | ||
b350dd2f GS |
2209 | If SIGNAL is zero, no signal is sent to the process. This is a |
2210 | useful way to check that the process is alive and hasn't changed | |
2211 | its UID. See L<perlport> for notes on the portability of this | |
2212 | construct. | |
2213 | ||
2214 | Unlike in the shell, if SIGNAL is negative, it kills | |
4633a7c4 LW |
2215 | process groups instead of processes. (On System V, a negative I<PROCESS> |
2216 | number will also kill process groups, but that's not portable.) That | |
2217 | means you usually want to use positive not negative signals. You may also | |
da0045b7 | 2218 | use a signal name in quotes. See L<perlipc/"Signals"> for details. |
a0d0e21e LW |
2219 | |
2220 | =item last LABEL | |
2221 | ||
2222 | =item last | |
2223 | ||
2224 | The C<last> command is like the C<break> statement in C (as used in | |
2225 | loops); it immediately exits the loop in question. If the LABEL is | |
2226 | omitted, the command refers to the innermost enclosing loop. The | |
2227 | C<continue> block, if any, is not executed: | |
2228 | ||
4633a7c4 LW |
2229 | LINE: while (<STDIN>) { |
2230 | last LINE if /^$/; # exit when done with header | |
5a964f20 | 2231 | #... |
a0d0e21e LW |
2232 | } |
2233 | ||
4968c1e4 | 2234 | C<last> cannot be used to exit a block which returns a value such as |
2b5ab1e7 TC |
2235 | C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit |
2236 | a grep() or map() operation. | |
4968c1e4 | 2237 | |
6c1372ed GS |
2238 | Note that a block by itself is semantically identical to a loop |
2239 | that executes once. Thus C<last> can be used to effect an early | |
2240 | exit out of such a block. | |
2241 | ||
98293880 JH |
2242 | See also L</continue> for an illustration of how C<last>, C<next>, and |
2243 | C<redo> work. | |
1d2dff63 | 2244 | |
a0d0e21e LW |
2245 | =item lc EXPR |
2246 | ||
54310121 | 2247 | =item lc |
bbce6d69 | 2248 | |
a0d0e21e | 2249 | Returns an lowercased version of EXPR. This is the internal function |
7660c0ab | 2250 | implementing the C<\L> escape in double-quoted strings. |
19799a22 GS |
2251 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale> |
2252 | and L<utf8>. | |
a0d0e21e | 2253 | |
7660c0ab | 2254 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 2255 | |
a0d0e21e LW |
2256 | =item lcfirst EXPR |
2257 | ||
54310121 | 2258 | =item lcfirst |
bbce6d69 | 2259 | |
a0d0e21e | 2260 | Returns the value of EXPR with the first character lowercased. This is |
7660c0ab | 2261 | the internal function implementing the C<\l> escape in double-quoted strings. |
a0ed51b3 | 2262 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. |
a0d0e21e | 2263 | |
7660c0ab | 2264 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 2265 | |
a0d0e21e LW |
2266 | =item length EXPR |
2267 | ||
54310121 | 2268 | =item length |
bbce6d69 | 2269 | |
a0ed51b3 | 2270 | Returns the length in characters of the value of EXPR. If EXPR is |
2b5ab1e7 TC |
2271 | omitted, returns length of C<$_>. Note that this cannot be used on |
2272 | an entire array or hash to find out how many elements these have. | |
2273 | For that, use C<scalar @array> and C<scalar keys %hash> respectively. | |
a0d0e21e LW |
2274 | |
2275 | =item link OLDFILE,NEWFILE | |
2276 | ||
19799a22 GS |
2277 | Creates a new filename linked to the old filename. Returns true for |
2278 | success, false otherwise. | |
a0d0e21e LW |
2279 | |
2280 | =item listen SOCKET,QUEUESIZE | |
2281 | ||
19799a22 GS |
2282 | Does the same thing that the listen system call does. Returns true if |
2283 | it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">. | |
a0d0e21e LW |
2284 | |
2285 | =item local EXPR | |
2286 | ||
19799a22 | 2287 | You really probably want to be using C<my> instead, because C<local> isn't |
2b5ab1e7 TC |
2288 | what most people think of as "local". See L<perlsub/"Private Variables |
2289 | via my()"> for details. | |
2290 | ||
5a964f20 TC |
2291 | A local modifies the listed variables to be local to the enclosing |
2292 | block, file, or eval. If more than one value is listed, the list must | |
2293 | be placed in parentheses. See L<perlsub/"Temporary Values via local()"> | |
2294 | for details, including issues with tied arrays and hashes. | |
a0d0e21e | 2295 | |
a0d0e21e LW |
2296 | =item localtime EXPR |
2297 | ||
19799a22 | 2298 | Converts a time as returned by the time function to a 9-element list |
5f05dabc | 2299 | with the time analyzed for the local time zone. Typically used as |
a0d0e21e LW |
2300 | follows: |
2301 | ||
54310121 | 2302 | # 0 1 2 3 4 5 6 7 8 |
a0d0e21e LW |
2303 | ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = |
2304 | localtime(time); | |
2305 | ||
19799a22 GS |
2306 | All list elements are numeric, and come straight out of a struct tm. |
2307 | In particular this means that $mon has the range C<0..11> and $wday | |
2308 | has the range C<0..6> with sunday as day C<0>. Also, $year is the | |
2309 | number of years since 1900, that is, $year is C<123> in year 2023, | |
2b5ab1e7 TC |
2310 | and I<not> simply the last two digits of the year. If you assume it is, |
2311 | then you create non-Y2K-compliant programs--and you wouldn't want to do | |
2312 | that, would you? | |
54310121 | 2313 | |
abd75f24 GS |
2314 | The proper way to get a complete 4-digit year is simply: |
2315 | ||
2316 | $year += 1900; | |
2317 | ||
2318 | And to get the last two digits of the year (e.g., '01' in 2001) do: | |
2319 | ||
2320 | $year = sprintf("%02d", $year % 100); | |
2321 | ||
54310121 | 2322 | If EXPR is omitted, uses the current time (C<localtime(time)>). |
a0d0e21e | 2323 | |
f86cebdf | 2324 | In scalar context, returns the ctime(3) value: |
a0d0e21e | 2325 | |
5f05dabc | 2326 | $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994" |
a0d0e21e | 2327 | |
a3cb178b | 2328 | This scalar value is B<not> locale dependent, see L<perllocale>, but |
68f8bed4 JH |
2329 | instead a Perl builtin. Also see the C<Time::Local> module |
2330 | (to convert the second, minutes, hours, ... back to seconds since the | |
2331 | stroke of midnight the 1st of January 1970, the value returned by | |
ca6e1c26 | 2332 | time()), and the strftime(3) and mktime(3) functions available via the |
68f8bed4 JH |
2333 | POSIX module. To get somewhat similar but locale dependent date |
2334 | strings, set up your locale environment variables appropriately | |
2335 | (please see L<perllocale>) and try for example: | |
a3cb178b | 2336 | |
5a964f20 | 2337 | use POSIX qw(strftime); |
2b5ab1e7 | 2338 | $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; |
a3cb178b GS |
2339 | |
2340 | Note that the C<%a> and C<%b>, the short forms of the day of the week | |
2341 | and the month of the year, may not necessarily be three characters wide. | |
a0d0e21e | 2342 | |
19799a22 GS |
2343 | =item lock |
2344 | ||
2345 | lock I<THING> | |
2346 | ||
2347 | This function places an advisory lock on a variable, subroutine, | |
2348 | or referenced object contained in I<THING> until the lock goes out | |
2349 | of scope. This is a built-in function only if your version of Perl | |
2350 | was built with threading enabled, and if you've said C<use Threads>. | |
2351 | Otherwise a user-defined function by this name will be called. See | |
2352 | L<Thread>. | |
2353 | ||
a0d0e21e LW |
2354 | =item log EXPR |
2355 | ||
54310121 | 2356 | =item log |
bbce6d69 | 2357 | |
2b5ab1e7 TC |
2358 | Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, |
2359 | returns log of C<$_>. To get the log of another base, use basic algebra: | |
19799a22 | 2360 | The base-N log of a number is equal to the natural log of that number |
2b5ab1e7 TC |
2361 | divided by the natural log of N. For example: |
2362 | ||
2363 | sub log10 { | |
2364 | my $n = shift; | |
2365 | return log($n)/log(10); | |
2366 | } | |
2367 | ||
2368 | See also L</exp> for the inverse operation. | |
a0d0e21e LW |
2369 | |
2370 | =item lstat FILEHANDLE | |
2371 | ||
2372 | =item lstat EXPR | |
2373 | ||
54310121 | 2374 | =item lstat |
bbce6d69 | 2375 | |
19799a22 | 2376 | Does the same thing as the C<stat> function (including setting the |
5a964f20 TC |
2377 | special C<_> filehandle) but stats a symbolic link instead of the file |
2378 | the symbolic link points to. If symbolic links are unimplemented on | |
19799a22 | 2379 | your system, a normal C<stat> is done. |
a0d0e21e | 2380 | |
7660c0ab | 2381 | If EXPR is omitted, stats C<$_>. |
bbce6d69 | 2382 | |
a0d0e21e LW |
2383 | =item m// |
2384 | ||
2385 | The match operator. See L<perlop>. | |
2386 | ||
2387 | =item map BLOCK LIST | |
2388 | ||
2389 | =item map EXPR,LIST | |
2390 | ||
19799a22 GS |
2391 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting |
2392 | C<$_> to each element) and returns the list value composed of the | |
2393 | results of each such evaluation. In scalar context, returns the | |
2394 | total number of elements so generated. Evaluates BLOCK or EXPR in | |
2395 | list context, so each element of LIST may produce zero, one, or | |
2396 | more elements in the returned value. | |
dd99ebda | 2397 | |
a0d0e21e LW |
2398 | @chars = map(chr, @nums); |
2399 | ||
2400 | translates a list of numbers to the corresponding characters. And | |
2401 | ||
4633a7c4 | 2402 | %hash = map { getkey($_) => $_ } @array; |
a0d0e21e LW |
2403 | |
2404 | is just a funny way to write | |
2405 | ||
2406 | %hash = (); | |
2407 | foreach $_ (@array) { | |
4633a7c4 | 2408 | $hash{getkey($_)} = $_; |
a0d0e21e LW |
2409 | } |
2410 | ||
2b5ab1e7 TC |
2411 | Note that, because C<$_> is a reference into the list value, it can |
2412 | be used to modify the elements of the array. While this is useful and | |
2413 | supported, it can cause bizarre results if the LIST is not a named array. | |
2414 | Using a regular C<foreach> loop for this purpose would be clearer in | |
2415 | most cases. See also L</grep> for an array composed of those items of | |
2416 | the original list for which the BLOCK or EXPR evaluates to true. | |
fb73857a | 2417 | |
19799a22 | 2418 | =item mkdir FILENAME,MASK |
a0d0e21e | 2419 | |
5a211162 GS |
2420 | =item mkdir FILENAME |
2421 | ||
0591cd52 | 2422 | Creates the directory specified by FILENAME, with permissions |
19799a22 GS |
2423 | specified by MASK (as modified by C<umask>). If it succeeds it |
2424 | returns true, otherwise it returns false and sets C<$!> (errno). | |
5a211162 | 2425 | If omitted, MASK defaults to 0777. |
0591cd52 | 2426 | |
19799a22 | 2427 | In general, it is better to create directories with permissive MASK, |
0591cd52 | 2428 | and let the user modify that with their C<umask>, than it is to supply |
19799a22 | 2429 | a restrictive MASK and give the user no way to be more permissive. |
0591cd52 NT |
2430 | The exceptions to this rule are when the file or directory should be |
2431 | kept private (mail files, for instance). The perlfunc(1) entry on | |
19799a22 | 2432 | C<umask> discusses the choice of MASK in more detail. |
a0d0e21e LW |
2433 | |
2434 | =item msgctl ID,CMD,ARG | |
2435 | ||
f86cebdf | 2436 | Calls the System V IPC function msgctl(2). You'll probably have to say |
0ade1984 JH |
2437 | |
2438 | use IPC::SysV; | |
2439 | ||
7660c0ab A |
2440 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, |
2441 | then ARG must be a variable which will hold the returned C<msqid_ds> | |
951ba7fe GS |
2442 | structure. Returns like C<ioctl>: the undefined value for error, |
2443 | C<"0 but true"> for zero, or the actual return value otherwise. See also | |
19799a22 | 2444 | C<IPC::SysV> and C<IPC::Semaphore> documentation. |
a0d0e21e LW |
2445 | |
2446 | =item msgget KEY,FLAGS | |
2447 | ||
f86cebdf | 2448 | Calls the System V IPC function msgget(2). Returns the message queue |
7660c0ab | 2449 | id, or the undefined value if there is an error. See also C<IPC::SysV> |
19799a22 | 2450 | and C<IPC::Msg> documentation. |
a0d0e21e LW |
2451 | |
2452 | =item msgsnd ID,MSG,FLAGS | |
2453 | ||
2454 | Calls the System V IPC function msgsnd to send the message MSG to the | |
2455 | message queue ID. MSG must begin with the long integer message type, | |
19799a22 GS |
2456 | which may be created with C<pack("l", $type)>. Returns true if |
2457 | successful, or false if there is an error. See also C<IPC::SysV> | |
7660c0ab | 2458 | and C<IPC::SysV::Msg> documentation. |
a0d0e21e LW |
2459 | |
2460 | =item msgrcv ID,VAR,SIZE,TYPE,FLAGS | |
2461 | ||
2462 | Calls the System V IPC function msgrcv to receive a message from | |
2463 | message queue ID into variable VAR with a maximum message size of | |
0ade1984 JH |
2464 | SIZE. Note that if a message is received, the message type will be |
2465 | the first thing in VAR, and the maximum length of VAR is SIZE plus the | |
19799a22 | 2466 | size of the message type. Returns true if successful, or false if |
7660c0ab | 2467 | there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation. |
a0d0e21e LW |
2468 | |
2469 | =item my EXPR | |
2470 | ||
09bef843 SB |
2471 | =item my EXPR : ATTRIBUTES |
2472 | ||
19799a22 GS |
2473 | A C<my> declares the listed variables to be local (lexically) to the |
2474 | enclosing block, file, or C<eval>. If | |
5f05dabc | 2475 | more than one value is listed, the list must be placed in parentheses. See |
cb1a09d0 | 2476 | L<perlsub/"Private Variables via my()"> for details. |
4633a7c4 | 2477 | |
a0d0e21e LW |
2478 | =item next LABEL |
2479 | ||
2480 | =item next | |
2481 | ||
2482 | The C<next> command is like the C<continue> statement in C; it starts | |
2483 | the next iteration of the loop: | |
2484 | ||
4633a7c4 LW |
2485 | LINE: while (<STDIN>) { |
2486 | next LINE if /^#/; # discard comments | |
5a964f20 | 2487 | #... |
a0d0e21e LW |
2488 | } |
2489 | ||
2490 | Note that if there were a C<continue> block on the above, it would get | |
2491 | executed even on discarded lines. If the LABEL is omitted, the command | |
2492 | refers to the innermost enclosing loop. | |
2493 | ||
4968c1e4 | 2494 | C<next> cannot be used to exit a block which returns a value such as |
2b5ab1e7 TC |
2495 | C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit |
2496 | a grep() or map() operation. | |
4968c1e4 | 2497 | |
6c1372ed GS |
2498 | Note that a block by itself is semantically identical to a loop |
2499 | that executes once. Thus C<next> will exit such a block early. | |
2500 | ||
98293880 JH |
2501 | See also L</continue> for an illustration of how C<last>, C<next>, and |
2502 | C<redo> work. | |
1d2dff63 | 2503 | |
a0d0e21e LW |
2504 | =item no Module LIST |
2505 | ||
7660c0ab | 2506 | See the L</use> function, which C<no> is the opposite of. |
a0d0e21e LW |
2507 | |
2508 | =item oct EXPR | |
2509 | ||
54310121 | 2510 | =item oct |
bbce6d69 | 2511 | |
4633a7c4 | 2512 | Interprets EXPR as an octal string and returns the corresponding |
4f19785b WSI |
2513 | value. (If EXPR happens to start off with C<0x>, interprets it as a |
2514 | hex string. If EXPR starts off with C<0b>, it is interpreted as a | |
2515 | binary string.) The following will handle decimal, binary, octal, and | |
4633a7c4 | 2516 | hex in the standard Perl or C notation: |
a0d0e21e LW |
2517 | |
2518 | $val = oct($val) if $val =~ /^0/; | |
2519 | ||
19799a22 GS |
2520 | If EXPR is omitted, uses C<$_>. To go the other way (produce a number |
2521 | in octal), use sprintf() or printf(): | |
2522 | ||
2523 | $perms = (stat("filename"))[2] & 07777; | |
2524 | $oct_perms = sprintf "%lo", $perms; | |
2525 | ||
2526 | The oct() function is commonly used when a string such as C<644> needs | |
2527 | to be converted into a file mode, for example. (Although perl will | |
2528 | automatically convert strings into numbers as needed, this automatic | |
2529 | conversion assumes base 10.) | |
a0d0e21e | 2530 | |
6170680b IZ |
2531 | =item open FILEHANDLE,MODE,EXPR |
2532 | ||
a0d0e21e LW |
2533 | =item open FILEHANDLE,EXPR |
2534 | ||
2535 | =item open FILEHANDLE | |
2536 | ||
2537 | Opens the file whose filename is given by EXPR, and associates it with | |
5f05dabc | 2538 | FILEHANDLE. If FILEHANDLE is an expression, its value is used as the |
2539 | name of the real filehandle wanted. If EXPR is omitted, the scalar | |
2540 | variable of the same name as the FILEHANDLE contains the filename. | |
19799a22 GS |
2541 | (Note that lexical variables--those declared with C<my>--will not work |
2542 | for this purpose; so if you're using C<my>, specify EXPR in your call | |
2b5ab1e7 TC |
2543 | to open.) See L<perlopentut> for a kinder, gentler explanation of opening |
2544 | files. | |
5f05dabc | 2545 | |
6170680b IZ |
2546 | If MODE is C<'E<lt>'> or nothing, the file is opened for input. |
2547 | If MODE is C<'E<gt>'>, the file is truncated and opened for | |
2548 | output, being created if necessary. If MODE is C<'E<gt>E<gt>'>, | |
fbb426e4 | 2549 | the file is opened for appending, again being created if necessary. |
7660c0ab A |
2550 | You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that |
2551 | you want both read and write access to the file; thus C<'+E<lt>'> is almost | |
2552 | always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the | |
5a964f20 TC |
2553 | file first. You can't usually use either read-write mode for updating |
2554 | textfiles, since they have variable length records. See the B<-i> | |
0591cd52 NT |
2555 | switch in L<perlrun> for a better approach. The file is created with |
2556 | permissions of C<0666> modified by the process' C<umask> value. | |
5a964f20 | 2557 | |
f86cebdf | 2558 | These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>, |
7660c0ab | 2559 | C<'w+'>, C<'a'>, and C<'a+'>. |
5f05dabc | 2560 | |
6170680b IZ |
2561 | In the 2-arguments (and 1-argument) form of the call the mode and |
2562 | filename should be concatenated (in this order), possibly separated by | |
2563 | spaces. It is possible to omit the mode if the mode is C<'E<lt>'>. | |
2564 | ||
7660c0ab | 2565 | If the filename begins with C<'|'>, the filename is interpreted as a |
5a964f20 | 2566 | command to which output is to be piped, and if the filename ends with a |
f244e06d GS |
2567 | C<'|'>, the filename is interpreted as a command which pipes output to |
2568 | us. See L<perlipc/"Using open() for IPC"> | |
19799a22 | 2569 | for more examples of this. (You are not allowed to C<open> to a command |
5a964f20 TC |
2570 | that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, |
2571 | and L<perlipc/"Bidirectional Communication"> for alternatives.) | |
cb1a09d0 | 2572 | |
6170680b IZ |
2573 | If MODE is C<'|-'>, the filename is interpreted as a |
2574 | command to which output is to be piped, and if MODE is | |
2575 | C<'-|'>, the filename is interpreted as a command which pipes output to | |
2576 | us. In the 2-arguments (and 1-argument) form one should replace dash | |
2577 | (C<'-'>) with the command. See L<perlipc/"Using open() for IPC"> | |
2578 | for more examples of this. (You are not allowed to C<open> to a command | |
2579 | that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, | |
2580 | and L<perlipc/"Bidirectional Communication"> for alternatives.) | |
2581 | ||
2582 | In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN | |
2583 | and opening C<'E<gt>-'> opens STDOUT. | |
2584 | ||
2585 | Open returns | |
19799a22 | 2586 | nonzero upon success, the undefined value otherwise. If the C<open> |
4633a7c4 | 2587 | involved a pipe, the return value happens to be the pid of the |
54310121 | 2588 | subprocess. |
cb1a09d0 AD |
2589 | |
2590 | If you're unfortunate enough to be running Perl on a system that | |
2591 | distinguishes between text files and binary files (modern operating | |
2592 | systems don't care), then you should check out L</binmode> for tips for | |
19799a22 | 2593 | dealing with this. The key distinction between systems that need C<binmode> |
5a964f20 TC |
2594 | and those that don't is their text file formats. Systems like Unix, MacOS, and |
2595 | Plan9, which delimit lines with a single character, and which encode that | |
19799a22 | 2596 | character in C as C<"\n">, do not need C<binmode>. The rest need it. |
cb1a09d0 | 2597 | |
fb73857a | 2598 | When opening a file, it's usually a bad idea to continue normal execution |
19799a22 GS |
2599 | if the request failed, so C<open> is frequently used in connection with |
2600 | C<die>. Even if C<die> won't do what you want (say, in a CGI script, | |
fb73857a | 2601 | where you want to make a nicely formatted error message (but there are |
5a964f20 | 2602 | modules that can help with that problem)) you should always check |
19799a22 | 2603 | the return value from opening a file. The infrequent exception is when |
fb73857a | 2604 | working with an unopened filehandle is actually what you want to do. |
2605 | ||
cb1a09d0 | 2606 | Examples: |
a0d0e21e LW |
2607 | |
2608 | $ARTICLE = 100; | |
2609 | open ARTICLE or die "Can't find article $ARTICLE: $!\n"; | |
2610 | while (<ARTICLE>) {... | |
2611 | ||
6170680b | 2612 | open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) |
fb73857a | 2613 | # if the open fails, output is discarded |
a0d0e21e | 2614 | |
6170680b | 2615 | open(DBASE, '+<', 'dbase.mine') # open for update |
fb73857a | 2616 | or die "Can't open 'dbase.mine' for update: $!"; |
cb1a09d0 | 2617 | |
6170680b IZ |
2618 | open(DBASE, '+<dbase.mine') # ditto |
2619 | or die "Can't open 'dbase.mine' for update: $!"; | |
2620 | ||
2621 | open(ARTICLE, '-|', "caesar <$article") # decrypt article | |
fb73857a | 2622 | or die "Can't start caesar: $!"; |
a0d0e21e | 2623 | |
6170680b IZ |
2624 | open(ARTICLE, "caesar <$article |") # ditto |
2625 | or die "Can't start caesar: $!"; | |
2626 | ||
2627 | open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id | |
fb73857a | 2628 | or die "Can't start sort: $!"; |
a0d0e21e LW |
2629 | |
2630 | # process argument list of files along with any includes | |
2631 | ||
2632 | foreach $file (@ARGV) { | |
2633 | process($file, 'fh00'); | |
2634 | } | |
2635 | ||
2636 | sub process { | |
5a964f20 | 2637 | my($filename, $input) = @_; |
a0d0e21e LW |
2638 | $input++; # this is a string increment |
2639 | unless (open($input, $filename)) { | |
2640 | print STDERR "Can't open $filename: $!\n"; | |
2641 | return; | |
2642 | } | |
2643 | ||
5a964f20 | 2644 | local $_; |
a0d0e21e LW |
2645 | while (<$input>) { # note use of indirection |
2646 | if (/^#include "(.*)"/) { | |
2647 | process($1, $input); | |
2648 | next; | |
2649 | } | |
5a964f20 | 2650 | #... # whatever |
a0d0e21e LW |
2651 | } |
2652 | } | |
2653 | ||
2654 | You may also, in the Bourne shell tradition, specify an EXPR beginning | |
7660c0ab | 2655 | with C<'E<gt>&'>, in which case the rest of the string is interpreted as the |
5a964f20 | 2656 | name of a filehandle (or file descriptor, if numeric) to be |
6170680b IZ |
2657 | duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, |
2658 | C<E<lt>>, C<+E<gt>>, C<+E<gt>E<gt>>, and C<+E<lt>>. The | |
a0d0e21e | 2659 | mode you specify should match the mode of the original filehandle. |
184e9718 | 2660 | (Duping a filehandle does not take into account any existing contents of |
6170680b IZ |
2661 | stdio buffers.) Duping file handles is not yet supported for 3-argument |
2662 | open(). | |
2663 | ||
a0d0e21e LW |
2664 | Here is a script that saves, redirects, and restores STDOUT and |
2665 | STDERR: | |
2666 | ||
2667 | #!/usr/bin/perl | |
5a964f20 TC |
2668 | open(OLDOUT, ">&STDOUT"); |
2669 | open(OLDERR, ">&STDERR"); | |
a0d0e21e | 2670 | |
6170680b IZ |
2671 | open(STDOUT, '>', "foo.out") || die "Can't redirect stdout"; |
2672 | open(STDERR, ">&STDOUT") || die "Can't dup stdout"; | |
a0d0e21e LW |
2673 | |
2674 | select(STDERR); $| = 1; # make unbuffered | |
2675 | select(STDOUT); $| = 1; # make unbuffered | |
2676 | ||
2677 | print STDOUT "stdout 1\n"; # this works for | |
2678 | print STDERR "stderr 1\n"; # subprocesses too | |
2679 | ||
2680 | close(STDOUT); | |
2681 | close(STDERR); | |
2682 | ||
5a964f20 TC |
2683 | open(STDOUT, ">&OLDOUT"); |
2684 | open(STDERR, ">&OLDERR"); | |
a0d0e21e LW |
2685 | |
2686 | print STDOUT "stdout 2\n"; | |
2687 | print STDERR "stderr 2\n"; | |
2688 | ||
7660c0ab | 2689 | If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an |
19799a22 | 2690 | equivalent of C's C<fdopen> of that file descriptor; this is more |
4633a7c4 | 2691 | parsimonious of file descriptors. For example: |
a0d0e21e LW |
2692 | |
2693 | open(FILEHANDLE, "<&=$fd") | |
2694 | ||
4af147f6 CS |
2695 | Note that this feature depends on the fdopen() C library function. |
2696 | On many UNIX systems, fdopen() is known to fail when file descriptors | |
2697 | exceed a certain value, typically 255. If you need more file | |
2698 | descriptors than that, consider rebuilding Perl to use the C<sfio> | |
2699 | library. | |
2700 | ||
6170680b IZ |
2701 | If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'> |
2702 | with 2-arguments (or 1-argument) form of open(), then | |
a0d0e21e | 2703 | there is an implicit fork done, and the return value of open is the pid |
7660c0ab | 2704 | of the child within the parent process, and C<0> within the child |
184e9718 | 2705 | process. (Use C<defined($pid)> to determine whether the open was successful.) |
a0d0e21e LW |
2706 | The filehandle behaves normally for the parent, but i/o to that |
2707 | filehandle is piped from/to the STDOUT/STDIN of the child process. | |
2708 | In the child process the filehandle isn't opened--i/o happens from/to | |
2709 | the new STDOUT or STDIN. Typically this is used like the normal | |
2710 | piped open when you want to exercise more control over just how the | |
2711 | pipe command gets executed, such as when you are running setuid, and | |
54310121 | 2712 | don't want to have to scan shell commands for metacharacters. |
6170680b | 2713 | The following triples are more or less equivalent: |
a0d0e21e LW |
2714 | |
2715 | open(FOO, "|tr '[a-z]' '[A-Z]'"); | |
6170680b IZ |
2716 | open(FOO, '|-', "tr '[a-z]' '[A-Z]'"); |
2717 | open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]'; | |
a0d0e21e LW |
2718 | |
2719 | open(FOO, "cat -n '$file'|"); | |
6170680b IZ |
2720 | open(FOO, '-|', "cat -n '$file'"); |
2721 | open(FOO, '-|') || exec 'cat', '-n', $file; | |
a0d0e21e | 2722 | |
4633a7c4 LW |
2723 | See L<perlipc/"Safe Pipe Opens"> for more examples of this. |
2724 | ||
45bc9206 GS |
2725 | NOTE: On any operation that may do a fork, all files opened for output |
2726 | are flushed before the fork is attempted. On systems that support a | |
2727 | close-on-exec flag on files, the flag will be set for the newly opened | |
2728 | file descriptor as determined by the value of $^F. See L<perlvar/$^F>. | |
a0d0e21e | 2729 | |
0dccf244 CS |
2730 | Closing any piped filehandle causes the parent process to wait for the |
2731 | child to finish, and returns the status value in C<$?>. | |
2732 | ||
6170680b IZ |
2733 | The filename passed to 2-argument (or 1-argument) form of open() |
2734 | will have leading and trailing | |
f86cebdf | 2735 | whitespace deleted, and the normal redirection characters |
5a964f20 TC |
2736 | honored. This property, known as "magic open", |
2737 | can often be used to good effect. A user could specify a filename of | |
7660c0ab | 2738 | F<"rsh cat file |">, or you could change certain filenames as needed: |
5a964f20 TC |
2739 | |
2740 | $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; | |
2741 | open(FH, $filename) or die "Can't open $filename: $!"; | |
2742 | ||
6170680b IZ |
2743 | Use 3-argument form to open a file with arbitrary weird characters in it, |
2744 | ||
2745 | open(FOO, '<', $file); | |
2746 | ||
2747 | otherwise it's necessary to protect any leading and trailing whitespace: | |
5a964f20 TC |
2748 | |
2749 | $file =~ s#^(\s)#./$1#; | |
2750 | open(FOO, "< $file\0"); | |
2751 | ||
6170680b IZ |
2752 | (this may not work on some bizzare filesystems). One should |
2753 | conscientiously choose between the the I<magic> and 3-arguments form | |
2754 | of open(): | |
2755 | ||
2756 | open IN, $ARGV[0]; | |
2757 | ||
2758 | will allow the user to specify an argument of the form C<"rsh cat file |">, | |
2759 | but will not work on a filename which happens to have a trailing space, while | |
2760 | ||
2761 | open IN, '<', $ARGV[0]; | |
2762 | ||
2763 | will have exactly the opposite restrictions. | |
2764 | ||
19799a22 | 2765 | If you want a "real" C C<open> (see L<open(2)> on your system), then you |
6170680b IZ |
2766 | should use the C<sysopen> function, which involves no such magic (but |
2767 | may use subtly different filemodes than Perl open(), which is mapped | |
2768 | to C fopen()). This is | |
5a964f20 TC |
2769 | another way to protect your filenames from interpretation. For example: |
2770 | ||
2771 | use IO::Handle; | |
2772 | sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL) | |
2773 | or die "sysopen $path: $!"; | |
2774 | $oldfh = select(HANDLE); $| = 1; select($oldfh); | |
2775 | print HANDLE "stuff $$\n"); | |
2776 | seek(HANDLE, 0, 0); | |
2777 | print "File contains: ", <HANDLE>; | |
2778 | ||
7660c0ab A |
2779 | Using the constructor from the C<IO::Handle> package (or one of its |
2780 | subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous | |
5a964f20 TC |
2781 | filehandles that have the scope of whatever variables hold references to |
2782 | them, and automatically close whenever and however you leave that scope: | |
c07a80fd | 2783 | |
5f05dabc | 2784 | use IO::File; |
5a964f20 | 2785 | #... |
c07a80fd | 2786 | sub read_myfile_munged { |
2787 | my $ALL = shift; | |
5f05dabc | 2788 | my $handle = new IO::File; |
c07a80fd | 2789 | open($handle, "myfile") or die "myfile: $!"; |
2790 | $first = <$handle> | |
2791 | or return (); # Automatically closed here. | |
2792 | mung $first or die "mung failed"; # Or here. | |
2793 | return $first, <$handle> if $ALL; # Or here. | |
2794 | $first; # Or here. | |
2795 | } | |
2796 | ||
b687b08b | 2797 | See L</seek> for some details about mixing reading and writing. |
a0d0e21e LW |
2798 | |
2799 | =item opendir DIRHANDLE,EXPR | |
2800 | ||
19799a22 GS |
2801 | Opens a directory named EXPR for processing by C<readdir>, C<telldir>, |
2802 | C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful. | |
a0d0e21e LW |
2803 | DIRHANDLEs have their own namespace separate from FILEHANDLEs. |
2804 | ||
2805 | =item ord EXPR | |
2806 | ||
54310121 | 2807 | =item ord |
bbce6d69 | 2808 | |
a0ed51b3 | 2809 | Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If |
7660c0ab | 2810 | EXPR is omitted, uses C<$_>. For the reverse, see L</chr>. |
2b5ab1e7 | 2811 | See L<utf8> for more about Unicode. |
a0d0e21e | 2812 | |
77ca0c92 LW |
2813 | =item our EXPR |
2814 | ||
2815 | An C<our> declares the listed variables to be valid globals within | |
2816 | the enclosing block, file, or C<eval>. That is, it has the same | |
2817 | scoping rules as a "my" declaration, but does not create a local | |
2818 | variable. If more than one value is listed, the list must be placed | |
2819 | in parentheses. The C<our> declaration has no semantic effect unless | |
2820 | "use strict vars" is in effect, in which case it lets you use the | |
2821 | declared global variable without qualifying it with a package name. | |
2822 | (But only within the lexical scope of the C<our> declaration. In this | |
2823 | it differs from "use vars", which is package scoped.) | |
2824 | ||
f472eb5c GS |
2825 | An C<our> declaration declares a global variable that will be visible |
2826 | across its entire lexical scope, even across package boundaries. The | |
2827 | package in which the variable is entered is determined at the point | |
2828 | of the declaration, not at the point of use. This means the following | |
2829 | behavior holds: | |
2830 | ||
2831 | package Foo; | |
2832 | our $bar; # declares $Foo::bar for rest of lexical scope | |
2833 | $bar = 20; | |
2834 | ||
2835 | package Bar; | |
2836 | print $bar; # prints 20 | |
2837 | ||
2838 | Multiple C<our> declarations in the same lexical scope are allowed | |
2839 | if they are in different packages. If they happened to be in the same | |
2840 | package, Perl will emit warnings if you have asked for them. | |
2841 | ||
2842 | use warnings; | |
2843 | package Foo; | |
2844 | our $bar; # declares $Foo::bar for rest of lexical scope | |
2845 | $bar = 20; | |
2846 | ||
2847 | package Bar; | |
2848 | our $bar = 30; # declares $Bar::bar for rest of lexical scope | |
2849 | print $bar; # prints 30 | |
2850 | ||
2851 | our $bar; # emits warning | |
2852 | ||
a0d0e21e LW |
2853 | =item pack TEMPLATE,LIST |
2854 | ||
2b6c5635 GS |
2855 | Takes a LIST of values and converts it into a string using the rules |
2856 | given by the TEMPLATE. The resulting string is the concatenation of | |
2857 | the converted values. Typically, each converted value looks | |
2858 | like its machine-level representation. For example, on 32-bit machines | |
2859 | a converted integer may be represented by a sequence of 4 bytes. | |
2860 | ||
2861 | The TEMPLATE is a | |
a0d0e21e LW |
2862 | sequence of characters that give the order and type of values, as |
2863 | follows: | |
2864 | ||
5a929a98 | 2865 | a A string with arbitrary binary data, will be null padded. |
a0d0e21e | 2866 | A An ascii string, will be space padded. |
5a929a98 VU |
2867 | Z A null terminated (asciz) string, will be null padded. |
2868 | ||
2b6c5635 GS |
2869 | b A bit string (ascending bit order inside each byte, like vec()). |
2870 | B A bit string (descending bit order inside each byte). | |
a0d0e21e LW |
2871 | h A hex string (low nybble first). |
2872 | H A hex string (high nybble first). | |
2873 | ||
2874 | c A signed char value. | |
a0ed51b3 | 2875 | C An unsigned char value. Only does bytes. See U for Unicode. |
96e4d5b1 | 2876 | |
a0d0e21e LW |
2877 | s A signed short value. |
2878 | S An unsigned short value. | |
96e4d5b1 | 2879 | (This 'short' is _exactly_ 16 bits, which may differ from |
851646ae JH |
2880 | what a local C compiler calls 'short'. If you want |
2881 | native-length shorts, use the '!' suffix.) | |
96e4d5b1 | 2882 | |
a0d0e21e LW |
2883 | i A signed integer value. |
2884 | I An unsigned integer value. | |
19799a22 | 2885 | (This 'integer' is _at_least_ 32 bits wide. Its exact |
f86cebdf GS |
2886 | size depends on what a local C compiler calls 'int', |
2887 | and may even be larger than the 'long' described in | |
2888 | the next item.) | |
96e4d5b1 | 2889 | |
a0d0e21e LW |
2890 | l A signed long value. |
2891 | L An unsigned long value. | |
96e4d5b1 | 2892 | (This 'long' is _exactly_ 32 bits, which may differ from |
851646ae JH |
2893 | what a local C compiler calls 'long'. If you want |
2894 | native-length longs, use the '!' suffix.) | |
a0d0e21e | 2895 | |
5d11dd56 MG |
2896 | n An unsigned short in "network" (big-endian) order. |
2897 | N An unsigned long in "network" (big-endian) order. | |
2898 | v An unsigned short in "VAX" (little-endian) order. | |
2899 | V An unsigned long in "VAX" (little-endian) order. | |
96e4d5b1 | 2900 | (These 'shorts' and 'longs' are _exactly_ 16 bits and |
2901 | _exactly_ 32 bits, respectively.) | |
a0d0e21e | 2902 | |
dae0da7a JH |
2903 | q A signed quad (64-bit) value. |
2904 | Q An unsigned quad value. | |
851646ae JH |
2905 | (Quads are available only if your system supports 64-bit |
2906 | integer values _and_ if Perl has been compiled to support those. | |
dae0da7a JH |
2907 | Causes a fatal error otherwise.) |
2908 | ||
a0d0e21e LW |
2909 | f A single-precision float in the native format. |
2910 | d A double-precision float in the native format. | |
2911 | ||
2912 | p A pointer to a null-terminated string. | |
2913 | P A pointer to a structure (fixed-length string). | |
2914 | ||
2915 | u A uuencoded string. | |
a0ed51b3 LW |
2916 | U A Unicode character number. Encodes to UTF-8 internally. |
2917 | Works even if C<use utf8> is not in effect. | |
a0d0e21e | 2918 | |
96e4d5b1 | 2919 | w A BER compressed integer. Its bytes represent an unsigned |
f86cebdf GS |
2920 | integer in base 128, most significant digit first, with as |
2921 | few digits as possible. Bit eight (the high bit) is set | |
2922 | on each byte except the last. | |
def98dd4 | 2923 | |
a0d0e21e LW |
2924 | x A null byte. |
2925 | X Back up a byte. | |
2926 | @ Null fill to absolute position. | |
2927 | ||
5a929a98 VU |
2928 | The following rules apply: |
2929 | ||
2930 | =over 8 | |
2931 | ||
2932 | =item * | |
2933 | ||
5a964f20 | 2934 | Each letter may optionally be followed by a number giving a repeat |
951ba7fe GS |
2935 | count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>, |
2936 | C<H>, and C<P> the pack function will gobble up that many values from | |
5a929a98 | 2937 | the LIST. A C<*> for the repeat count means to use however many items are |
951ba7fe GS |
2938 | left, except for C<@>, C<x>, C<X>, where it is equivalent |
2939 | to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the | |
2b6c5635 GS |
2940 | same). |
2941 | ||
951ba7fe | 2942 | When used with C<Z>, C<*> results in the addition of a trailing null |
2b6c5635 GS |
2943 | byte (so the packed result will be one longer than the byte C<length> |
2944 | of the item). | |
2945 | ||
951ba7fe | 2946 | The repeat count for C<u> is interpreted as the maximal number of bytes |
2b6c5635 | 2947 | to encode per line of output, with 0 and 1 replaced by 45. |
5a929a98 VU |
2948 | |
2949 | =item * | |
2950 | ||
951ba7fe | 2951 | The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a |
5a929a98 | 2952 | string of length count, padding with nulls or spaces as necessary. When |
951ba7fe GS |
2953 | unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything |
2954 | after the first null, and C<a> returns data verbatim. When packing, | |
2955 | C<a>, and C<Z> are equivalent. | |
2b6c5635 GS |
2956 | |
2957 | If the value-to-pack is too long, it is truncated. If too long and an | |
951ba7fe GS |
2958 | explicit count is provided, C<Z> packs only C<$count-1> bytes, followed |
2959 | by a null byte. Thus C<Z> always packs a trailing null byte under | |
2b6c5635 | 2960 | all circumstances. |
5a929a98 VU |
2961 | |
2962 | =item * | |
2963 | ||
951ba7fe | 2964 | Likewise, the C<b> and C<B> fields pack a string that many bits long. |
c73032f5 IZ |
2965 | Each byte of the input field of pack() generates 1 bit of the result. |
2966 | Each result bit is based on the least-significant bit of the corresponding | |
2967 | input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and | |
2968 | C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">. | |
2969 | ||
2970 | Starting from the beginning of the input string of pack(), each 8-tuple | |
951ba7fe | 2971 | of bytes is converted to 1 byte of output. With format C<b> |
c73032f5 | 2972 | the first byte of the 8-tuple determines the least-significant bit of a |
951ba7fe | 2973 | byte, and with format C<B> it determines the most-significant bit of |
c73032f5 IZ |
2974 | a byte. |
2975 | ||
2976 | If the length of the input string is not exactly divisible by 8, the | |
2977 | remainder is packed as if the input string were padded by null bytes | |
2978 | at the end. Similarly, during unpack()ing the "extra" bits are ignored. | |
2979 | ||
2980 | If the input string of pack() is longer than needed, extra bytes are ignored. | |
2b6c5635 GS |
2981 | A C<*> for the repeat count of pack() means to use all the bytes of |
2982 | the input field. On unpack()ing the bits are converted to a string | |
2983 | of C<"0">s and C<"1">s. | |
5a929a98 VU |
2984 | |
2985 | =item * | |
2986 | ||
951ba7fe | 2987 | The C<h> and C<H> fields pack a string that many nybbles (4-bit groups, |
851646ae | 2988 | representable as hexadecimal digits, 0-9a-f) long. |
5a929a98 | 2989 | |
c73032f5 IZ |
2990 | Each byte of the input field of pack() generates 4 bits of the result. |
2991 | For non-alphabetical bytes the result is based on the 4 least-significant | |
2992 | bits of the input byte, i.e., on C<ord($byte)%16>. In particular, | |
2993 | bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes | |
2994 | C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result | |
2995 | is compatible with the usual hexadecimal digits, so that C<"a"> and | |
2996 | C<"A"> both generate the nybble C<0xa==10>. The result for bytes | |
2997 | C<"g".."z"> and C<"G".."Z"> is not well-defined. | |
2998 | ||
2999 | Starting from the beginning of the input string of pack(), each pair | |
951ba7fe | 3000 | of bytes is converted to 1 byte of output. With format C<h> the |
c73032f5 | 3001 | first byte of the pair determines the least-significant nybble of the |
951ba7fe | 3002 | output byte, and with format C<H> it determines the most-significant |
c73032f5 IZ |
3003 | nybble. |
3004 | ||
3005 | If the length of the input string is not even, it behaves as if padded | |
3006 | by a null byte at the end. Similarly, during unpack()ing the "extra" | |
3007 | nybbles are ignored. | |
3008 | ||
3009 | If the input string of pack() is longer than needed, extra bytes are ignored. | |
3010 | A C<*> for the repeat count of pack() means to use all the bytes of | |
3011 | the input field. On unpack()ing the bits are converted to a string | |
3012 | of hexadecimal digits. | |
3013 | ||
5a929a98 VU |
3014 | =item * |
3015 | ||
951ba7fe | 3016 | The C<p> type packs a pointer to a null-terminated string. You are |
5a929a98 VU |
3017 | responsible for ensuring the string is not a temporary value (which can |
3018 | potentially get deallocated before you get around to using the packed result). | |
951ba7fe GS |
3019 | The C<P> type packs a pointer to a structure of the size indicated by the |
3020 | length. A NULL pointer is created if the corresponding value for C<p> or | |
3021 | C<P> is C<undef>, similarly for unpack(). | |
5a929a98 VU |
3022 | |
3023 | =item * | |
3024 | ||
951ba7fe GS |
3025 | The C</> template character allows packing and unpacking of strings where |
3026 | the packed structure contains a byte count followed by the string itself. | |
17f4a12d | 3027 | You write I<length-item>C</>I<string-item>. |
43192e07 IP |
3028 | |
3029 | The I<length-item> can be any C<pack> template letter, | |
3030 | and describes how the length value is packed. | |
3031 | The ones likely to be of most use are integer-packing ones like | |
951ba7fe GS |
3032 | C<n> (for Java strings), C<w> (for ASN.1 or SNMP) |
3033 | and C<N> (for Sun XDR). | |
43192e07 IP |
3034 | |
3035 | The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">. | |
3036 | For C<unpack> the length of the string is obtained from the I<length-item>, | |
3037 | but if you put in the '*' it will be ignored. | |
3038 | ||
17f4a12d IZ |
3039 | unpack 'C/a', "\04Gurusamy"; gives 'Guru' |
3040 | unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J') | |
3041 | pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world" | |
43192e07 IP |
3042 | |
3043 | The I<length-item> is not returned explicitly from C<unpack>. | |
3044 | ||
951ba7fe GS |
3045 | Adding a count to the I<length-item> letter is unlikely to do anything |
3046 | useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a | |
3047 | I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters, | |
43192e07 IP |
3048 | which Perl does not regard as legal in numeric strings. |
3049 | ||
3050 | =item * | |
3051 | ||
951ba7fe GS |
3052 | The integer types C<s>, C<S>, C<l>, and C<L> may be |
3053 | immediately followed by a C<!> suffix to signify native shorts or | |
3054 | longs--as you can see from above for example a bare C<l> does mean | |
851646ae JH |
3055 | exactly 32 bits, the native C<long> (as seen by the local C compiler) |
3056 | may be larger. This is an issue mainly in 64-bit platforms. You can | |
951ba7fe | 3057 | see whether using C<!> makes any difference by |
726ea183 | 3058 | |
4d0c1c44 GS |
3059 | print length(pack("s")), " ", length(pack("s!")), "\n"; |
3060 | print length(pack("l")), " ", length(pack("l!")), "\n"; | |
ef54e1a4 | 3061 | |
951ba7fe GS |
3062 | C<i!> and C<I!> also work but only because of completeness; |
3063 | they are identical to C<i> and C<I>. | |
ef54e1a4 | 3064 | |
19799a22 GS |
3065 | The actual sizes (in bytes) of native shorts, ints, longs, and long |
3066 | longs on the platform where Perl was built are also available via | |
3067 | L<Config>: | |
3068 | ||
3069 | use Config; | |
3070 | print $Config{shortsize}, "\n"; | |
3071 | print $Config{intsize}, "\n"; | |
3072 | print $Config{longsize}, "\n"; | |
3073 | print $Config{longlongsize}, "\n"; | |
ef54e1a4 | 3074 | |
5074e145 | 3075 | (The C<$Config{longlongsize}> will be undefine if your system does |
851646ae JH |
3076 | not support long longs.) |
3077 | ||
ef54e1a4 JH |
3078 | =item * |
3079 | ||
951ba7fe | 3080 | The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L> |
ef54e1a4 JH |
3081 | are inherently non-portable between processors and operating systems |
3082 | because they obey the native byteorder and endianness. For example a | |
140cb37e | 3083 | 4-byte integer 0x12345678 (305419896 decimal) be ordered natively |
ef54e1a4 JH |
3084 | (arranged in and handled by the CPU registers) into bytes as |
3085 | ||
719a3cf5 JH |
3086 | 0x12 0x34 0x56 0x78 # little-endian |
3087 | 0x78 0x56 0x34 0x12 # big-endian | |
ef54e1a4 | 3088 | |
5d11dd56 | 3089 | Basically, the Intel, Alpha, and VAX CPUs are little-endian, while |
719a3cf5 JH |
3090 | everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA, |
3091 | Power, and Cray are big-endian. MIPS can be either: Digital used it | |
19799a22 | 3092 | in little-endian mode; SGI uses it in big-endian mode. |
719a3cf5 | 3093 | |
19799a22 | 3094 | The names `big-endian' and `little-endian' are comic references to |
ef54e1a4 JH |
3095 | the classic "Gulliver's Travels" (via the paper "On Holy Wars and a |
3096 | Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and | |
19799a22 | 3097 | the egg-eating habits of the Lilliputians. |
ef54e1a4 | 3098 | |
140cb37e | 3099 | Some systems may have even weirder byte orders such as |
ef54e1a4 JH |
3100 | |
3101 | 0x56 0x78 0x12 0x34 | |
3102 | 0x34 0x12 0x78 0x56 | |
3103 | ||
3104 | You can see your system's preference with | |
3105 | ||
3106 | print join(" ", map { sprintf "%#02x", $_ } | |
3107 | unpack("C*",pack("L",0x12345678))), "\n"; | |
3108 | ||
d99ad34e | 3109 | The byteorder on the platform where Perl was built is also available |
726ea183 | 3110 | via L<Config>: |
ef54e1a4 JH |
3111 | |
3112 | use Config; | |
3113 | print $Config{byteorder}, "\n"; | |
3114 | ||
d99ad34e JH |
3115 | Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'> |
3116 | and C<'87654321'> are big-endian. | |
719a3cf5 | 3117 | |
951ba7fe GS |
3118 | If you want portable packed integers use the formats C<n>, C<N>, |
3119 | C<v>, and C<V>, their byte endianness and size is known. | |
851646ae | 3120 | See also L<perlport>. |
ef54e1a4 JH |
3121 | |
3122 | =item * | |
3123 | ||
5a929a98 VU |
3124 | Real numbers (floats and doubles) are in the native machine format only; |
3125 | due to the multiplicity of floating formats around, and the lack of a | |
3126 | standard "network" representation, no facility for interchange has been | |
3127 | made. This means that packed floating point data written on one machine | |
3128 | may not be readable on another - even if both use IEEE floating point | |
3129 | arithmetic (as the endian-ness of the memory representation is not part | |
851646ae | 3130 | of the IEEE spec). See also L<perlport>. |
5a929a98 VU |
3131 | |
3132 | Note that Perl uses doubles internally for all numeric calculation, and | |
3133 | converting from double into float and thence back to double again will | |
3134 | lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general | |
19799a22 | 3135 | equal $foo). |
5a929a98 | 3136 | |
851646ae JH |
3137 | =item * |
3138 | ||
3139 | You must yourself do any alignment or padding by inserting for example | |
9ccd05c0 JH |
3140 | enough C<'x'>es while packing. There is no way to pack() and unpack() |
3141 | could know where the bytes are going to or coming from. Therefore | |
3142 | C<pack> (and C<unpack>) handle their output and input as flat | |
3143 | sequences of bytes. | |
851646ae | 3144 | |
17f4a12d IZ |
3145 | =item * |
3146 | ||
3147 | A comment in a TEMPLATE starts with C<#> and goes to the end of line. | |
3148 | ||
2b6c5635 GS |
3149 | =item * |
3150 | ||
3151 | If TEMPLATE requires more arguments to pack() than actually given, pack() | |
3152 | assumes additional C<""> arguments. If TEMPLATE requires less arguments | |
3153 | to pack() than actually given, extra arguments are ignored. | |
3154 | ||
5a929a98 | 3155 | =back |
a0d0e21e LW |
3156 | |
3157 | Examples: | |
3158 | ||
a0ed51b3 | 3159 | $foo = pack("CCCC",65,66,67,68); |
a0d0e21e | 3160 | # foo eq "ABCD" |
a0ed51b3 | 3161 | $foo = pack("C4",65,66,67,68); |
a0d0e21e | 3162 | # same thing |
a0ed51b3 LW |
3163 | $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9); |
3164 | # same thing with Unicode circled letters | |
a0d0e21e LW |
3165 | |
3166 | $foo = pack("ccxxcc",65,66,67,68); | |
3167 | # foo eq "AB\0\0CD" | |
3168 | ||
9ccd05c0 JH |
3169 | # note: the above examples featuring "C" and "c" are true |
3170 | # only on ASCII and ASCII-derived systems such as ISO Latin 1 | |
3171 | # and UTF-8. In EBCDIC the first example would be | |
3172 | # $foo = pack("CCCC",193,194,195,196); | |
3173 | ||
a0d0e21e LW |
3174 | $foo = pack("s2",1,2); |
3175 | # "\1\0\2\0" on little-endian | |
3176 | # "\0\1\0\2" on big-endian | |
3177 | ||
3178 | $foo = pack("a4","abcd","x","y","z"); | |
3179 | # "abcd" | |
3180 | ||
3181 | $foo = pack("aaaa","abcd","x","y","z"); | |
3182 | # "axyz" | |
3183 | ||
3184 | $foo = pack("a14","abcdefg"); | |
3185 | # "abcdefg\0\0\0\0\0\0\0" | |
3186 | ||
3187 | $foo = pack("i9pl", gmtime); | |
3188 | # a real struct tm (on my system anyway) | |
3189 | ||
5a929a98 VU |
3190 | $utmp_template = "Z8 Z8 Z16 L"; |
3191 | $utmp = pack($utmp_template, @utmp1); | |
3192 | # a struct utmp (BSDish) | |
3193 | ||
3194 | @utmp2 = unpack($utmp_template, $utmp); | |
3195 | # "@utmp1" eq "@utmp2" | |
3196 | ||
a0d0e21e LW |
3197 | sub bintodec { |
3198 | unpack("N", pack("B32", substr("0" x 32 . shift, -32))); | |
3199 | } | |
3200 | ||
851646ae JH |
3201 | $foo = pack('sx2l', 12, 34); |
3202 | # short 12, two zero bytes padding, long 34 | |
3203 | $bar = pack('s@4l', 12, 34); | |
3204 | # short 12, zero fill to position 4, long 34 | |
3205 | # $foo eq $bar | |
3206 | ||
5a929a98 | 3207 | The same template may generally also be used in unpack(). |
a0d0e21e | 3208 | |
5a964f20 TC |
3209 | =item package |
3210 | ||
cb1a09d0 AD |
3211 | =item package NAMESPACE |
3212 | ||
3213 | Declares the compilation unit as being in the given namespace. The scope | |
2b5ab1e7 | 3214 | of the package declaration is from the declaration itself through the end |
19799a22 | 3215 | of the enclosing block, file, or eval (the same as the C<my> operator). |
2b5ab1e7 TC |
3216 | All further unqualified dynamic identifiers will be in this namespace. |
3217 | A package statement affects only dynamic variables--including those | |
19799a22 GS |
3218 | you've used C<local> on--but I<not> lexical variables, which are created |
3219 | with C<my>. Typically it would be the first declaration in a file to | |
2b5ab1e7 TC |
3220 | be included by the C<require> or C<use> operator. You can switch into a |
3221 | package in more than one place; it merely influences which symbol table | |
3222 | is used by the compiler for the rest of that block. You can refer to | |
3223 | variables and filehandles in other packages by prefixing the identifier | |
3224 | with the package name and a double colon: C<$Package::Variable>. | |
3225 | If the package name is null, the C<main> package as assumed. That is, | |
3226 | C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>, | |
3227 | still seen in older code). | |
cb1a09d0 | 3228 | |
5a964f20 TC |
3229 | If NAMESPACE is omitted, then there is no current package, and all |
3230 | identifiers must be fully qualified or lexicals. This is stricter | |
3231 | than C<use strict>, since it also extends to function names. | |
3232 | ||
cb1a09d0 AD |
3233 | See L<perlmod/"Packages"> for more information about packages, modules, |
3234 | and classes. See L<perlsub> for other scoping issues. | |
3235 | ||
a0d0e21e LW |
3236 | =item pipe READHANDLE,WRITEHANDLE |
3237 | ||
3238 | Opens a pair of connected pipes like the corresponding system call. | |
3239 | Note that if you set up a loop of piped processes, deadlock can occur | |
3240 | unless you are very careful. In addition, note that Perl's pipes use | |
184e9718 | 3241 | stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE |
a0d0e21e LW |
3242 | after each command, depending on the application. |
3243 | ||
7e1af8bc | 3244 | See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication"> |
4633a7c4 LW |
3245 | for examples of such things. |
3246 | ||
4771b018 GS |
3247 | On systems that support a close-on-exec flag on files, the flag will be set |
3248 | for the newly opened file descriptors as determined by the value of $^F. | |
3249 | See L<perlvar/$^F>. | |
3250 | ||
a0d0e21e LW |
3251 | =item pop ARRAY |
3252 | ||
54310121 | 3253 | =item pop |
28757baa | 3254 | |
a0d0e21e | 3255 | Pops and returns the last value of the array, shortening the array by |
19799a22 | 3256 | one element. Has an effect similar to |
a0d0e21e | 3257 | |
19799a22 | 3258 | $ARRAY[$#ARRAY--] |
a0d0e21e | 3259 | |
19799a22 GS |
3260 | If there are no elements in the array, returns the undefined value |
3261 | (although this may happen at other times as well). If ARRAY is | |
3262 | omitted, pops the C<@ARGV> array in the main program, and the C<@_> | |
3263 | array in subroutines, just like C<shift>. | |
a0d0e21e LW |
3264 | |
3265 | =item pos SCALAR | |
3266 | ||
54310121 | 3267 | =item pos |
bbce6d69 | 3268 | |
4633a7c4 | 3269 | Returns the offset of where the last C<m//g> search left off for the variable |
7660c0ab | 3270 | is in question (C<$_> is used when the variable is not specified). May be |
44a8e56a | 3271 | modified to change that offset. Such modification will also influence |
3272 | the C<\G> zero-width assertion in regular expressions. See L<perlre> and | |
3273 | L<perlop>. | |
a0d0e21e LW |
3274 | |
3275 | =item print FILEHANDLE LIST | |
3276 | ||
3277 | =item print LIST | |
3278 | ||
3279 | =item print | |
3280 | ||
19799a22 GS |
3281 | Prints a string or a list of strings. Returns true if successful. |
3282 | FILEHANDLE may be a scalar variable name, in which case the variable | |
3283 | contains the name of or a reference to the filehandle, thus introducing | |
3284 | one level of indirection. (NOTE: If FILEHANDLE is a variable and | |
3285 | the next token is a term, it may be misinterpreted as an operator | |
2b5ab1e7 | 3286 | unless you interpose a C<+> or put parentheses around the arguments.) |
19799a22 GS |
3287 | If FILEHANDLE is omitted, prints by default to standard output (or |
3288 | to the last selected output channel--see L</select>). If LIST is | |
3289 | also omitted, prints C<$_> to the currently selected output channel. | |
3290 | To set the default output channel to something other than STDOUT | |
3291 | use the select operation. The current value of C<$,> (if any) is | |
3292 | printed between each LIST item. The current value of C<$\> (if | |
3293 | any) is printed after the entire LIST has been printed. Because | |
3294 | print takes a LIST, anything in the LIST is evaluated in list | |
3295 | context, and any subroutine that you call will have one or more of | |
3296 | its expressions evaluated in list context. Also be careful not to | |
3297 | follow the print keyword with a left parenthesis unless you want | |
3298 | the corresponding right parenthesis to terminate the arguments to | |
3299 | the print--interpose a C<+> or put parentheses around all the | |
3300 | arguments. | |
a0d0e21e | 3301 | |
4633a7c4 | 3302 | Note that if you're storing FILEHANDLES in an array or other expression, |
da0045b7 | 3303 | you will have to use a block returning its value instead: |
4633a7c4 LW |
3304 | |
3305 | print { $files[$i] } "stuff\n"; | |
3306 | print { $OK ? STDOUT : STDERR } "stuff\n"; | |
3307 | ||
5f05dabc | 3308 | =item printf FILEHANDLE FORMAT, LIST |
a0d0e21e | 3309 | |
5f05dabc | 3310 | =item printf FORMAT, LIST |
a0d0e21e | 3311 | |
7660c0ab | 3312 | Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\> |
a3cb178b | 3313 | (the output record separator) is not appended. The first argument |
19799a22 | 3314 | of the list will be interpreted as the C<printf> format. If C<use locale> is |
a034a98d DD |
3315 | in effect, the character used for the decimal point in formatted real numbers |
3316 | is affected by the LC_NUMERIC locale. See L<perllocale>. | |
a0d0e21e | 3317 | |
19799a22 GS |
3318 | Don't fall into the trap of using a C<printf> when a simple |
3319 | C<print> would do. The C<print> is more efficient and less | |
28757baa | 3320 | error prone. |
3321 | ||
da0045b7 | 3322 | =item prototype FUNCTION |
3323 | ||
3324 | Returns the prototype of a function as a string (or C<undef> if the | |
5f05dabc | 3325 | function has no prototype). FUNCTION is a reference to, or the name of, |
3326 | the function whose prototype you want to retrieve. | |
da0045b7 | 3327 | |
2b5ab1e7 TC |
3328 | If FUNCTION is a string starting with C<CORE::>, the rest is taken as a |
3329 | name for Perl builtin. If the builtin is not I<overridable> (such as | |
ab4f32c2 | 3330 | C<qw//>) or its arguments cannot be expressed by a prototype (such as |
19799a22 | 3331 | C<system>) returns C<undef> because the builtin does not really behave |
2b5ab1e7 TC |
3332 | like a Perl function. Otherwise, the string describing the equivalent |
3333 | prototype is returned. | |
b6c543e3 | 3334 | |
a0d0e21e LW |
3335 | =item push ARRAY,LIST |
3336 | ||
3337 | Treats ARRAY as a stack, and pushes the values of LIST | |
3338 | onto the end of ARRAY. The length of ARRAY increases by the length of | |
3339 | LIST. Has the same effect as | |
3340 | ||
3341 | for $value (LIST) { | |
3342 | $ARRAY[++$#ARRAY] = $value; | |
3343 | } | |
3344 | ||
3345 | but is more efficient. Returns the new number of elements in the array. | |
3346 | ||
3347 | =item q/STRING/ | |
3348 | ||
3349 | =item qq/STRING/ | |
3350 | ||
8782bef2 GB |
3351 | =item qr/STRING/ |
3352 | ||
a0d0e21e LW |
3353 | =item qx/STRING/ |
3354 | ||
3355 | =item qw/STRING/ | |
3356 | ||
4b6a7270 | 3357 | Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">. |
a0d0e21e LW |
3358 | |
3359 | =item quotemeta EXPR | |
3360 | ||
54310121 | 3361 | =item quotemeta |
bbce6d69 | 3362 | |
68dc0745 | 3363 | Returns the value of EXPR with all non-alphanumeric |
a034a98d DD |
3364 | characters backslashed. (That is, all characters not matching |
3365 | C</[A-Za-z_0-9]/> will be preceded by a backslash in the | |
3366 | returned string, regardless of any locale settings.) | |
3367 | This is the internal function implementing | |
7660c0ab | 3368 | the C<\Q> escape in double-quoted strings. |
a0d0e21e | 3369 | |
7660c0ab | 3370 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 3371 | |
a0d0e21e LW |
3372 | =item rand EXPR |
3373 | ||
3374 | =item rand | |
3375 | ||
7660c0ab | 3376 | Returns a random fractional number greater than or equal to C<0> and less |
3e3baf6d | 3377 | than the value of EXPR. (EXPR should be positive.) If EXPR is |
19799a22 GS |
3378 | omitted, the value C<1> is used. Automatically calls C<srand> unless |
3379 | C<srand> has already been called. See also C<srand>. | |
a0d0e21e | 3380 | |
2f9daede | 3381 | (Note: If your rand function consistently returns numbers that are too |
a0d0e21e | 3382 | large or too small, then your version of Perl was probably compiled |
2f9daede | 3383 | with the wrong number of RANDBITS.) |
a0d0e21e LW |
3384 | |
3385 | =item read FILEHANDLE,SCALAR,LENGTH,OFFSET | |
3386 | ||
3387 | =item read FILEHANDLE,SCALAR,LENGTH | |
3388 | ||
3389 | Attempts to read LENGTH bytes of data into variable SCALAR from the | |
3b02c43c GS |
3390 | specified FILEHANDLE. Returns the number of bytes actually read, |
3391 | C<0> at end of file, or undef if there was an error. SCALAR will be grown | |
3392 | or shrunk to the length actually read. An OFFSET may be specified to | |
3393 | place the read data at some other place than the beginning of the | |
f86cebdf | 3394 | string. This call is actually implemented in terms of stdio's fread(3) |
19799a22 | 3395 | call. To get a true read(2) system call, see C<sysread>. |
a0d0e21e LW |
3396 | |
3397 | =item readdir DIRHANDLE | |
3398 | ||
19799a22 | 3399 | Returns the next directory entry for a directory opened by C<opendir>. |
5a964f20 | 3400 | If used in list context, returns all the rest of the entries in the |
a0d0e21e | 3401 | directory. If there are no more entries, returns an undefined value in |
5a964f20 | 3402 | scalar context or a null list in list context. |
a0d0e21e | 3403 | |
19799a22 | 3404 | If you're planning to filetest the return values out of a C<readdir>, you'd |
5f05dabc | 3405 | better prepend the directory in question. Otherwise, because we didn't |
19799a22 | 3406 | C<chdir> there, it would have been testing the wrong file. |
cb1a09d0 AD |
3407 | |
3408 | opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!"; | |
3409 | @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR); | |
3410 | closedir DIR; | |
3411 | ||
84902520 TB |
3412 | =item readline EXPR |
3413 | ||
fbad3eb5 GS |
3414 | Reads from the filehandle whose typeglob is contained in EXPR. In scalar |
3415 | context, each call reads and returns the next line, until end-of-file is | |
3416 | reached, whereupon the subsequent call returns undef. In list context, | |
3417 | reads until end-of-file is reached and returns a list of lines. Note that | |
3418 | the notion of "line" used here is however you may have defined it | |
3419 | with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">. | |
3420 | ||
2b5ab1e7 | 3421 | When C<$/> is set to C<undef>, when readline() is in scalar |
449bc448 GS |
3422 | context (i.e. file slurp mode), and when an empty file is read, it |
3423 | returns C<''> the first time, followed by C<undef> subsequently. | |
fbad3eb5 | 3424 | |
84902520 TB |
3425 | This is the internal function implementing the C<E<lt>EXPRE<gt>> |
3426 | operator, but you can use it directly. The C<E<lt>EXPRE<gt>> | |
3427 | operator is discussed in more detail in L<perlop/"I/O Operators">. | |
3428 | ||
5a964f20 TC |
3429 | $line = <STDIN>; |
3430 | $line = readline(*STDIN); # same thing | |
3431 | ||
a0d0e21e LW |
3432 | =item readlink EXPR |
3433 | ||
54310121 | 3434 | =item readlink |
bbce6d69 | 3435 | |
a0d0e21e LW |
3436 | Returns the value of a symbolic link, if symbolic links are |
3437 | implemented. If not, gives a fatal error. If there is some system | |
184e9718 | 3438 | error, returns the undefined value and sets C<$!> (errno). If EXPR is |
7660c0ab | 3439 | omitted, uses C<$_>. |
a0d0e21e | 3440 | |
84902520 TB |
3441 | =item readpipe EXPR |
3442 | ||
5a964f20 | 3443 | EXPR is executed as a system command. |
84902520 TB |
3444 | The collected standard output of the command is returned. |
3445 | In scalar context, it comes back as a single (potentially | |
3446 | multi-line) string. In list context, returns a list of lines | |
7660c0ab | 3447 | (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>). |
84902520 TB |
3448 | This is the internal function implementing the C<qx/EXPR/> |
3449 | operator, but you can use it directly. The C<qx/EXPR/> | |
3450 | operator is discussed in more detail in L<perlop/"I/O Operators">. | |
3451 | ||
399388f4 | 3452 | =item recv SOCKET,SCALAR,LENGTH,FLAGS |
a0d0e21e LW |
3453 | |
3454 | Receives a message on a socket. Attempts to receive LENGTH bytes of | |
478234b4 GS |
3455 | data into variable SCALAR from the specified SOCKET filehandle. SCALAR |
3456 | will be grown or shrunk to the length actually read. Takes the same | |
3457 | flags as the system call of the same name. Returns the address of the | |
3458 | sender if SOCKET's protocol supports this; returns an empty string | |
3459 | otherwise. If there's an error, returns the undefined value. This call | |
3460 | is actually implemented in terms of recvfrom(2) system call. See | |
3461 | L<perlipc/"UDP: Message Passing"> for examples. | |
a0d0e21e LW |
3462 | |
3463 | =item redo LABEL | |
3464 | ||
3465 | =item redo | |
3466 | ||
3467 | The C<redo> command restarts the loop block without evaluating the | |
98293880 | 3468 | conditional again. The C<continue> block, if any, is not executed. If |
a0d0e21e LW |
3469 | the LABEL is omitted, the command refers to the innermost enclosing |
3470 | loop. This command is normally used by programs that want to lie to | |
3471 | themselves about what was just input: | |
3472 | ||
3473 | # a simpleminded Pascal comment stripper | |
3474 | # (warning: assumes no { or } in strings) | |
4633a7c4 | 3475 | LINE: while (<STDIN>) { |
a0d0e21e LW |
3476 | while (s|({.*}.*){.*}|$1 |) {} |
3477 | s|{.*}| |; | |
3478 | if (s|{.*| |) { | |
3479 | $front = $_; | |
3480 | while (<STDIN>) { | |
3481 | if (/}/) { # end of comment? | |
5a964f20 | 3482 | s|^|$front\{|; |
4633a7c4 | 3483 | redo LINE; |
a0d0e21e LW |
3484 | } |
3485 | } | |
3486 | } | |
3487 | print; | |
3488 | } | |
3489 | ||
4968c1e4 | 3490 | C<redo> cannot be used to retry a block which returns a value such as |
2b5ab1e7 TC |
3491 | C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit |
3492 | a grep() or map() operation. | |
4968c1e4 | 3493 | |
6c1372ed GS |
3494 | Note that a block by itself is semantically identical to a loop |
3495 | that executes once. Thus C<redo> inside such a block will effectively | |
3496 | turn it into a looping construct. | |
3497 | ||
98293880 | 3498 | See also L</continue> for an illustration of how C<last>, C<next>, and |
1d2dff63 GS |
3499 | C<redo> work. |
3500 | ||
a0d0e21e LW |
3501 | =item ref EXPR |
3502 | ||
54310121 | 3503 | =item ref |
bbce6d69 | 3504 | |
19799a22 | 3505 | Returns a true value if EXPR is a reference, false otherwise. If EXPR |
7660c0ab | 3506 | is not specified, C<$_> will be used. The value returned depends on the |
bbce6d69 | 3507 | type of thing the reference is a reference to. |
a0d0e21e LW |
3508 | Builtin types include: |
3509 | ||
a0d0e21e LW |
3510 | SCALAR |
3511 | ARRAY | |
3512 | HASH | |
3513 | CODE | |
19799a22 | 3514 | REF |
a0d0e21e | 3515 | GLOB |
19799a22 | 3516 | LVALUE |
a0d0e21e | 3517 | |
54310121 | 3518 | If the referenced object has been blessed into a package, then that package |
19799a22 | 3519 | name is returned instead. You can think of C<ref> as a C<typeof> operator. |
a0d0e21e LW |
3520 | |
3521 | if (ref($r) eq "HASH") { | |
aa689395 | 3522 | print "r is a reference to a hash.\n"; |
54310121 | 3523 | } |
2b5ab1e7 | 3524 | unless (ref($r)) { |
a0d0e21e | 3525 | print "r is not a reference at all.\n"; |
54310121 | 3526 | } |
2b5ab1e7 TC |
3527 | if (UNIVERSAL::isa($r, "HASH")) { # for subclassing |
3528 | print "r is a reference to something that isa hash.\n"; | |
3529 | } | |
a0d0e21e LW |
3530 | |
3531 | See also L<perlref>. | |
3532 | ||
3533 | =item rename OLDNAME,NEWNAME | |
3534 | ||
19799a22 GS |
3535 | Changes the name of a file; an existing file NEWNAME will be |
3536 | clobbered. Returns true for success, false otherwise. | |
3537 | ||
2b5ab1e7 TC |
3538 | Behavior of this function varies wildly depending on your system |
3539 | implementation. For example, it will usually not work across file system | |
3540 | boundaries, even though the system I<mv> command sometimes compensates | |
3541 | for this. Other restrictions include whether it works on directories, | |
3542 | open files, or pre-existing files. Check L<perlport> and either the | |
3543 | rename(2) manpage or equivalent system documentation for details. | |
a0d0e21e | 3544 | |
16070b82 GS |
3545 | =item require VERSION |
3546 | ||
a0d0e21e LW |
3547 | =item require EXPR |
3548 | ||
3549 | =item require | |
3550 | ||
7660c0ab | 3551 | Demands some semantics specified by EXPR, or by C<$_> if EXPR is not |
16070b82 GS |
3552 | supplied. If a version number or tuple is specified, or if EXPR is |
3553 | numeric, demands that the current version of Perl | |
3554 | (C<$^V> or C<$]> or $PERL_VERSION) be equal or greater than EXPR. | |
a0d0e21e LW |
3555 | |
3556 | Otherwise, demands that a library file be included if it hasn't already | |
3557 | been included. The file is included via the do-FILE mechanism, which is | |
19799a22 | 3558 | essentially just a variety of C<eval>. Has semantics similar to the following |
a0d0e21e LW |
3559 | subroutine: |
3560 | ||
3561 | sub require { | |
5a964f20 | 3562 | my($filename) = @_; |
a0d0e21e | 3563 | return 1 if $INC{$filename}; |
5a964f20 | 3564 | my($realfilename,$result); |
a0d0e21e LW |
3565 | ITER: { |
3566 | foreach $prefix (@INC) { | |
3567 | $realfilename = "$prefix/$filename"; | |
3568 | if (-f $realfilename) { | |
f784dfa3 | 3569 | $INC{$filename} = $realfilename; |
a0d0e21e LW |
3570 | $result = do $realfilename; |
3571 | last ITER; | |
3572 | } | |
3573 | } | |
3574 | die "Can't find $filename in \@INC"; | |
3575 | } | |
f784dfa3 | 3576 | delete $INC{$filename} if $@ || !$result; |
a0d0e21e LW |
3577 | die $@ if $@; |
3578 | die "$filename did not return true value" unless $result; | |
5a964f20 | 3579 | return $result; |
a0d0e21e LW |
3580 | } |
3581 | ||
3582 | Note that the file will not be included twice under the same specified | |
19799a22 | 3583 | name. The file must return true as the last statement to indicate |
a0d0e21e | 3584 | successful execution of any initialization code, so it's customary to |
19799a22 GS |
3585 | end such a file with C<1;> unless you're sure it'll return true |
3586 | otherwise. But it's better just to put the C<1;>, in case you add more | |
a0d0e21e LW |
3587 | statements. |
3588 | ||
54310121 | 3589 | If EXPR is a bareword, the require assumes a "F<.pm>" extension and |
da0045b7 | 3590 | replaces "F<::>" with "F</>" in the filename for you, |
54310121 | 3591 | to make it easy to load standard modules. This form of loading of |
a0d0e21e LW |
3592 | modules does not risk altering your namespace. |
3593 | ||
ee580363 GS |
3594 | In other words, if you try this: |
3595 | ||
f86cebdf | 3596 | require Foo::Bar; # a splendid bareword |
ee580363 | 3597 | |
7660c0ab A |
3598 | The require function will actually look for the "F<Foo/Bar.pm>" file in the |
3599 | directories specified in the C<@INC> array. | |
ee580363 | 3600 | |
5a964f20 | 3601 | But if you try this: |
ee580363 GS |
3602 | |
3603 | $class = 'Foo::Bar'; | |
f86cebdf | 3604 | require $class; # $class is not a bareword |
5a964f20 | 3605 | #or |
f86cebdf | 3606 | require "Foo::Bar"; # not a bareword because of the "" |
ee580363 | 3607 | |
7660c0ab | 3608 | The require function will look for the "F<Foo::Bar>" file in the @INC array and |
19799a22 | 3609 | will complain about not finding "F<Foo::Bar>" there. In this case you can do: |
ee580363 GS |
3610 | |
3611 | eval "require $class"; | |
3612 | ||
3613 | For a yet-more-powerful import facility, see L</use> and L<perlmod>. | |
a0d0e21e LW |
3614 | |
3615 | =item reset EXPR | |
3616 | ||
3617 | =item reset | |
3618 | ||
3619 | Generally used in a C<continue> block at the end of a loop to clear | |
7660c0ab | 3620 | variables and reset C<??> searches so that they work again. The |
a0d0e21e LW |
3621 | expression is interpreted as a list of single characters (hyphens |
3622 | allowed for ranges). All variables and arrays beginning with one of | |
3623 | those letters are reset to their pristine state. If the expression is | |
7660c0ab | 3624 | omitted, one-match searches (C<?pattern?>) are reset to match again. Resets |
5f05dabc | 3625 | only variables or searches in the current package. Always returns |
a0d0e21e LW |
3626 | 1. Examples: |
3627 | ||
3628 | reset 'X'; # reset all X variables | |
3629 | reset 'a-z'; # reset lower case variables | |
2b5ab1e7 | 3630 | reset; # just reset ?one-time? searches |
a0d0e21e | 3631 | |
7660c0ab | 3632 | Resetting C<"A-Z"> is not recommended because you'll wipe out your |
2b5ab1e7 TC |
3633 | C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package |
3634 | variables--lexical variables are unaffected, but they clean themselves | |
3635 | up on scope exit anyway, so you'll probably want to use them instead. | |
3636 | See L</my>. | |
a0d0e21e | 3637 | |
54310121 | 3638 | =item return EXPR |
3639 | ||
3640 | =item return | |
3641 | ||
19799a22 | 3642 | Returns from a subroutine, C<eval>, or C<do FILE> with the value |
5a964f20 | 3643 | given in EXPR. Evaluation of EXPR may be in list, scalar, or void |
54310121 | 3644 | context, depending on how the return value will be used, and the context |
19799a22 | 3645 | may vary from one execution to the next (see C<wantarray>). If no EXPR |
2b5ab1e7 TC |
3646 | is given, returns an empty list in list context, the undefined value in |
3647 | scalar context, and (of course) nothing at all in a void context. | |
a0d0e21e | 3648 | |
2b5ab1e7 TC |
3649 | (Note that in the absence of a explicit C<return>, a subroutine, eval, |
3650 | or do FILE will automatically return the value of the last expression | |
3651 | evaluated.) | |
a0d0e21e LW |
3652 | |
3653 | =item reverse LIST | |
3654 | ||
5a964f20 TC |
3655 | In list context, returns a list value consisting of the elements |
3656 | of LIST in the opposite order. In scalar context, concatenates the | |
2b5ab1e7 | 3657 | elements of LIST and returns a string value with all characters |
a0ed51b3 | 3658 | in the opposite order. |
4633a7c4 | 3659 | |
2f9daede | 3660 | print reverse <>; # line tac, last line first |
4633a7c4 | 3661 | |
2f9daede | 3662 | undef $/; # for efficiency of <> |
a0ed51b3 | 3663 | print scalar reverse <>; # character tac, last line tsrif |
2f9daede TP |
3664 | |
3665 | This operator is also handy for inverting a hash, although there are some | |
3666 | caveats. If a value is duplicated in the original hash, only one of those | |
3667 | can be represented as a key in the inverted hash. Also, this has to | |
3668 | unwind one hash and build a whole new one, which may take some time | |
2b5ab1e7 | 3669 | on a large hash, such as from a DBM file. |
2f9daede TP |
3670 | |
3671 | %by_name = reverse %by_address; # Invert the hash | |
a0d0e21e LW |
3672 | |
3673 | =item rewinddir DIRHANDLE | |
3674 | ||
3675 | Sets the current position to the beginning of the directory for the | |
19799a22 | 3676 | C<readdir> routine on DIRHANDLE. |
a0d0e21e LW |
3677 | |
3678 | =item rindex STR,SUBSTR,POSITION | |
3679 | ||
3680 | =item rindex STR,SUBSTR | |
3681 | ||
2b5ab1e7 | 3682 | Works just like index() except that it returns the position of the LAST |
a0d0e21e LW |
3683 | occurrence of SUBSTR in STR. If POSITION is specified, returns the |
3684 | last occurrence at or before that position. | |
3685 | ||
3686 | =item rmdir FILENAME | |
3687 | ||
54310121 | 3688 | =item rmdir |
bbce6d69 | 3689 | |
5a964f20 | 3690 | Deletes the directory specified by FILENAME if that directory is empty. If it |
19799a22 | 3691 | succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If |
7660c0ab | 3692 | FILENAME is omitted, uses C<$_>. |
a0d0e21e LW |
3693 | |
3694 | =item s/// | |
3695 | ||
3696 | The substitution operator. See L<perlop>. | |
3697 | ||
3698 | =item scalar EXPR | |
3699 | ||
5a964f20 | 3700 | Forces EXPR to be interpreted in scalar context and returns the value |
54310121 | 3701 | of EXPR. |
cb1a09d0 AD |
3702 | |
3703 | @counts = ( scalar @a, scalar @b, scalar @c ); | |
3704 | ||
54310121 | 3705 | There is no equivalent operator to force an expression to |
2b5ab1e7 | 3706 | be interpolated in list context because in practice, this is never |
cb1a09d0 AD |
3707 | needed. If you really wanted to do so, however, you could use |
3708 | the construction C<@{[ (some expression) ]}>, but usually a simple | |
3709 | C<(some expression)> suffices. | |
a0d0e21e | 3710 | |
19799a22 | 3711 | Because C<scalar> is unary operator, if you accidentally use for EXPR a |
2b5ab1e7 TC |
3712 | parenthesized list, this behaves as a scalar comma expression, evaluating |
3713 | all but the last element in void context and returning the final element | |
3714 | evaluated in scalar context. This is seldom what you want. | |
62c18ce2 GS |
3715 | |
3716 | The following single statement: | |
3717 | ||
3718 | print uc(scalar(&foo,$bar)),$baz; | |
3719 | ||
3720 | is the moral equivalent of these two: | |
3721 | ||
3722 | &foo; | |
3723 | print(uc($bar),$baz); | |
3724 | ||
3725 | See L<perlop> for more details on unary operators and the comma operator. | |
3726 | ||
a0d0e21e LW |
3727 | =item seek FILEHANDLE,POSITION,WHENCE |
3728 | ||
19799a22 | 3729 | Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>. |
8903cb82 | 3730 | FILEHANDLE may be an expression whose value gives the name of the |
7660c0ab | 3731 | filehandle. The values for WHENCE are C<0> to set the new position to |
ac88732c JH |
3732 | POSITION, C<1> to set it to the current position plus POSITION, and |
3733 | C<2> to set it to EOF plus POSITION (typically negative). For WHENCE | |
3734 | you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> | |
ca6e1c26 JH |
3735 | (start of the file, current position, end of the file) from the Fcntl |
3736 | module. Returns C<1> upon success, C<0> otherwise. | |
8903cb82 | 3737 | |
19799a22 GS |
3738 | If you want to position file for C<sysread> or C<syswrite>, don't use |
3739 | C<seek>--buffering makes its effect on the file's system position | |
3740 | unpredictable and non-portable. Use C<sysseek> instead. | |
a0d0e21e | 3741 | |
2b5ab1e7 TC |
3742 | Due to the rules and rigors of ANSI C, on some systems you have to do a |
3743 | seek whenever you switch between reading and writing. Amongst other | |
3744 | things, this may have the effect of calling stdio's clearerr(3). | |
3745 | A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position: | |
cb1a09d0 AD |
3746 | |
3747 | seek(TEST,0,1); | |
3748 | ||
3749 | This is also useful for applications emulating C<tail -f>. Once you hit | |
3750 | EOF on your read, and then sleep for a while, you might have to stick in a | |
19799a22 | 3751 | seek() to reset things. The C<seek> doesn't change the current position, |
8903cb82 | 3752 | but it I<does> clear the end-of-file condition on the handle, so that the |
3753 | next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope. | |
cb1a09d0 AD |
3754 | |
3755 | If that doesn't work (some stdios are particularly cantankerous), then | |
3756 | you may need something more like this: | |
3757 | ||
3758 | for (;;) { | |
f86cebdf GS |
3759 | for ($curpos = tell(FILE); $_ = <FILE>; |
3760 | $curpos = tell(FILE)) { | |
cb1a09d0 AD |
3761 | # search for some stuff and put it into files |
3762 | } | |
3763 | sleep($for_a_while); | |
3764 | seek(FILE, $curpos, 0); | |
3765 | } | |
3766 | ||
a0d0e21e LW |
3767 | =item seekdir DIRHANDLE,POS |
3768 | ||
19799a22 GS |
3769 | Sets the current position for the C<readdir> routine on DIRHANDLE. POS |
3770 | must be a value returned by C<telldir>. Has the same caveats about | |
a0d0e21e LW |
3771 | possible directory compaction as the corresponding system library |
3772 | routine. | |
3773 | ||
3774 | =item select FILEHANDLE | |
3775 | ||
3776 | =item select | |
3777 | ||
3778 | Returns the currently selected filehandle. Sets the current default | |
3779 | filehandle for output, if FILEHANDLE is supplied. This has two | |
19799a22 | 3780 | effects: first, a C<write> or a C<print> without a filehandle will |
a0d0e21e LW |
3781 | default to this FILEHANDLE. Second, references to variables related to |
3782 | output will refer to this output channel. For example, if you have to | |
3783 | set the top of form format for more than one output channel, you might | |
3784 | do the following: | |
3785 | ||
3786 | select(REPORT1); | |
3787 | $^ = 'report1_top'; | |
3788 | select(REPORT2); | |
3789 | $^ = 'report2_top'; | |
3790 | ||
3791 | FILEHANDLE may be an expression whose value gives the name of the | |
3792 | actual filehandle. Thus: | |
3793 | ||
3794 | $oldfh = select(STDERR); $| = 1; select($oldfh); | |
3795 | ||
4633a7c4 LW |
3796 | Some programmers may prefer to think of filehandles as objects with |
3797 | methods, preferring to write the last example as: | |
a0d0e21e | 3798 | |
28757baa | 3799 | use IO::Handle; |
a0d0e21e LW |
3800 | STDERR->autoflush(1); |
3801 | ||
3802 | =item select RBITS,WBITS,EBITS,TIMEOUT | |
3803 | ||
f86cebdf | 3804 | This calls the select(2) system call with the bit masks specified, which |
19799a22 | 3805 | can be constructed using C<fileno> and C<vec>, along these lines: |
a0d0e21e LW |
3806 | |
3807 | $rin = $win = $ein = ''; | |
3808 | vec($rin,fileno(STDIN),1) = 1; | |
3809 | vec($win,fileno(STDOUT),1) = 1; | |
3810 | $ein = $rin | $win; | |
3811 | ||
3812 | If you want to select on many filehandles you might wish to write a | |
3813 | subroutine: | |
3814 | ||
3815 | sub fhbits { | |
5a964f20 TC |
3816 | my(@fhlist) = split(' ',$_[0]); |
3817 | my($bits); | |
a0d0e21e LW |
3818 | for (@fhlist) { |
3819 | vec($bits,fileno($_),1) = 1; | |
3820 | } | |
3821 | $bits; | |
3822 | } | |
4633a7c4 | 3823 | $rin = fhbits('STDIN TTY SOCK'); |
a0d0e21e LW |
3824 | |
3825 | The usual idiom is: | |
3826 | ||
3827 | ($nfound,$timeleft) = | |
3828 | select($rout=$rin, $wout=$win, $eout=$ein, $timeout); | |
3829 | ||
54310121 | 3830 | or to block until something becomes ready just do this |
a0d0e21e LW |
3831 | |
3832 | $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef); | |
3833 | ||
19799a22 GS |
3834 | Most systems do not bother to return anything useful in $timeleft, so |
3835 | calling select() in scalar context just returns $nfound. | |
c07a80fd | 3836 | |
5f05dabc | 3837 | Any of the bit masks can also be undef. The timeout, if specified, is |
a0d0e21e | 3838 | in seconds, which may be fractional. Note: not all implementations are |
19799a22 GS |
3839 | capable of returning the$timeleft. If not, they always return |
3840 | $timeleft equal to the supplied $timeout. | |
a0d0e21e | 3841 | |
ff68c719 | 3842 | You can effect a sleep of 250 milliseconds this way: |
a0d0e21e LW |
3843 | |
3844 | select(undef, undef, undef, 0.25); | |
3845 | ||
19799a22 GS |
3846 | B<WARNING>: One should not attempt to mix buffered I/O (like C<read> |
3847 | or E<lt>FHE<gt>) with C<select>, except as permitted by POSIX, and even | |
3848 | then only on POSIX systems. You have to use C<sysread> instead. | |
a0d0e21e LW |
3849 | |
3850 | =item semctl ID,SEMNUM,CMD,ARG | |
3851 | ||
19799a22 | 3852 | Calls the System V IPC function C<semctl>. You'll probably have to say |
0ade1984 JH |
3853 | |
3854 | use IPC::SysV; | |
3855 | ||
3856 | first to get the correct constant definitions. If CMD is IPC_STAT or | |
3857 | GETALL, then ARG must be a variable which will hold the returned | |
19799a22 GS |
3858 | semid_ds structure or semaphore value array. Returns like C<ioctl>: the |
3859 | undefined value for error, "C<0 but true>" for zero, or the actual return | |
7660c0ab | 3860 | value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation. |
a0d0e21e LW |
3861 | |
3862 | =item semget KEY,NSEMS,FLAGS | |
3863 | ||
3864 | Calls the System V IPC function semget. Returns the semaphore id, or | |
7660c0ab A |
3865 | the undefined value if there is an error. See also C<IPC::SysV> and |
3866 | C<IPC::SysV::Semaphore> documentation. | |
a0d0e21e LW |
3867 | |
3868 | =item semop KEY,OPSTRING | |
3869 | ||
3870 | Calls the System V IPC function semop to perform semaphore operations | |
3871 | such as signaling and waiting. OPSTRING must be a packed array of | |
3872 | semop structures. Each semop structure can be generated with | |
3873 | C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore | |
19799a22 GS |
3874 | operations is implied by the length of OPSTRING. Returns true if |
3875 | successful, or false if there is an error. As an example, the | |
3876 | following code waits on semaphore $semnum of semaphore id $semid: | |
a0d0e21e LW |
3877 | |
3878 | $semop = pack("sss", $semnum, -1, 0); | |
3879 | die "Semaphore trouble: $!\n" unless semop($semid, $semop); | |
3880 | ||
7660c0ab A |
3881 | To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV> |
3882 | and C<IPC::SysV::Semaphore> documentation. | |
a0d0e21e LW |
3883 | |
3884 | =item send SOCKET,MSG,FLAGS,TO | |
3885 | ||
3886 | =item send SOCKET,MSG,FLAGS | |
3887 | ||
3888 | Sends a message on a socket. Takes the same flags as the system call | |
3889 | of the same name. On unconnected sockets you must specify a | |
19799a22 | 3890 | destination to send TO, in which case it does a C C<sendto>. Returns |
a0d0e21e | 3891 | the number of characters sent, or the undefined value if there is an |
2b5ab1e7 | 3892 | error. The C system call sendmsg(2) is currently unimplemented. |
4633a7c4 | 3893 | See L<perlipc/"UDP: Message Passing"> for examples. |
a0d0e21e LW |
3894 | |
3895 | =item setpgrp PID,PGRP | |
3896 | ||
7660c0ab | 3897 | Sets the current process group for the specified PID, C<0> for the current |
a0d0e21e | 3898 | process. Will produce a fatal error if used on a machine that doesn't |
81777298 GS |
3899 | implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted, |
3900 | it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not | |
3901 | accept any arguments, so only C<setpgrp(0,0)> is portable. See also | |
3902 | C<POSIX::setsid()>. | |
a0d0e21e LW |
3903 | |
3904 | =item setpriority WHICH,WHO,PRIORITY | |
3905 | ||
3906 | Sets the current priority for a process, a process group, or a user. | |
f86cebdf GS |
3907 | (See setpriority(2).) Will produce a fatal error if used on a machine |
3908 | that doesn't implement setpriority(2). | |
a0d0e21e LW |
3909 | |
3910 | =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL | |
3911 | ||
3912 | Sets the socket option requested. Returns undefined if there is an | |
7660c0ab | 3913 | error. OPTVAL may be specified as C<undef> if you don't want to pass an |
a0d0e21e LW |
3914 | argument. |
3915 | ||
3916 | =item shift ARRAY | |
3917 | ||
3918 | =item shift | |
3919 | ||
3920 | Shifts the first value of the array off and returns it, shortening the | |
3921 | array by 1 and moving everything down. If there are no elements in the | |
3922 | array, returns the undefined value. If ARRAY is omitted, shifts the | |
7660c0ab A |
3923 | C<@_> array within the lexical scope of subroutines and formats, and the |
3924 | C<@ARGV> array at file scopes or within the lexical scopes established by | |
7d30b5c4 | 3925 | the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}> |
4f25aa18 GS |
3926 | constructs. |
3927 | ||
19799a22 GS |
3928 | See also C<unshift>, C<push>, and C<pop>. C<Shift()> and C<unshift> do the |
3929 | same thing to the left end of an array that C<pop> and C<push> do to the | |
977336f5 | 3930 | right end. |
a0d0e21e LW |
3931 | |
3932 | =item shmctl ID,CMD,ARG | |
3933 | ||
0ade1984 JH |
3934 | Calls the System V IPC function shmctl. You'll probably have to say |
3935 | ||
3936 | use IPC::SysV; | |
3937 | ||
7660c0ab A |
3938 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, |
3939 | then ARG must be a variable which will hold the returned C<shmid_ds> | |
3940 | structure. Returns like ioctl: the undefined value for error, "C<0> but | |
0ade1984 | 3941 | true" for zero, or the actual return value otherwise. |
7660c0ab | 3942 | See also C<IPC::SysV> documentation. |
a0d0e21e LW |
3943 | |
3944 | =item shmget KEY,SIZE,FLAGS | |
3945 | ||
3946 | Calls the System V IPC function shmget. Returns the shared memory | |
3947 | segment id, or the undefined value if there is an error. | |
7660c0ab | 3948 | See also C<IPC::SysV> documentation. |
a0d0e21e LW |
3949 | |
3950 | =item shmread ID,VAR,POS,SIZE | |
3951 | ||
3952 | =item shmwrite ID,STRING,POS,SIZE | |
3953 | ||
3954 | Reads or writes the System V shared memory segment ID starting at | |
3955 | position POS for size SIZE by attaching to it, copying in/out, and | |
5a964f20 | 3956 | detaching from it. When reading, VAR must be a variable that will |
a0d0e21e LW |
3957 | hold the data read. When writing, if STRING is too long, only SIZE |
3958 | bytes are used; if STRING is too short, nulls are written to fill out | |
19799a22 | 3959 | SIZE bytes. Return true if successful, or false if there is an error. |
2b5ab1e7 TC |
3960 | See also C<IPC::SysV> documentation and the C<IPC::Shareable> module |
3961 | from CPAN. | |
a0d0e21e LW |
3962 | |
3963 | =item shutdown SOCKET,HOW | |
3964 | ||
3965 | Shuts down a socket connection in the manner indicated by HOW, which | |
3966 | has the same interpretation as in the system call of the same name. | |
3967 | ||
f86cebdf GS |
3968 | shutdown(SOCKET, 0); # I/we have stopped reading data |
3969 | shutdown(SOCKET, 1); # I/we have stopped writing data | |
3970 | shutdown(SOCKET, 2); # I/we have stopped using this socket | |
5a964f20 TC |
3971 | |
3972 | This is useful with sockets when you want to tell the other | |
3973 | side you're done writing but not done reading, or vice versa. | |
3974 | It's also a more insistent form of close because it also | |
19799a22 | 3975 | disables the file descriptor in any forked copies in other |
5a964f20 TC |
3976 | processes. |
3977 | ||
a0d0e21e LW |
3978 | =item sin EXPR |
3979 | ||
54310121 | 3980 | =item sin |
bbce6d69 | 3981 | |
a0d0e21e | 3982 | Returns the sine of EXPR (expressed in radians). If EXPR is omitted, |
7660c0ab | 3983 | returns sine of C<$_>. |
a0d0e21e | 3984 | |
ca6e1c26 | 3985 | For the inverse sine operation, you may use the C<Math::Trig::asin> |
28757baa | 3986 | function, or use this relation: |
3987 | ||
3988 | sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) } | |
3989 | ||
a0d0e21e LW |
3990 | =item sleep EXPR |
3991 | ||
3992 | =item sleep | |
3993 | ||
3994 | Causes the script to sleep for EXPR seconds, or forever if no EXPR. | |
7660c0ab | 3995 | May be interrupted if the process receives a signal such as C<SIGALRM>. |
1d3434b8 | 3996 | Returns the number of seconds actually slept. You probably cannot |
19799a22 GS |
3997 | mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented |
3998 | using C<alarm>. | |
a0d0e21e LW |
3999 | |
4000 | On some older systems, it may sleep up to a full second less than what | |
4001 | you requested, depending on how it counts seconds. Most modern systems | |
5a964f20 TC |
4002 | always sleep the full amount. They may appear to sleep longer than that, |
4003 | however, because your process might not be scheduled right away in a | |
4004 | busy multitasking system. | |
a0d0e21e | 4005 | |
cb1a09d0 | 4006 | For delays of finer granularity than one second, you may use Perl's |
68f8bed4 JH |
4007 | C<syscall> interface to access setitimer(2) if your system supports |
4008 | it, or else see L</select> above. The Time::HiRes module from CPAN | |
4009 | may also help. | |
cb1a09d0 | 4010 | |
19799a22 | 4011 | See also the POSIX module's C<sigpause> function. |
5f05dabc | 4012 | |
a0d0e21e LW |
4013 | =item socket SOCKET,DOMAIN,TYPE,PROTOCOL |
4014 | ||
4015 | Opens a socket of the specified kind and attaches it to filehandle | |
19799a22 GS |
4016 | SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for |
4017 | the system call of the same name. You should C<use Socket> first | |
4018 | to get the proper definitions imported. See the examples in | |
4019 | L<perlipc/"Sockets: Client/Server Communication">. | |
a0d0e21e | 4020 | |
8d2a6795 GS |
4021 | On systems that support a close-on-exec flag on files, the flag will |
4022 | be set for the newly opened file descriptor, as determined by the | |
4023 | value of $^F. See L<perlvar/$^F>. | |
4024 | ||
a0d0e21e LW |
4025 | =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL |
4026 | ||
4027 | Creates an unnamed pair of sockets in the specified domain, of the | |
5f05dabc | 4028 | specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as |
a0d0e21e | 4029 | for the system call of the same name. If unimplemented, yields a fatal |
19799a22 | 4030 | error. Returns true if successful. |
a0d0e21e | 4031 | |
8d2a6795 GS |
4032 | On systems that support a close-on-exec flag on files, the flag will |
4033 | be set for the newly opened file descriptors, as determined by the value | |
4034 | of $^F. See L<perlvar/$^F>. | |
4035 | ||
19799a22 | 4036 | Some systems defined C<pipe> in terms of C<socketpair>, in which a call |
5a964f20 TC |
4037 | to C<pipe(Rdr, Wtr)> is essentially: |
4038 | ||
4039 | use Socket; | |
4040 | socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC); | |
4041 | shutdown(Rdr, 1); # no more writing for reader | |
4042 | shutdown(Wtr, 0); # no more reading for writer | |
4043 | ||
4044 | See L<perlipc> for an example of socketpair use. | |
4045 | ||
a0d0e21e LW |
4046 | =item sort SUBNAME LIST |
4047 | ||
4048 | =item sort BLOCK LIST | |
4049 | ||
4050 | =item sort LIST | |
4051 | ||
2f9daede | 4052 | Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK |
19799a22 | 4053 | is omitted, C<sort>s in standard string comparison order. If SUBNAME is |
2f9daede | 4054 | specified, it gives the name of a subroutine that returns an integer |
7660c0ab | 4055 | less than, equal to, or greater than C<0>, depending on how the elements |
19799a22 | 4056 | of the list are to be ordered. (The C<E<lt>=E<gt>> and C<cmp> |
2f9daede | 4057 | operators are extremely useful in such routines.) SUBNAME may be a |
1d3434b8 GS |
4058 | scalar variable name (unsubscripted), in which case the value provides |
4059 | the name of (or a reference to) the actual subroutine to use. In place | |
4060 | of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort | |
4061 | subroutine. | |
a0d0e21e | 4062 | |
43481408 GS |
4063 | If the subroutine's prototype is C<($$)>, the elements to be compared |
4064 | are passed by reference in C<@_>, as for a normal subroutine. If not, | |
4065 | the normal calling code for subroutines is bypassed in the interests of | |
4066 | efficiency, and the elements to be compared are passed into the subroutine | |
4067 | as the package global variables $a and $b (see example below). Note that | |
4068 | in the latter case, it is usually counter-productive to declare $a and | |
4069 | $b as lexicals. | |
4070 | ||
4071 | In either case, the subroutine may not be recursive. The values to be | |
4072 | compared are always passed by reference, so don't modify them. | |
a0d0e21e | 4073 | |
0a753a76 | 4074 | You also cannot exit out of the sort block or subroutine using any of the |
19799a22 | 4075 | loop control operators described in L<perlsyn> or with C<goto>. |
0a753a76 | 4076 | |
a034a98d DD |
4077 | When C<use locale> is in effect, C<sort LIST> sorts LIST according to the |
4078 | current collation locale. See L<perllocale>. | |
4079 | ||
a0d0e21e LW |
4080 | Examples: |
4081 | ||
4082 | # sort lexically | |
4083 | @articles = sort @files; | |
4084 | ||
4085 | # same thing, but with explicit sort routine | |
4086 | @articles = sort {$a cmp $b} @files; | |
4087 | ||
cb1a09d0 | 4088 | # now case-insensitively |
54310121 | 4089 | @articles = sort {uc($a) cmp uc($b)} @files; |
cb1a09d0 | 4090 | |
a0d0e21e LW |
4091 | # same thing in reversed order |
4092 | @articles = sort {$b cmp $a} @files; | |
4093 | ||
4094 | # sort numerically ascending | |
4095 | @articles = sort {$a <=> $b} @files; | |
4096 | ||
4097 | # sort numerically descending | |
4098 | @articles = sort {$b <=> $a} @files; | |
4099 | ||
19799a22 GS |
4100 | # this sorts the %age hash by value instead of key |
4101 | # using an in-line function | |
4102 | @eldest = sort { $age{$b} <=> $age{$a} } keys %age; | |
4103 | ||
a0d0e21e LW |
4104 | # sort using explicit subroutine name |
4105 | sub byage { | |
2f9daede | 4106 | $age{$a} <=> $age{$b}; # presuming numeric |
a0d0e21e LW |
4107 | } |
4108 | @sortedclass = sort byage @class; | |
4109 | ||
19799a22 GS |
4110 | sub backwards { $b cmp $a } |
4111 | @harry = qw(dog cat x Cain Abel); | |
4112 | @george = qw(gone chased yz Punished Axed); | |
a0d0e21e LW |
4113 | print sort @harry; |
4114 | # prints AbelCaincatdogx | |
4115 | print sort backwards @harry; | |
4116 | # prints xdogcatCainAbel | |
4117 | print sort @george, 'to', @harry; | |
4118 | # prints AbelAxedCainPunishedcatchaseddoggonetoxyz | |
4119 | ||
54310121 | 4120 | # inefficiently sort by descending numeric compare using |
4121 | # the first integer after the first = sign, or the | |
cb1a09d0 AD |
4122 | # whole record case-insensitively otherwise |
4123 | ||
4124 | @new = sort { | |
4125 | ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] | |
4126 | || | |
4127 | uc($a) cmp uc($b) | |
4128 | } @old; | |
4129 | ||
4130 | # same thing, but much more efficiently; | |
4131 | # we'll build auxiliary indices instead | |
4132 | # for speed | |
4133 | @nums = @caps = (); | |
54310121 | 4134 | for (@old) { |
cb1a09d0 AD |
4135 | push @nums, /=(\d+)/; |
4136 | push @caps, uc($_); | |
54310121 | 4137 | } |
cb1a09d0 AD |
4138 | |
4139 | @new = @old[ sort { | |
4140 | $nums[$b] <=> $nums[$a] | |
4141 | || | |
4142 | $caps[$a] cmp $caps[$b] | |
4143 | } 0..$#old | |
4144 | ]; | |
4145 | ||
19799a22 | 4146 | # same thing, but without any temps |
cb1a09d0 | 4147 | @new = map { $_->[0] } |
19799a22 GS |
4148 | sort { $b->[1] <=> $a->[1] |
4149 | || | |
4150 | $a->[2] cmp $b->[2] | |
4151 | } map { [$_, /=(\d+)/, uc($_)] } @old; | |
43481408 GS |
4152 | |
4153 | # using a prototype allows you to use any comparison subroutine | |
4154 | # as a sort subroutine (including other package's subroutines) | |
4155 | package other; | |
4156 | sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here | |
4157 | ||
4158 | package main; | |
4159 | @new = sort other::backwards @old; | |
cb1a09d0 | 4160 | |
19799a22 GS |
4161 | If you're using strict, you I<must not> declare $a |
4162 | and $b as lexicals. They are package globals. That means | |
cb1a09d0 AD |
4163 | if you're in the C<main> package, it's |
4164 | ||
4165 | @articles = sort {$main::b <=> $main::a} @files; | |
4166 | ||
4167 | or just | |
4168 | ||
4169 | @articles = sort {$::b <=> $::a} @files; | |
4170 | ||
4171 | but if you're in the C<FooPack> package, it's | |
4172 | ||
4173 | @articles = sort {$FooPack::b <=> $FooPack::a} @files; | |
4174 | ||
55497cff | 4175 | The comparison function is required to behave. If it returns |
7660c0ab A |
4176 | inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and |
4177 | sometimes saying the opposite, for example) the results are not | |
4178 | well-defined. | |
55497cff | 4179 | |
a0d0e21e LW |
4180 | =item splice ARRAY,OFFSET,LENGTH,LIST |
4181 | ||
4182 | =item splice ARRAY,OFFSET,LENGTH | |
4183 | ||
4184 | =item splice ARRAY,OFFSET | |
4185 | ||
453f9044 GS |
4186 | =item splice ARRAY |
4187 | ||
a0d0e21e | 4188 | Removes the elements designated by OFFSET and LENGTH from an array, and |
5a964f20 TC |
4189 | replaces them with the elements of LIST, if any. In list context, |
4190 | returns the elements removed from the array. In scalar context, | |
43051805 | 4191 | returns the last element removed, or C<undef> if no elements are |
48cdf507 | 4192 | removed. The array grows or shrinks as necessary. |
19799a22 | 4193 | If OFFSET is negative then it starts that far from the end of the array. |
48cdf507 | 4194 | If LENGTH is omitted, removes everything from OFFSET onward. |
453f9044 GS |
4195 | If LENGTH is negative, leaves that many elements off the end of the array. |
4196 | If both OFFSET and LENGTH are omitted, removes everything. | |
4197 | ||
48cdf507 | 4198 | The following equivalences hold (assuming C<$[ == 0>): |
a0d0e21e | 4199 | |
48cdf507 | 4200 | push(@a,$x,$y) splice(@a,@a,0,$x,$y) |
a0d0e21e LW |
4201 | pop(@a) splice(@a,-1) |
4202 | shift(@a) splice(@a,0,1) | |
4203 | unshift(@a,$x,$y) splice(@a,0,0,$x,$y) | |
5a964f20 | 4204 | $a[$x] = $y splice(@a,$x,1,$y) |
a0d0e21e LW |
4205 | |
4206 | Example, assuming array lengths are passed before arrays: | |
4207 | ||
4208 | sub aeq { # compare two list values | |
5a964f20 TC |
4209 | my(@a) = splice(@_,0,shift); |
4210 | my(@b) = splice(@_,0,shift); | |
a0d0e21e LW |
4211 | return 0 unless @a == @b; # same len? |
4212 | while (@a) { | |
4213 | return 0 if pop(@a) ne pop(@b); | |
4214 | } | |
4215 | return 1; | |
4216 | } | |
4217 | if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } | |
4218 | ||
4219 | =item split /PATTERN/,EXPR,LIMIT | |
4220 | ||
4221 | =item split /PATTERN/,EXPR | |
4222 | ||
4223 | =item split /PATTERN/ | |
4224 | ||
4225 | =item split | |
4226 | ||
19799a22 | 4227 | Splits a string into a list of strings and returns that list. By default, |
5a964f20 | 4228 | empty leading fields are preserved, and empty trailing ones are deleted. |
a0d0e21e | 4229 | |
5a964f20 | 4230 | If not in list context, returns the number of fields found and splits into |
7660c0ab | 4231 | the C<@_> array. (In list context, you can force the split into C<@_> by |
1d2dff63 | 4232 | using C<??> as the pattern delimiters, but it still returns the list |
7660c0ab | 4233 | value.) The use of implicit split to C<@_> is deprecated, however, because |
5a964f20 | 4234 | it clobbers your subroutine arguments. |
a0d0e21e | 4235 | |
7660c0ab | 4236 | If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted, |
4633a7c4 LW |
4237 | splits on whitespace (after skipping any leading whitespace). Anything |
4238 | matching PATTERN is taken to be a delimiter separating the fields. (Note | |
fb73857a | 4239 | that the delimiter may be longer than one character.) |
4240 | ||
5a964f20 | 4241 | If LIMIT is specified and positive, splits into no more than that |
7b8d334a GS |
4242 | many fields (though it may split into fewer). If LIMIT is unspecified |
4243 | or zero, trailing null fields are stripped (which potential users | |
19799a22 | 4244 | of C<pop> would do well to remember). If LIMIT is negative, it is |
fb73857a | 4245 | treated as if an arbitrarily large LIMIT had been specified. |
a0d0e21e LW |
4246 | |
4247 | A pattern matching the null string (not to be confused with | |
748a9306 | 4248 | a null pattern C<//>, which is just one member of the set of patterns |
a0d0e21e LW |
4249 | matching a null string) will split the value of EXPR into separate |
4250 | characters at each point it matches that way. For example: | |
4251 | ||
4252 | print join(':', split(/ */, 'hi there')); | |
4253 | ||
4254 | produces the output 'h:i:t:h:e:r:e'. | |
4255 | ||
5f05dabc | 4256 | The LIMIT parameter can be used to split a line partially |
a0d0e21e LW |
4257 | |
4258 | ($login, $passwd, $remainder) = split(/:/, $_, 3); | |
4259 | ||
4260 | When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT | |
4261 | one larger than the number of variables in the list, to avoid | |
4262 | unnecessary work. For the list above LIMIT would have been 4 by | |
4263 | default. In time critical applications it behooves you not to split | |
4264 | into more fields than you really need. | |
4265 | ||
19799a22 | 4266 | If the PATTERN contains parentheses, additional list elements are |
a0d0e21e LW |
4267 | created from each matching substring in the delimiter. |
4268 | ||
da0045b7 | 4269 | split(/([,-])/, "1-10,20", 3); |
a0d0e21e LW |
4270 | |
4271 | produces the list value | |
4272 | ||
4273 | (1, '-', 10, ',', 20) | |
4274 | ||
19799a22 | 4275 | If you had the entire header of a normal Unix email message in $header, |
4633a7c4 LW |
4276 | you could split it up into fields and their values this way: |
4277 | ||
4278 | $header =~ s/\n\s+/ /g; # fix continuation lines | |
fb73857a | 4279 | %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header); |
4633a7c4 | 4280 | |
a0d0e21e LW |
4281 | The pattern C</PATTERN/> may be replaced with an expression to specify |
4282 | patterns that vary at runtime. (To do runtime compilation only once, | |
748a9306 LW |
4283 | use C</$variable/o>.) |
4284 | ||
4285 | As a special case, specifying a PATTERN of space (C<' '>) will split on | |
19799a22 | 4286 | white space just as C<split> with no arguments does. Thus, C<split(' ')> can |
748a9306 LW |
4287 | be used to emulate B<awk>'s default behavior, whereas C<split(/ /)> |
4288 | will give you as many null initial fields as there are leading spaces. | |
19799a22 GS |
4289 | A C<split> on C</\s+/> is like a C<split(' ')> except that any leading |
4290 | whitespace produces a null first field. A C<split> with no arguments | |
748a9306 | 4291 | really does a C<split(' ', $_)> internally. |
a0d0e21e LW |
4292 | |
4293 | Example: | |
4294 | ||
5a964f20 TC |
4295 | open(PASSWD, '/etc/passwd'); |
4296 | while (<PASSWD>) { | |
f86cebdf GS |
4297 | ($login, $passwd, $uid, $gid, |
4298 | $gcos, $home, $shell) = split(/:/); | |
5a964f20 | 4299 | #... |
a0d0e21e LW |
4300 | } |
4301 | ||
19799a22 | 4302 | (Note that $shell above will still have a newline on it. See L</chop>, |
a0d0e21e LW |
4303 | L</chomp>, and L</join>.) |
4304 | ||
5f05dabc | 4305 | =item sprintf FORMAT, LIST |
a0d0e21e | 4306 | |
19799a22 GS |
4307 | Returns a string formatted by the usual C<printf> conventions of the |
4308 | C library function C<sprintf>. See L<sprintf(3)> or L<printf(3)> | |
74a77017 CS |
4309 | on your system for an explanation of the general principles. |
4310 | ||
19799a22 GS |
4311 | Perl does its own C<sprintf> formatting--it emulates the C |
4312 | function C<sprintf>, but it doesn't use it (except for floating-point | |
74a77017 | 4313 | numbers, and even then only the standard modifiers are allowed). As a |
19799a22 | 4314 | result, any non-standard extensions in your local C<sprintf> are not |
74a77017 CS |
4315 | available from Perl. |
4316 | ||
19799a22 | 4317 | Perl's C<sprintf> permits the following universally-known conversions: |
74a77017 CS |
4318 | |
4319 | %% a percent sign | |
4320 | %c a character with the given number | |
4321 | %s a string | |
4322 | %d a signed integer, in decimal | |
4323 | %u an unsigned integer, in decimal | |
4324 | %o an unsigned integer, in octal | |
4325 | %x an unsigned integer, in hexadecimal | |
4326 | %e a floating-point number, in scientific notation | |
4327 | %f a floating-point number, in fixed decimal notation | |
4328 | %g a floating-point number, in %e or %f notation | |
4329 | ||
1b3f7d21 | 4330 | In addition, Perl permits the following widely-supported conversions: |
74a77017 | 4331 | |
74a77017 CS |
4332 | %X like %x, but using upper-case letters |
4333 | %E like %e, but using an upper-case "E" | |
4334 | %G like %g, but with an upper-case "E" (if applicable) | |
4f19785b | 4335 | %b an unsigned integer, in binary |
74a77017 | 4336 | %p a pointer (outputs the Perl value's address in hexadecimal) |
1b3f7d21 CS |
4337 | %n special: *stores* the number of characters output so far |
4338 | into the next variable in the parameter list | |
74a77017 | 4339 | |
1b3f7d21 CS |
4340 | Finally, for backward (and we do mean "backward") compatibility, Perl |
4341 | permits these unnecessary but widely-supported conversions: | |
74a77017 | 4342 | |
1b3f7d21 | 4343 | %i a synonym for %d |
74a77017 CS |
4344 | %D a synonym for %ld |
4345 | %U a synonym for %lu | |
4346 | %O a synonym for %lo | |
4347 | %F a synonym for %f | |
4348 | ||
4349 | Perl permits the following universally-known flags between the C<%> | |
4350 | and the conversion letter: | |
4351 | ||
4352 | space prefix positive number with a space | |
4353 | + prefix positive number with a plus sign | |
4354 | - left-justify within the field | |
4355 | 0 use zeros, not spaces, to right-justify | |
a3cb178b | 4356 | # prefix non-zero octal with "0", non-zero hex with "0x" |
74a77017 | 4357 | number minimum field width |
f86cebdf GS |
4358 | .number "precision": digits after decimal point for |
4359 | floating-point, max length for string, minimum length | |
4360 | for integer | |
74a77017 | 4361 | l interpret integer as C type "long" or "unsigned long" |
74a77017 | 4362 | h interpret integer as C type "short" or "unsigned short" |
661cc6a6 | 4363 | If no flags, interpret integer as C type "int" or "unsigned" |
74a77017 | 4364 | |
4628e4f8 | 4365 | There are also two Perl-specific flags: |
74a77017 CS |
4366 | |
4367 | V interpret integer as Perl's standard integer type | |
b22c7a20 GS |
4368 | v interpret string as a vector of integers, output as |
4369 | numbers separated either by dots, or by an arbitrary | |
4370 | string received from the argument list when the flag | |
4371 | is preceded by C<*> | |
74a77017 | 4372 | |
19799a22 | 4373 | Where a number would appear in the flags, an asterisk (C<*>) may be |
74a77017 CS |
4374 | used instead, in which case Perl uses the next item in the parameter |
4375 | list as the given number (that is, as the field width or precision). | |
19799a22 GS |
4376 | If a field width obtained through C<*> is negative, it has the same |
4377 | effect as the C<-> flag: left-justification. | |
74a77017 | 4378 | |
b22c7a20 GS |
4379 | The C<v> flag is useful for displaying ordinal values of characters |
4380 | in arbitrary strings: | |
4381 | ||
4382 | printf "version is v%vd\n", $^V; # Perl's version | |
4383 | printf "address is %*vX\n", ":", $addr; # IPv6 address | |
4384 | printf "bits are %*vb\n", "", $bits; # random bitstring | |
4385 | ||
74a77017 CS |
4386 | If C<use locale> is in effect, the character used for the decimal |
4387 | point in formatted real numbers is affected by the LC_NUMERIC locale. | |
4388 | See L<perllocale>. | |
a0d0e21e | 4389 | |
07158430 | 4390 | If Perl understands "quads" (64-bit integers) (this requires |
a8764340 GS |
4391 | either that the platform natively support quads or that Perl |
4392 | be specifically compiled to support quads), the characters | |
07158430 JH |
4393 | |
4394 | d u o x X b i D U O | |
4395 | ||
4396 | print quads, and they may optionally be preceded by | |
4397 | ||
4398 | ll L q | |
4399 | ||
4400 | For example | |
4401 | ||
4402 | %lld %16LX %qo | |
4403 | ||
46465067 | 4404 | You can find out whether your Perl supports quads via L<Config>: |
07158430 JH |
4405 | |
4406 | use Config; | |
46465067 JH |
4407 | ($Config{use64bits} eq 'define' || $Config{longsize} == 8) && |
4408 | print "quads\n"; | |
07158430 JH |
4409 | |
4410 | If Perl understands "long doubles" (this requires that the platform | |
a8764340 | 4411 | support long doubles), the flags |
07158430 JH |
4412 | |
4413 | e f g E F G | |
4414 | ||
4415 | may optionally be preceded by | |
4416 | ||
4417 | ll L | |
4418 | ||
4419 | For example | |
4420 | ||
4421 | %llf %Lg | |
4422 | ||
4423 | You can find out whether your Perl supports long doubles via L<Config>: | |
4424 | ||
4425 | use Config; | |
46465067 | 4426 | $Config{d_longdbl} eq 'define' && print "long doubles\n"; |
07158430 | 4427 | |
a0d0e21e LW |
4428 | =item sqrt EXPR |
4429 | ||
54310121 | 4430 | =item sqrt |
bbce6d69 | 4431 | |
a0d0e21e | 4432 | Return the square root of EXPR. If EXPR is omitted, returns square |
2b5ab1e7 TC |
4433 | root of C<$_>. Only works on non-negative operands, unless you've |
4434 | loaded the standard Math::Complex module. | |
4435 | ||
4436 | use Math::Complex; | |
4437 | print sqrt(-2); # prints 1.4142135623731i | |
a0d0e21e LW |
4438 | |
4439 | =item srand EXPR | |
4440 | ||
93dc8474 CS |
4441 | =item srand |
4442 | ||
19799a22 | 4443 | Sets the random number seed for the C<rand> operator. If EXPR is |
73c60299 RS |
4444 | omitted, uses a semi-random value supplied by the kernel (if it supports |
4445 | the F</dev/urandom> device) or based on the current time and process | |
93dc8474 | 4446 | ID, among other things. In versions of Perl prior to 5.004 the default |
19799a22 | 4447 | seed was just the current C<time>. This isn't a particularly good seed, |
93dc8474 | 4448 | so many old programs supply their own seed value (often C<time ^ $$> or |
7b8d334a | 4449 | C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more. |
93dc8474 | 4450 | |
19799a22 | 4451 | In fact, it's usually not necessary to call C<srand> at all, because if |
93dc8474 | 4452 | it is not called explicitly, it is called implicitly at the first use of |
19799a22 | 4453 | the C<rand> operator. However, this was not the case in version of Perl |
2f9daede | 4454 | before 5.004, so if your script will run under older Perl versions, it |
19799a22 | 4455 | should call C<srand>. |
93dc8474 | 4456 | |
2f9daede TP |
4457 | Note that you need something much more random than the default seed for |
4458 | cryptographic purposes. Checksumming the compressed output of one or more | |
4459 | rapidly changing operating system status programs is the usual method. For | |
4460 | example: | |
28757baa | 4461 | |
4462 | srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`); | |
4463 | ||
7660c0ab | 4464 | If you're particularly concerned with this, see the C<Math::TrulyRandom> |
0078ec44 RS |
4465 | module in CPAN. |
4466 | ||
19799a22 | 4467 | Do I<not> call C<srand> multiple times in your program unless you know |
28757baa | 4468 | exactly what you're doing and why you're doing it. The point of the |
19799a22 | 4469 | function is to "seed" the C<rand> function so that C<rand> can produce |
28757baa | 4470 | a different sequence each time you run your program. Just do it once at the |
19799a22 | 4471 | top of your program, or you I<won't> get random numbers out of C<rand>! |
28757baa | 4472 | |
54310121 | 4473 | Frequently called programs (like CGI scripts) that simply use |
28757baa | 4474 | |
4475 | time ^ $$ | |
4476 | ||
54310121 | 4477 | for a seed can fall prey to the mathematical property that |
28757baa | 4478 | |
4479 | a^b == (a+1)^(b+1) | |
4480 | ||
0078ec44 | 4481 | one-third of the time. So don't do that. |
f86702cc | 4482 | |
a0d0e21e LW |
4483 | =item stat FILEHANDLE |
4484 | ||
4485 | =item stat EXPR | |
4486 | ||
54310121 | 4487 | =item stat |
bbce6d69 | 4488 | |
1d2dff63 GS |
4489 | Returns a 13-element list giving the status info for a file, either |
4490 | the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, | |
7660c0ab | 4491 | it stats C<$_>. Returns a null list if the stat fails. Typically used |
1d2dff63 | 4492 | as follows: |
a0d0e21e LW |
4493 | |
4494 | ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, | |
4495 | $atime,$mtime,$ctime,$blksize,$blocks) | |
4496 | = stat($filename); | |
4497 | ||
54310121 | 4498 | Not all fields are supported on all filesystem types. Here are the |
c07a80fd | 4499 | meaning of the fields: |
4500 | ||
54310121 | 4501 | 0 dev device number of filesystem |
4502 | 1 ino inode number | |
4503 | 2 mode file mode (type and permissions) | |
4504 | 3 nlink number of (hard) links to the file | |
4505 | 4 uid numeric user ID of file's owner | |
4506 | 5 gid numeric group ID of file's owner | |
4507 | 6 rdev the device identifier (special files only) | |
4508 | 7 size total size of file, in bytes | |
1c74f1bd GS |
4509 | 8 atime last access time in seconds since the epoch |
4510 | 9 mtime last modify time in seconds since the epoch | |
4511 | 10 ctime inode change time (NOT creation time!) in seconds since the epoch | |
54310121 | 4512 | 11 blksize preferred block size for file system I/O |
4513 | 12 blocks actual number of blocks allocated | |
c07a80fd | 4514 | |
4515 | (The epoch was at 00:00 January 1, 1970 GMT.) | |
4516 | ||
a0d0e21e LW |
4517 | If stat is passed the special filehandle consisting of an underline, no |
4518 | stat is done, but the current contents of the stat structure from the | |
4519 | last stat or filetest are returned. Example: | |
4520 | ||
4521 | if (-x $file && (($d) = stat(_)) && $d < 0) { | |
4522 | print "$file is executable NFS file\n"; | |
4523 | } | |
4524 | ||
ca6e1c26 JH |
4525 | (This works on machines only for which the device number is negative |
4526 | under NFS.) | |
a0d0e21e | 4527 | |
2b5ab1e7 TC |
4528 | Because the mode contains both the file type and its permissions, you |
4529 | should mask off the file type portion and (s)printf using a C<"%o"> | |
4530 | if you want to see the real permissions. | |
4531 | ||
4532 | $mode = (stat($filename))[2]; | |
4533 | printf "Permissions are %04o\n", $mode & 07777; | |
4534 | ||
19799a22 | 4535 | In scalar context, C<stat> returns a boolean value indicating success |
1d2dff63 GS |
4536 | or failure, and, if successful, sets the information associated with |
4537 | the special filehandle C<_>. | |
4538 | ||
2b5ab1e7 TC |
4539 | The File::stat module provides a convenient, by-name access mechanism: |
4540 | ||
4541 | use File::stat; | |
4542 | $sb = stat($filename); | |
4543 | printf "File is %s, size is %s, perm %04o, mtime %s\n", | |
4544 | $filename, $sb->size, $sb->mode & 07777, | |
4545 | scalar localtime $sb->mtime; | |
4546 | ||
ca6e1c26 JH |
4547 | You can import symbolic mode constants (C<S_IF*>) and functions |
4548 | (C<S_IS*>) from the Fcntl module: | |
4549 | ||
4550 | use Fcntl ':mode'; | |
4551 | ||
4552 | $mode = (stat($filename))[2]; | |
4553 | ||
4554 | $user_rwx = ($mode & S_IRWXU) >> 6; | |
4555 | $group_read = ($mode & S_IRGRP) >> 3; | |
4556 | $other_execute = $mode & S_IXOTH; | |
4557 | ||
4558 | printf "Permissions are %04o\n", S_ISMODE($mode), "\n"; | |
4559 | ||
4560 | $is_setuid = $mode & S_ISUID; | |
4561 | $is_setgid = S_ISDIR($mode); | |
4562 | ||
4563 | You could write the last two using the C<-u> and C<-d> operators. | |
4564 | The commonly available S_IF* constants are | |
4565 | ||
4566 | # Permissions: read, write, execute, for user, group, others. | |
4567 | ||
4568 | S_IRWXU S_IRUSR S_IWUSR S_IXUSR | |
4569 | S_IRWXG S_IRGRP S_IWGRP S_IXGRP | |
4570 | S_IRWXO S_IROTH S_IWOTH S_IXOTH | |
4571 | ||
4572 | # Setuid/Setgid/Stickiness. | |
4573 | ||
4574 | S_ISUID S_ISGID S_ISVTX S_ISTXT | |
4575 | ||
4576 | # File types. Not necessarily all are available on your system. | |
4577 | ||
4578 | S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT | |
4579 | ||
4580 | # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR. | |
4581 | ||
4582 | S_IREAD S_IWRITE S_IEXEC | |
4583 | ||
4584 | and the S_IF* functions are | |
4585 | ||
4586 | S_IFMODE($mode) the part of $mode containg the permission bits | |
4587 | and the setuid/setgid/sticky bits | |
4588 | ||
4589 | S_IFMT($mode) the part of $mode containing the file type | |
4590 | which can be bit-anded with e.g. S_IFREG | |
4591 | or with the following functions | |
4592 | ||
4593 | # The operators -f, -d, -l, -b, -c, -p, and -s. | |
4594 | ||
4595 | S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode) | |
4596 | S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode) | |
4597 | ||
4598 | # No direct -X operator counterpart, but for the first one | |
4599 | # the -g operator is often equivalent. The ENFMT stands for | |
4600 | # record flocking enforcement, a platform-dependent feature. | |
4601 | ||
4602 | S_ISENFMT($mode) S_ISWHT($mode) | |
4603 | ||
4604 | See your native chmod(2) and stat(2) documentation for more details | |
4605 | about the S_* constants. | |
4606 | ||
a0d0e21e LW |
4607 | =item study SCALAR |
4608 | ||
4609 | =item study | |
4610 | ||
184e9718 | 4611 | Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of |
a0d0e21e LW |
4612 | doing many pattern matches on the string before it is next modified. |
4613 | This may or may not save time, depending on the nature and number of | |
4614 | patterns you are searching on, and on the distribution of character | |
19799a22 | 4615 | frequencies in the string to be searched--you probably want to compare |
5f05dabc | 4616 | run times with and without it to see which runs faster. Those loops |
a0d0e21e LW |
4617 | which scan for many short constant strings (including the constant |
4618 | parts of more complex patterns) will benefit most. You may have only | |
19799a22 GS |
4619 | one C<study> active at a time--if you study a different scalar the first |
4620 | is "unstudied". (The way C<study> works is this: a linked list of every | |
a0d0e21e | 4621 | character in the string to be searched is made, so we know, for |
7660c0ab | 4622 | example, where all the C<'k'> characters are. From each search string, |
a0d0e21e LW |
4623 | the rarest character is selected, based on some static frequency tables |
4624 | constructed from some C programs and English text. Only those places | |
4625 | that contain this "rarest" character are examined.) | |
4626 | ||
5a964f20 | 4627 | For example, here is a loop that inserts index producing entries |
a0d0e21e LW |
4628 | before any line containing a certain pattern: |
4629 | ||
4630 | while (<>) { | |
4631 | study; | |
2b5ab1e7 TC |
4632 | print ".IX foo\n" if /\bfoo\b/; |
4633 | print ".IX bar\n" if /\bbar\b/; | |
4634 | print ".IX blurfl\n" if /\bblurfl\b/; | |
5a964f20 | 4635 | # ... |
a0d0e21e LW |
4636 | print; |
4637 | } | |
4638 | ||
951ba7fe GS |
4639 | In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f> |
4640 | will be looked at, because C<f> is rarer than C<o>. In general, this is | |
a0d0e21e LW |
4641 | a big win except in pathological cases. The only question is whether |
4642 | it saves you more time than it took to build the linked list in the | |
4643 | first place. | |
4644 | ||
4645 | Note that if you have to look for strings that you don't know till | |
19799a22 | 4646 | runtime, you can build an entire loop as a string and C<eval> that to |
a0d0e21e | 4647 | avoid recompiling all your patterns all the time. Together with |
7660c0ab | 4648 | undefining C<$/> to input entire files as one record, this can be very |
f86cebdf | 4649 | fast, often faster than specialized programs like fgrep(1). The following |
184e9718 | 4650 | scans a list of files (C<@files>) for a list of words (C<@words>), and prints |
a0d0e21e LW |
4651 | out the names of those files that contain a match: |
4652 | ||
4653 | $search = 'while (<>) { study;'; | |
4654 | foreach $word (@words) { | |
4655 | $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n"; | |
4656 | } | |
4657 | $search .= "}"; | |
4658 | @ARGV = @files; | |
4659 | undef $/; | |
4660 | eval $search; # this screams | |
5f05dabc | 4661 | $/ = "\n"; # put back to normal input delimiter |
a0d0e21e LW |
4662 | foreach $file (sort keys(%seen)) { |
4663 | print $file, "\n"; | |
4664 | } | |
4665 | ||
cb1a09d0 AD |
4666 | =item sub BLOCK |
4667 | ||
4668 | =item sub NAME | |
4669 | ||
4670 | =item sub NAME BLOCK | |
4671 | ||
4672 | This is subroutine definition, not a real function I<per se>. With just a | |
09bef843 SB |
4673 | NAME (and possibly prototypes or attributes), it's just a forward declaration. |
4674 | Without a NAME, it's an anonymous function declaration, and does actually | |
4675 | return a value: the CODE ref of the closure you just created. See L<perlsub> | |
4676 | and L<perlref> for details. | |
cb1a09d0 | 4677 | |
87275199 | 4678 | =item substr EXPR,OFFSET,LENGTH,REPLACEMENT |
7b8d334a | 4679 | |
87275199 | 4680 | =item substr EXPR,OFFSET,LENGTH |
a0d0e21e LW |
4681 | |
4682 | =item substr EXPR,OFFSET | |
4683 | ||
4684 | Extracts a substring out of EXPR and returns it. First character is at | |
7660c0ab | 4685 | offset C<0>, or whatever you've set C<$[> to (but don't do that). |
84902520 | 4686 | If OFFSET is negative (or more precisely, less than C<$[>), starts |
87275199 GS |
4687 | that far from the end of the string. If LENGTH is omitted, returns |
4688 | everything to the end of the string. If LENGTH is negative, leaves that | |
748a9306 LW |
4689 | many characters off the end of the string. |
4690 | ||
2b5ab1e7 | 4691 | You can use the substr() function as an lvalue, in which case EXPR |
87275199 GS |
4692 | must itself be an lvalue. If you assign something shorter than LENGTH, |
4693 | the string will shrink, and if you assign something longer than LENGTH, | |
2b5ab1e7 | 4694 | the string will grow to accommodate it. To keep the string the same |
19799a22 | 4695 | length you may need to pad or chop your value using C<sprintf>. |
a0d0e21e | 4696 | |
87275199 GS |
4697 | If OFFSET and LENGTH specify a substring that is partly outside the |
4698 | string, only the part within the string is returned. If the substring | |
4699 | is beyond either end of the string, substr() returns the undefined | |
4700 | value and produces a warning. When used as an lvalue, specifying a | |
4701 | substring that is entirely outside the string is a fatal error. | |
4702 | Here's an example showing the behavior for boundary cases: | |
4703 | ||
4704 | my $name = 'fred'; | |
4705 | substr($name, 4) = 'dy'; # $name is now 'freddy' | |
4706 | my $null = substr $name, 6, 2; # returns '' (no warning) | |
4707 | my $oops = substr $name, 7; # returns undef, with warning | |
4708 | substr($name, 7) = 'gap'; # fatal error | |
4709 | ||
2b5ab1e7 | 4710 | An alternative to using substr() as an lvalue is to specify the |
7b8d334a | 4711 | replacement string as the 4th argument. This allows you to replace |
2b5ab1e7 TC |
4712 | parts of the EXPR and return what was there before in one operation, |
4713 | just as you can with splice(). | |
7b8d334a | 4714 | |
a0d0e21e LW |
4715 | =item symlink OLDFILE,NEWFILE |
4716 | ||
4717 | Creates a new filename symbolically linked to the old filename. | |
7660c0ab | 4718 | Returns C<1> for success, C<0> otherwise. On systems that don't support |
a0d0e21e LW |
4719 | symbolic links, produces a fatal error at run time. To check for that, |
4720 | use eval: | |
4721 | ||
2b5ab1e7 | 4722 | $symlink_exists = eval { symlink("",""); 1 }; |
a0d0e21e LW |
4723 | |
4724 | =item syscall LIST | |
4725 | ||
4726 | Calls the system call specified as the first element of the list, | |
4727 | passing the remaining elements as arguments to the system call. If | |
4728 | unimplemented, produces a fatal error. The arguments are interpreted | |
4729 | as follows: if a given argument is numeric, the argument is passed as | |
4730 | an int. If not, the pointer to the string value is passed. You are | |
4731 | responsible to make sure a string is pre-extended long enough to | |
a3cb178b | 4732 | receive any result that might be written into a string. You can't use a |
19799a22 | 4733 | string literal (or other read-only string) as an argument to C<syscall> |
a3cb178b GS |
4734 | because Perl has to assume that any string pointer might be written |
4735 | through. If your | |
a0d0e21e | 4736 | integer arguments are not literals and have never been interpreted in a |
7660c0ab | 4737 | numeric context, you may need to add C<0> to them to force them to look |
19799a22 | 4738 | like numbers. This emulates the C<syswrite> function (or vice versa): |
a0d0e21e LW |
4739 | |
4740 | require 'syscall.ph'; # may need to run h2ph | |
a3cb178b GS |
4741 | $s = "hi there\n"; |
4742 | syscall(&SYS_write, fileno(STDOUT), $s, length $s); | |
a0d0e21e | 4743 | |
5f05dabc | 4744 | Note that Perl supports passing of up to only 14 arguments to your system call, |
a0d0e21e LW |
4745 | which in practice should usually suffice. |
4746 | ||
fb73857a | 4747 | Syscall returns whatever value returned by the system call it calls. |
19799a22 | 4748 | If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno). |
7660c0ab | 4749 | Note that some system calls can legitimately return C<-1>. The proper |
fb73857a | 4750 | way to handle such calls is to assign C<$!=0;> before the call and |
7660c0ab | 4751 | check the value of C<$!> if syscall returns C<-1>. |
fb73857a | 4752 | |
4753 | There's a problem with C<syscall(&SYS_pipe)>: it returns the file | |
4754 | number of the read end of the pipe it creates. There is no way | |
4755 | to retrieve the file number of the other end. You can avoid this | |
19799a22 | 4756 | problem by using C<pipe> instead. |
fb73857a | 4757 | |
c07a80fd | 4758 | =item sysopen FILEHANDLE,FILENAME,MODE |
4759 | ||
4760 | =item sysopen FILEHANDLE,FILENAME,MODE,PERMS | |
4761 | ||
4762 | Opens the file whose filename is given by FILENAME, and associates it | |
4763 | with FILEHANDLE. If FILEHANDLE is an expression, its value is used as | |
4764 | the name of the real filehandle wanted. This function calls the | |
19799a22 | 4765 | underlying operating system's C<open> function with the parameters |
c07a80fd | 4766 | FILENAME, MODE, PERMS. |
4767 | ||
4768 | The possible values and flag bits of the MODE parameter are | |
4769 | system-dependent; they are available via the standard module C<Fcntl>. | |
ea2b5ef6 JH |
4770 | See the documentation of your operating system's C<open> to see which |
4771 | values and flag bits are available. You may combine several flags | |
4772 | using the C<|>-operator. | |
4773 | ||
4774 | Some of the most common values are C<O_RDONLY> for opening the file in | |
4775 | read-only mode, C<O_WRONLY> for opening the file in write-only mode, | |
4776 | and C<O_RDWR> for opening the file in read-write mode, and. | |
4777 | ||
adf5897a DF |
4778 | For historical reasons, some values work on almost every system |
4779 | supported by perl: zero means read-only, one means write-only, and two | |
4780 | means read/write. We know that these values do I<not> work under | |
7c5ffed3 | 4781 | OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to |
4af147f6 | 4782 | use them in new code. |
c07a80fd | 4783 | |
19799a22 | 4784 | If the file named by FILENAME does not exist and the C<open> call creates |
7660c0ab | 4785 | it (typically because MODE includes the C<O_CREAT> flag), then the value of |
5a964f20 | 4786 | PERMS specifies the permissions of the newly created file. If you omit |
19799a22 | 4787 | the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>. |
5a964f20 | 4788 | These permission values need to be in octal, and are modified by your |
0591cd52 NT |
4789 | process's current C<umask>. |
4790 | ||
ea2b5ef6 JH |
4791 | In many systems the C<O_EXCL> flag is available for opening files in |
4792 | exclusive mode. This is B<not> locking: exclusiveness means here that | |
4793 | if the file already exists, sysopen() fails. The C<O_EXCL> wins | |
4794 | C<O_TRUNC>. | |
4795 | ||
4796 | Sometimes you may want to truncate an already-existing file: C<O_TRUNC>. | |
4797 | ||
19799a22 | 4798 | You should seldom if ever use C<0644> as argument to C<sysopen>, because |
2b5ab1e7 TC |
4799 | that takes away the user's option to have a more permissive umask. |
4800 | Better to omit it. See the perlfunc(1) entry on C<umask> for more | |
4801 | on this. | |
c07a80fd | 4802 | |
4af147f6 CS |
4803 | Note that C<sysopen> depends on the fdopen() C library function. |
4804 | On many UNIX systems, fdopen() is known to fail when file descriptors | |
4805 | exceed a certain value, typically 255. If you need more file | |
4806 | descriptors than that, consider rebuilding Perl to use the C<sfio> | |
4807 | library, or perhaps using the POSIX::open() function. | |
4808 | ||
2b5ab1e7 | 4809 | See L<perlopentut> for a kinder, gentler explanation of opening files. |
28757baa | 4810 | |
a0d0e21e LW |
4811 | =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET |
4812 | ||
4813 | =item sysread FILEHANDLE,SCALAR,LENGTH | |
4814 | ||
4815 | Attempts to read LENGTH bytes of data into variable SCALAR from the | |
b43ceaf2 | 4816 | specified FILEHANDLE, using the system call read(2). It bypasses stdio, |
19799a22 GS |
4817 | so mixing this with other kinds of reads, C<print>, C<write>, |
4818 | C<seek>, C<tell>, or C<eof> can cause confusion because stdio | |
b43ceaf2 AB |
4819 | usually buffers data. Returns the number of bytes actually read, C<0> |
4820 | at end of file, or undef if there was an error. SCALAR will be grown or | |
4821 | shrunk so that the last byte actually read is the last byte of the | |
4822 | scalar after the read. | |
ff68c719 | 4823 | |
4824 | An OFFSET may be specified to place the read data at some place in the | |
4825 | string other than the beginning. A negative OFFSET specifies | |
4826 | placement at that many bytes counting backwards from the end of the | |
4827 | string. A positive OFFSET greater than the length of SCALAR results | |
7660c0ab | 4828 | in the string being padded to the required size with C<"\0"> bytes before |
ff68c719 | 4829 | the result of the read is appended. |
a0d0e21e | 4830 | |
2b5ab1e7 TC |
4831 | There is no syseof() function, which is ok, since eof() doesn't work |
4832 | very well on device files (like ttys) anyway. Use sysread() and check | |
19799a22 | 4833 | for a return value for 0 to decide whether you're done. |
2b5ab1e7 | 4834 | |
137443ea | 4835 | =item sysseek FILEHANDLE,POSITION,WHENCE |
4836 | ||
f86cebdf | 4837 | Sets FILEHANDLE's system position using the system call lseek(2). It |
19799a22 | 4838 | bypasses stdio, so mixing this with reads (other than C<sysread>), |
ac88732c JH |
4839 | C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion. |
4840 | FILEHANDLE may be an expression whose value gives the name of the | |
4841 | filehandle. The values for WHENCE are C<0> to set the new position to | |
4842 | POSITION, C<1> to set the it to the current position plus POSITION, | |
4843 | and C<2> to set it to EOF plus POSITION (typically negative). For | |
4844 | WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and | |
4845 | C<SEEK_END> (start of the file, current position, end of the file) | |
ca6e1c26 | 4846 | from the Fcntl module. |
8903cb82 | 4847 | |
4848 | Returns the new position, or the undefined value on failure. A position | |
19799a22 GS |
4849 | of zero is returned as the string C<"0 but true">; thus C<sysseek> returns |
4850 | true on success and false on failure, yet you can still easily determine | |
8903cb82 | 4851 | the new position. |
137443ea | 4852 | |
a0d0e21e LW |
4853 | =item system LIST |
4854 | ||
8bf3b016 GS |
4855 | =item system PROGRAM LIST |
4856 | ||
19799a22 GS |
4857 | Does exactly the same thing as C<exec LIST>, except that a fork is |
4858 | done first, and the parent process waits for the child process to | |
4859 | complete. Note that argument processing varies depending on the | |
4860 | number of arguments. If there is more than one argument in LIST, | |
4861 | or if LIST is an array with more than one value, starts the program | |
4862 | given by the first element of the list with arguments given by the | |
4863 | rest of the list. If there is only one scalar argument, the argument | |
4864 | is checked for shell metacharacters, and if there are any, the | |
4865 | entire argument is passed to the system's command shell for parsing | |
4866 | (this is C</bin/sh -c> on Unix platforms, but varies on other | |
4867 | platforms). If there are no shell metacharacters in the argument, | |
4868 | it is split into words and passed directly to C<execvp>, which is | |
4869 | more efficient. | |
4870 | ||
4871 | All files opened for output are flushed before attempting the exec(). | |
a2008d6d GS |
4872 | |
4873 | The return value is the exit status of the program as | |
19799a22 GS |
4874 | returned by the C<wait> call. To get the actual exit value divide by |
4875 | 256. See also L</exec>. This is I<not> what you want to use to capture | |
54310121 | 4876 | the output from a command, for that you should use merely backticks or |
d5a9bfb0 IZ |
4877 | C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1 |
4878 | indicates a failure to start the program (inspect $! for the reason). | |
a0d0e21e | 4879 | |
19799a22 GS |
4880 | Like C<exec>, C<system> allows you to lie to a program about its name if |
4881 | you use the C<system PROGRAM LIST> syntax. Again, see L</exec>. | |
8bf3b016 | 4882 | |
19799a22 | 4883 | Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the |
28757baa | 4884 | program they're running doesn't actually interrupt your program. |
4885 | ||
4886 | @args = ("command", "arg1", "arg2"); | |
54310121 | 4887 | system(@args) == 0 |
4888 | or die "system @args failed: $?" | |
28757baa | 4889 | |
5a964f20 TC |
4890 | You can check all the failure possibilities by inspecting |
4891 | C<$?> like this: | |
28757baa | 4892 | |
5a964f20 TC |
4893 | $exit_value = $? >> 8; |
4894 | $signal_num = $? & 127; | |
4895 | $dumped_core = $? & 128; | |
f86702cc | 4896 | |
c8db1d39 TC |
4897 | When the arguments get executed via the system shell, results |
4898 | and return codes will be subject to its quirks and capabilities. | |
4899 | See L<perlop/"`STRING`"> and L</exec> for details. | |
bb32b41a | 4900 | |
a0d0e21e LW |
4901 | =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET |
4902 | ||
4903 | =item syswrite FILEHANDLE,SCALAR,LENGTH | |
4904 | ||
145d37e2 GA |
4905 | =item syswrite FILEHANDLE,SCALAR |
4906 | ||
a0d0e21e | 4907 | Attempts to write LENGTH bytes of data from variable SCALAR to the |
19799a22 GS |
4908 | specified FILEHANDLE, using the system call write(2). If LENGTH |
4909 | is not specified, writes whole SCALAR. It bypasses stdio, so mixing | |
4910 | this with reads (other than C<sysread())>, C<print>, C<write>, | |
4911 | C<seek>, C<tell>, or C<eof> may cause confusion because stdio | |
4912 | usually buffers data. Returns the number of bytes actually written, | |
4913 | or C<undef> if there was an error. If the LENGTH is greater than | |
4914 | the available data in the SCALAR after the OFFSET, only as much | |
4915 | data as is available will be written. | |
ff68c719 | 4916 | |
4917 | An OFFSET may be specified to write the data from some part of the | |
4918 | string other than the beginning. A negative OFFSET specifies writing | |
fb73857a | 4919 | that many bytes counting backwards from the end of the string. In the |
4920 | case the SCALAR is empty you can use OFFSET but only zero offset. | |
a0d0e21e LW |
4921 | |
4922 | =item tell FILEHANDLE | |
4923 | ||
4924 | =item tell | |
4925 | ||
8903cb82 | 4926 | Returns the current position for FILEHANDLE. FILEHANDLE may be an |
a0d0e21e | 4927 | expression whose value gives the name of the actual filehandle. If |
2b5ab1e7 TC |
4928 | FILEHANDLE is omitted, assumes the file last read. |
4929 | ||
19799a22 | 4930 | There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that. |
a0d0e21e LW |
4931 | |
4932 | =item telldir DIRHANDLE | |
4933 | ||
19799a22 GS |
4934 | Returns the current position of the C<readdir> routines on DIRHANDLE. |
4935 | Value may be given to C<seekdir> to access a particular location in a | |
a0d0e21e LW |
4936 | directory. Has the same caveats about possible directory compaction as |
4937 | the corresponding system library routine. | |
4938 | ||
4633a7c4 | 4939 | =item tie VARIABLE,CLASSNAME,LIST |
a0d0e21e | 4940 | |
4633a7c4 LW |
4941 | This function binds a variable to a package class that will provide the |
4942 | implementation for the variable. VARIABLE is the name of the variable | |
4943 | to be enchanted. CLASSNAME is the name of a class implementing objects | |
19799a22 | 4944 | of correct type. Any additional arguments are passed to the C<new> |
8a059744 GS |
4945 | method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>, |
4946 | or C<TIEHASH>). Typically these are arguments such as might be passed | |
19799a22 GS |
4947 | to the C<dbm_open()> function of C. The object returned by the C<new> |
4948 | method is also returned by the C<tie> function, which would be useful | |
8a059744 | 4949 | if you want to access other methods in CLASSNAME. |
a0d0e21e | 4950 | |
19799a22 | 4951 | Note that functions such as C<keys> and C<values> may return huge lists |
1d2dff63 | 4952 | when used on large objects, like DBM files. You may prefer to use the |
19799a22 | 4953 | C<each> function to iterate over such. Example: |
a0d0e21e LW |
4954 | |
4955 | # print out history file offsets | |
4633a7c4 | 4956 | use NDBM_File; |
da0045b7 | 4957 | tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); |
a0d0e21e LW |
4958 | while (($key,$val) = each %HIST) { |
4959 | print $key, ' = ', unpack('L',$val), "\n"; | |
4960 | } | |
4961 | untie(%HIST); | |
4962 | ||
aa689395 | 4963 | A class implementing a hash should have the following methods: |
a0d0e21e | 4964 | |
4633a7c4 | 4965 | TIEHASH classname, LIST |
a0d0e21e LW |
4966 | FETCH this, key |
4967 | STORE this, key, value | |
4968 | DELETE this, key | |
8a059744 | 4969 | CLEAR this |
a0d0e21e LW |
4970 | EXISTS this, key |
4971 | FIRSTKEY this | |
4972 | NEXTKEY this, lastkey | |
8a059744 | 4973 | DESTROY this |
a0d0e21e | 4974 | |
4633a7c4 | 4975 | A class implementing an ordinary array should have the following methods: |
a0d0e21e | 4976 | |
4633a7c4 | 4977 | TIEARRAY classname, LIST |
a0d0e21e LW |
4978 | FETCH this, key |
4979 | STORE this, key, value | |
8a059744 GS |
4980 | FETCHSIZE this |
4981 | STORESIZE this, count | |
4982 | CLEAR this | |
4983 | PUSH this, LIST | |
4984 | POP this | |
4985 | SHIFT this | |
4986 | UNSHIFT this, LIST | |
4987 | SPLICE this, offset, length, LIST | |
4988 | EXTEND this, count | |
4989 | DESTROY this | |
4990 | ||
4991 | A class implementing a file handle should have the following methods: | |
4992 | ||
4993 | TIEHANDLE classname, LIST | |
4994 | READ this, scalar, length, offset | |
4995 | READLINE this | |
4996 | GETC this | |
4997 | WRITE this, scalar, length, offset | |
4998 | PRINT this, LIST | |
4999 | PRINTF this, format, LIST | |
5000 | CLOSE this | |
5001 | DESTROY this | |
a0d0e21e | 5002 | |
4633a7c4 | 5003 | A class implementing a scalar should have the following methods: |
a0d0e21e | 5004 | |
4633a7c4 | 5005 | TIESCALAR classname, LIST |
54310121 | 5006 | FETCH this, |
a0d0e21e | 5007 | STORE this, value |
8a059744 GS |
5008 | DESTROY this |
5009 | ||
5010 | Not all methods indicated above need be implemented. See L<perltie>, | |
2b5ab1e7 | 5011 | L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>. |
a0d0e21e | 5012 | |
19799a22 | 5013 | Unlike C<dbmopen>, the C<tie> function will not use or require a module |
4633a7c4 | 5014 | for you--you need to do that explicitly yourself. See L<DB_File> |
19799a22 | 5015 | or the F<Config> module for interesting C<tie> implementations. |
4633a7c4 | 5016 | |
b687b08b | 5017 | For further details see L<perltie>, L<"tied VARIABLE">. |
cc6b7395 | 5018 | |
f3cbc334 RS |
5019 | =item tied VARIABLE |
5020 | ||
5021 | Returns a reference to the object underlying VARIABLE (the same value | |
19799a22 | 5022 | that was originally returned by the C<tie> call that bound the variable |
f3cbc334 RS |
5023 | to a package.) Returns the undefined value if VARIABLE isn't tied to a |
5024 | package. | |
5025 | ||
a0d0e21e LW |
5026 | =item time |
5027 | ||
da0045b7 | 5028 | Returns the number of non-leap seconds since whatever time the system |
5029 | considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS, | |
5030 | and 00:00:00 UTC, January 1, 1970 for most other systems). | |
19799a22 | 5031 | Suitable for feeding to C<gmtime> and C<localtime>. |
a0d0e21e | 5032 | |
68f8bed4 JH |
5033 | For measuring time in better granularity than one second, |
5034 | you may use either the Time::HiRes module from CPAN, or | |
5035 | if you have gettimeofday(2), you may be able to use the | |
5036 | C<syscall> interface of Perl, see L<perlfaq8> for details. | |
5037 | ||
a0d0e21e LW |
5038 | =item times |
5039 | ||
1d2dff63 | 5040 | Returns a four-element list giving the user and system times, in |
a0d0e21e LW |
5041 | seconds, for this process and the children of this process. |
5042 | ||
5043 | ($user,$system,$cuser,$csystem) = times; | |
5044 | ||
5045 | =item tr/// | |
5046 | ||
19799a22 | 5047 | The transliteration operator. Same as C<y///>. See L<perlop>. |
a0d0e21e LW |
5048 | |
5049 | =item truncate FILEHANDLE,LENGTH | |
5050 | ||
5051 | =item truncate EXPR,LENGTH | |
5052 | ||
5053 | Truncates the file opened on FILEHANDLE, or named by EXPR, to the | |
5054 | specified length. Produces a fatal error if truncate isn't implemented | |
19799a22 | 5055 | on your system. Returns true if successful, the undefined value |
a3cb178b | 5056 | otherwise. |
a0d0e21e LW |
5057 | |
5058 | =item uc EXPR | |
5059 | ||
54310121 | 5060 | =item uc |
bbce6d69 | 5061 | |
a0d0e21e | 5062 | Returns an uppercased version of EXPR. This is the internal function |
7660c0ab | 5063 | implementing the C<\U> escape in double-quoted strings. |
a034a98d | 5064 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. |
a0ed51b3 | 5065 | Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It |
19799a22 | 5066 | does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.) |
a0d0e21e | 5067 | |
7660c0ab | 5068 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 5069 | |
a0d0e21e LW |
5070 | =item ucfirst EXPR |
5071 | ||
54310121 | 5072 | =item ucfirst |
bbce6d69 | 5073 | |
a0ed51b3 LW |
5074 | Returns the value of EXPR with the first character |
5075 | in uppercase (titlecase in Unicode). This is | |
7660c0ab | 5076 | the internal function implementing the C<\u> escape in double-quoted strings. |
2b5ab1e7 TC |
5077 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale> |
5078 | and L<utf8>. | |
a0d0e21e | 5079 | |
7660c0ab | 5080 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 5081 | |
a0d0e21e LW |
5082 | =item umask EXPR |
5083 | ||
5084 | =item umask | |
5085 | ||
2f9daede | 5086 | Sets the umask for the process to EXPR and returns the previous value. |
eec2d3df GS |
5087 | If EXPR is omitted, merely returns the current umask. |
5088 | ||
0591cd52 NT |
5089 | The Unix permission C<rwxr-x---> is represented as three sets of three |
5090 | bits, or three octal digits: C<0750> (the leading 0 indicates octal | |
b5a41e52 | 5091 | and isn't one of the digits). The C<umask> value is such a number |
0591cd52 NT |
5092 | representing disabled permissions bits. The permission (or "mode") |
5093 | values you pass C<mkdir> or C<sysopen> are modified by your umask, so | |
5094 | even if you tell C<sysopen> to create a file with permissions C<0777>, | |
5095 | if your umask is C<0022> then the file will actually be created with | |
5096 | permissions C<0755>. If your C<umask> were C<0027> (group can't | |
5097 | write; others can't read, write, or execute), then passing | |
19799a22 | 5098 | C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~ |
0591cd52 NT |
5099 | 027> is C<0640>). |
5100 | ||
5101 | Here's some advice: supply a creation mode of C<0666> for regular | |
19799a22 GS |
5102 | files (in C<sysopen>) and one of C<0777> for directories (in |
5103 | C<mkdir>) and executable files. This gives users the freedom of | |
0591cd52 NT |
5104 | choice: if they want protected files, they might choose process umasks |
5105 | of C<022>, C<027>, or even the particularly antisocial mask of C<077>. | |
5106 | Programs should rarely if ever make policy decisions better left to | |
5107 | the user. The exception to this is when writing files that should be | |
5108 | kept private: mail files, web browser cookies, I<.rhosts> files, and | |
5109 | so on. | |
5110 | ||
f86cebdf | 5111 | If umask(2) is not implemented on your system and you are trying to |
eec2d3df | 5112 | restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a |
f86cebdf | 5113 | fatal error at run time. If umask(2) is not implemented and you are |
eec2d3df GS |
5114 | not trying to restrict access for yourself, returns C<undef>. |
5115 | ||
5116 | Remember that a umask is a number, usually given in octal; it is I<not> a | |
5117 | string of octal digits. See also L</oct>, if all you have is a string. | |
a0d0e21e LW |
5118 | |
5119 | =item undef EXPR | |
5120 | ||
5121 | =item undef | |
5122 | ||
54310121 | 5123 | Undefines the value of EXPR, which must be an lvalue. Use only on a |
19799a22 GS |
5124 | scalar value, an array (using C<@>), a hash (using C<%>), a subroutine |
5125 | (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}> | |
20408e3c GS |
5126 | will probably not do what you expect on most predefined variables or |
5127 | DBM list values, so don't do that; see L<delete>.) Always returns the | |
5128 | undefined value. You can omit the EXPR, in which case nothing is | |
5129 | undefined, but you still get an undefined value that you could, for | |
5130 | instance, return from a subroutine, assign to a variable or pass as a | |
5131 | parameter. Examples: | |
a0d0e21e LW |
5132 | |
5133 | undef $foo; | |
f86cebdf | 5134 | undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'}; |
a0d0e21e | 5135 | undef @ary; |
aa689395 | 5136 | undef %hash; |
a0d0e21e | 5137 | undef &mysub; |
20408e3c | 5138 | undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc. |
54310121 | 5139 | return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it; |
2f9daede TP |
5140 | select undef, undef, undef, 0.25; |
5141 | ($a, $b, undef, $c) = &foo; # Ignore third value returned | |
a0d0e21e | 5142 | |
5a964f20 TC |
5143 | Note that this is a unary operator, not a list operator. |
5144 | ||
a0d0e21e LW |
5145 | =item unlink LIST |
5146 | ||
54310121 | 5147 | =item unlink |
bbce6d69 | 5148 | |
a0d0e21e LW |
5149 | Deletes a list of files. Returns the number of files successfully |
5150 | deleted. | |
5151 | ||
5152 | $cnt = unlink 'a', 'b', 'c'; | |
5153 | unlink @goners; | |
5154 | unlink <*.bak>; | |
5155 | ||
19799a22 | 5156 | Note: C<unlink> will not delete directories unless you are superuser and |
a0d0e21e LW |
5157 | the B<-U> flag is supplied to Perl. Even if these conditions are |
5158 | met, be warned that unlinking a directory can inflict damage on your | |
19799a22 | 5159 | filesystem. Use C<rmdir> instead. |
a0d0e21e | 5160 | |
7660c0ab | 5161 | If LIST is omitted, uses C<$_>. |
bbce6d69 | 5162 | |
a0d0e21e LW |
5163 | =item unpack TEMPLATE,EXPR |
5164 | ||
19799a22 | 5165 | C<unpack> does the reverse of C<pack>: it takes a string |
2b6c5635 | 5166 | and expands it out into a list of values. |
19799a22 | 5167 | (In scalar context, it returns merely the first value produced.) |
2b6c5635 GS |
5168 | |
5169 | The string is broken into chunks described by the TEMPLATE. Each chunk | |
5170 | is converted separately to a value. Typically, either the string is a result | |
5171 | of C<pack>, or the bytes of the string represent a C structure of some | |
5172 | kind. | |
5173 | ||
19799a22 | 5174 | The TEMPLATE has the same format as in the C<pack> function. |
a0d0e21e LW |
5175 | Here's a subroutine that does substring: |
5176 | ||
5177 | sub substr { | |
5a964f20 | 5178 | my($what,$where,$howmuch) = @_; |
a0d0e21e LW |
5179 | unpack("x$where a$howmuch", $what); |
5180 | } | |
5181 | ||
5182 | and then there's | |
5183 | ||
5184 | sub ordinal { unpack("c",$_[0]); } # same as ord() | |
5185 | ||
2b6c5635 GS |
5186 | In addition to fields allowed in pack(), you may prefix a field with |
5187 | a %E<lt>numberE<gt> to indicate that | |
184e9718 | 5188 | you want a E<lt>numberE<gt>-bit checksum of the items instead of the items |
2b6c5635 GS |
5189 | themselves. Default is a 16-bit checksum. Checksum is calculated by |
5190 | summing numeric values of expanded values (for string fields the sum of | |
5191 | C<ord($char)> is taken, for bit fields the sum of zeroes and ones). | |
5192 | ||
5193 | For example, the following | |
a0d0e21e LW |
5194 | computes the same number as the System V sum program: |
5195 | ||
19799a22 GS |
5196 | $checksum = do { |
5197 | local $/; # slurp! | |
5198 | unpack("%32C*",<>) % 65535; | |
5199 | }; | |
a0d0e21e LW |
5200 | |
5201 | The following efficiently counts the number of set bits in a bit vector: | |
5202 | ||
5203 | $setbits = unpack("%32b*", $selectmask); | |
5204 | ||
951ba7fe | 5205 | The C<p> and C<P> formats should be used with care. Since Perl |
3160c391 GS |
5206 | has no way of checking whether the value passed to C<unpack()> |
5207 | corresponds to a valid memory location, passing a pointer value that's | |
5208 | not known to be valid is likely to have disastrous consequences. | |
5209 | ||
2b6c5635 GS |
5210 | If the repeat count of a field is larger than what the remainder of |
5211 | the input string allows, repeat count is decreased. If the input string | |
5212 | is longer than one described by the TEMPLATE, the rest is ignored. | |
5213 | ||
851646ae | 5214 | See L</pack> for more examples and notes. |
5a929a98 | 5215 | |
98293880 JH |
5216 | =item untie VARIABLE |
5217 | ||
19799a22 | 5218 | Breaks the binding between a variable and a package. (See C<tie>.) |
98293880 | 5219 | |
a0d0e21e LW |
5220 | =item unshift ARRAY,LIST |
5221 | ||
19799a22 | 5222 | Does the opposite of a C<shift>. Or the opposite of a C<push>, |
a0d0e21e LW |
5223 | depending on how you look at it. Prepends list to the front of the |
5224 | array, and returns the new number of elements in the array. | |
5225 | ||
5226 | unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/; | |
5227 | ||
5228 | Note the LIST is prepended whole, not one element at a time, so the | |
19799a22 | 5229 | prepended elements stay in the same order. Use C<reverse> to do the |
a0d0e21e LW |
5230 | reverse. |
5231 | ||
5232 | =item use Module LIST | |
5233 | ||
5234 | =item use Module | |
5235 | ||
da0045b7 | 5236 | =item use Module VERSION LIST |
5237 | ||
5238 | =item use VERSION | |
5239 | ||
a0d0e21e LW |
5240 | Imports some semantics into the current package from the named module, |
5241 | generally by aliasing certain subroutine or variable names into your | |
5242 | package. It is exactly equivalent to | |
5243 | ||
5244 | BEGIN { require Module; import Module LIST; } | |
5245 | ||
54310121 | 5246 | except that Module I<must> be a bareword. |
da0045b7 | 5247 | |
16070b82 GS |
5248 | If the first argument to C<use> is a number or a version tuple, it is |
5249 | treated as a version instead of a module name. If the version | |
5250 | of the Perl interpreter is less than VERSION, then an error message | |
5251 | is printed and Perl exits immediately. | |
5252 | ||
5253 | use 5.005_03; # version number | |
5254 | use v5.6.0; # version tuple | |
5255 | ||
5256 | This is often useful if you need to check the current Perl version before | |
5257 | C<use>ing library modules that have changed in incompatible ways from | |
5258 | older versions of Perl. (We try not to do this more than we have to.) | |
da0045b7 | 5259 | |
19799a22 | 5260 | The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The |
7660c0ab | 5261 | C<require> makes sure the module is loaded into memory if it hasn't been |
19799a22 GS |
5262 | yet. The C<import> is not a builtin--it's just an ordinary static method |
5263 | call into the C<Module> package to tell the module to import the list of | |
a0d0e21e | 5264 | features back into the current package. The module can implement its |
19799a22 GS |
5265 | C<import> method any way it likes, though most modules just choose to |
5266 | derive their C<import> method via inheritance from the C<Exporter> class that | |
5267 | is defined in the C<Exporter> module. See L<Exporter>. If no C<import> | |
10696ff6 | 5268 | method can be found then the call is skipped. |
cb1a09d0 AD |
5269 | |
5270 | If you don't want your namespace altered, explicitly supply an empty list: | |
5271 | ||
5272 | use Module (); | |
5273 | ||
5274 | That is exactly equivalent to | |
5275 | ||
5a964f20 | 5276 | BEGIN { require Module } |
a0d0e21e | 5277 | |
da0045b7 | 5278 | If the VERSION argument is present between Module and LIST, then the |
71be2cbc | 5279 | C<use> will call the VERSION method in class Module with the given |
5280 | version as an argument. The default VERSION method, inherited from | |
5281 | the Universal class, croaks if the given version is larger than the | |
7660c0ab | 5282 | value of the variable C<$Module::VERSION>. (Note that there is not a |
71be2cbc | 5283 | comma after VERSION!) |
da0045b7 | 5284 | |
a0d0e21e LW |
5285 | Because this is a wide-open interface, pragmas (compiler directives) |
5286 | are also implemented this way. Currently implemented pragmas are: | |
5287 | ||
5288 | use integer; | |
4633a7c4 | 5289 | use diagnostics; |
4438c4b7 JH |
5290 | use sigtrap qw(SEGV BUS); |
5291 | use strict qw(subs vars refs); | |
5292 | use subs qw(afunc blurfl); | |
5293 | use warnings qw(all); | |
a0d0e21e | 5294 | |
19799a22 | 5295 | Some of these pseudo-modules import semantics into the current |
5a964f20 TC |
5296 | block scope (like C<strict> or C<integer>, unlike ordinary modules, |
5297 | which import symbols into the current package (which are effective | |
5298 | through the end of the file). | |
a0d0e21e | 5299 | |
19799a22 GS |
5300 | There's a corresponding C<no> command that unimports meanings imported |
5301 | by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>. | |
a0d0e21e LW |
5302 | |
5303 | no integer; | |
5304 | no strict 'refs'; | |
4438c4b7 | 5305 | no warnings; |
a0d0e21e | 5306 | |
19799a22 | 5307 | If no C<unimport> method can be found the call fails with a fatal error. |
55497cff | 5308 | |
a0d0e21e LW |
5309 | See L<perlmod> for a list of standard modules and pragmas. |
5310 | ||
5311 | =item utime LIST | |
5312 | ||
5313 | Changes the access and modification times on each file of a list of | |
5314 | files. The first two elements of the list must be the NUMERICAL access | |
5315 | and modification times, in that order. Returns the number of files | |
46cdf678 | 5316 | successfully changed. The inode change time of each file is set |
19799a22 | 5317 | to the current time. This code has the same effect as the C<touch> |
a3cb178b | 5318 | command if the files already exist: |
a0d0e21e LW |
5319 | |
5320 | #!/usr/bin/perl | |
5321 | $now = time; | |
5322 | utime $now, $now, @ARGV; | |
5323 | ||
aa689395 | 5324 | =item values HASH |
a0d0e21e | 5325 | |
1d2dff63 GS |
5326 | Returns a list consisting of all the values of the named hash. (In a |
5327 | scalar context, returns the number of values.) The values are | |
ab192400 GS |
5328 | returned in an apparently random order. The actual random order is |
5329 | subject to change in future versions of perl, but it is guaranteed to | |
19799a22 | 5330 | be the same order as either the C<keys> or C<each> function would |
ab192400 GS |
5331 | produce on the same (unmodified) hash. |
5332 | ||
2b5ab1e7 TC |
5333 | Note that you cannot modify the values of a hash this way, because the |
5334 | returned list is just a copy. You need to use a hash slice for that, | |
5335 | since it's lvaluable in a way that values() is not. | |
5336 | ||
5337 | for (values %hash) { s/foo/bar/g } # FAILS! | |
5338 | for (@hash{keys %hash}) { s/foo/bar/g } # ok | |
5339 | ||
5340 | As a side effect, calling values() resets the HASH's internal iterator. | |
19799a22 | 5341 | See also C<keys>, C<each>, and C<sort>. |
a0d0e21e LW |
5342 | |
5343 | =item vec EXPR,OFFSET,BITS | |
5344 | ||
e69129f1 GS |
5345 | Treats the string in EXPR as a bit vector made up of elements of |
5346 | width BITS, and returns the value of the element specified by OFFSET | |
5347 | as an unsigned integer. BITS therefore specifies the number of bits | |
5348 | that are reserved for each element in the bit vector. This must | |
5349 | be a power of two from 1 to 32 (or 64, if your platform supports | |
5350 | that). | |
c5a0f51a | 5351 | |
c73032f5 IZ |
5352 | If BITS is 8, "elements" coincide with bytes of the input string. |
5353 | ||
5354 | If BITS is 16 or more, bytes of the input string are grouped into chunks | |
5355 | of size BITS/8, and each group is converted to a number as with | |
5356 | pack()/unpack() with big-endian formats C<n>/C<N> (and analoguously | |
5357 | for BITS==64). See L<"pack"> for details. | |
5358 | ||
5359 | If bits is 4 or less, the string is broken into bytes, then the bits | |
5360 | of each byte are broken into 8/BITS groups. Bits of a byte are | |
5361 | numbered in a little-endian-ish way, as in C<0x01>, C<0x02>, | |
5362 | C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example, | |
5363 | breaking the single input byte C<chr(0x36)> into two groups gives a list | |
5364 | C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>. | |
5365 | ||
81e118e0 JH |
5366 | C<vec> may also be assigned to, in which case parentheses are needed |
5367 | to give the expression the correct precedence as in | |
22dc801b | 5368 | |
5369 | vec($image, $max_x * $x + $y, 8) = 3; | |
a0d0e21e | 5370 | |
fac70343 GS |
5371 | If the selected element is off the end of the string, the value 0 is |
5372 | returned. If an element off the end of the string is written to, | |
5373 | Perl will first extend the string with sufficiently many zero bytes. | |
5374 | ||
5375 | Strings created with C<vec> can also be manipulated with the logical | |
5376 | operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit | |
5377 | vector operation is desired when both operands are strings. | |
c5a0f51a | 5378 | See L<perlop/"Bitwise String Operators">. |
a0d0e21e | 5379 | |
7660c0ab | 5380 | The following code will build up an ASCII string saying C<'PerlPerlPerl'>. |
19799a22 | 5381 | The comments show the string after each step. Note that this code works |
cca87523 GS |
5382 | in the same way on big-endian or little-endian machines. |
5383 | ||
5384 | my $foo = ''; | |
5385 | vec($foo, 0, 32) = 0x5065726C; # 'Perl' | |
e69129f1 GS |
5386 | |
5387 | # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits | |
5388 | print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P') | |
5389 | ||
cca87523 GS |
5390 | vec($foo, 2, 16) = 0x5065; # 'PerlPe' |
5391 | vec($foo, 3, 16) = 0x726C; # 'PerlPerl' | |
5392 | vec($foo, 8, 8) = 0x50; # 'PerlPerlP' | |
5393 | vec($foo, 9, 8) = 0x65; # 'PerlPerlPe' | |
5394 | vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02" | |
f86cebdf GS |
5395 | vec($foo, 21, 4) = 7; # 'PerlPerlPer' |
5396 | # 'r' is "\x72" | |
cca87523 GS |
5397 | vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c" |
5398 | vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c" | |
f86cebdf GS |
5399 | vec($foo, 94, 1) = 1; # 'PerlPerlPerl' |
5400 | # 'l' is "\x6c" | |
cca87523 | 5401 | |
19799a22 | 5402 | To transform a bit vector into a string or list of 0's and 1's, use these: |
a0d0e21e LW |
5403 | |
5404 | $bits = unpack("b*", $vector); | |
5405 | @bits = split(//, unpack("b*", $vector)); | |
5406 | ||
7660c0ab | 5407 | If you know the exact length in bits, it can be used in place of the C<*>. |
a0d0e21e | 5408 | |
e69129f1 GS |
5409 | Here is an example to illustrate how the bits actually fall in place: |
5410 | ||
5411 | #!/usr/bin/perl -wl | |
5412 | ||
5413 | print <<'EOT'; | |
5414 | 0 1 2 3 | |
5415 | unpack("V",$_) 01234567890123456789012345678901 | |
5416 | ------------------------------------------------------------------ | |
5417 | EOT | |
5418 | ||
5419 | for $w (0..3) { | |
5420 | $width = 2**$w; | |
5421 | for ($shift=0; $shift < $width; ++$shift) { | |
5422 | for ($off=0; $off < 32/$width; ++$off) { | |
5423 | $str = pack("B*", "0"x32); | |
5424 | $bits = (1<<$shift); | |
5425 | vec($str, $off, $width) = $bits; | |
5426 | $res = unpack("b*",$str); | |
5427 | $val = unpack("V", $str); | |
5428 | write; | |
5429 | } | |
5430 | } | |
5431 | } | |
5432 | ||
5433 | format STDOUT = | |
5434 | vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> | |
5435 | $off, $width, $bits, $val, $res | |
5436 | . | |
5437 | __END__ | |
5438 | ||
5439 | Regardless of the machine architecture on which it is run, the above | |
5440 | example should print the following table: | |
5441 | ||
5442 | 0 1 2 3 | |
5443 | unpack("V",$_) 01234567890123456789012345678901 | |
5444 | ------------------------------------------------------------------ | |
5445 | vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000 | |
5446 | vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000 | |
5447 | vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000 | |
5448 | vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000 | |
5449 | vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000 | |
5450 | vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000 | |
5451 | vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000 | |
5452 | vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000 | |
5453 | vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000 | |
5454 | vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000 | |
5455 | vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000 | |
5456 | vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000 | |
5457 | vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000 | |
5458 | vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000 | |
5459 | vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000 | |
5460 | vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000 | |
5461 | vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000 | |
5462 | vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000 | |
5463 | vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000 | |
5464 | vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000 | |
5465 | vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000 | |
5466 | vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000 | |
5467 | vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000 | |
5468 | vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000 | |
5469 | vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000 | |
5470 | vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000 | |
5471 | vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000 | |
5472 | vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000 | |
5473 | vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000 | |
5474 | vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100 | |
5475 | vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010 | |
5476 | vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001 | |
5477 | vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000 | |
5478 | vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000 | |
5479 | vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000 | |
5480 | vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000 | |
5481 | vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000 | |
5482 | vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000 | |
5483 | vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000 | |
5484 | vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000 | |
5485 | vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000 | |
5486 | vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000 | |
5487 | vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000 | |
5488 | vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000 | |
5489 | vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000 | |
5490 | vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000 | |
5491 | vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000 | |
5492 | vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010 | |
5493 | vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000 | |
5494 | vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000 | |
5495 | vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000 | |
5496 | vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000 | |
5497 | vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000 | |
5498 | vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000 | |
5499 | vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000 | |
5500 | vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000 | |
5501 | vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000 | |
5502 | vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000 | |
5503 | vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000 | |
5504 | vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000 | |
5505 | vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000 | |
5506 | vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000 | |
5507 | vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100 | |
5508 | vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001 | |
5509 | vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000 | |
5510 | vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000 | |
5511 | vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000 | |
5512 | vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000 | |
5513 | vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000 | |
5514 | vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000 | |
5515 | vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000 | |
5516 | vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000 | |
5517 | vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000 | |
5518 | vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000 | |
5519 | vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000 | |
5520 | vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000 | |
5521 | vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000 | |
5522 | vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000 | |
5523 | vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000 | |
5524 | vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100 | |
5525 | vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000 | |
5526 | vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000 | |
5527 | vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000 | |
5528 | vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000 | |
5529 | vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000 | |
5530 | vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000 | |
5531 | vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000 | |
5532 | vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010 | |
5533 | vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000 | |
5534 | vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000 | |
5535 | vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000 | |
5536 | vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000 | |
5537 | vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000 | |
5538 | vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000 | |
5539 | vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000 | |
5540 | vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001 | |
5541 | vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000 | |
5542 | vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000 | |
5543 | vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000 | |
5544 | vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000 | |
5545 | vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000 | |
5546 | vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000 | |
5547 | vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000 | |
5548 | vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000 | |
5549 | vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000 | |
5550 | vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000 | |
5551 | vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000 | |
5552 | vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000 | |
5553 | vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000 | |
5554 | vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000 | |
5555 | vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000 | |
5556 | vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000 | |
5557 | vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000 | |
5558 | vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000 | |
5559 | vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000 | |
5560 | vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000 | |
5561 | vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000 | |
5562 | vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000 | |
5563 | vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000 | |
5564 | vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100 | |
5565 | vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000 | |
5566 | vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000 | |
5567 | vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000 | |
5568 | vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010 | |
5569 | vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000 | |
5570 | vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000 | |
5571 | vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000 | |
5572 | vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001 | |
5573 | ||
a0d0e21e LW |
5574 | =item wait |
5575 | ||
2b5ab1e7 TC |
5576 | Behaves like the wait(2) system call on your system: it waits for a child |
5577 | process to terminate and returns the pid of the deceased process, or | |
19799a22 | 5578 | C<-1> if there are no child processes. The status is returned in C<$?>. |
2b5ab1e7 TC |
5579 | Note that a return value of C<-1> could mean that child processes are |
5580 | being automatically reaped, as described in L<perlipc>. | |
a0d0e21e LW |
5581 | |
5582 | =item waitpid PID,FLAGS | |
5583 | ||
2b5ab1e7 TC |
5584 | Waits for a particular child process to terminate and returns the pid of |
5585 | the deceased process, or C<-1> if there is no such child process. On some | |
5586 | systems, a value of 0 indicates that there are processes still running. | |
5587 | The status is returned in C<$?>. If you say | |
a0d0e21e | 5588 | |
5f05dabc | 5589 | use POSIX ":sys_wait_h"; |
5a964f20 | 5590 | #... |
2b5ab1e7 TC |
5591 | do { |
5592 | $kid = waitpid(-1,&WNOHANG); | |
5593 | } until $kid == -1; | |
a0d0e21e | 5594 | |
2b5ab1e7 TC |
5595 | then you can do a non-blocking wait for all pending zombie processes. |
5596 | Non-blocking wait is available on machines supporting either the | |
5597 | waitpid(2) or wait4(2) system calls. However, waiting for a particular | |
5598 | pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the | |
5599 | system call by remembering the status values of processes that have | |
5600 | exited but have not been harvested by the Perl script yet.) | |
a0d0e21e | 5601 | |
2b5ab1e7 TC |
5602 | Note that on some systems, a return value of C<-1> could mean that child |
5603 | processes are being automatically reaped. See L<perlipc> for details, | |
5604 | and for other examples. | |
5a964f20 | 5605 | |
a0d0e21e LW |
5606 | =item wantarray |
5607 | ||
19799a22 GS |
5608 | Returns true if the context of the currently executing subroutine is |
5609 | looking for a list value. Returns false if the context is looking | |
54310121 | 5610 | for a scalar. Returns the undefined value if the context is looking |
5611 | for no value (void context). | |
a0d0e21e | 5612 | |
54310121 | 5613 | return unless defined wantarray; # don't bother doing more |
5614 | my @a = complex_calculation(); | |
5615 | return wantarray ? @a : "@a"; | |
a0d0e21e | 5616 | |
19799a22 GS |
5617 | This function should have been named wantlist() instead. |
5618 | ||
a0d0e21e LW |
5619 | =item warn LIST |
5620 | ||
19799a22 | 5621 | Produces a message on STDERR just like C<die>, but doesn't exit or throw |
774d564b | 5622 | an exception. |
5623 | ||
7660c0ab A |
5624 | If LIST is empty and C<$@> already contains a value (typically from a |
5625 | previous eval) that value is used after appending C<"\t...caught"> | |
19799a22 GS |
5626 | to C<$@>. This is useful for staying almost, but not entirely similar to |
5627 | C<die>. | |
43051805 | 5628 | |
7660c0ab | 5629 | If C<$@> is empty then the string C<"Warning: Something's wrong"> is used. |
43051805 | 5630 | |
774d564b | 5631 | No message is printed if there is a C<$SIG{__WARN__}> handler |
5632 | installed. It is the handler's responsibility to deal with the message | |
19799a22 | 5633 | as it sees fit (like, for instance, converting it into a C<die>). Most |
774d564b | 5634 | handlers must therefore make arrangements to actually display the |
19799a22 | 5635 | warnings that they are not prepared to deal with, by calling C<warn> |
774d564b | 5636 | again in the handler. Note that this is quite safe and will not |
5637 | produce an endless loop, since C<__WARN__> hooks are not called from | |
5638 | inside one. | |
5639 | ||
5640 | You will find this behavior is slightly different from that of | |
5641 | C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can | |
19799a22 | 5642 | instead call C<die> again to change it). |
774d564b | 5643 | |
5644 | Using a C<__WARN__> handler provides a powerful way to silence all | |
5645 | warnings (even the so-called mandatory ones). An example: | |
5646 | ||
5647 | # wipe out *all* compile-time warnings | |
5648 | BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } } | |
5649 | my $foo = 10; | |
5650 | my $foo = 20; # no warning about duplicate my $foo, | |
5651 | # but hey, you asked for it! | |
5652 | # no compile-time or run-time warnings before here | |
5653 | $DOWARN = 1; | |
5654 | ||
5655 | # run-time warnings enabled after here | |
5656 | warn "\$foo is alive and $foo!"; # does show up | |
5657 | ||
5658 | See L<perlvar> for details on setting C<%SIG> entries, and for more | |
2b5ab1e7 TC |
5659 | examples. See the Carp module for other kinds of warnings using its |
5660 | carp() and cluck() functions. | |
a0d0e21e LW |
5661 | |
5662 | =item write FILEHANDLE | |
5663 | ||
5664 | =item write EXPR | |
5665 | ||
5666 | =item write | |
5667 | ||
5a964f20 | 5668 | Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, |
a0d0e21e | 5669 | using the format associated with that file. By default the format for |
54310121 | 5670 | a file is the one having the same name as the filehandle, but the |
19799a22 | 5671 | format for the current output channel (see the C<select> function) may be set |
184e9718 | 5672 | explicitly by assigning the name of the format to the C<$~> variable. |
a0d0e21e LW |
5673 | |
5674 | Top of form processing is handled automatically: if there is | |
5675 | insufficient room on the current page for the formatted record, the | |
5676 | page is advanced by writing a form feed, a special top-of-page format | |
5677 | is used to format the new page header, and then the record is written. | |
5678 | By default the top-of-page format is the name of the filehandle with | |
5679 | "_TOP" appended, but it may be dynamically set to the format of your | |
184e9718 | 5680 | choice by assigning the name to the C<$^> variable while the filehandle is |
a0d0e21e | 5681 | selected. The number of lines remaining on the current page is in |
7660c0ab | 5682 | variable C<$->, which can be set to C<0> to force a new page. |
a0d0e21e LW |
5683 | |
5684 | If FILEHANDLE is unspecified, output goes to the current default output | |
5685 | channel, which starts out as STDOUT but may be changed by the | |
19799a22 | 5686 | C<select> operator. If the FILEHANDLE is an EXPR, then the expression |
a0d0e21e LW |
5687 | is evaluated and the resulting string is used to look up the name of |
5688 | the FILEHANDLE at run time. For more on formats, see L<perlform>. | |
5689 | ||
19799a22 | 5690 | Note that write is I<not> the opposite of C<read>. Unfortunately. |
a0d0e21e LW |
5691 | |
5692 | =item y/// | |
5693 | ||
7660c0ab | 5694 | The transliteration operator. Same as C<tr///>. See L<perlop>. |
a0d0e21e LW |
5695 | |
5696 | =back |