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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 | |
61eff3bc | 338 | characters with the high bit set. If too many strange characters (>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 | 441 | |
1c1fc3ea GS |
442 | =item binmode FILEHANDLE, DISCIPLINE |
443 | ||
a0d0e21e LW |
444 | =item binmode FILEHANDLE |
445 | ||
19799a22 | 446 | Arranges for FILEHANDLE to be read or written in "binary" mode on |
30168b04 GS |
447 | systems where the run-time libraries distinguish between binary and |
448 | text files. If FILEHANDLE is an expression, the value is taken as the | |
449 | name of the filehandle. binmode() should be called after open() but | |
450 | before any I/O is done on the filehandle. The only way to reset | |
451 | binary mode on a filehandle is to reopen the file. | |
452 | ||
453 | On many systems binmode() has no effect, and on some systems it is | |
454 | necessary when you're not working with a text file. For the sake of | |
455 | portability it is a good idea to always use it when appropriate, and | |
456 | to never use it when it isn't appropriate. | |
457 | ||
458 | In other words: Regardless of platform, use binmode() on binary | |
459 | files, and do not use binmode() on text files. | |
19799a22 GS |
460 | |
461 | The operating system, device drivers, C libraries, and Perl run-time | |
30168b04 GS |
462 | system all work together to let the programmer treat a single |
463 | character (C<\n>) as the line terminator, irrespective of the external | |
464 | representation. On many operating systems, the native text file | |
465 | representation matches the internal representation, but on some | |
466 | platforms the external representation of C<\n> is made up of more than | |
467 | one character. | |
468 | ||
469 | Mac OS and all variants of Unix use a single character to end each line | |
470 | in the external representation of text (even though that single | |
471 | character is not necessarily the same across these platforms). | |
472 | Consequently binmode() has no effect on these operating systems. In | |
473 | other systems like VMS, MS-DOS and the various flavors of MS-Windows | |
474 | your program sees a C<\n> as a simple C<\cJ>, but what's stored in text | |
475 | files are the two characters C<\cM\cJ>. That means that, if you don't | |
476 | use binmode() on these systems, C<\cM\cJ> sequences on disk will be | |
477 | converted to C<\n> on input, and any C<\n> in your program will be | |
478 | converted back to C<\cM\cJ> on output. This is what you want for text | |
479 | files, but it can be disastrous for binary files. | |
480 | ||
481 | Another consequence of using binmode() (on some systems) is that | |
482 | special end-of-file markers will be seen as part of the data stream. | |
483 | For systems from the Microsoft family this means that if your binary | |
484 | data contains C<\cZ>, the I/O subsystem will ragard it as the end of | |
485 | the file, unless you use binmode(). | |
486 | ||
487 | binmode() is not only important for readline() and print() operations, | |
488 | but also when using read(), seek(), sysread(), syswrite() and tell() | |
489 | (see L<perlport> for more details). See the C<$/> and C<$\> variables | |
490 | in L<perlvar> for how to manually set your input and output | |
491 | line-termination sequences. | |
a0d0e21e | 492 | |
4633a7c4 | 493 | =item bless REF,CLASSNAME |
a0d0e21e LW |
494 | |
495 | =item bless REF | |
496 | ||
2b5ab1e7 TC |
497 | This function tells the thingy referenced by REF that it is now an object |
498 | in the CLASSNAME package. If CLASSNAME is omitted, the current package | |
19799a22 | 499 | is used. Because a C<bless> is often the last thing in a constructor, |
2b5ab1e7 TC |
500 | it returns the reference for convenience. Always use the two-argument |
501 | version if the function doing the blessing might be inherited by a | |
502 | derived class. See L<perltoot> and L<perlobj> for more about the blessing | |
503 | (and blessings) of objects. | |
a0d0e21e | 504 | |
57668c4d | 505 | Consider always blessing objects in CLASSNAMEs that are mixed case. |
2b5ab1e7 TC |
506 | Namespaces with all lowercase names are considered reserved for |
507 | Perl pragmata. Builtin types have all uppercase names, so to prevent | |
508 | confusion, you may wish to avoid such package names as well. Make sure | |
509 | that CLASSNAME is a true value. | |
60ad88b8 GS |
510 | |
511 | See L<perlmod/"Perl Modules">. | |
512 | ||
a0d0e21e LW |
513 | =item caller EXPR |
514 | ||
515 | =item caller | |
516 | ||
5a964f20 | 517 | Returns the context of the current subroutine call. In scalar context, |
28757baa | 518 | returns the caller's package name if there is a caller, that is, if |
19799a22 | 519 | we're in a subroutine or C<eval> or C<require>, and the undefined value |
5a964f20 | 520 | otherwise. In list context, returns |
a0d0e21e | 521 | |
748a9306 | 522 | ($package, $filename, $line) = caller; |
a0d0e21e LW |
523 | |
524 | With EXPR, it returns some extra information that the debugger uses to | |
525 | print a stack trace. The value of EXPR indicates how many call frames | |
526 | to go back before the current one. | |
527 | ||
f3aa04c2 | 528 | ($package, $filename, $line, $subroutine, $hasargs, |
e476b1b5 | 529 | $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i); |
e7ea3e70 | 530 | |
951ba7fe | 531 | Here $subroutine may be C<(eval)> if the frame is not a subroutine |
19799a22 | 532 | call, but an C<eval>. In such a case additional elements $evaltext and |
7660c0ab | 533 | C<$is_require> are set: C<$is_require> is true if the frame is created by a |
19799a22 | 534 | C<require> or C<use> statement, $evaltext contains the text of the |
dc848c6f | 535 | C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement, |
951ba7fe | 536 | $filename is C<(eval)>, but $evaltext is undefined. (Note also that |
dc848c6f | 537 | each C<use> statement creates a C<require> frame inside an C<eval EXPR>) |
e476b1b5 GS |
538 | frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller |
539 | was compiled with. The C<$hints> and C<$bitmask> values are subject to | |
540 | change between versions of Perl, and are not meant for external use. | |
748a9306 LW |
541 | |
542 | Furthermore, when called from within the DB package, caller returns more | |
7660c0ab | 543 | detailed information: it sets the list variable C<@DB::args> to be the |
54310121 | 544 | arguments with which the subroutine was invoked. |
748a9306 | 545 | |
7660c0ab | 546 | Be aware that the optimizer might have optimized call frames away before |
19799a22 | 547 | C<caller> had a chance to get the information. That means that C<caller(N)> |
7660c0ab | 548 | might not return information about the call frame you expect it do, for |
61eff3bc | 549 | C<< N > 1 >>. In particular, C<@DB::args> might have information from the |
19799a22 | 550 | previous time C<caller> was called. |
7660c0ab | 551 | |
a0d0e21e LW |
552 | =item chdir EXPR |
553 | ||
2b5ab1e7 | 554 | Changes the working directory to EXPR, if possible. If EXPR is omitted, |
0bfc1ec4 GS |
555 | changes to the directory specified by C<$ENV{HOME}>, if set; if not, |
556 | changes to the directory specified by C<$ENV{LOGDIR}>. If neither is | |
557 | set, C<chdir> does nothing. It returns true upon success, false | |
558 | otherwise. See the example under C<die>. | |
a0d0e21e LW |
559 | |
560 | =item chmod LIST | |
561 | ||
562 | Changes the permissions of a list of files. The first element of the | |
4633a7c4 | 563 | list must be the numerical mode, which should probably be an octal |
2f9daede TP |
564 | number, and which definitely should I<not> a string of octal digits: |
565 | C<0644> is okay, C<'0644'> is not. Returns the number of files | |
dc848c6f | 566 | successfully changed. See also L</oct>, if all you have is a string. |
a0d0e21e LW |
567 | |
568 | $cnt = chmod 0755, 'foo', 'bar'; | |
569 | chmod 0755, @executables; | |
f86cebdf GS |
570 | $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to |
571 | # --w----r-T | |
2f9daede TP |
572 | $mode = '0644'; chmod oct($mode), 'foo'; # this is better |
573 | $mode = 0644; chmod $mode, 'foo'; # this is best | |
a0d0e21e | 574 | |
ca6e1c26 JH |
575 | You can also import the symbolic C<S_I*> constants from the Fcntl |
576 | module: | |
577 | ||
578 | use Fcntl ':mode'; | |
579 | ||
580 | chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables; | |
581 | # This is identical to the chmod 0755 of the above example. | |
582 | ||
a0d0e21e LW |
583 | =item chomp VARIABLE |
584 | ||
585 | =item chomp LIST | |
586 | ||
587 | =item chomp | |
588 | ||
2b5ab1e7 TC |
589 | This safer version of L</chop> removes any trailing string |
590 | that corresponds to the current value of C<$/> (also known as | |
28757baa | 591 | $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total |
592 | number of characters removed from all its arguments. It's often used to | |
593 | remove the newline from the end of an input record when you're worried | |
2b5ab1e7 TC |
594 | that the final record may be missing its newline. When in paragraph |
595 | mode (C<$/ = "">), it removes all trailing newlines from the string. | |
4c5a6083 GS |
596 | When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is |
597 | a reference to an integer or the like, see L<perlvar>) chomp() won't | |
19799a22 GS |
598 | remove anything. |
599 | If VARIABLE is omitted, it chomps C<$_>. Example: | |
a0d0e21e LW |
600 | |
601 | while (<>) { | |
602 | chomp; # avoid \n on last field | |
603 | @array = split(/:/); | |
5a964f20 | 604 | # ... |
a0d0e21e LW |
605 | } |
606 | ||
607 | You can actually chomp anything that's an lvalue, including an assignment: | |
608 | ||
609 | chomp($cwd = `pwd`); | |
610 | chomp($answer = <STDIN>); | |
611 | ||
612 | If you chomp a list, each element is chomped, and the total number of | |
613 | characters removed is returned. | |
614 | ||
615 | =item chop VARIABLE | |
616 | ||
617 | =item chop LIST | |
618 | ||
619 | =item chop | |
620 | ||
621 | Chops off the last character of a string and returns the character | |
622 | chopped. It's used primarily to remove the newline from the end of an | |
623 | input record, but is much more efficient than C<s/\n//> because it neither | |
7660c0ab | 624 | scans nor copies the string. If VARIABLE is omitted, chops C<$_>. |
a0d0e21e LW |
625 | Example: |
626 | ||
627 | while (<>) { | |
628 | chop; # avoid \n on last field | |
629 | @array = split(/:/); | |
5a964f20 | 630 | #... |
a0d0e21e LW |
631 | } |
632 | ||
633 | You can actually chop anything that's an lvalue, including an assignment: | |
634 | ||
635 | chop($cwd = `pwd`); | |
636 | chop($answer = <STDIN>); | |
637 | ||
638 | If you chop a list, each element is chopped. Only the value of the | |
19799a22 | 639 | last C<chop> is returned. |
a0d0e21e | 640 | |
19799a22 | 641 | Note that C<chop> returns the last character. To return all but the last |
748a9306 LW |
642 | character, use C<substr($string, 0, -1)>. |
643 | ||
a0d0e21e LW |
644 | =item chown LIST |
645 | ||
646 | Changes the owner (and group) of a list of files. The first two | |
19799a22 GS |
647 | elements of the list must be the I<numeric> uid and gid, in that |
648 | order. A value of -1 in either position is interpreted by most | |
649 | systems to leave that value unchanged. Returns the number of files | |
650 | successfully changed. | |
a0d0e21e LW |
651 | |
652 | $cnt = chown $uid, $gid, 'foo', 'bar'; | |
653 | chown $uid, $gid, @filenames; | |
654 | ||
54310121 | 655 | Here's an example that looks up nonnumeric uids in the passwd file: |
a0d0e21e LW |
656 | |
657 | print "User: "; | |
19799a22 | 658 | chomp($user = <STDIN>); |
5a964f20 | 659 | print "Files: "; |
19799a22 | 660 | chomp($pattern = <STDIN>); |
a0d0e21e LW |
661 | |
662 | ($login,$pass,$uid,$gid) = getpwnam($user) | |
663 | or die "$user not in passwd file"; | |
664 | ||
5a964f20 | 665 | @ary = glob($pattern); # expand filenames |
a0d0e21e LW |
666 | chown $uid, $gid, @ary; |
667 | ||
54310121 | 668 | On most systems, you are not allowed to change the ownership of the |
4633a7c4 LW |
669 | file unless you're the superuser, although you should be able to change |
670 | the group to any of your secondary groups. On insecure systems, these | |
671 | restrictions may be relaxed, but this is not a portable assumption. | |
19799a22 GS |
672 | On POSIX systems, you can detect this condition this way: |
673 | ||
674 | use POSIX qw(sysconf _PC_CHOWN_RESTRICTED); | |
675 | $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED); | |
4633a7c4 | 676 | |
a0d0e21e LW |
677 | =item chr NUMBER |
678 | ||
54310121 | 679 | =item chr |
bbce6d69 | 680 | |
a0d0e21e | 681 | Returns the character represented by that NUMBER in the character set. |
a0ed51b3 | 682 | For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and |
2b5ab1e7 TC |
683 | chr(0x263a) is a Unicode smiley face (but only within the scope of |
684 | a C<use utf8>). For the reverse, use L</ord>. | |
685 | See L<utf8> for more about Unicode. | |
a0d0e21e | 686 | |
7660c0ab | 687 | If NUMBER is omitted, uses C<$_>. |
bbce6d69 | 688 | |
a0d0e21e LW |
689 | =item chroot FILENAME |
690 | ||
54310121 | 691 | =item chroot |
bbce6d69 | 692 | |
5a964f20 | 693 | This function works like the system call by the same name: it makes the |
4633a7c4 | 694 | named directory the new root directory for all further pathnames that |
951ba7fe | 695 | begin with a C</> by your process and all its children. (It doesn't |
28757baa | 696 | change your current working directory, which is unaffected.) For security |
4633a7c4 | 697 | reasons, this call is restricted to the superuser. If FILENAME is |
19799a22 | 698 | omitted, does a C<chroot> to C<$_>. |
a0d0e21e LW |
699 | |
700 | =item close FILEHANDLE | |
701 | ||
6a518fbc TP |
702 | =item close |
703 | ||
19799a22 | 704 | Closes the file or pipe associated with the file handle, returning true |
a0d0e21e | 705 | only if stdio successfully flushes buffers and closes the system file |
19799a22 | 706 | descriptor. Closes the currently selected filehandle if the argument |
6a518fbc | 707 | is omitted. |
fb73857a | 708 | |
709 | You don't have to close FILEHANDLE if you are immediately going to do | |
19799a22 GS |
710 | another C<open> on it, because C<open> will close it for you. (See |
711 | C<open>.) However, an explicit C<close> on an input file resets the line | |
712 | counter (C<$.>), while the implicit close done by C<open> does not. | |
fb73857a | 713 | |
19799a22 GS |
714 | If the file handle came from a piped open C<close> will additionally |
715 | return false if one of the other system calls involved fails or if the | |
fb73857a | 716 | program exits with non-zero status. (If the only problem was that the |
2b5ab1e7 TC |
717 | program exited non-zero C<$!> will be set to C<0>.) Closing a pipe |
718 | also waits for the process executing on the pipe to complete, in case you | |
719 | want to look at the output of the pipe afterwards, and | |
720 | implicitly puts the exit status value of that command into C<$?>. | |
5a964f20 | 721 | |
73689b13 GS |
722 | Prematurely closing the read end of a pipe (i.e. before the process |
723 | writing to it at the other end has closed it) will result in a | |
724 | SIGPIPE being delivered to the writer. If the other end can't | |
725 | handle that, be sure to read all the data before closing the pipe. | |
726 | ||
fb73857a | 727 | Example: |
a0d0e21e | 728 | |
fb73857a | 729 | open(OUTPUT, '|sort >foo') # pipe to sort |
730 | or die "Can't start sort: $!"; | |
5a964f20 | 731 | #... # print stuff to output |
fb73857a | 732 | close OUTPUT # wait for sort to finish |
733 | or warn $! ? "Error closing sort pipe: $!" | |
734 | : "Exit status $? from sort"; | |
735 | open(INPUT, 'foo') # get sort's results | |
736 | or die "Can't open 'foo' for input: $!"; | |
a0d0e21e | 737 | |
5a964f20 TC |
738 | FILEHANDLE may be an expression whose value can be used as an indirect |
739 | filehandle, usually the real filehandle name. | |
a0d0e21e LW |
740 | |
741 | =item closedir DIRHANDLE | |
742 | ||
19799a22 | 743 | Closes a directory opened by C<opendir> and returns the success of that |
5a964f20 TC |
744 | system call. |
745 | ||
746 | DIRHANDLE may be an expression whose value can be used as an indirect | |
747 | dirhandle, usually the real dirhandle name. | |
a0d0e21e LW |
748 | |
749 | =item connect SOCKET,NAME | |
750 | ||
751 | Attempts to connect to a remote socket, just as the connect system call | |
19799a22 | 752 | does. Returns true if it succeeded, false otherwise. NAME should be a |
4633a7c4 LW |
753 | packed address of the appropriate type for the socket. See the examples in |
754 | L<perlipc/"Sockets: Client/Server Communication">. | |
a0d0e21e | 755 | |
cb1a09d0 AD |
756 | =item continue BLOCK |
757 | ||
758 | Actually a flow control statement rather than a function. If there is a | |
98293880 JH |
759 | C<continue> BLOCK attached to a BLOCK (typically in a C<while> or |
760 | C<foreach>), it is always executed just before the conditional is about to | |
761 | be evaluated again, just like the third part of a C<for> loop in C. Thus | |
cb1a09d0 AD |
762 | it can be used to increment a loop variable, even when the loop has been |
763 | continued via the C<next> statement (which is similar to the C C<continue> | |
764 | statement). | |
765 | ||
98293880 | 766 | C<last>, C<next>, or C<redo> may appear within a C<continue> |
19799a22 GS |
767 | block. C<last> and C<redo> will behave as if they had been executed within |
768 | the main block. So will C<next>, but since it will execute a C<continue> | |
1d2dff63 GS |
769 | block, it may be more entertaining. |
770 | ||
771 | while (EXPR) { | |
772 | ### redo always comes here | |
773 | do_something; | |
774 | } continue { | |
775 | ### next always comes here | |
776 | do_something_else; | |
777 | # then back the top to re-check EXPR | |
778 | } | |
779 | ### last always comes here | |
780 | ||
781 | Omitting the C<continue> section is semantically equivalent to using an | |
19799a22 | 782 | empty one, logically enough. In that case, C<next> goes directly back |
1d2dff63 GS |
783 | to check the condition at the top of the loop. |
784 | ||
a0d0e21e LW |
785 | =item cos EXPR |
786 | ||
5a964f20 | 787 | Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, |
7660c0ab | 788 | takes cosine of C<$_>. |
a0d0e21e | 789 | |
ca6e1c26 | 790 | For the inverse cosine operation, you may use the C<Math::Trig::acos()> |
28757baa | 791 | function, or use this relation: |
792 | ||
793 | sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) } | |
794 | ||
a0d0e21e LW |
795 | =item crypt PLAINTEXT,SALT |
796 | ||
f86cebdf | 797 | Encrypts a string exactly like the crypt(3) function in the C library |
4633a7c4 LW |
798 | (assuming that you actually have a version there that has not been |
799 | extirpated as a potential munition). This can prove useful for checking | |
800 | the password file for lousy passwords, amongst other things. Only the | |
801 | guys wearing white hats should do this. | |
a0d0e21e | 802 | |
19799a22 | 803 | Note that C<crypt> is intended to be a one-way function, much like breaking |
11155c91 CS |
804 | eggs to make an omelette. There is no (known) corresponding decrypt |
805 | function. As a result, this function isn't all that useful for | |
806 | cryptography. (For that, see your nearby CPAN mirror.) | |
2f9daede | 807 | |
e71965be RS |
808 | When verifying an existing encrypted string you should use the encrypted |
809 | text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This | |
19799a22 | 810 | allows your code to work with the standard C<crypt> and with more |
e71965be RS |
811 | exotic implementations. When choosing a new salt create a random two |
812 | character string whose characters come from the set C<[./0-9A-Za-z]> | |
813 | (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). | |
814 | ||
a0d0e21e LW |
815 | Here's an example that makes sure that whoever runs this program knows |
816 | their own password: | |
817 | ||
818 | $pwd = (getpwuid($<))[1]; | |
a0d0e21e LW |
819 | |
820 | system "stty -echo"; | |
821 | print "Password: "; | |
e71965be | 822 | chomp($word = <STDIN>); |
a0d0e21e LW |
823 | print "\n"; |
824 | system "stty echo"; | |
825 | ||
e71965be | 826 | if (crypt($word, $pwd) ne $pwd) { |
a0d0e21e LW |
827 | die "Sorry...\n"; |
828 | } else { | |
829 | print "ok\n"; | |
54310121 | 830 | } |
a0d0e21e | 831 | |
9f8f0c9d | 832 | Of course, typing in your own password to whoever asks you |
748a9306 | 833 | for it is unwise. |
a0d0e21e | 834 | |
19799a22 GS |
835 | The L<crypt> function is unsuitable for encrypting large quantities |
836 | of data, not least of all because you can't get the information | |
837 | back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories | |
838 | on your favorite CPAN mirror for a slew of potentially useful | |
839 | modules. | |
840 | ||
aa689395 | 841 | =item dbmclose HASH |
a0d0e21e | 842 | |
19799a22 | 843 | [This function has been largely superseded by the C<untie> function.] |
a0d0e21e | 844 | |
aa689395 | 845 | Breaks the binding between a DBM file and a hash. |
a0d0e21e | 846 | |
19799a22 | 847 | =item dbmopen HASH,DBNAME,MASK |
a0d0e21e | 848 | |
19799a22 | 849 | [This function has been largely superseded by the C<tie> function.] |
a0d0e21e | 850 | |
7b8d334a | 851 | This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a |
19799a22 GS |
852 | hash. HASH is the name of the hash. (Unlike normal C<open>, the first |
853 | argument is I<not> a filehandle, even though it looks like one). DBNAME | |
aa689395 | 854 | is the name of the database (without the F<.dir> or F<.pag> extension if |
855 | any). If the database does not exist, it is created with protection | |
19799a22 GS |
856 | specified by MASK (as modified by the C<umask>). If your system supports |
857 | only the older DBM functions, you may perform only one C<dbmopen> in your | |
aa689395 | 858 | program. In older versions of Perl, if your system had neither DBM nor |
19799a22 | 859 | ndbm, calling C<dbmopen> produced a fatal error; it now falls back to |
aa689395 | 860 | sdbm(3). |
861 | ||
862 | If you don't have write access to the DBM file, you can only read hash | |
863 | variables, not set them. If you want to test whether you can write, | |
19799a22 | 864 | either use file tests or try setting a dummy hash entry inside an C<eval>, |
aa689395 | 865 | which will trap the error. |
a0d0e21e | 866 | |
19799a22 GS |
867 | Note that functions such as C<keys> and C<values> may return huge lists |
868 | when used on large DBM files. You may prefer to use the C<each> | |
a0d0e21e LW |
869 | function to iterate over large DBM files. Example: |
870 | ||
871 | # print out history file offsets | |
872 | dbmopen(%HIST,'/usr/lib/news/history',0666); | |
873 | while (($key,$val) = each %HIST) { | |
874 | print $key, ' = ', unpack('L',$val), "\n"; | |
875 | } | |
876 | dbmclose(%HIST); | |
877 | ||
cb1a09d0 | 878 | See also L<AnyDBM_File> for a more general description of the pros and |
184e9718 | 879 | cons of the various dbm approaches, as well as L<DB_File> for a particularly |
cb1a09d0 | 880 | rich implementation. |
4633a7c4 | 881 | |
2b5ab1e7 TC |
882 | You can control which DBM library you use by loading that library |
883 | before you call dbmopen(): | |
884 | ||
885 | use DB_File; | |
886 | dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db") | |
887 | or die "Can't open netscape history file: $!"; | |
888 | ||
a0d0e21e LW |
889 | =item defined EXPR |
890 | ||
54310121 | 891 | =item defined |
bbce6d69 | 892 | |
2f9daede TP |
893 | Returns a Boolean value telling whether EXPR has a value other than |
894 | the undefined value C<undef>. If EXPR is not present, C<$_> will be | |
895 | checked. | |
896 | ||
897 | Many operations return C<undef> to indicate failure, end of file, | |
898 | system error, uninitialized variable, and other exceptional | |
899 | conditions. This function allows you to distinguish C<undef> from | |
900 | other values. (A simple Boolean test will not distinguish among | |
7660c0ab | 901 | C<undef>, zero, the empty string, and C<"0">, which are all equally |
2f9daede | 902 | false.) Note that since C<undef> is a valid scalar, its presence |
19799a22 | 903 | doesn't I<necessarily> indicate an exceptional condition: C<pop> |
2f9daede TP |
904 | returns C<undef> when its argument is an empty array, I<or> when the |
905 | element to return happens to be C<undef>. | |
906 | ||
f10b0346 GS |
907 | You may also use C<defined(&func)> to check whether subroutine C<&func> |
908 | has ever been defined. The return value is unaffected by any forward | |
909 | declarations of C<&foo>. | |
910 | ||
911 | Use of C<defined> on aggregates (hashes and arrays) is deprecated. It | |
912 | used to report whether memory for that aggregate has ever been | |
913 | allocated. This behavior may disappear in future versions of Perl. | |
914 | You should instead use a simple test for size: | |
915 | ||
916 | if (@an_array) { print "has array elements\n" } | |
917 | if (%a_hash) { print "has hash members\n" } | |
2f9daede TP |
918 | |
919 | When used on a hash element, it tells you whether the value is defined, | |
dc848c6f | 920 | not whether the key exists in the hash. Use L</exists> for the latter |
2f9daede | 921 | purpose. |
a0d0e21e LW |
922 | |
923 | Examples: | |
924 | ||
925 | print if defined $switch{'D'}; | |
926 | print "$val\n" while defined($val = pop(@ary)); | |
927 | die "Can't readlink $sym: $!" | |
928 | unless defined($value = readlink $sym); | |
a0d0e21e | 929 | sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } |
2f9daede | 930 | $debugging = 0 unless defined $debugging; |
a0d0e21e | 931 | |
19799a22 | 932 | Note: Many folks tend to overuse C<defined>, and then are surprised to |
7660c0ab | 933 | discover that the number C<0> and C<""> (the zero-length string) are, in fact, |
2f9daede | 934 | defined values. For example, if you say |
a5f75d66 AD |
935 | |
936 | "ab" =~ /a(.*)b/; | |
937 | ||
7660c0ab | 938 | The pattern match succeeds, and C<$1> is defined, despite the fact that it |
a5f75d66 | 939 | matched "nothing". But it didn't really match nothing--rather, it |
2b5ab1e7 | 940 | matched something that happened to be zero characters long. This is all |
a5f75d66 | 941 | very above-board and honest. When a function returns an undefined value, |
2f9daede | 942 | it's an admission that it couldn't give you an honest answer. So you |
19799a22 | 943 | should use C<defined> only when you're questioning the integrity of what |
7660c0ab | 944 | you're trying to do. At other times, a simple comparison to C<0> or C<""> is |
2f9daede TP |
945 | what you want. |
946 | ||
dc848c6f | 947 | See also L</undef>, L</exists>, L</ref>. |
2f9daede | 948 | |
a0d0e21e LW |
949 | =item delete EXPR |
950 | ||
01020589 GS |
951 | Given an expression that specifies a hash element, array element, hash slice, |
952 | or array slice, deletes the specified element(s) from the hash or array. | |
8216c1fd GS |
953 | In the case of an array, if the array elements happen to be at the end, |
954 | the size of the array will shrink to the highest element that tests | |
955 | true for exists() (or 0 if no such element exists). | |
a0d0e21e | 956 | |
01020589 GS |
957 | Returns each element so deleted or the undefined value if there was no such |
958 | element. Deleting from C<$ENV{}> modifies the environment. Deleting from | |
959 | a hash tied to a DBM file deletes the entry from the DBM file. Deleting | |
960 | from a C<tie>d hash or array may not necessarily return anything. | |
961 | ||
8ea97a1e GS |
962 | Deleting an array element effectively returns that position of the array |
963 | to its initial, uninitialized state. Subsequently testing for the same | |
8216c1fd GS |
964 | element with exists() will return false. Note that deleting array |
965 | elements in the middle of an array will not shift the index of the ones | |
966 | after them down--use splice() for that. See L</exists>. | |
8ea97a1e | 967 | |
01020589 | 968 | The following (inefficiently) deletes all the values of %HASH and @ARRAY: |
a0d0e21e | 969 | |
5f05dabc | 970 | foreach $key (keys %HASH) { |
971 | delete $HASH{$key}; | |
a0d0e21e LW |
972 | } |
973 | ||
01020589 GS |
974 | foreach $index (0 .. $#ARRAY) { |
975 | delete $ARRAY[$index]; | |
976 | } | |
977 | ||
978 | And so do these: | |
5f05dabc | 979 | |
01020589 GS |
980 | delete @HASH{keys %HASH}; |
981 | ||
9740c838 | 982 | delete @ARRAY[0 .. $#ARRAY]; |
5f05dabc | 983 | |
2b5ab1e7 | 984 | But both of these are slower than just assigning the empty list |
01020589 GS |
985 | or undefining %HASH or @ARRAY: |
986 | ||
987 | %HASH = (); # completely empty %HASH | |
988 | undef %HASH; # forget %HASH ever existed | |
2b5ab1e7 | 989 | |
01020589 GS |
990 | @ARRAY = (); # completely empty @ARRAY |
991 | undef @ARRAY; # forget @ARRAY ever existed | |
2b5ab1e7 TC |
992 | |
993 | Note that the EXPR can be arbitrarily complicated as long as the final | |
01020589 GS |
994 | operation is a hash element, array element, hash slice, or array slice |
995 | lookup: | |
a0d0e21e LW |
996 | |
997 | delete $ref->[$x][$y]{$key}; | |
5f05dabc | 998 | delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys}; |
a0d0e21e | 999 | |
01020589 GS |
1000 | delete $ref->[$x][$y][$index]; |
1001 | delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices]; | |
1002 | ||
a0d0e21e LW |
1003 | =item die LIST |
1004 | ||
19799a22 GS |
1005 | Outside an C<eval>, prints the value of LIST to C<STDERR> and |
1006 | exits with the current value of C<$!> (errno). If C<$!> is C<0>, | |
61eff3bc JH |
1007 | exits with the value of C<<< ($? >> 8) >>> (backtick `command` |
1008 | status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside | |
19799a22 GS |
1009 | an C<eval(),> the error message is stuffed into C<$@> and the |
1010 | C<eval> is terminated with the undefined value. This makes | |
1011 | C<die> the way to raise an exception. | |
a0d0e21e LW |
1012 | |
1013 | Equivalent examples: | |
1014 | ||
1015 | die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; | |
54310121 | 1016 | chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" |
a0d0e21e LW |
1017 | |
1018 | If the value of EXPR does not end in a newline, the current script line | |
1019 | number and input line number (if any) are also printed, and a newline | |
883faa13 GS |
1020 | is supplied. Note that the "input line number" (also known as "chunk") |
1021 | is subject to whatever notion of "line" happens to be currently in | |
1022 | effect, and is also available as the special variable C<$.>. | |
1023 | See L<perlvar/"$/"> and L<perlvar/"$.">. | |
1024 | ||
1025 | Hint: sometimes appending C<", stopped"> to your message | |
7660c0ab | 1026 | will cause it to make better sense when the string C<"at foo line 123"> is |
a0d0e21e LW |
1027 | appended. Suppose you are running script "canasta". |
1028 | ||
1029 | die "/etc/games is no good"; | |
1030 | die "/etc/games is no good, stopped"; | |
1031 | ||
1032 | produce, respectively | |
1033 | ||
1034 | /etc/games is no good at canasta line 123. | |
1035 | /etc/games is no good, stopped at canasta line 123. | |
1036 | ||
2b5ab1e7 | 1037 | See also exit(), warn(), and the Carp module. |
a0d0e21e | 1038 | |
7660c0ab A |
1039 | If LIST is empty and C<$@> already contains a value (typically from a |
1040 | previous eval) that value is reused after appending C<"\t...propagated">. | |
fb73857a | 1041 | This is useful for propagating exceptions: |
1042 | ||
1043 | eval { ... }; | |
1044 | die unless $@ =~ /Expected exception/; | |
1045 | ||
7660c0ab | 1046 | If C<$@> is empty then the string C<"Died"> is used. |
fb73857a | 1047 | |
52531d10 GS |
1048 | die() can also be called with a reference argument. If this happens to be |
1049 | trapped within an eval(), $@ contains the reference. This behavior permits | |
1050 | a more elaborate exception handling implementation using objects that | |
1051 | maintain arbitary state about the nature of the exception. Such a scheme | |
1052 | is sometimes preferable to matching particular string values of $@ using | |
1053 | regular expressions. Here's an example: | |
1054 | ||
1055 | eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) }; | |
1056 | if ($@) { | |
1057 | if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) { | |
1058 | # handle Some::Module::Exception | |
1059 | } | |
1060 | else { | |
1061 | # handle all other possible exceptions | |
1062 | } | |
1063 | } | |
1064 | ||
19799a22 | 1065 | Because perl will stringify uncaught exception messages before displaying |
52531d10 GS |
1066 | them, you may want to overload stringification operations on such custom |
1067 | exception objects. See L<overload> for details about that. | |
1068 | ||
19799a22 GS |
1069 | You can arrange for a callback to be run just before the C<die> |
1070 | does its deed, by setting the C<$SIG{__DIE__}> hook. The associated | |
1071 | handler will be called with the error text and can change the error | |
1072 | message, if it sees fit, by calling C<die> again. See | |
1073 | L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and | |
1074 | L<"eval BLOCK"> for some examples. Although this feature was meant | |
1075 | to be run only right before your program was to exit, this is not | |
1076 | currently the case--the C<$SIG{__DIE__}> hook is currently called | |
1077 | even inside eval()ed blocks/strings! If one wants the hook to do | |
1078 | nothing in such situations, put | |
fb73857a | 1079 | |
1080 | die @_ if $^S; | |
1081 | ||
19799a22 GS |
1082 | as the first line of the handler (see L<perlvar/$^S>). Because |
1083 | this promotes strange action at a distance, this counterintuitive | |
1084 | behavior may be fixed in a future release. | |
774d564b | 1085 | |
a0d0e21e LW |
1086 | =item do BLOCK |
1087 | ||
1088 | Not really a function. Returns the value of the last command in the | |
1089 | sequence of commands indicated by BLOCK. When modified by a loop | |
98293880 JH |
1090 | modifier, executes the BLOCK once before testing the loop condition. |
1091 | (On other statements the loop modifiers test the conditional first.) | |
a0d0e21e | 1092 | |
4968c1e4 | 1093 | C<do BLOCK> does I<not> count as a loop, so the loop control statements |
2b5ab1e7 TC |
1094 | C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. |
1095 | See L<perlsyn> for alternative strategies. | |
4968c1e4 | 1096 | |
a0d0e21e LW |
1097 | =item do SUBROUTINE(LIST) |
1098 | ||
1099 | A deprecated form of subroutine call. See L<perlsub>. | |
1100 | ||
1101 | =item do EXPR | |
1102 | ||
1103 | Uses the value of EXPR as a filename and executes the contents of the | |
1104 | file as a Perl script. Its primary use is to include subroutines | |
1105 | from a Perl subroutine library. | |
1106 | ||
1107 | do 'stat.pl'; | |
1108 | ||
1109 | is just like | |
1110 | ||
fb73857a | 1111 | scalar eval `cat stat.pl`; |
a0d0e21e | 1112 | |
2b5ab1e7 TC |
1113 | except that it's more efficient and concise, keeps track of the current |
1114 | filename for error messages, searches the @INC libraries, and updates | |
1115 | C<%INC> if the file is found. See L<perlvar/Predefined Names> for these | |
1116 | variables. It also differs in that code evaluated with C<do FILENAME> | |
1117 | cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the | |
1118 | same, however, in that it does reparse the file every time you call it, | |
1119 | so you probably don't want to do this inside a loop. | |
a0d0e21e | 1120 | |
8e30cc93 | 1121 | If C<do> cannot read the file, it returns undef and sets C<$!> to the |
2b5ab1e7 | 1122 | error. If C<do> can read the file but cannot compile it, it |
8e30cc93 MG |
1123 | returns undef and sets an error message in C<$@>. If the file is |
1124 | successfully compiled, C<do> returns the value of the last expression | |
1125 | evaluated. | |
1126 | ||
a0d0e21e | 1127 | Note that inclusion of library modules is better done with the |
19799a22 | 1128 | C<use> and C<require> operators, which also do automatic error checking |
4633a7c4 | 1129 | and raise an exception if there's a problem. |
a0d0e21e | 1130 | |
5a964f20 TC |
1131 | You might like to use C<do> to read in a program configuration |
1132 | file. Manual error checking can be done this way: | |
1133 | ||
1134 | # read in config files: system first, then user | |
f86cebdf | 1135 | for $file ("/share/prog/defaults.rc", |
2b5ab1e7 TC |
1136 | "$ENV{HOME}/.someprogrc") |
1137 | { | |
5a964f20 | 1138 | unless ($return = do $file) { |
f86cebdf GS |
1139 | warn "couldn't parse $file: $@" if $@; |
1140 | warn "couldn't do $file: $!" unless defined $return; | |
1141 | warn "couldn't run $file" unless $return; | |
5a964f20 TC |
1142 | } |
1143 | } | |
1144 | ||
a0d0e21e LW |
1145 | =item dump LABEL |
1146 | ||
1614b0e3 JD |
1147 | =item dump |
1148 | ||
19799a22 GS |
1149 | This function causes an immediate core dump. See also the B<-u> |
1150 | command-line switch in L<perlrun>, which does the same thing. | |
1151 | Primarily this is so that you can use the B<undump> program (not | |
1152 | supplied) to turn your core dump into an executable binary after | |
1153 | having initialized all your variables at the beginning of the | |
1154 | program. When the new binary is executed it will begin by executing | |
1155 | a C<goto LABEL> (with all the restrictions that C<goto> suffers). | |
1156 | Think of it as a goto with an intervening core dump and reincarnation. | |
1157 | If C<LABEL> is omitted, restarts the program from the top. | |
1158 | ||
1159 | B<WARNING>: Any files opened at the time of the dump will I<not> | |
1160 | be open any more when the program is reincarnated, with possible | |
1161 | resulting confusion on the part of Perl. | |
1162 | ||
1163 | This function is now largely obsolete, partly because it's very | |
1164 | hard to convert a core file into an executable, and because the | |
1165 | real compiler backends for generating portable bytecode and compilable | |
1166 | C code have superseded it. | |
1167 | ||
1168 | If you're looking to use L<dump> to speed up your program, consider | |
1169 | generating bytecode or native C code as described in L<perlcc>. If | |
1170 | you're just trying to accelerate a CGI script, consider using the | |
1171 | C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI. | |
1172 | You might also consider autoloading or selfloading, which at least | |
1173 | make your program I<appear> to run faster. | |
5a964f20 | 1174 | |
aa689395 | 1175 | =item each HASH |
1176 | ||
5a964f20 | 1177 | When called in list context, returns a 2-element list consisting of the |
aa689395 | 1178 | key and value for the next element of a hash, so that you can iterate over |
5a964f20 | 1179 | it. When called in scalar context, returns the key for only the "next" |
e902a979 | 1180 | element in the hash. |
2f9daede | 1181 | |
ab192400 GS |
1182 | Entries are returned in an apparently random order. The actual random |
1183 | order is subject to change in future versions of perl, but it is guaranteed | |
19799a22 | 1184 | to be in the same order as either the C<keys> or C<values> function |
ab192400 GS |
1185 | would produce on the same (unmodified) hash. |
1186 | ||
1187 | When the hash is entirely read, a null array is returned in list context | |
19799a22 GS |
1188 | (which when assigned produces a false (C<0>) value), and C<undef> in |
1189 | scalar context. The next call to C<each> after that will start iterating | |
1190 | again. There is a single iterator for each hash, shared by all C<each>, | |
1191 | C<keys>, and C<values> function calls in the program; it can be reset by | |
2f9daede TP |
1192 | reading all the elements from the hash, or by evaluating C<keys HASH> or |
1193 | C<values HASH>. If you add or delete elements of a hash while you're | |
1194 | iterating over it, you may get entries skipped or duplicated, so don't. | |
aa689395 | 1195 | |
f86cebdf | 1196 | The following prints out your environment like the printenv(1) program, |
aa689395 | 1197 | only in a different order: |
a0d0e21e LW |
1198 | |
1199 | while (($key,$value) = each %ENV) { | |
1200 | print "$key=$value\n"; | |
1201 | } | |
1202 | ||
19799a22 | 1203 | See also C<keys>, C<values> and C<sort>. |
a0d0e21e LW |
1204 | |
1205 | =item eof FILEHANDLE | |
1206 | ||
4633a7c4 LW |
1207 | =item eof () |
1208 | ||
a0d0e21e LW |
1209 | =item eof |
1210 | ||
1211 | Returns 1 if the next read on FILEHANDLE will return end of file, or if | |
1212 | FILEHANDLE is not open. FILEHANDLE may be an expression whose value | |
5a964f20 | 1213 | gives the real filehandle. (Note that this function actually |
19799a22 | 1214 | reads a character and then C<ungetc>s it, so isn't very useful in an |
748a9306 | 1215 | interactive context.) Do not read from a terminal file (or call |
19799a22 | 1216 | C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such |
748a9306 LW |
1217 | as terminals may lose the end-of-file condition if you do. |
1218 | ||
820475bd GS |
1219 | An C<eof> without an argument uses the last file read. Using C<eof()> |
1220 | with empty parentheses is very different. It refers to the pseudo file | |
1221 | formed from the files listed on the command line and accessed via the | |
61eff3bc JH |
1222 | C<< <> >> operator. Since C<< <> >> isn't explicitly opened, |
1223 | as a normal filehandle is, an C<eof()> before C<< <> >> has been | |
820475bd GS |
1224 | used will cause C<@ARGV> to be examined to determine if input is |
1225 | available. | |
1226 | ||
61eff3bc | 1227 | In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to |
820475bd GS |
1228 | detect the end of each file, C<eof()> will only detect the end of the |
1229 | last file. Examples: | |
a0d0e21e | 1230 | |
748a9306 LW |
1231 | # reset line numbering on each input file |
1232 | while (<>) { | |
5a964f20 | 1233 | next if /^\s*#/; # skip comments |
748a9306 | 1234 | print "$.\t$_"; |
5a964f20 TC |
1235 | } continue { |
1236 | close ARGV if eof; # Not eof()! | |
748a9306 LW |
1237 | } |
1238 | ||
a0d0e21e LW |
1239 | # insert dashes just before last line of last file |
1240 | while (<>) { | |
5a964f20 | 1241 | if (eof()) { # check for end of current file |
a0d0e21e | 1242 | print "--------------\n"; |
2b5ab1e7 | 1243 | close(ARGV); # close or last; is needed if we |
748a9306 | 1244 | # are reading from the terminal |
a0d0e21e LW |
1245 | } |
1246 | print; | |
1247 | } | |
1248 | ||
a0d0e21e | 1249 | Practical hint: you almost never need to use C<eof> in Perl, because the |
3ce0d271 GS |
1250 | input operators typically return C<undef> when they run out of data, or if |
1251 | there was an error. | |
a0d0e21e LW |
1252 | |
1253 | =item eval EXPR | |
1254 | ||
1255 | =item eval BLOCK | |
1256 | ||
c7cc6f1c GS |
1257 | In the first form, the return value of EXPR is parsed and executed as if it |
1258 | were a little Perl program. The value of the expression (which is itself | |
5a964f20 | 1259 | determined within scalar context) is first parsed, and if there weren't any |
c7cc6f1c | 1260 | errors, executed in the context of the current Perl program, so that any |
5f05dabc | 1261 | variable settings or subroutine and format definitions remain afterwards. |
c7cc6f1c GS |
1262 | Note that the value is parsed every time the eval executes. If EXPR is |
1263 | omitted, evaluates C<$_>. This form is typically used to delay parsing | |
1264 | and subsequent execution of the text of EXPR until run time. | |
1265 | ||
1266 | In the second form, the code within the BLOCK is parsed only once--at the | |
1267 | same time the code surrounding the eval itself was parsed--and executed | |
1268 | within the context of the current Perl program. This form is typically | |
1269 | used to trap exceptions more efficiently than the first (see below), while | |
1270 | also providing the benefit of checking the code within BLOCK at compile | |
1271 | time. | |
1272 | ||
1273 | The final semicolon, if any, may be omitted from the value of EXPR or within | |
1274 | the BLOCK. | |
1275 | ||
1276 | In both forms, the value returned is the value of the last expression | |
5a964f20 | 1277 | evaluated inside the mini-program; a return statement may be also used, just |
c7cc6f1c | 1278 | as with subroutines. The expression providing the return value is evaluated |
5a964f20 | 1279 | in void, scalar, or list context, depending on the context of the eval itself. |
c7cc6f1c | 1280 | See L</wantarray> for more on how the evaluation context can be determined. |
a0d0e21e | 1281 | |
19799a22 GS |
1282 | If there is a syntax error or runtime error, or a C<die> statement is |
1283 | executed, an undefined value is returned by C<eval>, and C<$@> is set to the | |
a0d0e21e | 1284 | error message. If there was no error, C<$@> is guaranteed to be a null |
19799a22 | 1285 | string. Beware that using C<eval> neither silences perl from printing |
c7cc6f1c GS |
1286 | warnings to STDERR, nor does it stuff the text of warning messages into C<$@>. |
1287 | To do either of those, you have to use the C<$SIG{__WARN__}> facility. See | |
1288 | L</warn> and L<perlvar>. | |
a0d0e21e | 1289 | |
19799a22 GS |
1290 | Note that, because C<eval> traps otherwise-fatal errors, it is useful for |
1291 | determining whether a particular feature (such as C<socket> or C<symlink>) | |
a0d0e21e LW |
1292 | is implemented. It is also Perl's exception trapping mechanism, where |
1293 | the die operator is used to raise exceptions. | |
1294 | ||
1295 | If the code to be executed doesn't vary, you may use the eval-BLOCK | |
1296 | form to trap run-time errors without incurring the penalty of | |
1297 | recompiling each time. The error, if any, is still returned in C<$@>. | |
1298 | Examples: | |
1299 | ||
54310121 | 1300 | # make divide-by-zero nonfatal |
a0d0e21e LW |
1301 | eval { $answer = $a / $b; }; warn $@ if $@; |
1302 | ||
1303 | # same thing, but less efficient | |
1304 | eval '$answer = $a / $b'; warn $@ if $@; | |
1305 | ||
1306 | # a compile-time error | |
5a964f20 | 1307 | eval { $answer = }; # WRONG |
a0d0e21e LW |
1308 | |
1309 | # a run-time error | |
1310 | eval '$answer ='; # sets $@ | |
1311 | ||
2b5ab1e7 TC |
1312 | Due to the current arguably broken state of C<__DIE__> hooks, when using |
1313 | the C<eval{}> form as an exception trap in libraries, you may wish not | |
1314 | to trigger any C<__DIE__> hooks that user code may have installed. | |
1315 | You can use the C<local $SIG{__DIE__}> construct for this purpose, | |
1316 | as shown in this example: | |
774d564b | 1317 | |
1318 | # a very private exception trap for divide-by-zero | |
f86cebdf GS |
1319 | eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; |
1320 | warn $@ if $@; | |
774d564b | 1321 | |
1322 | This is especially significant, given that C<__DIE__> hooks can call | |
19799a22 | 1323 | C<die> again, which has the effect of changing their error messages: |
774d564b | 1324 | |
1325 | # __DIE__ hooks may modify error messages | |
1326 | { | |
f86cebdf GS |
1327 | local $SIG{'__DIE__'} = |
1328 | sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; | |
c7cc6f1c GS |
1329 | eval { die "foo lives here" }; |
1330 | print $@ if $@; # prints "bar lives here" | |
774d564b | 1331 | } |
1332 | ||
19799a22 | 1333 | Because this promotes action at a distance, this counterintuitive behavior |
2b5ab1e7 TC |
1334 | may be fixed in a future release. |
1335 | ||
19799a22 | 1336 | With an C<eval>, you should be especially careful to remember what's |
a0d0e21e LW |
1337 | being looked at when: |
1338 | ||
1339 | eval $x; # CASE 1 | |
1340 | eval "$x"; # CASE 2 | |
1341 | ||
1342 | eval '$x'; # CASE 3 | |
1343 | eval { $x }; # CASE 4 | |
1344 | ||
5a964f20 | 1345 | eval "\$$x++"; # CASE 5 |
a0d0e21e LW |
1346 | $$x++; # CASE 6 |
1347 | ||
2f9daede | 1348 | Cases 1 and 2 above behave identically: they run the code contained in |
19799a22 | 1349 | the variable $x. (Although case 2 has misleading double quotes making |
2f9daede | 1350 | the reader wonder what else might be happening (nothing is).) Cases 3 |
7660c0ab | 1351 | and 4 likewise behave in the same way: they run the code C<'$x'>, which |
19799a22 | 1352 | does nothing but return the value of $x. (Case 4 is preferred for |
2f9daede TP |
1353 | purely visual reasons, but it also has the advantage of compiling at |
1354 | compile-time instead of at run-time.) Case 5 is a place where | |
19799a22 | 1355 | normally you I<would> like to use double quotes, except that in this |
2f9daede TP |
1356 | particular situation, you can just use symbolic references instead, as |
1357 | in case 6. | |
a0d0e21e | 1358 | |
4968c1e4 | 1359 | C<eval BLOCK> does I<not> count as a loop, so the loop control statements |
2b5ab1e7 | 1360 | C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. |
4968c1e4 | 1361 | |
a0d0e21e LW |
1362 | =item exec LIST |
1363 | ||
8bf3b016 GS |
1364 | =item exec PROGRAM LIST |
1365 | ||
19799a22 GS |
1366 | The C<exec> function executes a system command I<and never returns>-- |
1367 | use C<system> instead of C<exec> if you want it to return. It fails and | |
1368 | returns false only if the command does not exist I<and> it is executed | |
fb73857a | 1369 | directly instead of via your system's command shell (see below). |
a0d0e21e | 1370 | |
19799a22 GS |
1371 | Since it's a common mistake to use C<exec> instead of C<system>, Perl |
1372 | warns you if there is a following statement which isn't C<die>, C<warn>, | |
1373 | or C<exit> (if C<-w> is set - but you always do that). If you | |
1374 | I<really> want to follow an C<exec> with some other statement, you | |
55d729e4 GS |
1375 | can use one of these styles to avoid the warning: |
1376 | ||
5a964f20 TC |
1377 | exec ('foo') or print STDERR "couldn't exec foo: $!"; |
1378 | { exec ('foo') }; print STDERR "couldn't exec foo: $!"; | |
55d729e4 | 1379 | |
5a964f20 | 1380 | If there is more than one argument in LIST, or if LIST is an array |
f86cebdf | 1381 | with more than one value, calls execvp(3) with the arguments in LIST. |
5a964f20 TC |
1382 | If there is only one scalar argument or an array with one element in it, |
1383 | the argument is checked for shell metacharacters, and if there are any, | |
1384 | the entire argument is passed to the system's command shell for parsing | |
1385 | (this is C</bin/sh -c> on Unix platforms, but varies on other platforms). | |
1386 | If there are no shell metacharacters in the argument, it is split into | |
19799a22 GS |
1387 | words and passed directly to C<execvp>, which is more efficient. |
1388 | Examples: | |
a0d0e21e | 1389 | |
19799a22 GS |
1390 | exec '/bin/echo', 'Your arguments are: ', @ARGV; |
1391 | exec "sort $outfile | uniq"; | |
a0d0e21e LW |
1392 | |
1393 | If you don't really want to execute the first argument, but want to lie | |
1394 | to the program you are executing about its own name, you can specify | |
1395 | the program you actually want to run as an "indirect object" (without a | |
1396 | comma) in front of the LIST. (This always forces interpretation of the | |
54310121 | 1397 | LIST as a multivalued list, even if there is only a single scalar in |
a0d0e21e LW |
1398 | the list.) Example: |
1399 | ||
1400 | $shell = '/bin/csh'; | |
1401 | exec $shell '-sh'; # pretend it's a login shell | |
1402 | ||
1403 | or, more directly, | |
1404 | ||
1405 | exec {'/bin/csh'} '-sh'; # pretend it's a login shell | |
1406 | ||
bb32b41a GS |
1407 | When the arguments get executed via the system shell, results will |
1408 | be subject to its quirks and capabilities. See L<perlop/"`STRING`"> | |
1409 | for details. | |
1410 | ||
19799a22 GS |
1411 | Using an indirect object with C<exec> or C<system> is also more |
1412 | secure. This usage (which also works fine with system()) forces | |
1413 | interpretation of the arguments as a multivalued list, even if the | |
1414 | list had just one argument. That way you're safe from the shell | |
1415 | expanding wildcards or splitting up words with whitespace in them. | |
5a964f20 TC |
1416 | |
1417 | @args = ( "echo surprise" ); | |
1418 | ||
2b5ab1e7 | 1419 | exec @args; # subject to shell escapes |
f86cebdf | 1420 | # if @args == 1 |
2b5ab1e7 | 1421 | exec { $args[0] } @args; # safe even with one-arg list |
5a964f20 TC |
1422 | |
1423 | The first version, the one without the indirect object, ran the I<echo> | |
1424 | program, passing it C<"surprise"> an argument. The second version | |
1425 | didn't--it tried to run a program literally called I<"echo surprise">, | |
1426 | didn't find it, and set C<$?> to a non-zero value indicating failure. | |
1427 | ||
19799a22 | 1428 | Note that C<exec> will not call your C<END> blocks, nor will it call |
7660c0ab A |
1429 | any C<DESTROY> methods in your objects. |
1430 | ||
a0d0e21e LW |
1431 | =item exists EXPR |
1432 | ||
01020589 | 1433 | Given an expression that specifies a hash element or array element, |
8ea97a1e GS |
1434 | returns true if the specified element in the hash or array has ever |
1435 | been initialized, even if the corresponding value is undefined. The | |
1436 | element is not autovivified if it doesn't exist. | |
a0d0e21e | 1437 | |
01020589 GS |
1438 | print "Exists\n" if exists $hash{$key}; |
1439 | print "Defined\n" if defined $hash{$key}; | |
1440 | print "True\n" if $hash{$key}; | |
1441 | ||
1442 | print "Exists\n" if exists $array[$index]; | |
1443 | print "Defined\n" if defined $array[$index]; | |
1444 | print "True\n" if $array[$index]; | |
a0d0e21e | 1445 | |
8ea97a1e | 1446 | A hash or array element can be true only if it's defined, and defined if |
a0d0e21e LW |
1447 | it exists, but the reverse doesn't necessarily hold true. |
1448 | ||
afebc493 GS |
1449 | Given an expression that specifies the name of a subroutine, |
1450 | returns true if the specified subroutine has ever been declared, even | |
1451 | if it is undefined. Mentioning a subroutine name for exists or defined | |
1452 | does not count as declaring it. | |
1453 | ||
1454 | print "Exists\n" if exists &subroutine; | |
1455 | print "Defined\n" if defined &subroutine; | |
1456 | ||
a0d0e21e | 1457 | Note that the EXPR can be arbitrarily complicated as long as the final |
afebc493 | 1458 | operation is a hash or array key lookup or subroutine name: |
a0d0e21e | 1459 | |
2b5ab1e7 TC |
1460 | if (exists $ref->{A}->{B}->{$key}) { } |
1461 | if (exists $hash{A}{B}{$key}) { } | |
1462 | ||
01020589 GS |
1463 | if (exists $ref->{A}->{B}->[$ix]) { } |
1464 | if (exists $hash{A}{B}[$ix]) { } | |
1465 | ||
afebc493 GS |
1466 | if (exists &{$ref->{A}{B}{$key}}) { } |
1467 | ||
01020589 GS |
1468 | Although the deepest nested array or hash will not spring into existence |
1469 | just because its existence was tested, any intervening ones will. | |
61eff3bc | 1470 | Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring |
01020589 GS |
1471 | into existence due to the existence test for the $key element above. |
1472 | This happens anywhere the arrow operator is used, including even: | |
5a964f20 | 1473 | |
2b5ab1e7 TC |
1474 | undef $ref; |
1475 | if (exists $ref->{"Some key"}) { } | |
1476 | print $ref; # prints HASH(0x80d3d5c) | |
1477 | ||
1478 | This surprising autovivification in what does not at first--or even | |
1479 | second--glance appear to be an lvalue context may be fixed in a future | |
5a964f20 | 1480 | release. |
a0d0e21e | 1481 | |
479ba383 GS |
1482 | See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics |
1483 | on how exists() acts when used on a pseudo-hash. | |
e0478e5a | 1484 | |
afebc493 GS |
1485 | Use of a subroutine call, rather than a subroutine name, as an argument |
1486 | to exists() is an error. | |
1487 | ||
1488 | exists ⊂ # OK | |
1489 | exists &sub(); # Error | |
1490 | ||
a0d0e21e LW |
1491 | =item exit EXPR |
1492 | ||
2b5ab1e7 | 1493 | Evaluates EXPR and exits immediately with that value. Example: |
a0d0e21e LW |
1494 | |
1495 | $ans = <STDIN>; | |
1496 | exit 0 if $ans =~ /^[Xx]/; | |
1497 | ||
19799a22 | 1498 | See also C<die>. If EXPR is omitted, exits with C<0> status. The only |
2b5ab1e7 TC |
1499 | universally recognized values for EXPR are C<0> for success and C<1> |
1500 | for error; other values are subject to interpretation depending on the | |
1501 | environment in which the Perl program is running. For example, exiting | |
1502 | 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause | |
1503 | the mailer to return the item undelivered, but that's not true everywhere. | |
a0d0e21e | 1504 | |
19799a22 GS |
1505 | Don't use C<exit> to abort a subroutine if there's any chance that |
1506 | someone might want to trap whatever error happened. Use C<die> instead, | |
1507 | which can be trapped by an C<eval>. | |
28757baa | 1508 | |
19799a22 | 1509 | The exit() function does not always exit immediately. It calls any |
2b5ab1e7 | 1510 | defined C<END> routines first, but these C<END> routines may not |
19799a22 | 1511 | themselves abort the exit. Likewise any object destructors that need to |
2b5ab1e7 TC |
1512 | be called are called before the real exit. If this is a problem, you |
1513 | can call C<POSIX:_exit($status)> to avoid END and destructor processing. | |
87275199 | 1514 | See L<perlmod> for details. |
5a964f20 | 1515 | |
a0d0e21e LW |
1516 | =item exp EXPR |
1517 | ||
54310121 | 1518 | =item exp |
bbce6d69 | 1519 | |
2b5ab1e7 | 1520 | Returns I<e> (the natural logarithm base) to the power of EXPR. |
a0d0e21e LW |
1521 | If EXPR is omitted, gives C<exp($_)>. |
1522 | ||
1523 | =item fcntl FILEHANDLE,FUNCTION,SCALAR | |
1524 | ||
f86cebdf | 1525 | Implements the fcntl(2) function. You'll probably have to say |
a0d0e21e LW |
1526 | |
1527 | use Fcntl; | |
1528 | ||
0ade1984 | 1529 | first to get the correct constant definitions. Argument processing and |
19799a22 | 1530 | value return works just like C<ioctl> below. |
a0d0e21e LW |
1531 | For example: |
1532 | ||
1533 | use Fcntl; | |
5a964f20 TC |
1534 | fcntl($filehandle, F_GETFL, $packed_return_buffer) |
1535 | or die "can't fcntl F_GETFL: $!"; | |
1536 | ||
19799a22 | 1537 | You don't have to check for C<defined> on the return from C<fnctl>. |
951ba7fe GS |
1538 | Like C<ioctl>, it maps a C<0> return from the system call into |
1539 | C<"0 but true"> in Perl. This string is true in boolean context and C<0> | |
2b5ab1e7 TC |
1540 | in numeric context. It is also exempt from the normal B<-w> warnings |
1541 | on improper numeric conversions. | |
5a964f20 | 1542 | |
19799a22 | 1543 | Note that C<fcntl> will produce a fatal error if used on a machine that |
2b5ab1e7 TC |
1544 | doesn't implement fcntl(2). See the Fcntl module or your fcntl(2) |
1545 | manpage to learn what functions are available on your system. | |
a0d0e21e LW |
1546 | |
1547 | =item fileno FILEHANDLE | |
1548 | ||
2b5ab1e7 TC |
1549 | Returns the file descriptor for a filehandle, or undefined if the |
1550 | filehandle is not open. This is mainly useful for constructing | |
19799a22 | 1551 | bitmaps for C<select> and low-level POSIX tty-handling operations. |
2b5ab1e7 TC |
1552 | If FILEHANDLE is an expression, the value is taken as an indirect |
1553 | filehandle, generally its name. | |
5a964f20 TC |
1554 | |
1555 | You can use this to find out whether two handles refer to the | |
1556 | same underlying descriptor: | |
1557 | ||
1558 | if (fileno(THIS) == fileno(THAT)) { | |
1559 | print "THIS and THAT are dups\n"; | |
1560 | } | |
a0d0e21e LW |
1561 | |
1562 | =item flock FILEHANDLE,OPERATION | |
1563 | ||
19799a22 GS |
1564 | Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true |
1565 | for success, false on failure. Produces a fatal error if used on a | |
2b5ab1e7 | 1566 | machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3). |
19799a22 | 1567 | C<flock> is Perl's portable file locking interface, although it locks |
2b5ab1e7 TC |
1568 | only entire files, not records. |
1569 | ||
1570 | Two potentially non-obvious but traditional C<flock> semantics are | |
1571 | that it waits indefinitely until the lock is granted, and that its locks | |
1572 | B<merely advisory>. Such discretionary locks are more flexible, but offer | |
19799a22 GS |
1573 | fewer guarantees. This means that files locked with C<flock> may be |
1574 | modified by programs that do not also use C<flock>. See L<perlport>, | |
2b5ab1e7 TC |
1575 | your port's specific documentation, or your system-specific local manpages |
1576 | for details. It's best to assume traditional behavior if you're writing | |
1577 | portable programs. (But if you're not, you should as always feel perfectly | |
1578 | free to write for your own system's idiosyncrasies (sometimes called | |
1579 | "features"). Slavish adherence to portability concerns shouldn't get | |
1580 | in the way of your getting your job done.) | |
a3cb178b | 1581 | |
8ebc5c01 | 1582 | OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with |
1583 | LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but | |
ea3105be | 1584 | you can use the symbolic names if you import them from the Fcntl module, |
68dc0745 | 1585 | either individually, or as a group using the ':flock' tag. LOCK_SH |
1586 | requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN | |
ea3105be GS |
1587 | releases a previously requested lock. If LOCK_NB is bitwise-or'ed with |
1588 | LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking | |
68dc0745 | 1589 | waiting for the lock (check the return status to see if you got it). |
1590 | ||
2b5ab1e7 TC |
1591 | To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE |
1592 | before locking or unlocking it. | |
8ebc5c01 | 1593 | |
f86cebdf | 1594 | Note that the emulation built with lockf(3) doesn't provide shared |
8ebc5c01 | 1595 | locks, and it requires that FILEHANDLE be open with write intent. These |
2b5ab1e7 | 1596 | are the semantics that lockf(3) implements. Most if not all systems |
f86cebdf | 1597 | implement lockf(3) in terms of fcntl(2) locking, though, so the |
8ebc5c01 | 1598 | differing semantics shouldn't bite too many people. |
1599 | ||
19799a22 GS |
1600 | Note also that some versions of C<flock> cannot lock things over the |
1601 | network; you would need to use the more system-specific C<fcntl> for | |
f86cebdf GS |
1602 | that. If you like you can force Perl to ignore your system's flock(2) |
1603 | function, and so provide its own fcntl(2)-based emulation, by passing | |
8ebc5c01 | 1604 | the switch C<-Ud_flock> to the F<Configure> program when you configure |
1605 | perl. | |
4633a7c4 LW |
1606 | |
1607 | Here's a mailbox appender for BSD systems. | |
a0d0e21e | 1608 | |
7e1af8bc | 1609 | use Fcntl ':flock'; # import LOCK_* constants |
a0d0e21e LW |
1610 | |
1611 | sub lock { | |
7e1af8bc | 1612 | flock(MBOX,LOCK_EX); |
a0d0e21e LW |
1613 | # and, in case someone appended |
1614 | # while we were waiting... | |
1615 | seek(MBOX, 0, 2); | |
1616 | } | |
1617 | ||
1618 | sub unlock { | |
7e1af8bc | 1619 | flock(MBOX,LOCK_UN); |
a0d0e21e LW |
1620 | } |
1621 | ||
1622 | open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}") | |
1623 | or die "Can't open mailbox: $!"; | |
1624 | ||
1625 | lock(); | |
1626 | print MBOX $msg,"\n\n"; | |
1627 | unlock(); | |
1628 | ||
2b5ab1e7 TC |
1629 | On systems that support a real flock(), locks are inherited across fork() |
1630 | calls, whereas those that must resort to the more capricious fcntl() | |
1631 | function lose the locks, making it harder to write servers. | |
1632 | ||
cb1a09d0 | 1633 | See also L<DB_File> for other flock() examples. |
a0d0e21e LW |
1634 | |
1635 | =item fork | |
1636 | ||
2b5ab1e7 TC |
1637 | Does a fork(2) system call to create a new process running the |
1638 | same program at the same point. It returns the child pid to the | |
1639 | parent process, C<0> to the child process, or C<undef> if the fork is | |
1640 | unsuccessful. File descriptors (and sometimes locks on those descriptors) | |
1641 | are shared, while everything else is copied. On most systems supporting | |
1642 | fork(), great care has gone into making it extremely efficient (for | |
1643 | example, using copy-on-write technology on data pages), making it the | |
1644 | dominant paradigm for multitasking over the last few decades. | |
5a964f20 | 1645 | |
45bc9206 | 1646 | All files opened for output are flushed before forking the child process. |
a0d0e21e | 1647 | |
19799a22 | 1648 | If you C<fork> without ever waiting on your children, you will |
2b5ab1e7 TC |
1649 | accumulate zombies. On some systems, you can avoid this by setting |
1650 | C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of | |
1651 | forking and reaping moribund children. | |
cb1a09d0 | 1652 | |
28757baa | 1653 | Note that if your forked child inherits system file descriptors like |
1654 | STDIN and STDOUT that are actually connected by a pipe or socket, even | |
2b5ab1e7 | 1655 | if you exit, then the remote server (such as, say, a CGI script or a |
19799a22 | 1656 | backgrounded job launched from a remote shell) won't think you're done. |
2b5ab1e7 | 1657 | You should reopen those to F</dev/null> if it's any issue. |
28757baa | 1658 | |
cb1a09d0 AD |
1659 | =item format |
1660 | ||
19799a22 | 1661 | Declare a picture format for use by the C<write> function. For |
cb1a09d0 AD |
1662 | example: |
1663 | ||
54310121 | 1664 | format Something = |
cb1a09d0 AD |
1665 | Test: @<<<<<<<< @||||| @>>>>> |
1666 | $str, $%, '$' . int($num) | |
1667 | . | |
1668 | ||
1669 | $str = "widget"; | |
184e9718 | 1670 | $num = $cost/$quantity; |
cb1a09d0 AD |
1671 | $~ = 'Something'; |
1672 | write; | |
1673 | ||
1674 | See L<perlform> for many details and examples. | |
1675 | ||
8903cb82 | 1676 | =item formline PICTURE,LIST |
a0d0e21e | 1677 | |
5a964f20 | 1678 | This is an internal function used by C<format>s, though you may call it, |
a0d0e21e LW |
1679 | too. It formats (see L<perlform>) a list of values according to the |
1680 | contents of PICTURE, placing the output into the format output | |
7660c0ab | 1681 | accumulator, C<$^A> (or C<$ACCUMULATOR> in English). |
19799a22 | 1682 | Eventually, when a C<write> is done, the contents of |
a0d0e21e | 1683 | C<$^A> are written to some filehandle, but you could also read C<$^A> |
7660c0ab | 1684 | yourself and then set C<$^A> back to C<"">. Note that a format typically |
19799a22 | 1685 | does one C<formline> per line of form, but the C<formline> function itself |
748a9306 | 1686 | doesn't care how many newlines are embedded in the PICTURE. This means |
4633a7c4 | 1687 | that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line. |
748a9306 LW |
1688 | You may therefore need to use multiple formlines to implement a single |
1689 | record format, just like the format compiler. | |
1690 | ||
19799a22 | 1691 | Be careful if you put double quotes around the picture, because an C<@> |
748a9306 | 1692 | character may be taken to mean the beginning of an array name. |
19799a22 | 1693 | C<formline> always returns true. See L<perlform> for other examples. |
a0d0e21e LW |
1694 | |
1695 | =item getc FILEHANDLE | |
1696 | ||
1697 | =item getc | |
1698 | ||
1699 | Returns the next character from the input file attached to FILEHANDLE, | |
2b5ab1e7 TC |
1700 | or the undefined value at end of file, or if there was an error. |
1701 | If FILEHANDLE is omitted, reads from STDIN. This is not particularly | |
1702 | efficient. However, it cannot be used by itself to fetch single | |
1703 | characters without waiting for the user to hit enter. For that, try | |
1704 | something more like: | |
4633a7c4 LW |
1705 | |
1706 | if ($BSD_STYLE) { | |
1707 | system "stty cbreak </dev/tty >/dev/tty 2>&1"; | |
1708 | } | |
1709 | else { | |
54310121 | 1710 | system "stty", '-icanon', 'eol', "\001"; |
4633a7c4 LW |
1711 | } |
1712 | ||
1713 | $key = getc(STDIN); | |
1714 | ||
1715 | if ($BSD_STYLE) { | |
1716 | system "stty -cbreak </dev/tty >/dev/tty 2>&1"; | |
1717 | } | |
1718 | else { | |
5f05dabc | 1719 | system "stty", 'icanon', 'eol', '^@'; # ASCII null |
4633a7c4 LW |
1720 | } |
1721 | print "\n"; | |
1722 | ||
54310121 | 1723 | Determination of whether $BSD_STYLE should be set |
1724 | is left as an exercise to the reader. | |
cb1a09d0 | 1725 | |
19799a22 | 1726 | The C<POSIX::getattr> function can do this more portably on |
2b5ab1e7 TC |
1727 | systems purporting POSIX compliance. See also the C<Term::ReadKey> |
1728 | module from your nearest CPAN site; details on CPAN can be found on | |
1729 | L<perlmodlib/CPAN>. | |
a0d0e21e LW |
1730 | |
1731 | =item getlogin | |
1732 | ||
5a964f20 TC |
1733 | Implements the C library function of the same name, which on most |
1734 | systems returns the current login from F</etc/utmp>, if any. If null, | |
19799a22 | 1735 | use C<getpwuid>. |
a0d0e21e | 1736 | |
f86702cc | 1737 | $login = getlogin || getpwuid($<) || "Kilroy"; |
a0d0e21e | 1738 | |
19799a22 GS |
1739 | Do not consider C<getlogin> for authentication: it is not as |
1740 | secure as C<getpwuid>. | |
4633a7c4 | 1741 | |
a0d0e21e LW |
1742 | =item getpeername SOCKET |
1743 | ||
1744 | Returns the packed sockaddr address of other end of the SOCKET connection. | |
1745 | ||
4633a7c4 LW |
1746 | use Socket; |
1747 | $hersockaddr = getpeername(SOCK); | |
19799a22 | 1748 | ($port, $iaddr) = sockaddr_in($hersockaddr); |
4633a7c4 LW |
1749 | $herhostname = gethostbyaddr($iaddr, AF_INET); |
1750 | $herstraddr = inet_ntoa($iaddr); | |
a0d0e21e LW |
1751 | |
1752 | =item getpgrp PID | |
1753 | ||
47e29363 | 1754 | Returns the current process group for the specified PID. Use |
7660c0ab | 1755 | a PID of C<0> to get the current process group for the |
4633a7c4 | 1756 | current process. Will raise an exception if used on a machine that |
f86cebdf | 1757 | doesn't implement getpgrp(2). If PID is omitted, returns process |
19799a22 | 1758 | group of current process. Note that the POSIX version of C<getpgrp> |
7660c0ab | 1759 | does not accept a PID argument, so only C<PID==0> is truly portable. |
a0d0e21e LW |
1760 | |
1761 | =item getppid | |
1762 | ||
1763 | Returns the process id of the parent process. | |
1764 | ||
1765 | =item getpriority WHICH,WHO | |
1766 | ||
4633a7c4 LW |
1767 | Returns the current priority for a process, a process group, or a user. |
1768 | (See L<getpriority(2)>.) Will raise a fatal exception if used on a | |
f86cebdf | 1769 | machine that doesn't implement getpriority(2). |
a0d0e21e LW |
1770 | |
1771 | =item getpwnam NAME | |
1772 | ||
1773 | =item getgrnam NAME | |
1774 | ||
1775 | =item gethostbyname NAME | |
1776 | ||
1777 | =item getnetbyname NAME | |
1778 | ||
1779 | =item getprotobyname NAME | |
1780 | ||
1781 | =item getpwuid UID | |
1782 | ||
1783 | =item getgrgid GID | |
1784 | ||
1785 | =item getservbyname NAME,PROTO | |
1786 | ||
1787 | =item gethostbyaddr ADDR,ADDRTYPE | |
1788 | ||
1789 | =item getnetbyaddr ADDR,ADDRTYPE | |
1790 | ||
1791 | =item getprotobynumber NUMBER | |
1792 | ||
1793 | =item getservbyport PORT,PROTO | |
1794 | ||
1795 | =item getpwent | |
1796 | ||
1797 | =item getgrent | |
1798 | ||
1799 | =item gethostent | |
1800 | ||
1801 | =item getnetent | |
1802 | ||
1803 | =item getprotoent | |
1804 | ||
1805 | =item getservent | |
1806 | ||
1807 | =item setpwent | |
1808 | ||
1809 | =item setgrent | |
1810 | ||
1811 | =item sethostent STAYOPEN | |
1812 | ||
1813 | =item setnetent STAYOPEN | |
1814 | ||
1815 | =item setprotoent STAYOPEN | |
1816 | ||
1817 | =item setservent STAYOPEN | |
1818 | ||
1819 | =item endpwent | |
1820 | ||
1821 | =item endgrent | |
1822 | ||
1823 | =item endhostent | |
1824 | ||
1825 | =item endnetent | |
1826 | ||
1827 | =item endprotoent | |
1828 | ||
1829 | =item endservent | |
1830 | ||
1831 | These routines perform the same functions as their counterparts in the | |
5a964f20 | 1832 | system library. In list context, the return values from the |
a0d0e21e LW |
1833 | various get routines are as follows: |
1834 | ||
1835 | ($name,$passwd,$uid,$gid, | |
6ee623d5 | 1836 | $quota,$comment,$gcos,$dir,$shell,$expire) = getpw* |
a0d0e21e LW |
1837 | ($name,$passwd,$gid,$members) = getgr* |
1838 | ($name,$aliases,$addrtype,$length,@addrs) = gethost* | |
1839 | ($name,$aliases,$addrtype,$net) = getnet* | |
1840 | ($name,$aliases,$proto) = getproto* | |
1841 | ($name,$aliases,$port,$proto) = getserv* | |
1842 | ||
1843 | (If the entry doesn't exist you get a null list.) | |
1844 | ||
5a964f20 | 1845 | In scalar context, you get the name, unless the function was a |
a0d0e21e LW |
1846 | lookup by name, in which case you get the other thing, whatever it is. |
1847 | (If the entry doesn't exist you get the undefined value.) For example: | |
1848 | ||
5a964f20 TC |
1849 | $uid = getpwnam($name); |
1850 | $name = getpwuid($num); | |
1851 | $name = getpwent(); | |
1852 | $gid = getgrnam($name); | |
1853 | $name = getgrgid($num; | |
1854 | $name = getgrent(); | |
1855 | #etc. | |
a0d0e21e | 1856 | |
19799a22 | 1857 | In I<getpw*()> the fields $quota, $comment, and $expire are |
2b5ab1e7 | 1858 | special cases in the sense that in many systems they are unsupported. |
19799a22 GS |
1859 | If the $quota is unsupported, it is an empty scalar. If it is |
1860 | supported, it usually encodes the disk quota. If the $comment | |
2b5ab1e7 TC |
1861 | field is unsupported, it is an empty scalar. If it is supported it |
1862 | usually encodes some administrative comment about the user. In some | |
19799a22 GS |
1863 | systems the $quota field may be $change or $age, fields that have |
1864 | to do with password aging. In some systems the $comment field may | |
1865 | be $class. The $expire field, if present, encodes the expiration | |
2b5ab1e7 TC |
1866 | period of the account or the password. For the availability and the |
1867 | exact meaning of these fields in your system, please consult your | |
1868 | getpwnam(3) documentation and your F<pwd.h> file. You can also find | |
19799a22 GS |
1869 | out from within Perl what your $quota and $comment fields mean |
1870 | and whether you have the $expire field by using the C<Config> module | |
2b5ab1e7 TC |
1871 | and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>, |
1872 | and C<d_pwexpire>. Shadow password files are only supported if your | |
1873 | vendor has implemented them in the intuitive fashion that calling the | |
1874 | regular C library routines gets the shadow versions if you're running | |
1875 | under privilege. Those that incorrectly implement a separate library | |
1876 | call are not supported. | |
6ee623d5 | 1877 | |
19799a22 | 1878 | The $members value returned by I<getgr*()> is a space separated list of |
a0d0e21e LW |
1879 | the login names of the members of the group. |
1880 | ||
1881 | For the I<gethost*()> functions, if the C<h_errno> variable is supported in | |
1882 | C, it will be returned to you via C<$?> if the function call fails. The | |
7660c0ab | 1883 | C<@addrs> value returned by a successful call is a list of the raw |
a0d0e21e LW |
1884 | addresses returned by the corresponding system library call. In the |
1885 | Internet domain, each address is four bytes long and you can unpack it | |
1886 | by saying something like: | |
1887 | ||
1888 | ($a,$b,$c,$d) = unpack('C4',$addr[0]); | |
1889 | ||
2b5ab1e7 TC |
1890 | The Socket library makes this slightly easier: |
1891 | ||
1892 | use Socket; | |
1893 | $iaddr = inet_aton("127.1"); # or whatever address | |
1894 | $name = gethostbyaddr($iaddr, AF_INET); | |
1895 | ||
1896 | # or going the other way | |
19799a22 | 1897 | $straddr = inet_ntoa($iaddr); |
2b5ab1e7 | 1898 | |
19799a22 GS |
1899 | If you get tired of remembering which element of the return list |
1900 | contains which return value, by-name interfaces are provided | |
1901 | in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>, | |
1902 | C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>, | |
1903 | and C<User::grent>. These override the normal built-ins, supplying | |
1904 | versions that return objects with the appropriate names | |
1905 | for each field. For example: | |
5a964f20 TC |
1906 | |
1907 | use File::stat; | |
1908 | use User::pwent; | |
1909 | $is_his = (stat($filename)->uid == pwent($whoever)->uid); | |
1910 | ||
1911 | Even though it looks like they're the same method calls (uid), | |
19799a22 GS |
1912 | they aren't, because a C<File::stat> object is different from |
1913 | a C<User::pwent> object. | |
5a964f20 | 1914 | |
a0d0e21e LW |
1915 | =item getsockname SOCKET |
1916 | ||
19799a22 GS |
1917 | Returns the packed sockaddr address of this end of the SOCKET connection, |
1918 | in case you don't know the address because you have several different | |
1919 | IPs that the connection might have come in on. | |
a0d0e21e | 1920 | |
4633a7c4 LW |
1921 | use Socket; |
1922 | $mysockaddr = getsockname(SOCK); | |
19799a22 GS |
1923 | ($port, $myaddr) = sockaddr_in($mysockaddr); |
1924 | printf "Connect to %s [%s]\n", | |
1925 | scalar gethostbyaddr($myaddr, AF_INET), | |
1926 | inet_ntoa($myaddr); | |
a0d0e21e LW |
1927 | |
1928 | =item getsockopt SOCKET,LEVEL,OPTNAME | |
1929 | ||
5a964f20 | 1930 | Returns the socket option requested, or undef if there is an error. |
a0d0e21e LW |
1931 | |
1932 | =item glob EXPR | |
1933 | ||
0a753a76 | 1934 | =item glob |
1935 | ||
2b5ab1e7 TC |
1936 | Returns the value of EXPR with filename expansions such as the |
1937 | standard Unix shell F</bin/csh> would do. This is the internal function | |
61eff3bc JH |
1938 | implementing the C<< <*.c> >> operator, but you can use it directly. |
1939 | If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is | |
2b5ab1e7 | 1940 | discussed in more detail in L<perlop/"I/O Operators">. |
a0d0e21e | 1941 | |
3a4b19e4 GS |
1942 | Beginning with v5.6.0, this operator is implemented using the standard |
1943 | C<File::Glob> extension. See L<File::Glob> for details. | |
1944 | ||
a0d0e21e LW |
1945 | =item gmtime EXPR |
1946 | ||
19799a22 | 1947 | Converts a time as returned by the time function to a 9-element list |
54310121 | 1948 | with the time localized for the standard Greenwich time zone. |
4633a7c4 | 1949 | Typically used as follows: |
a0d0e21e | 1950 | |
54310121 | 1951 | # 0 1 2 3 4 5 6 7 8 |
a0d0e21e LW |
1952 | ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = |
1953 | gmtime(time); | |
1954 | ||
19799a22 GS |
1955 | All list elements are numeric, and come straight out of a struct tm. |
1956 | In particular this means that $mon has the range C<0..11> and $wday | |
1957 | has the range C<0..6> with sunday as day C<0>. Also, $year is the | |
1958 | number of years since 1900, that is, $year is C<123> in year 2023, | |
2b5ab1e7 TC |
1959 | I<not> simply the last two digits of the year. If you assume it is, |
1960 | then you create non-Y2K-compliant programs--and you wouldn't want to do | |
1961 | that, would you? | |
2f9daede | 1962 | |
abd75f24 GS |
1963 | The proper way to get a complete 4-digit year is simply: |
1964 | ||
1965 | $year += 1900; | |
1966 | ||
1967 | And to get the last two digits of the year (e.g., '01' in 2001) do: | |
1968 | ||
1969 | $year = sprintf("%02d", $year % 100); | |
1970 | ||
2f9daede | 1971 | If EXPR is omitted, does C<gmtime(time())>. |
a0d0e21e | 1972 | |
f86cebdf | 1973 | In scalar context, returns the ctime(3) value: |
0a753a76 | 1974 | |
1975 | $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994" | |
1976 | ||
19799a22 | 1977 | Also see the C<timegm> function provided by the C<Time::Local> module, |
f86cebdf | 1978 | and the strftime(3) function available via the POSIX module. |
7660c0ab | 1979 | |
2b5ab1e7 TC |
1980 | This scalar value is B<not> locale dependent (see L<perllocale>), but |
1981 | is instead a Perl builtin. Also see the C<Time::Local> module, and the | |
1982 | strftime(3) and mktime(3) functions available via the POSIX module. To | |
7660c0ab A |
1983 | get somewhat similar but locale dependent date strings, set up your |
1984 | locale environment variables appropriately (please see L<perllocale>) | |
1985 | and try for example: | |
1986 | ||
1987 | use POSIX qw(strftime); | |
2b5ab1e7 | 1988 | $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime; |
7660c0ab | 1989 | |
2b5ab1e7 TC |
1990 | Note that the C<%a> and C<%b> escapes, which represent the short forms |
1991 | of the day of the week and the month of the year, may not necessarily | |
1992 | be three characters wide in all locales. | |
0a753a76 | 1993 | |
a0d0e21e LW |
1994 | =item goto LABEL |
1995 | ||
748a9306 LW |
1996 | =item goto EXPR |
1997 | ||
a0d0e21e LW |
1998 | =item goto &NAME |
1999 | ||
7660c0ab | 2000 | The C<goto-LABEL> form finds the statement labeled with LABEL and resumes |
a0d0e21e | 2001 | execution there. It may not be used to go into any construct that |
7660c0ab | 2002 | requires initialization, such as a subroutine or a C<foreach> loop. It |
0a753a76 | 2003 | also can't be used to go into a construct that is optimized away, |
19799a22 | 2004 | or to get out of a block or subroutine given to C<sort>. |
0a753a76 | 2005 | It can be used to go almost anywhere else within the dynamic scope, |
a0d0e21e | 2006 | including out of subroutines, but it's usually better to use some other |
19799a22 | 2007 | construct such as C<last> or C<die>. The author of Perl has never felt the |
7660c0ab | 2008 | need to use this form of C<goto> (in Perl, that is--C is another matter). |
a0d0e21e | 2009 | |
7660c0ab A |
2010 | The C<goto-EXPR> form expects a label name, whose scope will be resolved |
2011 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
748a9306 LW |
2012 | necessarily recommended if you're optimizing for maintainability: |
2013 | ||
2014 | goto ("FOO", "BAR", "GLARCH")[$i]; | |
2015 | ||
6cb9131c GS |
2016 | The C<goto-&NAME> form is quite different from the other forms of C<goto>. |
2017 | In fact, it isn't a goto in the normal sense at all, and doesn't have | |
2018 | the stigma associated with other gotos. Instead, it | |
2019 | substitutes a call to the named subroutine for the currently running | |
2020 | subroutine. This is used by C<AUTOLOAD> subroutines that wish to load | |
2021 | another subroutine and then pretend that the other subroutine had been | |
2022 | called in the first place (except that any modifications to C<@_> | |
2023 | in the current subroutine are propagated to the other subroutine.) | |
2024 | After the C<goto>, not even C<caller> will be able to tell that this | |
2025 | routine was called first. | |
2026 | ||
2027 | NAME needn't be the name of a subroutine; it can be a scalar variable | |
2028 | containing a code reference, or a block which evaluates to a code | |
2029 | reference. | |
a0d0e21e LW |
2030 | |
2031 | =item grep BLOCK LIST | |
2032 | ||
2033 | =item grep EXPR,LIST | |
2034 | ||
2b5ab1e7 TC |
2035 | This is similar in spirit to, but not the same as, grep(1) and its |
2036 | relatives. In particular, it is not limited to using regular expressions. | |
2f9daede | 2037 | |
a0d0e21e | 2038 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting |
7660c0ab | 2039 | C<$_> to each element) and returns the list value consisting of those |
19799a22 GS |
2040 | elements for which the expression evaluated to true. In scalar |
2041 | context, returns the number of times the expression was true. | |
a0d0e21e LW |
2042 | |
2043 | @foo = grep(!/^#/, @bar); # weed out comments | |
2044 | ||
2045 | or equivalently, | |
2046 | ||
2047 | @foo = grep {!/^#/} @bar; # weed out comments | |
2048 | ||
2b5ab1e7 TC |
2049 | Note that, because C<$_> is a reference into the list value, it can |
2050 | be used to modify the elements of the array. While this is useful and | |
2051 | supported, it can cause bizarre results if the LIST is not a named array. | |
2052 | Similarly, grep returns aliases into the original list, much as a for | |
2053 | loop's index variable aliases the list elements. That is, modifying an | |
19799a22 GS |
2054 | element of a list returned by grep (for example, in a C<foreach>, C<map> |
2055 | or another C<grep>) actually modifies the element in the original list. | |
2b5ab1e7 | 2056 | This is usually something to be avoided when writing clear code. |
a0d0e21e | 2057 | |
19799a22 | 2058 | See also L</map> for a list composed of the results of the BLOCK or EXPR. |
38325410 | 2059 | |
a0d0e21e LW |
2060 | =item hex EXPR |
2061 | ||
54310121 | 2062 | =item hex |
bbce6d69 | 2063 | |
2b5ab1e7 TC |
2064 | Interprets EXPR as a hex string and returns the corresponding value. |
2065 | (To convert strings that might start with either 0, 0x, or 0b, see | |
2066 | L</oct>.) If EXPR is omitted, uses C<$_>. | |
2f9daede TP |
2067 | |
2068 | print hex '0xAf'; # prints '175' | |
2069 | print hex 'aF'; # same | |
a0d0e21e | 2070 | |
19799a22 | 2071 | Hex strings may only represent integers. Strings that would cause |
c6edd1b7 | 2072 | integer overflow trigger a warning. |
19799a22 | 2073 | |
a0d0e21e LW |
2074 | =item import |
2075 | ||
19799a22 | 2076 | There is no builtin C<import> function. It is just an ordinary |
4633a7c4 | 2077 | method (subroutine) defined (or inherited) by modules that wish to export |
19799a22 | 2078 | names to another module. The C<use> function calls the C<import> method |
54310121 | 2079 | for the package used. See also L</use()>, L<perlmod>, and L<Exporter>. |
a0d0e21e LW |
2080 | |
2081 | =item index STR,SUBSTR,POSITION | |
2082 | ||
2083 | =item index STR,SUBSTR | |
2084 | ||
2b5ab1e7 TC |
2085 | The index function searches for one string within another, but without |
2086 | the wildcard-like behavior of a full regular-expression pattern match. | |
2087 | It returns the position of the first occurrence of SUBSTR in STR at | |
2088 | or after POSITION. If POSITION is omitted, starts searching from the | |
2089 | beginning of the string. The return value is based at C<0> (or whatever | |
2090 | you've set the C<$[> variable to--but don't do that). If the substring | |
2091 | is not found, returns one less than the base, ordinarily C<-1>. | |
a0d0e21e LW |
2092 | |
2093 | =item int EXPR | |
2094 | ||
54310121 | 2095 | =item int |
bbce6d69 | 2096 | |
7660c0ab | 2097 | Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>. |
2b5ab1e7 TC |
2098 | You should not use this function for rounding: one because it truncates |
2099 | towards C<0>, and two because machine representations of floating point | |
2100 | numbers can sometimes produce counterintuitive results. For example, | |
2101 | C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's | |
2102 | because it's really more like -268.99999999999994315658 instead. Usually, | |
19799a22 | 2103 | the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil> |
2b5ab1e7 | 2104 | functions will serve you better than will int(). |
a0d0e21e LW |
2105 | |
2106 | =item ioctl FILEHANDLE,FUNCTION,SCALAR | |
2107 | ||
2b5ab1e7 | 2108 | Implements the ioctl(2) function. You'll probably first have to say |
a0d0e21e | 2109 | |
4633a7c4 | 2110 | require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph |
a0d0e21e | 2111 | |
2b5ab1e7 | 2112 | to get the correct function definitions. If F<ioctl.ph> doesn't |
a0d0e21e | 2113 | exist or doesn't have the correct definitions you'll have to roll your |
61eff3bc | 2114 | own, based on your C header files such as F<< <sys/ioctl.h> >>. |
5a964f20 | 2115 | (There is a Perl script called B<h2ph> that comes with the Perl kit that |
54310121 | 2116 | may help you in this, but it's nontrivial.) SCALAR will be read and/or |
4633a7c4 | 2117 | written depending on the FUNCTION--a pointer to the string value of SCALAR |
19799a22 | 2118 | will be passed as the third argument of the actual C<ioctl> call. (If SCALAR |
4633a7c4 LW |
2119 | has no string value but does have a numeric value, that value will be |
2120 | passed rather than a pointer to the string value. To guarantee this to be | |
19799a22 GS |
2121 | true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack> |
2122 | functions may be needed to manipulate the values of structures used by | |
2123 | C<ioctl>. | |
a0d0e21e | 2124 | |
19799a22 | 2125 | The return value of C<ioctl> (and C<fcntl>) is as follows: |
a0d0e21e LW |
2126 | |
2127 | if OS returns: then Perl returns: | |
2128 | -1 undefined value | |
2129 | 0 string "0 but true" | |
2130 | anything else that number | |
2131 | ||
19799a22 | 2132 | Thus Perl returns true on success and false on failure, yet you can |
a0d0e21e LW |
2133 | still easily determine the actual value returned by the operating |
2134 | system: | |
2135 | ||
2b5ab1e7 | 2136 | $retval = ioctl(...) || -1; |
a0d0e21e LW |
2137 | printf "System returned %d\n", $retval; |
2138 | ||
c2611fb3 | 2139 | The special string "C<0> but true" is exempt from B<-w> complaints |
5a964f20 TC |
2140 | about improper numeric conversions. |
2141 | ||
19799a22 GS |
2142 | Here's an example of setting a filehandle named C<REMOTE> to be |
2143 | non-blocking at the system level. You'll have to negotiate C<$|> | |
2144 | on your own, though. | |
2145 | ||
2146 | use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK); | |
2147 | ||
2148 | $flags = fcntl(REMOTE, F_GETFL, 0) | |
2149 | or die "Can't get flags for the socket: $!\n"; | |
2150 | ||
2151 | $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK) | |
2152 | or die "Can't set flags for the socket: $!\n"; | |
2153 | ||
a0d0e21e LW |
2154 | =item join EXPR,LIST |
2155 | ||
2b5ab1e7 TC |
2156 | Joins the separate strings of LIST into a single string with fields |
2157 | separated by the value of EXPR, and returns that new string. Example: | |
a0d0e21e | 2158 | |
2b5ab1e7 | 2159 | $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); |
a0d0e21e | 2160 | |
eb6e2d6f GS |
2161 | Beware that unlike C<split>, C<join> doesn't take a pattern as its |
2162 | first argument. Compare L</split>. | |
a0d0e21e | 2163 | |
aa689395 | 2164 | =item keys HASH |
2165 | ||
19799a22 | 2166 | Returns a list consisting of all the keys of the named hash. (In |
1d2dff63 | 2167 | scalar context, returns the number of keys.) The keys are returned in |
ab192400 GS |
2168 | an apparently random order. The actual random order is subject to |
2169 | change in future versions of perl, but it is guaranteed to be the same | |
19799a22 | 2170 | order as either the C<values> or C<each> function produces (given |
ab192400 GS |
2171 | that the hash has not been modified). As a side effect, it resets |
2172 | HASH's iterator. | |
a0d0e21e | 2173 | |
aa689395 | 2174 | Here is yet another way to print your environment: |
a0d0e21e LW |
2175 | |
2176 | @keys = keys %ENV; | |
2177 | @values = values %ENV; | |
19799a22 | 2178 | while (@keys) { |
a0d0e21e LW |
2179 | print pop(@keys), '=', pop(@values), "\n"; |
2180 | } | |
2181 | ||
2182 | or how about sorted by key: | |
2183 | ||
2184 | foreach $key (sort(keys %ENV)) { | |
2185 | print $key, '=', $ENV{$key}, "\n"; | |
2186 | } | |
2187 | ||
19799a22 | 2188 | To sort a hash by value, you'll need to use a C<sort> function. |
aa689395 | 2189 | Here's a descending numeric sort of a hash by its values: |
4633a7c4 | 2190 | |
5a964f20 | 2191 | foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { |
4633a7c4 LW |
2192 | printf "%4d %s\n", $hash{$key}, $key; |
2193 | } | |
2194 | ||
19799a22 | 2195 | As an lvalue C<keys> allows you to increase the number of hash buckets |
aa689395 | 2196 | allocated for the given hash. This can gain you a measure of efficiency if |
2197 | you know the hash is going to get big. (This is similar to pre-extending | |
2198 | an array by assigning a larger number to $#array.) If you say | |
55497cff | 2199 | |
2200 | keys %hash = 200; | |
2201 | ||
ab192400 GS |
2202 | then C<%hash> will have at least 200 buckets allocated for it--256 of them, |
2203 | in fact, since it rounds up to the next power of two. These | |
55497cff | 2204 | buckets will be retained even if you do C<%hash = ()>, use C<undef |
2205 | %hash> if you want to free the storage while C<%hash> is still in scope. | |
2206 | You can't shrink the number of buckets allocated for the hash using | |
19799a22 | 2207 | C<keys> in this way (but you needn't worry about doing this by accident, |
55497cff | 2208 | as trying has no effect). |
2209 | ||
19799a22 | 2210 | See also C<each>, C<values> and C<sort>. |
ab192400 | 2211 | |
b350dd2f | 2212 | =item kill SIGNAL, LIST |
a0d0e21e | 2213 | |
b350dd2f | 2214 | Sends a signal to a list of processes. Returns the number of |
517db077 GS |
2215 | processes successfully signaled (which is not necessarily the |
2216 | same as the number actually killed). | |
a0d0e21e LW |
2217 | |
2218 | $cnt = kill 1, $child1, $child2; | |
2219 | kill 9, @goners; | |
2220 | ||
b350dd2f GS |
2221 | If SIGNAL is zero, no signal is sent to the process. This is a |
2222 | useful way to check that the process is alive and hasn't changed | |
2223 | its UID. See L<perlport> for notes on the portability of this | |
2224 | construct. | |
2225 | ||
2226 | Unlike in the shell, if SIGNAL is negative, it kills | |
4633a7c4 LW |
2227 | process groups instead of processes. (On System V, a negative I<PROCESS> |
2228 | number will also kill process groups, but that's not portable.) That | |
2229 | means you usually want to use positive not negative signals. You may also | |
da0045b7 | 2230 | use a signal name in quotes. See L<perlipc/"Signals"> for details. |
a0d0e21e LW |
2231 | |
2232 | =item last LABEL | |
2233 | ||
2234 | =item last | |
2235 | ||
2236 | The C<last> command is like the C<break> statement in C (as used in | |
2237 | loops); it immediately exits the loop in question. If the LABEL is | |
2238 | omitted, the command refers to the innermost enclosing loop. The | |
2239 | C<continue> block, if any, is not executed: | |
2240 | ||
4633a7c4 LW |
2241 | LINE: while (<STDIN>) { |
2242 | last LINE if /^$/; # exit when done with header | |
5a964f20 | 2243 | #... |
a0d0e21e LW |
2244 | } |
2245 | ||
4968c1e4 | 2246 | C<last> cannot be used to exit a block which returns a value such as |
2b5ab1e7 TC |
2247 | C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit |
2248 | a grep() or map() operation. | |
4968c1e4 | 2249 | |
6c1372ed GS |
2250 | Note that a block by itself is semantically identical to a loop |
2251 | that executes once. Thus C<last> can be used to effect an early | |
2252 | exit out of such a block. | |
2253 | ||
98293880 JH |
2254 | See also L</continue> for an illustration of how C<last>, C<next>, and |
2255 | C<redo> work. | |
1d2dff63 | 2256 | |
a0d0e21e LW |
2257 | =item lc EXPR |
2258 | ||
54310121 | 2259 | =item lc |
bbce6d69 | 2260 | |
a0d0e21e | 2261 | Returns an lowercased version of EXPR. This is the internal function |
7660c0ab | 2262 | implementing the C<\L> escape in double-quoted strings. |
19799a22 GS |
2263 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale> |
2264 | and L<utf8>. | |
a0d0e21e | 2265 | |
7660c0ab | 2266 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 2267 | |
a0d0e21e LW |
2268 | =item lcfirst EXPR |
2269 | ||
54310121 | 2270 | =item lcfirst |
bbce6d69 | 2271 | |
a0d0e21e | 2272 | Returns the value of EXPR with the first character lowercased. This is |
7660c0ab | 2273 | the internal function implementing the C<\l> escape in double-quoted strings. |
a0ed51b3 | 2274 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. |
a0d0e21e | 2275 | |
7660c0ab | 2276 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 2277 | |
a0d0e21e LW |
2278 | =item length EXPR |
2279 | ||
54310121 | 2280 | =item length |
bbce6d69 | 2281 | |
a0ed51b3 | 2282 | Returns the length in characters of the value of EXPR. If EXPR is |
2b5ab1e7 TC |
2283 | omitted, returns length of C<$_>. Note that this cannot be used on |
2284 | an entire array or hash to find out how many elements these have. | |
2285 | For that, use C<scalar @array> and C<scalar keys %hash> respectively. | |
a0d0e21e LW |
2286 | |
2287 | =item link OLDFILE,NEWFILE | |
2288 | ||
19799a22 GS |
2289 | Creates a new filename linked to the old filename. Returns true for |
2290 | success, false otherwise. | |
a0d0e21e LW |
2291 | |
2292 | =item listen SOCKET,QUEUESIZE | |
2293 | ||
19799a22 GS |
2294 | Does the same thing that the listen system call does. Returns true if |
2295 | it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">. | |
a0d0e21e LW |
2296 | |
2297 | =item local EXPR | |
2298 | ||
19799a22 | 2299 | You really probably want to be using C<my> instead, because C<local> isn't |
2b5ab1e7 TC |
2300 | what most people think of as "local". See L<perlsub/"Private Variables |
2301 | via my()"> for details. | |
2302 | ||
5a964f20 TC |
2303 | A local modifies the listed variables to be local to the enclosing |
2304 | block, file, or eval. If more than one value is listed, the list must | |
2305 | be placed in parentheses. See L<perlsub/"Temporary Values via local()"> | |
2306 | for details, including issues with tied arrays and hashes. | |
a0d0e21e | 2307 | |
a0d0e21e LW |
2308 | =item localtime EXPR |
2309 | ||
19799a22 | 2310 | Converts a time as returned by the time function to a 9-element list |
5f05dabc | 2311 | with the time analyzed for the local time zone. Typically used as |
a0d0e21e LW |
2312 | follows: |
2313 | ||
54310121 | 2314 | # 0 1 2 3 4 5 6 7 8 |
a0d0e21e LW |
2315 | ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = |
2316 | localtime(time); | |
2317 | ||
19799a22 GS |
2318 | All list elements are numeric, and come straight out of a struct tm. |
2319 | In particular this means that $mon has the range C<0..11> and $wday | |
2320 | has the range C<0..6> with sunday as day C<0>. Also, $year is the | |
2321 | number of years since 1900, that is, $year is C<123> in year 2023, | |
2b5ab1e7 TC |
2322 | and I<not> simply the last two digits of the year. If you assume it is, |
2323 | then you create non-Y2K-compliant programs--and you wouldn't want to do | |
2324 | that, would you? | |
54310121 | 2325 | |
abd75f24 GS |
2326 | The proper way to get a complete 4-digit year is simply: |
2327 | ||
2328 | $year += 1900; | |
2329 | ||
2330 | And to get the last two digits of the year (e.g., '01' in 2001) do: | |
2331 | ||
2332 | $year = sprintf("%02d", $year % 100); | |
2333 | ||
54310121 | 2334 | If EXPR is omitted, uses the current time (C<localtime(time)>). |
a0d0e21e | 2335 | |
f86cebdf | 2336 | In scalar context, returns the ctime(3) value: |
a0d0e21e | 2337 | |
5f05dabc | 2338 | $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994" |
a0d0e21e | 2339 | |
a3cb178b | 2340 | This scalar value is B<not> locale dependent, see L<perllocale>, but |
68f8bed4 JH |
2341 | instead a Perl builtin. Also see the C<Time::Local> module |
2342 | (to convert the second, minutes, hours, ... back to seconds since the | |
2343 | stroke of midnight the 1st of January 1970, the value returned by | |
ca6e1c26 | 2344 | time()), and the strftime(3) and mktime(3) functions available via the |
68f8bed4 JH |
2345 | POSIX module. To get somewhat similar but locale dependent date |
2346 | strings, set up your locale environment variables appropriately | |
2347 | (please see L<perllocale>) and try for example: | |
a3cb178b | 2348 | |
5a964f20 | 2349 | use POSIX qw(strftime); |
2b5ab1e7 | 2350 | $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; |
a3cb178b GS |
2351 | |
2352 | Note that the C<%a> and C<%b>, the short forms of the day of the week | |
2353 | and the month of the year, may not necessarily be three characters wide. | |
a0d0e21e | 2354 | |
19799a22 GS |
2355 | =item lock |
2356 | ||
2357 | lock I<THING> | |
2358 | ||
2359 | This function places an advisory lock on a variable, subroutine, | |
2360 | or referenced object contained in I<THING> until the lock goes out | |
2361 | of scope. This is a built-in function only if your version of Perl | |
2362 | was built with threading enabled, and if you've said C<use Threads>. | |
2363 | Otherwise a user-defined function by this name will be called. See | |
2364 | L<Thread>. | |
2365 | ||
a0d0e21e LW |
2366 | =item log EXPR |
2367 | ||
54310121 | 2368 | =item log |
bbce6d69 | 2369 | |
2b5ab1e7 TC |
2370 | Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, |
2371 | returns log of C<$_>. To get the log of another base, use basic algebra: | |
19799a22 | 2372 | The base-N log of a number is equal to the natural log of that number |
2b5ab1e7 TC |
2373 | divided by the natural log of N. For example: |
2374 | ||
2375 | sub log10 { | |
2376 | my $n = shift; | |
2377 | return log($n)/log(10); | |
2378 | } | |
2379 | ||
2380 | See also L</exp> for the inverse operation. | |
a0d0e21e LW |
2381 | |
2382 | =item lstat FILEHANDLE | |
2383 | ||
2384 | =item lstat EXPR | |
2385 | ||
54310121 | 2386 | =item lstat |
bbce6d69 | 2387 | |
19799a22 | 2388 | Does the same thing as the C<stat> function (including setting the |
5a964f20 TC |
2389 | special C<_> filehandle) but stats a symbolic link instead of the file |
2390 | the symbolic link points to. If symbolic links are unimplemented on | |
19799a22 | 2391 | your system, a normal C<stat> is done. |
a0d0e21e | 2392 | |
7660c0ab | 2393 | If EXPR is omitted, stats C<$_>. |
bbce6d69 | 2394 | |
a0d0e21e LW |
2395 | =item m// |
2396 | ||
2397 | The match operator. See L<perlop>. | |
2398 | ||
2399 | =item map BLOCK LIST | |
2400 | ||
2401 | =item map EXPR,LIST | |
2402 | ||
19799a22 GS |
2403 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting |
2404 | C<$_> to each element) and returns the list value composed of the | |
2405 | results of each such evaluation. In scalar context, returns the | |
2406 | total number of elements so generated. Evaluates BLOCK or EXPR in | |
2407 | list context, so each element of LIST may produce zero, one, or | |
2408 | more elements in the returned value. | |
dd99ebda | 2409 | |
a0d0e21e LW |
2410 | @chars = map(chr, @nums); |
2411 | ||
2412 | translates a list of numbers to the corresponding characters. And | |
2413 | ||
4633a7c4 | 2414 | %hash = map { getkey($_) => $_ } @array; |
a0d0e21e LW |
2415 | |
2416 | is just a funny way to write | |
2417 | ||
2418 | %hash = (); | |
2419 | foreach $_ (@array) { | |
4633a7c4 | 2420 | $hash{getkey($_)} = $_; |
a0d0e21e LW |
2421 | } |
2422 | ||
2b5ab1e7 TC |
2423 | Note that, because C<$_> is a reference into the list value, it can |
2424 | be used to modify the elements of the array. While this is useful and | |
2425 | supported, it can cause bizarre results if the LIST is not a named array. | |
2426 | Using a regular C<foreach> loop for this purpose would be clearer in | |
2427 | most cases. See also L</grep> for an array composed of those items of | |
2428 | the original list for which the BLOCK or EXPR evaluates to true. | |
fb73857a | 2429 | |
19799a22 | 2430 | =item mkdir FILENAME,MASK |
a0d0e21e | 2431 | |
5a211162 GS |
2432 | =item mkdir FILENAME |
2433 | ||
0591cd52 | 2434 | Creates the directory specified by FILENAME, with permissions |
19799a22 GS |
2435 | specified by MASK (as modified by C<umask>). If it succeeds it |
2436 | returns true, otherwise it returns false and sets C<$!> (errno). | |
5a211162 | 2437 | If omitted, MASK defaults to 0777. |
0591cd52 | 2438 | |
19799a22 | 2439 | In general, it is better to create directories with permissive MASK, |
0591cd52 | 2440 | and let the user modify that with their C<umask>, than it is to supply |
19799a22 | 2441 | a restrictive MASK and give the user no way to be more permissive. |
0591cd52 NT |
2442 | The exceptions to this rule are when the file or directory should be |
2443 | kept private (mail files, for instance). The perlfunc(1) entry on | |
19799a22 | 2444 | C<umask> discusses the choice of MASK in more detail. |
a0d0e21e LW |
2445 | |
2446 | =item msgctl ID,CMD,ARG | |
2447 | ||
f86cebdf | 2448 | Calls the System V IPC function msgctl(2). You'll probably have to say |
0ade1984 JH |
2449 | |
2450 | use IPC::SysV; | |
2451 | ||
7660c0ab A |
2452 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, |
2453 | then ARG must be a variable which will hold the returned C<msqid_ds> | |
951ba7fe GS |
2454 | structure. Returns like C<ioctl>: the undefined value for error, |
2455 | C<"0 but true"> for zero, or the actual return value otherwise. See also | |
19799a22 | 2456 | C<IPC::SysV> and C<IPC::Semaphore> documentation. |
a0d0e21e LW |
2457 | |
2458 | =item msgget KEY,FLAGS | |
2459 | ||
f86cebdf | 2460 | Calls the System V IPC function msgget(2). Returns the message queue |
7660c0ab | 2461 | id, or the undefined value if there is an error. See also C<IPC::SysV> |
19799a22 | 2462 | and C<IPC::Msg> documentation. |
a0d0e21e LW |
2463 | |
2464 | =item msgsnd ID,MSG,FLAGS | |
2465 | ||
2466 | Calls the System V IPC function msgsnd to send the message MSG to the | |
e4038a1f MS |
2467 | message queue ID. MSG must begin with the native long integer message |
2468 | type, which may be created with C<pack("l!", $type)>. Returns true if | |
2469 | successful, or false if there is an error. See also C<IPC::SysV> and | |
2470 | C<IPC::SysV::Msg> documentation. | |
a0d0e21e LW |
2471 | |
2472 | =item msgrcv ID,VAR,SIZE,TYPE,FLAGS | |
2473 | ||
2474 | Calls the System V IPC function msgrcv to receive a message from | |
2475 | message queue ID into variable VAR with a maximum message size of | |
0ade1984 JH |
2476 | SIZE. Note that if a message is received, the message type will be |
2477 | the first thing in VAR, and the maximum length of VAR is SIZE plus the | |
19799a22 | 2478 | size of the message type. Returns true if successful, or false if |
7660c0ab | 2479 | there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation. |
a0d0e21e LW |
2480 | |
2481 | =item my EXPR | |
2482 | ||
09bef843 SB |
2483 | =item my EXPR : ATTRIBUTES |
2484 | ||
19799a22 GS |
2485 | A C<my> declares the listed variables to be local (lexically) to the |
2486 | enclosing block, file, or C<eval>. If | |
5f05dabc | 2487 | more than one value is listed, the list must be placed in parentheses. See |
cb1a09d0 | 2488 | L<perlsub/"Private Variables via my()"> for details. |
4633a7c4 | 2489 | |
a0d0e21e LW |
2490 | =item next LABEL |
2491 | ||
2492 | =item next | |
2493 | ||
2494 | The C<next> command is like the C<continue> statement in C; it starts | |
2495 | the next iteration of the loop: | |
2496 | ||
4633a7c4 LW |
2497 | LINE: while (<STDIN>) { |
2498 | next LINE if /^#/; # discard comments | |
5a964f20 | 2499 | #... |
a0d0e21e LW |
2500 | } |
2501 | ||
2502 | Note that if there were a C<continue> block on the above, it would get | |
2503 | executed even on discarded lines. If the LABEL is omitted, the command | |
2504 | refers to the innermost enclosing loop. | |
2505 | ||
4968c1e4 | 2506 | C<next> cannot be used to exit a block which returns a value such as |
2b5ab1e7 TC |
2507 | C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit |
2508 | a grep() or map() operation. | |
4968c1e4 | 2509 | |
6c1372ed GS |
2510 | Note that a block by itself is semantically identical to a loop |
2511 | that executes once. Thus C<next> will exit such a block early. | |
2512 | ||
98293880 JH |
2513 | See also L</continue> for an illustration of how C<last>, C<next>, and |
2514 | C<redo> work. | |
1d2dff63 | 2515 | |
a0d0e21e LW |
2516 | =item no Module LIST |
2517 | ||
7660c0ab | 2518 | See the L</use> function, which C<no> is the opposite of. |
a0d0e21e LW |
2519 | |
2520 | =item oct EXPR | |
2521 | ||
54310121 | 2522 | =item oct |
bbce6d69 | 2523 | |
4633a7c4 | 2524 | Interprets EXPR as an octal string and returns the corresponding |
4f19785b WSI |
2525 | value. (If EXPR happens to start off with C<0x>, interprets it as a |
2526 | hex string. If EXPR starts off with C<0b>, it is interpreted as a | |
2527 | binary string.) The following will handle decimal, binary, octal, and | |
4633a7c4 | 2528 | hex in the standard Perl or C notation: |
a0d0e21e LW |
2529 | |
2530 | $val = oct($val) if $val =~ /^0/; | |
2531 | ||
19799a22 GS |
2532 | If EXPR is omitted, uses C<$_>. To go the other way (produce a number |
2533 | in octal), use sprintf() or printf(): | |
2534 | ||
2535 | $perms = (stat("filename"))[2] & 07777; | |
2536 | $oct_perms = sprintf "%lo", $perms; | |
2537 | ||
2538 | The oct() function is commonly used when a string such as C<644> needs | |
2539 | to be converted into a file mode, for example. (Although perl will | |
2540 | automatically convert strings into numbers as needed, this automatic | |
2541 | conversion assumes base 10.) | |
a0d0e21e | 2542 | |
1c1fc3ea | 2543 | =item open FILEHANDLE,MODE,LIST |
6170680b | 2544 | |
a0d0e21e LW |
2545 | =item open FILEHANDLE,EXPR |
2546 | ||
2547 | =item open FILEHANDLE | |
2548 | ||
2549 | Opens the file whose filename is given by EXPR, and associates it with | |
5f05dabc | 2550 | FILEHANDLE. If FILEHANDLE is an expression, its value is used as the |
2551 | name of the real filehandle wanted. If EXPR is omitted, the scalar | |
2552 | variable of the same name as the FILEHANDLE contains the filename. | |
19799a22 GS |
2553 | (Note that lexical variables--those declared with C<my>--will not work |
2554 | for this purpose; so if you're using C<my>, specify EXPR in your call | |
2b5ab1e7 TC |
2555 | to open.) See L<perlopentut> for a kinder, gentler explanation of opening |
2556 | files. | |
5f05dabc | 2557 | |
61eff3bc JH |
2558 | If MODE is C<< '<' >> or nothing, the file is opened for input. |
2559 | If MODE is C<< '>' >>, the file is truncated and opened for | |
2560 | output, being created if necessary. If MODE is C<<< '>>' >>>, | |
fbb426e4 | 2561 | the file is opened for appending, again being created if necessary. |
61eff3bc JH |
2562 | You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that |
2563 | you want both read and write access to the file; thus C<< '+<' >> is almost | |
2564 | always preferred for read/write updates--the C<< '+>' >> mode would clobber the | |
5a964f20 TC |
2565 | file first. You can't usually use either read-write mode for updating |
2566 | textfiles, since they have variable length records. See the B<-i> | |
0591cd52 NT |
2567 | switch in L<perlrun> for a better approach. The file is created with |
2568 | permissions of C<0666> modified by the process' C<umask> value. | |
5a964f20 | 2569 | |
61eff3bc JH |
2570 | These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, |
2571 | C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>. | |
5f05dabc | 2572 | |
6170680b IZ |
2573 | In the 2-arguments (and 1-argument) form of the call the mode and |
2574 | filename should be concatenated (in this order), possibly separated by | |
61eff3bc | 2575 | spaces. It is possible to omit the mode if the mode is C<< '<' >>. |
6170680b | 2576 | |
7660c0ab | 2577 | If the filename begins with C<'|'>, the filename is interpreted as a |
5a964f20 | 2578 | command to which output is to be piped, and if the filename ends with a |
f244e06d GS |
2579 | C<'|'>, the filename is interpreted as a command which pipes output to |
2580 | us. See L<perlipc/"Using open() for IPC"> | |
19799a22 | 2581 | for more examples of this. (You are not allowed to C<open> to a command |
5a964f20 TC |
2582 | that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, |
2583 | and L<perlipc/"Bidirectional Communication"> for alternatives.) | |
cb1a09d0 | 2584 | |
6170680b IZ |
2585 | If MODE is C<'|-'>, the filename is interpreted as a |
2586 | command to which output is to be piped, and if MODE is | |
2587 | C<'-|'>, the filename is interpreted as a command which pipes output to | |
2588 | us. In the 2-arguments (and 1-argument) form one should replace dash | |
2589 | (C<'-'>) with the command. See L<perlipc/"Using open() for IPC"> | |
2590 | for more examples of this. (You are not allowed to C<open> to a command | |
2591 | that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, | |
2592 | and L<perlipc/"Bidirectional Communication"> for alternatives.) | |
2593 | ||
2594 | In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN | |
61eff3bc | 2595 | and opening C<< '>-' >> opens STDOUT. |
6170680b IZ |
2596 | |
2597 | Open returns | |
19799a22 | 2598 | nonzero upon success, the undefined value otherwise. If the C<open> |
4633a7c4 | 2599 | involved a pipe, the return value happens to be the pid of the |
54310121 | 2600 | subprocess. |
cb1a09d0 AD |
2601 | |
2602 | If you're unfortunate enough to be running Perl on a system that | |
2603 | distinguishes between text files and binary files (modern operating | |
2604 | systems don't care), then you should check out L</binmode> for tips for | |
19799a22 | 2605 | dealing with this. The key distinction between systems that need C<binmode> |
5a964f20 TC |
2606 | and those that don't is their text file formats. Systems like Unix, MacOS, and |
2607 | Plan9, which delimit lines with a single character, and which encode that | |
19799a22 | 2608 | character in C as C<"\n">, do not need C<binmode>. The rest need it. |
cb1a09d0 | 2609 | |
fb73857a | 2610 | When opening a file, it's usually a bad idea to continue normal execution |
19799a22 GS |
2611 | if the request failed, so C<open> is frequently used in connection with |
2612 | C<die>. Even if C<die> won't do what you want (say, in a CGI script, | |
fb73857a | 2613 | where you want to make a nicely formatted error message (but there are |
5a964f20 | 2614 | modules that can help with that problem)) you should always check |
19799a22 | 2615 | the return value from opening a file. The infrequent exception is when |
fb73857a | 2616 | working with an unopened filehandle is actually what you want to do. |
2617 | ||
cb1a09d0 | 2618 | Examples: |
a0d0e21e LW |
2619 | |
2620 | $ARTICLE = 100; | |
2621 | open ARTICLE or die "Can't find article $ARTICLE: $!\n"; | |
2622 | while (<ARTICLE>) {... | |
2623 | ||
6170680b | 2624 | open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) |
fb73857a | 2625 | # if the open fails, output is discarded |
a0d0e21e | 2626 | |
6170680b | 2627 | open(DBASE, '+<', 'dbase.mine') # open for update |
fb73857a | 2628 | or die "Can't open 'dbase.mine' for update: $!"; |
cb1a09d0 | 2629 | |
6170680b IZ |
2630 | open(DBASE, '+<dbase.mine') # ditto |
2631 | or die "Can't open 'dbase.mine' for update: $!"; | |
2632 | ||
2633 | open(ARTICLE, '-|', "caesar <$article") # decrypt article | |
fb73857a | 2634 | or die "Can't start caesar: $!"; |
a0d0e21e | 2635 | |
6170680b IZ |
2636 | open(ARTICLE, "caesar <$article |") # ditto |
2637 | or die "Can't start caesar: $!"; | |
2638 | ||
2639 | open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id | |
fb73857a | 2640 | or die "Can't start sort: $!"; |
a0d0e21e LW |
2641 | |
2642 | # process argument list of files along with any includes | |
2643 | ||
2644 | foreach $file (@ARGV) { | |
2645 | process($file, 'fh00'); | |
2646 | } | |
2647 | ||
2648 | sub process { | |
5a964f20 | 2649 | my($filename, $input) = @_; |
a0d0e21e LW |
2650 | $input++; # this is a string increment |
2651 | unless (open($input, $filename)) { | |
2652 | print STDERR "Can't open $filename: $!\n"; | |
2653 | return; | |
2654 | } | |
2655 | ||
5a964f20 | 2656 | local $_; |
a0d0e21e LW |
2657 | while (<$input>) { # note use of indirection |
2658 | if (/^#include "(.*)"/) { | |
2659 | process($1, $input); | |
2660 | next; | |
2661 | } | |
5a964f20 | 2662 | #... # whatever |
a0d0e21e LW |
2663 | } |
2664 | } | |
2665 | ||
2666 | You may also, in the Bourne shell tradition, specify an EXPR beginning | |
61eff3bc | 2667 | with C<< '>&' >>, in which case the rest of the string is interpreted as the |
5a964f20 | 2668 | name of a filehandle (or file descriptor, if numeric) to be |
61eff3bc JH |
2669 | duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>, |
2670 | C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The | |
a0d0e21e | 2671 | mode you specify should match the mode of the original filehandle. |
184e9718 | 2672 | (Duping a filehandle does not take into account any existing contents of |
6170680b IZ |
2673 | stdio buffers.) Duping file handles is not yet supported for 3-argument |
2674 | open(). | |
2675 | ||
a0d0e21e LW |
2676 | Here is a script that saves, redirects, and restores STDOUT and |
2677 | STDERR: | |
2678 | ||
2679 | #!/usr/bin/perl | |
5a964f20 TC |
2680 | open(OLDOUT, ">&STDOUT"); |
2681 | open(OLDERR, ">&STDERR"); | |
a0d0e21e | 2682 | |
6170680b IZ |
2683 | open(STDOUT, '>', "foo.out") || die "Can't redirect stdout"; |
2684 | open(STDERR, ">&STDOUT") || die "Can't dup stdout"; | |
a0d0e21e LW |
2685 | |
2686 | select(STDERR); $| = 1; # make unbuffered | |
2687 | select(STDOUT); $| = 1; # make unbuffered | |
2688 | ||
2689 | print STDOUT "stdout 1\n"; # this works for | |
2690 | print STDERR "stderr 1\n"; # subprocesses too | |
2691 | ||
2692 | close(STDOUT); | |
2693 | close(STDERR); | |
2694 | ||
5a964f20 TC |
2695 | open(STDOUT, ">&OLDOUT"); |
2696 | open(STDERR, ">&OLDERR"); | |
a0d0e21e LW |
2697 | |
2698 | print STDOUT "stdout 2\n"; | |
2699 | print STDERR "stderr 2\n"; | |
2700 | ||
61eff3bc | 2701 | If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an |
19799a22 | 2702 | equivalent of C's C<fdopen> of that file descriptor; this is more |
4633a7c4 | 2703 | parsimonious of file descriptors. For example: |
a0d0e21e LW |
2704 | |
2705 | open(FILEHANDLE, "<&=$fd") | |
2706 | ||
4af147f6 CS |
2707 | Note that this feature depends on the fdopen() C library function. |
2708 | On many UNIX systems, fdopen() is known to fail when file descriptors | |
2709 | exceed a certain value, typically 255. If you need more file | |
2710 | descriptors than that, consider rebuilding Perl to use the C<sfio> | |
2711 | library. | |
2712 | ||
6170680b IZ |
2713 | If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'> |
2714 | with 2-arguments (or 1-argument) form of open(), then | |
a0d0e21e | 2715 | there is an implicit fork done, and the return value of open is the pid |
7660c0ab | 2716 | of the child within the parent process, and C<0> within the child |
184e9718 | 2717 | process. (Use C<defined($pid)> to determine whether the open was successful.) |
a0d0e21e LW |
2718 | The filehandle behaves normally for the parent, but i/o to that |
2719 | filehandle is piped from/to the STDOUT/STDIN of the child process. | |
2720 | In the child process the filehandle isn't opened--i/o happens from/to | |
2721 | the new STDOUT or STDIN. Typically this is used like the normal | |
2722 | piped open when you want to exercise more control over just how the | |
2723 | pipe command gets executed, such as when you are running setuid, and | |
54310121 | 2724 | don't want to have to scan shell commands for metacharacters. |
6170680b | 2725 | The following triples are more or less equivalent: |
a0d0e21e LW |
2726 | |
2727 | open(FOO, "|tr '[a-z]' '[A-Z]'"); | |
6170680b IZ |
2728 | open(FOO, '|-', "tr '[a-z]' '[A-Z]'"); |
2729 | open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]'; | |
a0d0e21e LW |
2730 | |
2731 | open(FOO, "cat -n '$file'|"); | |
6170680b IZ |
2732 | open(FOO, '-|', "cat -n '$file'"); |
2733 | open(FOO, '-|') || exec 'cat', '-n', $file; | |
a0d0e21e | 2734 | |
4633a7c4 LW |
2735 | See L<perlipc/"Safe Pipe Opens"> for more examples of this. |
2736 | ||
45bc9206 GS |
2737 | NOTE: On any operation that may do a fork, all files opened for output |
2738 | are flushed before the fork is attempted. On systems that support a | |
2739 | close-on-exec flag on files, the flag will be set for the newly opened | |
2740 | file descriptor as determined by the value of $^F. See L<perlvar/$^F>. | |
a0d0e21e | 2741 | |
0dccf244 CS |
2742 | Closing any piped filehandle causes the parent process to wait for the |
2743 | child to finish, and returns the status value in C<$?>. | |
2744 | ||
6170680b IZ |
2745 | The filename passed to 2-argument (or 1-argument) form of open() |
2746 | will have leading and trailing | |
f86cebdf | 2747 | whitespace deleted, and the normal redirection characters |
5a964f20 TC |
2748 | honored. This property, known as "magic open", |
2749 | can often be used to good effect. A user could specify a filename of | |
7660c0ab | 2750 | F<"rsh cat file |">, or you could change certain filenames as needed: |
5a964f20 TC |
2751 | |
2752 | $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; | |
2753 | open(FH, $filename) or die "Can't open $filename: $!"; | |
2754 | ||
6170680b IZ |
2755 | Use 3-argument form to open a file with arbitrary weird characters in it, |
2756 | ||
2757 | open(FOO, '<', $file); | |
2758 | ||
2759 | otherwise it's necessary to protect any leading and trailing whitespace: | |
5a964f20 TC |
2760 | |
2761 | $file =~ s#^(\s)#./$1#; | |
2762 | open(FOO, "< $file\0"); | |
2763 | ||
6170680b IZ |
2764 | (this may not work on some bizzare filesystems). One should |
2765 | conscientiously choose between the the I<magic> and 3-arguments form | |
2766 | of open(): | |
2767 | ||
2768 | open IN, $ARGV[0]; | |
2769 | ||
2770 | will allow the user to specify an argument of the form C<"rsh cat file |">, | |
2771 | but will not work on a filename which happens to have a trailing space, while | |
2772 | ||
2773 | open IN, '<', $ARGV[0]; | |
2774 | ||
2775 | will have exactly the opposite restrictions. | |
2776 | ||
19799a22 | 2777 | If you want a "real" C C<open> (see L<open(2)> on your system), then you |
6170680b IZ |
2778 | should use the C<sysopen> function, which involves no such magic (but |
2779 | may use subtly different filemodes than Perl open(), which is mapped | |
2780 | to C fopen()). This is | |
5a964f20 TC |
2781 | another way to protect your filenames from interpretation. For example: |
2782 | ||
2783 | use IO::Handle; | |
2784 | sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL) | |
2785 | or die "sysopen $path: $!"; | |
2786 | $oldfh = select(HANDLE); $| = 1; select($oldfh); | |
2787 | print HANDLE "stuff $$\n"); | |
2788 | seek(HANDLE, 0, 0); | |
2789 | print "File contains: ", <HANDLE>; | |
2790 | ||
7660c0ab A |
2791 | Using the constructor from the C<IO::Handle> package (or one of its |
2792 | subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous | |
5a964f20 TC |
2793 | filehandles that have the scope of whatever variables hold references to |
2794 | them, and automatically close whenever and however you leave that scope: | |
c07a80fd | 2795 | |
5f05dabc | 2796 | use IO::File; |
5a964f20 | 2797 | #... |
c07a80fd | 2798 | sub read_myfile_munged { |
2799 | my $ALL = shift; | |
5f05dabc | 2800 | my $handle = new IO::File; |
c07a80fd | 2801 | open($handle, "myfile") or die "myfile: $!"; |
2802 | $first = <$handle> | |
2803 | or return (); # Automatically closed here. | |
2804 | mung $first or die "mung failed"; # Or here. | |
2805 | return $first, <$handle> if $ALL; # Or here. | |
2806 | $first; # Or here. | |
2807 | } | |
2808 | ||
b687b08b | 2809 | See L</seek> for some details about mixing reading and writing. |
a0d0e21e LW |
2810 | |
2811 | =item opendir DIRHANDLE,EXPR | |
2812 | ||
19799a22 GS |
2813 | Opens a directory named EXPR for processing by C<readdir>, C<telldir>, |
2814 | C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful. | |
a0d0e21e LW |
2815 | DIRHANDLEs have their own namespace separate from FILEHANDLEs. |
2816 | ||
2817 | =item ord EXPR | |
2818 | ||
54310121 | 2819 | =item ord |
bbce6d69 | 2820 | |
a0ed51b3 | 2821 | Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If |
7660c0ab | 2822 | EXPR is omitted, uses C<$_>. For the reverse, see L</chr>. |
2b5ab1e7 | 2823 | See L<utf8> for more about Unicode. |
a0d0e21e | 2824 | |
77ca0c92 LW |
2825 | =item our EXPR |
2826 | ||
2827 | An C<our> declares the listed variables to be valid globals within | |
2828 | the enclosing block, file, or C<eval>. That is, it has the same | |
2829 | scoping rules as a "my" declaration, but does not create a local | |
2830 | variable. If more than one value is listed, the list must be placed | |
2831 | in parentheses. The C<our> declaration has no semantic effect unless | |
2832 | "use strict vars" is in effect, in which case it lets you use the | |
2833 | declared global variable without qualifying it with a package name. | |
2834 | (But only within the lexical scope of the C<our> declaration. In this | |
2835 | it differs from "use vars", which is package scoped.) | |
2836 | ||
f472eb5c GS |
2837 | An C<our> declaration declares a global variable that will be visible |
2838 | across its entire lexical scope, even across package boundaries. The | |
2839 | package in which the variable is entered is determined at the point | |
2840 | of the declaration, not at the point of use. This means the following | |
2841 | behavior holds: | |
2842 | ||
2843 | package Foo; | |
2844 | our $bar; # declares $Foo::bar for rest of lexical scope | |
2845 | $bar = 20; | |
2846 | ||
2847 | package Bar; | |
2848 | print $bar; # prints 20 | |
2849 | ||
2850 | Multiple C<our> declarations in the same lexical scope are allowed | |
2851 | if they are in different packages. If they happened to be in the same | |
2852 | package, Perl will emit warnings if you have asked for them. | |
2853 | ||
2854 | use warnings; | |
2855 | package Foo; | |
2856 | our $bar; # declares $Foo::bar for rest of lexical scope | |
2857 | $bar = 20; | |
2858 | ||
2859 | package Bar; | |
2860 | our $bar = 30; # declares $Bar::bar for rest of lexical scope | |
2861 | print $bar; # prints 30 | |
2862 | ||
2863 | our $bar; # emits warning | |
2864 | ||
a0d0e21e LW |
2865 | =item pack TEMPLATE,LIST |
2866 | ||
2b6c5635 GS |
2867 | Takes a LIST of values and converts it into a string using the rules |
2868 | given by the TEMPLATE. The resulting string is the concatenation of | |
2869 | the converted values. Typically, each converted value looks | |
2870 | like its machine-level representation. For example, on 32-bit machines | |
2871 | a converted integer may be represented by a sequence of 4 bytes. | |
2872 | ||
2873 | The TEMPLATE is a | |
a0d0e21e LW |
2874 | sequence of characters that give the order and type of values, as |
2875 | follows: | |
2876 | ||
5a929a98 | 2877 | a A string with arbitrary binary data, will be null padded. |
a0d0e21e | 2878 | A An ascii string, will be space padded. |
5a929a98 VU |
2879 | Z A null terminated (asciz) string, will be null padded. |
2880 | ||
2b6c5635 GS |
2881 | b A bit string (ascending bit order inside each byte, like vec()). |
2882 | B A bit string (descending bit order inside each byte). | |
a0d0e21e LW |
2883 | h A hex string (low nybble first). |
2884 | H A hex string (high nybble first). | |
2885 | ||
2886 | c A signed char value. | |
a0ed51b3 | 2887 | C An unsigned char value. Only does bytes. See U for Unicode. |
96e4d5b1 | 2888 | |
a0d0e21e LW |
2889 | s A signed short value. |
2890 | S An unsigned short value. | |
96e4d5b1 | 2891 | (This 'short' is _exactly_ 16 bits, which may differ from |
851646ae JH |
2892 | what a local C compiler calls 'short'. If you want |
2893 | native-length shorts, use the '!' suffix.) | |
96e4d5b1 | 2894 | |
a0d0e21e LW |
2895 | i A signed integer value. |
2896 | I An unsigned integer value. | |
19799a22 | 2897 | (This 'integer' is _at_least_ 32 bits wide. Its exact |
f86cebdf GS |
2898 | size depends on what a local C compiler calls 'int', |
2899 | and may even be larger than the 'long' described in | |
2900 | the next item.) | |
96e4d5b1 | 2901 | |
a0d0e21e LW |
2902 | l A signed long value. |
2903 | L An unsigned long value. | |
96e4d5b1 | 2904 | (This 'long' is _exactly_ 32 bits, which may differ from |
851646ae JH |
2905 | what a local C compiler calls 'long'. If you want |
2906 | native-length longs, use the '!' suffix.) | |
a0d0e21e | 2907 | |
5d11dd56 MG |
2908 | n An unsigned short in "network" (big-endian) order. |
2909 | N An unsigned long in "network" (big-endian) order. | |
2910 | v An unsigned short in "VAX" (little-endian) order. | |
2911 | V An unsigned long in "VAX" (little-endian) order. | |
96e4d5b1 | 2912 | (These 'shorts' and 'longs' are _exactly_ 16 bits and |
2913 | _exactly_ 32 bits, respectively.) | |
a0d0e21e | 2914 | |
dae0da7a JH |
2915 | q A signed quad (64-bit) value. |
2916 | Q An unsigned quad value. | |
851646ae JH |
2917 | (Quads are available only if your system supports 64-bit |
2918 | integer values _and_ if Perl has been compiled to support those. | |
dae0da7a JH |
2919 | Causes a fatal error otherwise.) |
2920 | ||
a0d0e21e LW |
2921 | f A single-precision float in the native format. |
2922 | d A double-precision float in the native format. | |
2923 | ||
2924 | p A pointer to a null-terminated string. | |
2925 | P A pointer to a structure (fixed-length string). | |
2926 | ||
2927 | u A uuencoded string. | |
a0ed51b3 LW |
2928 | U A Unicode character number. Encodes to UTF-8 internally. |
2929 | Works even if C<use utf8> is not in effect. | |
a0d0e21e | 2930 | |
96e4d5b1 | 2931 | w A BER compressed integer. Its bytes represent an unsigned |
f86cebdf GS |
2932 | integer in base 128, most significant digit first, with as |
2933 | few digits as possible. Bit eight (the high bit) is set | |
2934 | on each byte except the last. | |
def98dd4 | 2935 | |
a0d0e21e LW |
2936 | x A null byte. |
2937 | X Back up a byte. | |
2938 | @ Null fill to absolute position. | |
2939 | ||
5a929a98 VU |
2940 | The following rules apply: |
2941 | ||
2942 | =over 8 | |
2943 | ||
2944 | =item * | |
2945 | ||
5a964f20 | 2946 | Each letter may optionally be followed by a number giving a repeat |
951ba7fe GS |
2947 | count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>, |
2948 | C<H>, and C<P> the pack function will gobble up that many values from | |
5a929a98 | 2949 | the LIST. A C<*> for the repeat count means to use however many items are |
951ba7fe GS |
2950 | left, except for C<@>, C<x>, C<X>, where it is equivalent |
2951 | to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the | |
2b6c5635 GS |
2952 | same). |
2953 | ||
951ba7fe | 2954 | When used with C<Z>, C<*> results in the addition of a trailing null |
2b6c5635 GS |
2955 | byte (so the packed result will be one longer than the byte C<length> |
2956 | of the item). | |
2957 | ||
951ba7fe | 2958 | The repeat count for C<u> is interpreted as the maximal number of bytes |
2b6c5635 | 2959 | to encode per line of output, with 0 and 1 replaced by 45. |
5a929a98 VU |
2960 | |
2961 | =item * | |
2962 | ||
951ba7fe | 2963 | The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a |
5a929a98 | 2964 | string of length count, padding with nulls or spaces as necessary. When |
951ba7fe GS |
2965 | unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything |
2966 | after the first null, and C<a> returns data verbatim. When packing, | |
2967 | C<a>, and C<Z> are equivalent. | |
2b6c5635 GS |
2968 | |
2969 | If the value-to-pack is too long, it is truncated. If too long and an | |
951ba7fe GS |
2970 | explicit count is provided, C<Z> packs only C<$count-1> bytes, followed |
2971 | by a null byte. Thus C<Z> always packs a trailing null byte under | |
2b6c5635 | 2972 | all circumstances. |
5a929a98 VU |
2973 | |
2974 | =item * | |
2975 | ||
951ba7fe | 2976 | Likewise, the C<b> and C<B> fields pack a string that many bits long. |
c73032f5 IZ |
2977 | Each byte of the input field of pack() generates 1 bit of the result. |
2978 | Each result bit is based on the least-significant bit of the corresponding | |
2979 | input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and | |
2980 | C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">. | |
2981 | ||
2982 | Starting from the beginning of the input string of pack(), each 8-tuple | |
951ba7fe | 2983 | of bytes is converted to 1 byte of output. With format C<b> |
c73032f5 | 2984 | the first byte of the 8-tuple determines the least-significant bit of a |
951ba7fe | 2985 | byte, and with format C<B> it determines the most-significant bit of |
c73032f5 IZ |
2986 | a byte. |
2987 | ||
2988 | If the length of the input string is not exactly divisible by 8, the | |
2989 | remainder is packed as if the input string were padded by null bytes | |
2990 | at the end. Similarly, during unpack()ing the "extra" bits are ignored. | |
2991 | ||
2992 | If the input string of pack() is longer than needed, extra bytes are ignored. | |
2b6c5635 GS |
2993 | A C<*> for the repeat count of pack() means to use all the bytes of |
2994 | the input field. On unpack()ing the bits are converted to a string | |
2995 | of C<"0">s and C<"1">s. | |
5a929a98 VU |
2996 | |
2997 | =item * | |
2998 | ||
951ba7fe | 2999 | The C<h> and C<H> fields pack a string that many nybbles (4-bit groups, |
851646ae | 3000 | representable as hexadecimal digits, 0-9a-f) long. |
5a929a98 | 3001 | |
c73032f5 IZ |
3002 | Each byte of the input field of pack() generates 4 bits of the result. |
3003 | For non-alphabetical bytes the result is based on the 4 least-significant | |
3004 | bits of the input byte, i.e., on C<ord($byte)%16>. In particular, | |
3005 | bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes | |
3006 | C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result | |
3007 | is compatible with the usual hexadecimal digits, so that C<"a"> and | |
3008 | C<"A"> both generate the nybble C<0xa==10>. The result for bytes | |
3009 | C<"g".."z"> and C<"G".."Z"> is not well-defined. | |
3010 | ||
3011 | Starting from the beginning of the input string of pack(), each pair | |
951ba7fe | 3012 | of bytes is converted to 1 byte of output. With format C<h> the |
c73032f5 | 3013 | first byte of the pair determines the least-significant nybble of the |
951ba7fe | 3014 | output byte, and with format C<H> it determines the most-significant |
c73032f5 IZ |
3015 | nybble. |
3016 | ||
3017 | If the length of the input string is not even, it behaves as if padded | |
3018 | by a null byte at the end. Similarly, during unpack()ing the "extra" | |
3019 | nybbles are ignored. | |
3020 | ||
3021 | If the input string of pack() is longer than needed, extra bytes are ignored. | |
3022 | A C<*> for the repeat count of pack() means to use all the bytes of | |
3023 | the input field. On unpack()ing the bits are converted to a string | |
3024 | of hexadecimal digits. | |
3025 | ||
5a929a98 VU |
3026 | =item * |
3027 | ||
951ba7fe | 3028 | The C<p> type packs a pointer to a null-terminated string. You are |
5a929a98 VU |
3029 | responsible for ensuring the string is not a temporary value (which can |
3030 | potentially get deallocated before you get around to using the packed result). | |
951ba7fe GS |
3031 | The C<P> type packs a pointer to a structure of the size indicated by the |
3032 | length. A NULL pointer is created if the corresponding value for C<p> or | |
3033 | C<P> is C<undef>, similarly for unpack(). | |
5a929a98 VU |
3034 | |
3035 | =item * | |
3036 | ||
951ba7fe GS |
3037 | The C</> template character allows packing and unpacking of strings where |
3038 | the packed structure contains a byte count followed by the string itself. | |
17f4a12d | 3039 | You write I<length-item>C</>I<string-item>. |
43192e07 IP |
3040 | |
3041 | The I<length-item> can be any C<pack> template letter, | |
3042 | and describes how the length value is packed. | |
3043 | The ones likely to be of most use are integer-packing ones like | |
951ba7fe GS |
3044 | C<n> (for Java strings), C<w> (for ASN.1 or SNMP) |
3045 | and C<N> (for Sun XDR). | |
43192e07 IP |
3046 | |
3047 | The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">. | |
3048 | For C<unpack> the length of the string is obtained from the I<length-item>, | |
3049 | but if you put in the '*' it will be ignored. | |
3050 | ||
17f4a12d IZ |
3051 | unpack 'C/a', "\04Gurusamy"; gives 'Guru' |
3052 | unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J') | |
3053 | pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world" | |
43192e07 IP |
3054 | |
3055 | The I<length-item> is not returned explicitly from C<unpack>. | |
3056 | ||
951ba7fe GS |
3057 | Adding a count to the I<length-item> letter is unlikely to do anything |
3058 | useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a | |
3059 | I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters, | |
43192e07 IP |
3060 | which Perl does not regard as legal in numeric strings. |
3061 | ||
3062 | =item * | |
3063 | ||
951ba7fe GS |
3064 | The integer types C<s>, C<S>, C<l>, and C<L> may be |
3065 | immediately followed by a C<!> suffix to signify native shorts or | |
3066 | longs--as you can see from above for example a bare C<l> does mean | |
851646ae JH |
3067 | exactly 32 bits, the native C<long> (as seen by the local C compiler) |
3068 | may be larger. This is an issue mainly in 64-bit platforms. You can | |
951ba7fe | 3069 | see whether using C<!> makes any difference by |
726ea183 | 3070 | |
4d0c1c44 GS |
3071 | print length(pack("s")), " ", length(pack("s!")), "\n"; |
3072 | print length(pack("l")), " ", length(pack("l!")), "\n"; | |
ef54e1a4 | 3073 | |
951ba7fe GS |
3074 | C<i!> and C<I!> also work but only because of completeness; |
3075 | they are identical to C<i> and C<I>. | |
ef54e1a4 | 3076 | |
19799a22 GS |
3077 | The actual sizes (in bytes) of native shorts, ints, longs, and long |
3078 | longs on the platform where Perl was built are also available via | |
3079 | L<Config>: | |
3080 | ||
3081 | use Config; | |
3082 | print $Config{shortsize}, "\n"; | |
3083 | print $Config{intsize}, "\n"; | |
3084 | print $Config{longsize}, "\n"; | |
3085 | print $Config{longlongsize}, "\n"; | |
ef54e1a4 | 3086 | |
5074e145 | 3087 | (The C<$Config{longlongsize}> will be undefine if your system does |
851646ae JH |
3088 | not support long longs.) |
3089 | ||
ef54e1a4 JH |
3090 | =item * |
3091 | ||
951ba7fe | 3092 | The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L> |
ef54e1a4 JH |
3093 | are inherently non-portable between processors and operating systems |
3094 | because they obey the native byteorder and endianness. For example a | |
140cb37e | 3095 | 4-byte integer 0x12345678 (305419896 decimal) be ordered natively |
ef54e1a4 | 3096 | (arranged in and handled by the CPU registers) into bytes as |
61eff3bc | 3097 | |
719a3cf5 JH |
3098 | 0x12 0x34 0x56 0x78 # little-endian |
3099 | 0x78 0x56 0x34 0x12 # big-endian | |
61eff3bc | 3100 | |
5d11dd56 | 3101 | Basically, the Intel, Alpha, and VAX CPUs are little-endian, while |
719a3cf5 JH |
3102 | everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA, |
3103 | Power, and Cray are big-endian. MIPS can be either: Digital used it | |
19799a22 | 3104 | in little-endian mode; SGI uses it in big-endian mode. |
719a3cf5 | 3105 | |
19799a22 | 3106 | The names `big-endian' and `little-endian' are comic references to |
ef54e1a4 JH |
3107 | the classic "Gulliver's Travels" (via the paper "On Holy Wars and a |
3108 | Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and | |
19799a22 | 3109 | the egg-eating habits of the Lilliputians. |
61eff3bc | 3110 | |
140cb37e | 3111 | Some systems may have even weirder byte orders such as |
61eff3bc | 3112 | |
ef54e1a4 JH |
3113 | 0x56 0x78 0x12 0x34 |
3114 | 0x34 0x12 0x78 0x56 | |
61eff3bc | 3115 | |
ef54e1a4 JH |
3116 | You can see your system's preference with |
3117 | ||
3118 | print join(" ", map { sprintf "%#02x", $_ } | |
3119 | unpack("C*",pack("L",0x12345678))), "\n"; | |
3120 | ||
d99ad34e | 3121 | The byteorder on the platform where Perl was built is also available |
726ea183 | 3122 | via L<Config>: |
ef54e1a4 JH |
3123 | |
3124 | use Config; | |
3125 | print $Config{byteorder}, "\n"; | |
3126 | ||
d99ad34e JH |
3127 | Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'> |
3128 | and C<'87654321'> are big-endian. | |
719a3cf5 | 3129 | |
951ba7fe GS |
3130 | If you want portable packed integers use the formats C<n>, C<N>, |
3131 | C<v>, and C<V>, their byte endianness and size is known. | |
851646ae | 3132 | See also L<perlport>. |
ef54e1a4 JH |
3133 | |
3134 | =item * | |
3135 | ||
5a929a98 VU |
3136 | Real numbers (floats and doubles) are in the native machine format only; |
3137 | due to the multiplicity of floating formats around, and the lack of a | |
3138 | standard "network" representation, no facility for interchange has been | |
3139 | made. This means that packed floating point data written on one machine | |
3140 | may not be readable on another - even if both use IEEE floating point | |
3141 | arithmetic (as the endian-ness of the memory representation is not part | |
851646ae | 3142 | of the IEEE spec). See also L<perlport>. |
5a929a98 VU |
3143 | |
3144 | Note that Perl uses doubles internally for all numeric calculation, and | |
3145 | converting from double into float and thence back to double again will | |
3146 | lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general | |
19799a22 | 3147 | equal $foo). |
5a929a98 | 3148 | |
851646ae JH |
3149 | =item * |
3150 | ||
3151 | You must yourself do any alignment or padding by inserting for example | |
9ccd05c0 JH |
3152 | enough C<'x'>es while packing. There is no way to pack() and unpack() |
3153 | could know where the bytes are going to or coming from. Therefore | |
3154 | C<pack> (and C<unpack>) handle their output and input as flat | |
3155 | sequences of bytes. | |
851646ae | 3156 | |
17f4a12d IZ |
3157 | =item * |
3158 | ||
3159 | A comment in a TEMPLATE starts with C<#> and goes to the end of line. | |
3160 | ||
2b6c5635 GS |
3161 | =item * |
3162 | ||
3163 | If TEMPLATE requires more arguments to pack() than actually given, pack() | |
3164 | assumes additional C<""> arguments. If TEMPLATE requires less arguments | |
3165 | to pack() than actually given, extra arguments are ignored. | |
3166 | ||
5a929a98 | 3167 | =back |
a0d0e21e LW |
3168 | |
3169 | Examples: | |
3170 | ||
a0ed51b3 | 3171 | $foo = pack("CCCC",65,66,67,68); |
a0d0e21e | 3172 | # foo eq "ABCD" |
a0ed51b3 | 3173 | $foo = pack("C4",65,66,67,68); |
a0d0e21e | 3174 | # same thing |
a0ed51b3 LW |
3175 | $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9); |
3176 | # same thing with Unicode circled letters | |
a0d0e21e LW |
3177 | |
3178 | $foo = pack("ccxxcc",65,66,67,68); | |
3179 | # foo eq "AB\0\0CD" | |
3180 | ||
9ccd05c0 JH |
3181 | # note: the above examples featuring "C" and "c" are true |
3182 | # only on ASCII and ASCII-derived systems such as ISO Latin 1 | |
3183 | # and UTF-8. In EBCDIC the first example would be | |
3184 | # $foo = pack("CCCC",193,194,195,196); | |
3185 | ||
a0d0e21e LW |
3186 | $foo = pack("s2",1,2); |
3187 | # "\1\0\2\0" on little-endian | |
3188 | # "\0\1\0\2" on big-endian | |
3189 | ||
3190 | $foo = pack("a4","abcd","x","y","z"); | |
3191 | # "abcd" | |
3192 | ||
3193 | $foo = pack("aaaa","abcd","x","y","z"); | |
3194 | # "axyz" | |
3195 | ||
3196 | $foo = pack("a14","abcdefg"); | |
3197 | # "abcdefg\0\0\0\0\0\0\0" | |
3198 | ||
3199 | $foo = pack("i9pl", gmtime); | |
3200 | # a real struct tm (on my system anyway) | |
3201 | ||
5a929a98 VU |
3202 | $utmp_template = "Z8 Z8 Z16 L"; |
3203 | $utmp = pack($utmp_template, @utmp1); | |
3204 | # a struct utmp (BSDish) | |
3205 | ||
3206 | @utmp2 = unpack($utmp_template, $utmp); | |
3207 | # "@utmp1" eq "@utmp2" | |
3208 | ||
a0d0e21e LW |
3209 | sub bintodec { |
3210 | unpack("N", pack("B32", substr("0" x 32 . shift, -32))); | |
3211 | } | |
3212 | ||
851646ae JH |
3213 | $foo = pack('sx2l', 12, 34); |
3214 | # short 12, two zero bytes padding, long 34 | |
3215 | $bar = pack('s@4l', 12, 34); | |
3216 | # short 12, zero fill to position 4, long 34 | |
3217 | # $foo eq $bar | |
3218 | ||
5a929a98 | 3219 | The same template may generally also be used in unpack(). |
a0d0e21e | 3220 | |
5a964f20 TC |
3221 | =item package |
3222 | ||
cb1a09d0 AD |
3223 | =item package NAMESPACE |
3224 | ||
3225 | Declares the compilation unit as being in the given namespace. The scope | |
2b5ab1e7 | 3226 | of the package declaration is from the declaration itself through the end |
19799a22 | 3227 | of the enclosing block, file, or eval (the same as the C<my> operator). |
2b5ab1e7 TC |
3228 | All further unqualified dynamic identifiers will be in this namespace. |
3229 | A package statement affects only dynamic variables--including those | |
19799a22 GS |
3230 | you've used C<local> on--but I<not> lexical variables, which are created |
3231 | with C<my>. Typically it would be the first declaration in a file to | |
2b5ab1e7 TC |
3232 | be included by the C<require> or C<use> operator. You can switch into a |
3233 | package in more than one place; it merely influences which symbol table | |
3234 | is used by the compiler for the rest of that block. You can refer to | |
3235 | variables and filehandles in other packages by prefixing the identifier | |
3236 | with the package name and a double colon: C<$Package::Variable>. | |
3237 | If the package name is null, the C<main> package as assumed. That is, | |
3238 | C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>, | |
3239 | still seen in older code). | |
cb1a09d0 | 3240 | |
5a964f20 TC |
3241 | If NAMESPACE is omitted, then there is no current package, and all |
3242 | identifiers must be fully qualified or lexicals. This is stricter | |
3243 | than C<use strict>, since it also extends to function names. | |
3244 | ||
cb1a09d0 AD |
3245 | See L<perlmod/"Packages"> for more information about packages, modules, |
3246 | and classes. See L<perlsub> for other scoping issues. | |
3247 | ||
a0d0e21e LW |
3248 | =item pipe READHANDLE,WRITEHANDLE |
3249 | ||
3250 | Opens a pair of connected pipes like the corresponding system call. | |
3251 | Note that if you set up a loop of piped processes, deadlock can occur | |
3252 | unless you are very careful. In addition, note that Perl's pipes use | |
184e9718 | 3253 | stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE |
a0d0e21e LW |
3254 | after each command, depending on the application. |
3255 | ||
7e1af8bc | 3256 | See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication"> |
4633a7c4 LW |
3257 | for examples of such things. |
3258 | ||
4771b018 GS |
3259 | On systems that support a close-on-exec flag on files, the flag will be set |
3260 | for the newly opened file descriptors as determined by the value of $^F. | |
3261 | See L<perlvar/$^F>. | |
3262 | ||
a0d0e21e LW |
3263 | =item pop ARRAY |
3264 | ||
54310121 | 3265 | =item pop |
28757baa | 3266 | |
a0d0e21e | 3267 | Pops and returns the last value of the array, shortening the array by |
19799a22 | 3268 | one element. Has an effect similar to |
a0d0e21e | 3269 | |
19799a22 | 3270 | $ARRAY[$#ARRAY--] |
a0d0e21e | 3271 | |
19799a22 GS |
3272 | If there are no elements in the array, returns the undefined value |
3273 | (although this may happen at other times as well). If ARRAY is | |
3274 | omitted, pops the C<@ARGV> array in the main program, and the C<@_> | |
3275 | array in subroutines, just like C<shift>. | |
a0d0e21e LW |
3276 | |
3277 | =item pos SCALAR | |
3278 | ||
54310121 | 3279 | =item pos |
bbce6d69 | 3280 | |
4633a7c4 | 3281 | Returns the offset of where the last C<m//g> search left off for the variable |
7660c0ab | 3282 | is in question (C<$_> is used when the variable is not specified). May be |
44a8e56a | 3283 | modified to change that offset. Such modification will also influence |
3284 | the C<\G> zero-width assertion in regular expressions. See L<perlre> and | |
3285 | L<perlop>. | |
a0d0e21e LW |
3286 | |
3287 | =item print FILEHANDLE LIST | |
3288 | ||
3289 | =item print LIST | |
3290 | ||
3291 | =item print | |
3292 | ||
19799a22 GS |
3293 | Prints a string or a list of strings. Returns true if successful. |
3294 | FILEHANDLE may be a scalar variable name, in which case the variable | |
3295 | contains the name of or a reference to the filehandle, thus introducing | |
3296 | one level of indirection. (NOTE: If FILEHANDLE is a variable and | |
3297 | the next token is a term, it may be misinterpreted as an operator | |
2b5ab1e7 | 3298 | unless you interpose a C<+> or put parentheses around the arguments.) |
19799a22 GS |