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1 | .\" This manpage is copyright (C) 1992 Drew Eckhardt, |
2 | .\" copyright (C) 1995 Michael Shields. | |
3 | .\" | |
93015253 | 4 | .\" %%%LICENSE_START(VERBATIM) |
fea681da MK |
5 | .\" Permission is granted to make and distribute verbatim copies of this |
6 | .\" manual provided the copyright notice and this permission notice are | |
7 | .\" preserved on all copies. | |
8 | .\" | |
9 | .\" Permission is granted to copy and distribute modified versions of this | |
10 | .\" manual under the conditions for verbatim copying, provided that the | |
11 | .\" entire resulting derived work is distributed under the terms of a | |
12 | .\" permission notice identical to this one. | |
c13182ef | 13 | .\" |
fea681da MK |
14 | .\" Since the Linux kernel and libraries are constantly changing, this |
15 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
16 | .\" responsibility for errors or omissions, or for damages resulting from | |
17 | .\" the use of the information contained herein. The author(s) may not | |
18 | .\" have taken the same level of care in the production of this manual, | |
19 | .\" which is licensed free of charge, as they might when working | |
20 | .\" professionally. | |
c13182ef | 21 | .\" |
fea681da MK |
22 | .\" Formatted or processed versions of this manual, if unaccompanied by |
23 | .\" the source, must acknowledge the copyright and authors of this work. | |
4b72fb64 | 24 | .\" %%%LICENSE_END |
fea681da MK |
25 | .\" |
26 | .\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu> | |
27 | .\" Modified 1995-05-18 by Jim Van Zandt <jrv@vanzandt.mv.com> | |
28 | .\" Sun Feb 11 14:07:00 MET 1996 Martin Schulze <joey@linux.de> | |
29 | .\" * layout slightly modified | |
30 | .\" | |
31 | .\" Modified Mon Oct 21 23:05:29 EDT 1996 by Eric S. Raymond <esr@thyrsus.com> | |
32 | .\" Modified Thu Feb 24 01:41:09 CET 2000 by aeb | |
33 | .\" Modified Thu Feb 9 22:32:09 CET 2001 by bert hubert <ahu@ds9a.nl>, aeb | |
34 | .\" Modified Mon Nov 11 14:35:00 PST 2002 by Ben Woodard <ben@zork.net> | |
d02aa9bc MK |
35 | .\" 2005-03-11, mtk, modified pselect() text (it is now a system |
36 | .\" call in 2.6.16. | |
fea681da | 37 | .\" |
31a1b45e | 38 | .TH SELECT 2 2017-03-13 "Linux" "Linux Programmer's Manual" |
fea681da | 39 | .SH NAME |
c13182ef | 40 | select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO \- |
35478399 | 41 | synchronous I/O multiplexing |
fea681da | 42 | .SH SYNOPSIS |
cc9befa9 | 43 | .nf |
e9419385 | 44 | /* According to POSIX.1-2001, POSIX.1-2008 */ |
fea681da MK |
45 | .br |
46 | .B #include <sys/select.h> | |
47 | .sp | |
48 | /* According to earlier standards */ | |
49 | .br | |
50 | .B #include <sys/time.h> | |
51 | .br | |
52 | .B #include <sys/types.h> | |
53 | .br | |
54 | .B #include <unistd.h> | |
55 | .sp | |
cc4615cc MK |
56 | .BI "int select(int " nfds ", fd_set *" readfds ", fd_set *" writefds , |
57 | .BI " fd_set *" exceptfds ", struct timeval *" timeout ); | |
fea681da | 58 | .sp |
071dbad9 | 59 | .BI "void FD_CLR(int " fd ", fd_set *" set ); |
fea681da | 60 | .br |
521bf584 | 61 | .BI "int FD_ISSET(int " fd ", fd_set *" set ); |
fea681da | 62 | .br |
071dbad9 | 63 | .BI "void FD_SET(int " fd ", fd_set *" set ); |
fea681da | 64 | .br |
071dbad9 | 65 | .BI "void FD_ZERO(fd_set *" set ); |
9b813741 | 66 | .sp |
9b813741 MK |
67 | .B #include <sys/select.h> |
68 | .sp | |
cc4615cc MK |
69 | .BI "int pselect(int " nfds ", fd_set *" readfds ", fd_set *" writefds , |
70 | .BI " fd_set *" exceptfds ", const struct timespec *" timeout , | |
71 | .BI " const sigset_t *" sigmask ); | |
fea681da | 72 | .fi |
cc4615cc MK |
73 | .sp |
74 | .in -4n | |
75 | Feature Test Macro Requirements for glibc (see | |
76 | .BR feature_test_macros (7)): | |
77 | .in | |
78 | .sp | |
79 | .BR pselect (): | |
a446ac0c | 80 | _POSIX_C_SOURCE\ >=\ 200112L |
fea681da | 81 | .SH DESCRIPTION |
e511ffb6 | 82 | .BR select () |
fea681da | 83 | and |
e511ffb6 | 84 | .BR pselect () |
39179b3e | 85 | allow a program to monitor multiple file descriptors, |
5e01a1de MK |
86 | waiting until one or more of the file descriptors become "ready" |
87 | for some class of I/O operation (e.g., input possible). | |
39179b3e | 88 | A file descriptor is considered ready if it is possible to |
d2e7d1bb RR |
89 | perform a corresponding I/O operation (e.g., |
90 | .BR read (2) | |
91 | without blocking, or a sufficiently small | |
92 | .BR write (2)). | |
fea681da | 93 | .PP |
6b6e9185 | 94 | .BR select () |
6c345305 MK |
95 | can monitor only file descriptors numbers that are less than |
96 | .BR FD_SETSIZE ; | |
97 | .BR poll (2) | |
6b6e9185 | 98 | does not have this limitation. |
6c345305 | 99 | See BUGS. |
6b6e9185 | 100 | .PP |
5e01a1de MK |
101 | The operation of |
102 | .BR select () | |
103 | and | |
104 | .BR pselect () | |
8c1d94ae | 105 | is identical, other than these three differences: |
fea681da MK |
106 | .TP |
107 | (i) | |
e511ffb6 | 108 | .BR select () |
cc9befa9 | 109 | uses a timeout that is a |
fea681da MK |
110 | .I struct timeval |
111 | (with seconds and microseconds), while | |
e511ffb6 | 112 | .BR pselect () |
fea681da MK |
113 | uses a |
114 | .I struct timespec | |
115 | (with seconds and nanoseconds). | |
116 | .TP | |
117 | (ii) | |
e511ffb6 | 118 | .BR select () |
cc9befa9 | 119 | may update the |
fea681da | 120 | .I timeout |
cc9befa9 | 121 | argument to indicate how much time was left. |
e511ffb6 | 122 | .BR pselect () |
cc9befa9 | 123 | does not change this argument. |
fea681da MK |
124 | .TP |
125 | (iii) | |
e511ffb6 | 126 | .BR select () |
cc9befa9 | 127 | has no |
fea681da | 128 | .I sigmask |
cc9befa9 | 129 | argument, and behaves as |
e511ffb6 | 130 | .BR pselect () |
fea681da MK |
131 | called with NULL |
132 | .IR sigmask . | |
133 | .PP | |
c13182ef | 134 | Three independent sets of file descriptors are watched. |
22f348ca | 135 | Those listed in |
fea681da MK |
136 | .I readfds |
137 | will be watched to see if characters become | |
138 | available for reading (more precisely, to see if a read will not | |
e9496f74 | 139 | block; in particular, a file descriptor is also ready on end-of-file), |
fea681da MK |
140 | those in |
141 | .I writefds | |
d2e7d1bb RR |
142 | will be watched to see if space is available for write (though a large |
143 | write may still block), and those in | |
fea681da | 144 | .I exceptfds |
c13182ef MK |
145 | will be watched for exceptions. |
146 | On exit, the sets are modified in place | |
22f348ca MK |
147 | to indicate which file descriptors actually changed status. |
148 | Each of the three file descriptor sets may be specified as NULL | |
c13182ef | 149 | if no file descriptors are to be watched for the corresponding class |
22f348ca | 150 | of events. |
fea681da MK |
151 | .PP |
152 | Four macros are provided to manipulate the sets. | |
e511ffb6 | 153 | .BR FD_ZERO () |
21045df8 | 154 | clears a set. |
e511ffb6 | 155 | .BR FD_SET () |
fea681da | 156 | and |
e511ffb6 | 157 | .BR FD_CLR () |
22f348ca | 158 | respectively add and remove a given file descriptor from a set. |
e511ffb6 | 159 | .BR FD_ISSET () |
c13182ef | 160 | tests to see if a file descriptor is part of the set; |
22f348ca | 161 | this is useful after |
e511ffb6 | 162 | .BR select () |
fea681da MK |
163 | returns. |
164 | .PP | |
6efed4df | 165 | .I nfds |
8b58a9d4 MK |
166 | should be set to the highest-numbered file descriptor in any |
167 | of the three sets, plus 1. | |
168 | The indicated file descriptors in each set are checked, up to this limit | |
169 | (but see BUGS). | |
fea681da | 170 | .PP |
8c121f40 | 171 | The |
fea681da | 172 | .I timeout |
073f0240 | 173 | argument specifies the interval that |
e511ffb6 | 174 | .BR select () |
8c121f40 | 175 | should block waiting for a file descriptor to become ready. |
40df3d00 MK |
176 | The call will block until either: |
177 | .IP * 3 | |
178 | a file descriptor becomes ready; | |
179 | .IP * | |
180 | the call is interrupted by a signal handler; or | |
181 | .IP * | |
71e7d7f1 | 182 | the timeout expires. |
40df3d00 MK |
183 | .PP |
184 | Note that the | |
185 | .I timeout | |
186 | interval will be rounded up to the system clock granularity, | |
8c121f40 | 187 | and kernel scheduling delays mean that the blocking interval |
073f0240 | 188 | may overrun by a small amount. |
485eb4ad MK |
189 | If both fields of the |
190 | .I timeval | |
c808bb16 | 191 | structure are zero, then |
e511ffb6 | 192 | .BR select () |
485eb4ad | 193 | returns immediately. |
c13182ef MK |
194 | (This is useful for polling.) |
195 | If | |
fea681da MK |
196 | .I timeout |
197 | is NULL (no timeout), | |
e511ffb6 | 198 | .BR select () |
fea681da MK |
199 | can block indefinitely. |
200 | .PP | |
201 | .I sigmask | |
202 | is a pointer to a signal mask (see | |
203 | .BR sigprocmask (2)); | |
204 | if it is not NULL, then | |
e511ffb6 | 205 | .BR pselect () |
fea681da MK |
206 | first replaces the current signal mask by the one pointed to by |
207 | .IR sigmask , | |
2d986c92 | 208 | then does the "select" function, and then restores the original |
cc9befa9 | 209 | signal mask. |
fea681da | 210 | .PP |
d02aa9bc MK |
211 | Other than the difference in the precision of the |
212 | .I timeout | |
c13182ef | 213 | argument, the following |
d02aa9bc MK |
214 | .BR pselect () |
215 | call: | |
216 | .nf | |
217 | ||
c13182ef | 218 | ready = pselect(nfds, &readfds, &writefds, &exceptfds, |
d02aa9bc MK |
219 | timeout, &sigmask); |
220 | ||
221 | .fi | |
222 | is equivalent to | |
223 | .I atomically | |
224 | executing the following calls: | |
225 | .nf | |
226 | ||
227 | sigset_t origmask; | |
228 | ||
2e72e991 | 229 | pthread_sigmask(SIG_SETMASK, &sigmask, &origmask); |
d02aa9bc | 230 | ready = select(nfds, &readfds, &writefds, &exceptfds, timeout); |
2e72e991 | 231 | pthread_sigmask(SIG_SETMASK, &origmask, NULL); |
d02aa9bc MK |
232 | .fi |
233 | .PP | |
c13182ef | 234 | The reason that |
e511ffb6 | 235 | .BR pselect () |
d02aa9bc MK |
236 | is needed is that if one wants to wait for either a signal |
237 | or for a file descriptor to become ready, then | |
c13182ef | 238 | an atomic test is needed to prevent race conditions. |
d02aa9bc | 239 | (Suppose the signal handler sets a global flag and |
c13182ef MK |
240 | returns. |
241 | Then a test of this global flag followed by a call of | |
fea681da MK |
242 | .BR select () |
243 | could hang indefinitely if the signal arrived just after the test | |
c13182ef | 244 | but just before the call. |
d02aa9bc | 245 | By contrast, |
e511ffb6 | 246 | .BR pselect () |
fea681da MK |
247 | allows one to first block signals, handle the signals that have come in, |
248 | then call | |
249 | .BR pselect () | |
250 | with the desired | |
251 | .IR sigmask , | |
252 | avoiding the race.) | |
73d8cece | 253 | .SS The timeout |
fea681da MK |
254 | The time structures involved are defined in |
255 | .I <sys/time.h> | |
256 | and look like | |
257 | ||
088a639b | 258 | .in +4n |
fea681da | 259 | .nf |
c13182ef | 260 | struct timeval { |
fea681da MK |
261 | long tv_sec; /* seconds */ |
262 | long tv_usec; /* microseconds */ | |
263 | }; | |
264 | .fi | |
a08ea57c | 265 | .in |
fea681da MK |
266 | |
267 | and | |
268 | ||
088a639b | 269 | .in +4n |
fea681da MK |
270 | .nf |
271 | struct timespec { | |
272 | long tv_sec; /* seconds */ | |
273 | long tv_nsec; /* nanoseconds */ | |
274 | }; | |
275 | .fi | |
a08ea57c | 276 | .in |
fea681da | 277 | |
e9419385 | 278 | (However, see below on the POSIX.1 versions.) |
fea681da MK |
279 | .PP |
280 | Some code calls | |
e511ffb6 | 281 | .BR select () |
fea681da | 282 | with all three sets empty, |
79b8032e | 283 | .I nfds |
22f348ca | 284 | zero, and a non-NULL |
fea681da MK |
285 | .I timeout |
286 | as a fairly portable way to sleep with subsecond precision. | |
287 | .PP | |
c13182ef | 288 | On Linux, |
e511ffb6 | 289 | .BR select () |
fea681da MK |
290 | modifies |
291 | .I timeout | |
292 | to reflect the amount of time not slept; most other implementations | |
77f00d75 | 293 | do not do this. |
e9419385 | 294 | (POSIX.1 permits either behavior.) |
77f00d75 | 295 | This causes problems both when Linux code which reads |
fea681da MK |
296 | .I timeout |
297 | is ported to other operating systems, and when code is ported to Linux | |
0c2ec4f1 | 298 | that reuses a \fIstruct timeval\fP for multiple |
e511ffb6 | 299 | .BR select ()s |
c13182ef MK |
300 | in a loop without reinitializing it. |
301 | Consider | |
fea681da MK |
302 | .I timeout |
303 | to be undefined after | |
e511ffb6 | 304 | .BR select () |
fea681da | 305 | returns. |
d9bfdb9c | 306 | .\" .PP - it is rumored that: |
fea681da MK |
307 | .\" On BSD, when a timeout occurs, the file descriptor bits are not changed. |
308 | .\" - it is certainly true that: | |
309 | .\" Linux follows SUSv2 and sets the bit masks to zero upon a timeout. | |
47297adb | 310 | .SH RETURN VALUE |
fea681da | 311 | On success, |
e511ffb6 | 312 | .BR select () |
fea681da | 313 | and |
e511ffb6 | 314 | .BR pselect () |
22f348ca MK |
315 | return the number of file descriptors contained in the three returned |
316 | descriptor sets (that is, the total number of bits that are set in | |
fea681da MK |
317 | .IR readfds , |
318 | .IR writefds , | |
319 | .IR exceptfds ) | |
320 | which may be zero if the timeout expires before anything interesting happens. | |
321 | On error, \-1 is returned, and | |
322 | .I errno | |
8dc33675 MK |
323 | is set to indicate the error; |
324 | the file descriptor sets are unmodified, | |
325 | and | |
fea681da | 326 | .I timeout |
8dc33675 | 327 | becomes undefined. |
fea681da MK |
328 | .SH ERRORS |
329 | .TP | |
330 | .B EBADF | |
331 | An invalid file descriptor was given in one of the sets. | |
c13182ef | 332 | (Perhaps a file descriptor that was already closed, |
6efed4df | 333 | or one on which an error has occurred.) |
8b58a9d4 | 334 | However, see BUGS. |
fea681da MK |
335 | .TP |
336 | .B EINTR | |
01538d0d MK |
337 | A signal was caught; see |
338 | .BR signal (7). | |
fea681da MK |
339 | .TP |
340 | .B EINVAL | |
6efed4df | 341 | .I nfds |
b9ebc9b7 MK |
342 | is negative or exceeds the |
343 | .BR RLIMIT_NOFILE | |
344 | resource limit (see | |
345 | .BR getrlimit (2)). | |
346 | .TP | |
347 | .B EINVAL | |
02959ce2 | 348 | The value contained within |
fea681da MK |
349 | .I timeout |
350 | is invalid. | |
351 | .TP | |
352 | .B ENOMEM | |
02959ce2 | 353 | Unable to allocate memory for internal tables. |
a1d5f77c MK |
354 | .SH VERSIONS |
355 | .BR pselect () | |
356 | was added to Linux in kernel 2.6.16. | |
357 | Prior to this, | |
358 | .BR pselect () | |
359 | was emulated in glibc (but see BUGS). | |
47297adb | 360 | .SH CONFORMING TO |
c13182ef | 361 | .BR select () |
e9419385 | 362 | conforms to POSIX.1-2001, POSIX.1-2008, and |
c13182ef | 363 | 4.4BSD |
cc9befa9 | 364 | .RB ( select () |
c13182ef MK |
365 | first appeared in 4.2BSD). |
366 | Generally portable to/from | |
fea681da | 367 | non-BSD systems supporting clones of the BSD socket layer (including |
efbfd7ec MK |
368 | System\ V variants). |
369 | However, note that the System\ V variant typically | |
fea681da MK |
370 | sets the timeout variable before exit, but the BSD variant does not. |
371 | .PP | |
e511ffb6 | 372 | .BR pselect () |
97c1eac8 | 373 | is defined in POSIX.1g, and in |
e9419385 | 374 | POSIX.1-2001 and POSIX.1-2008. |
fea681da | 375 | .SH NOTES |
c13182ef MK |
376 | An |
377 | .I fd_set | |
378 | is a fixed size buffer. | |
379 | Executing | |
22f348ca | 380 | .BR FD_CLR () |
c13182ef | 381 | or |
22f348ca MK |
382 | .BR FD_SET () |
383 | with a value of | |
fea681da | 384 | .I fd |
682edefb MK |
385 | that is negative or is equal to or larger than |
386 | .B FD_SETSIZE | |
387 | will result | |
c13182ef MK |
388 | in undefined behavior. |
389 | Moreover, POSIX requires | |
fea681da MK |
390 | .I fd |
391 | to be a valid file descriptor. | |
392 | ||
20cc8fa8 MK |
393 | On some other UNIX systems, |
394 | .\" Darwin, according to a report by Jeremy Sequoia, relayed by Josh Triplett | |
395 | .BR select () | |
396 | can fail with the error | |
397 | .B EAGAIN | |
398 | if the system fails to allocate kernel-internal resources, rather than | |
399 | .B ENOMEM | |
400 | as Linux does. | |
401 | POSIX specifies this error for | |
402 | .BR poll (2), | |
403 | but not for | |
404 | .BR select (). | |
405 | Portable programs may wish to check for | |
406 | .B EAGAIN | |
407 | and loop, just as with | |
408 | .BR EINTR . | |
409 | ||
3116bbe0 MK |
410 | On systems that lack |
411 | .BR pselect (), | |
412 | reliable (and more portable) signal trapping can be achieved | |
413 | using the self-pipe trick. | |
414 | In this technique, | |
415 | a signal handler writes a byte to a pipe whose other end | |
416 | is monitored by | |
417 | .BR select () | |
418 | in the main program. | |
419 | (To avoid possibly blocking when writing to a pipe that may be full | |
420 | or reading from a pipe that may be empty, | |
421 | nonblocking I/O is used when reading from and writing to the pipe.) | |
422 | ||
fea681da | 423 | Concerning the types involved, the classical situation is that |
22f348ca | 424 | the two fields of a |
c13182ef | 425 | .I timeval |
e3e25559 MK |
426 | structure are typed as |
427 | .I long | |
428 | (as shown above), and the structure is defined in | |
fea681da | 429 | .IR <sys/time.h> . |
e9419385 | 430 | The POSIX.1 situation is |
fea681da | 431 | |
088a639b | 432 | .in +4n |
fea681da MK |
433 | .nf |
434 | struct timeval { | |
435 | time_t tv_sec; /* seconds */ | |
436 | suseconds_t tv_usec; /* microseconds */ | |
437 | }; | |
438 | .fi | |
a08ea57c | 439 | .in |
fea681da | 440 | |
22f348ca | 441 | where the structure is defined in |
fea681da | 442 | .I <sys/select.h> |
c13182ef MK |
443 | and the data types |
444 | .I time_t | |
445 | and | |
446 | .I suseconds_t | |
22f348ca | 447 | are defined in |
fea681da MK |
448 | .IR <sys/types.h> . |
449 | .LP | |
450 | Concerning prototypes, the classical situation is that one should | |
451 | include | |
452 | .I <time.h> | |
453 | for | |
e511ffb6 | 454 | .BR select (). |
e9419385 | 455 | The POSIX.1 situation is that one should include |
fea681da MK |
456 | .I <sys/select.h> |
457 | for | |
e511ffb6 | 458 | .BR select () |
fea681da | 459 | and |
e511ffb6 | 460 | .BR pselect (). |
8b98e6fc | 461 | |
083e5b2f MK |
462 | Under glibc 2.0, |
463 | .I <sys/select.h> | |
464 | gives the wrong prototype for | |
8b98e6fc | 465 | .BR pselect (). |
565ee767 | 466 | Under glibc 2.1 to 2.2.1, it gives |
e511ffb6 | 467 | .BR pselect () |
fea681da MK |
468 | when |
469 | .B _GNU_SOURCE | |
8b98e6fc | 470 | is defined. |
565ee767 | 471 | Since glibc 2.2.2, the requirements are as shown in the SYNOPSIS. |
10ed041e MK |
472 | .SS Multithreaded applications |
473 | If a file descriptor being monitored by | |
474 | .BR select () | |
475 | is closed in another thread, the result is unspecified. | |
476 | On some UNIX systems, | |
477 | .BR select () | |
478 | unblocks and returns, with an indication that the file descriptor is ready | |
479 | (a subsequent I/O operation will likely fail with an error, | |
480 | unless another the file descriptor reopened between the time | |
481 | .BR select () | |
482 | returned and the I/O operations was performed). | |
483 | On Linux (and some other systems), | |
484 | closing the file descriptor in another thread has no effect on | |
485 | .BR select (). | |
486 | In summary, any application that relies on a particular behavior | |
487 | in this scenario must be considered buggy. | |
17ec2d27 | 488 | .\" |
0722a578 | 489 | .SS C library/kernel differences |
6c345305 MK |
490 | The Linux kernel allows file descriptor sets of arbitrary size, |
491 | determining the length of the sets to be checked from the value of | |
492 | .IR nfds . | |
493 | However, in the glibc implementation, the | |
494 | .IR fd_set | |
495 | type is fixed in size. | |
496 | See also BUGS. | |
497 | ||
a16eec1e | 498 | The |
77f00d75 | 499 | .BR pselect () |
a16eec1e MK |
500 | interface described in this page is implemented by glibc. |
501 | The underlying Linux system call is named | |
502 | .BR pselect6 (). | |
a59e64be | 503 | This system call has somewhat different behavior from the glibc |
a16eec1e MK |
504 | wrapper function. |
505 | ||
506 | The Linux | |
507 | .BR pselect6 () | |
c13182ef MK |
508 | system call modifies its |
509 | .I timeout | |
2f11acf5 | 510 | argument. |
d9bfdb9c | 511 | However, the glibc wrapper function hides this behavior |
77f00d75 MK |
512 | by using a local variable for the timeout argument that |
513 | is passed to the system call. | |
c13182ef | 514 | Thus, the glibc |
77f00d75 | 515 | .BR pselect () |
d53de2a7 MK |
516 | function does not modify its |
517 | .I timeout | |
518 | argument; | |
d9bfdb9c | 519 | this is the behavior required by POSIX.1-2001. |
a16eec1e MK |
520 | |
521 | The final argument of the | |
02ace852 | 522 | .BR pselect6 () |
0ab8aeec | 523 | system call is not a |
a16eec1e MK |
524 | .I "sigset_t\ *" |
525 | pointer, but is instead a structure of the form: | |
526 | .in +4 | |
527 | .nf | |
528 | ||
529 | struct { | |
1a116ea0 MK |
530 | const kernel_sigset_t *ss; /* Pointer to signal set */ |
531 | size_t ss_len; /* Size (in bytes) of object | |
532 | pointed to by 'ss' */ | |
a16eec1e MK |
533 | }; |
534 | ||
535 | .fi | |
536 | .in | |
537 | This allows the system call to obtain both | |
538 | a pointer to the signal set and its size, | |
539 | while allowing for the fact that most architectures | |
f8dcca84 MK |
540 | support a maximum of 6 arguments to a system call. |
541 | See | |
542 | .BR sigprocmask (2) | |
f25ea51b N |
543 | for a discussion of the difference between the kernel and libc |
544 | notion of the signal set. | |
fea681da | 545 | .SH BUGS |
6c345305 MK |
546 | POSIX allows an implementation to define an upper limit, |
547 | advertised via the constant | |
548 | .BR FD_SETSIZE , | |
549 | on the range of file descriptors that can be specified | |
550 | in a file descriptor set. | |
551 | The Linux kernel imposes no fixed limit, but the glibc implementation makes | |
552 | .IR fd_set | |
553 | a fixed-size type, with | |
554 | .BR FD_SETSIZE | |
555 | defined as 1024, and the | |
556 | .BR FD_* () | |
557 | macros operating according to that limit. | |
558 | To monitor file descriptors greater than 1023, use | |
559 | .BR poll (2) | |
560 | instead. | |
561 | ||
8b58a9d4 MK |
562 | According to POSIX, |
563 | .BR select () | |
564 | should check all specified file descriptors in the three file descriptor sets, | |
565 | up to the limit | |
566 | .IR nfds\-1 . | |
567 | However, the current implementation ignores any file descriptor in | |
568 | these sets that is greater than the maximum file descriptor number | |
569 | that the process currently has open. | |
570 | According to POSIX, any such file descriptor that is specified in one | |
571 | of the sets should result in the error | |
572 | .BR EBADF . | |
573 | ||
cc9befa9 | 574 | Glibc 2.0 provided a version of |
e511ffb6 | 575 | .BR pselect () |
c13182ef MK |
576 | that did not take a |
577 | .I sigmask | |
cc9befa9 MK |
578 | argument. |
579 | ||
3fa2e4b9 | 580 | Starting with version 2.1, glibc provided an emulation of |
c13182ef | 581 | .BR pselect () |
3fa2e4b9 | 582 | that was implemented using |
cc9befa9 MK |
583 | .BR sigprocmask (2) |
584 | and | |
585 | .BR select (). | |
3fa2e4b9 | 586 | This implementation remained vulnerable to the very race condition that |
cc9befa9 MK |
587 | .BR pselect () |
588 | was designed to prevent. | |
3fa2e4b9 MK |
589 | Modern versions of glibc use the (race-free) |
590 | .BR pselect () | |
591 | system call on kernels where it is provided. | |
592 | ||
fea681da | 593 | Under Linux, |
e511ffb6 | 594 | .BR select () |
fea681da | 595 | may report a socket file descriptor as "ready for reading", while |
c13182ef MK |
596 | nevertheless a subsequent read blocks. |
597 | This could for example | |
fea681da | 598 | happen when data has arrived but upon examination has wrong |
c13182ef MK |
599 | checksum and is discarded. |
600 | There may be other circumstances | |
2f11acf5 | 601 | in which a file descriptor is spuriously reported as ready. |
fea681da MK |
602 | .\" Stevens discusses a case where accept can block after select |
603 | .\" returns successfully because of an intervening RST from the client. | |
682edefb MK |
604 | Thus it may be safer to use |
605 | .B O_NONBLOCK | |
606 | on sockets that should not block. | |
fea681da | 607 | .\" Maybe the kernel should have returned EIO in such a situation? |
5766b196 MK |
608 | |
609 | On Linux, | |
610 | .BR select () | |
611 | also modifies | |
612 | .I timeout | |
613 | if the call is interrupted by a signal handler (i.e., the | |
614 | .B EINTR | |
615 | error return). | |
e9419385 | 616 | This is not permitted by POSIX.1. |
5766b196 | 617 | The Linux |
2777b1ca | 618 | .BR pselect () |
5766b196 MK |
619 | system call has the same behavior, |
620 | but the glibc wrapper hides this behavior by internally copying the | |
621 | .I timeout | |
622 | to a local variable and passing that variable to the system call. | |
2b2581ee MK |
623 | .SH EXAMPLE |
624 | .nf | |
625 | #include <stdio.h> | |
af9c7ff2 | 626 | #include <stdlib.h> |
2b2581ee MK |
627 | #include <sys/time.h> |
628 | #include <sys/types.h> | |
629 | #include <unistd.h> | |
630 | ||
631 | int | |
632 | main(void) | |
633 | { | |
634 | fd_set rfds; | |
635 | struct timeval tv; | |
636 | int retval; | |
637 | ||
638 | /* Watch stdin (fd 0) to see when it has input. */ | |
887f19e8 | 639 | |
2b2581ee MK |
640 | FD_ZERO(&rfds); |
641 | FD_SET(0, &rfds); | |
642 | ||
643 | /* Wait up to five seconds. */ | |
887f19e8 | 644 | |
2b2581ee MK |
645 | tv.tv_sec = 5; |
646 | tv.tv_usec = 0; | |
647 | ||
648 | retval = select(1, &rfds, NULL, NULL, &tv); | |
649 | /* Don't rely on the value of tv now! */ | |
650 | ||
651 | if (retval == \-1) | |
652 | perror("select()"); | |
653 | else if (retval) | |
654 | printf("Data is available now.\\n"); | |
655 | /* FD_ISSET(0, &rfds) will be true. */ | |
656 | else | |
657 | printf("No data within five seconds.\\n"); | |
658 | ||
659 | exit(EXIT_SUCCESS); | |
660 | } | |
661 | .fi | |
47297adb | 662 | .SH SEE ALSO |
fea681da MK |
663 | .BR accept (2), |
664 | .BR connect (2), | |
665 | .BR poll (2), | |
666 | .BR read (2), | |
667 | .BR recv (2), | |
25a7bfe6 | 668 | .BR restart_syscall (2), |
fea681da MK |
669 | .BR send (2), |
670 | .BR sigprocmask (2), | |
50e5322c | 671 | .BR write (2), |
1d7c4d16 MK |
672 | .BR epoll (7), |
673 | .BR time (7) | |
173fe7e7 DP |
674 | |
675 | For a tutorial with discussion and examples, see | |
676 | .BR select_tut (2). |