1 .\" Hey Emacs! This file is -*- nroff -*- source.
3 .\" This manpage is copyright (C) 1992 Drew Eckhardt,
4 .\" copyright (C) 1995 Michael Shields.
6 .\" Permission is granted to make and distribute verbatim copies of this
7 .\" manual provided the copyright notice and this permission notice are
8 .\" preserved on all copies.
10 .\" Permission is granted to copy and distribute modified versions of this
11 .\" manual under the conditions for verbatim copying, provided that the
12 .\" entire resulting derived work is distributed under the terms of a
13 .\" permission notice identical to this one.
15 .\" Since the Linux kernel and libraries are constantly changing, this
16 .\" manual page may be incorrect or out-of-date. The author(s) assume no
17 .\" responsibility for errors or omissions, or for damages resulting from
18 .\" the use of the information contained herein. The author(s) may not
19 .\" have taken the same level of care in the production of this manual,
20 .\" which is licensed free of charge, as they might when working
23 .\" Formatted or processed versions of this manual, if unaccompanied by
24 .\" the source, must acknowledge the copyright and authors of this work.
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
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>
35 .\" 2005-03-11, mtk, modified pselect() text (it is now a system
38 .TH SELECT 2 2010-08-31 "Linux" "Linux Programmer's Manual"
40 select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO \-
41 synchronous I/O multiplexing
44 /* According to POSIX.1-2001 */
46 .B #include <sys/select.h>
48 /* According to earlier standards */
50 .B #include <sys/time.h>
52 .B #include <sys/types.h>
54 .B #include <unistd.h>
56 .BI "int select(int " nfds ", fd_set *" readfds ", fd_set *" writefds ,
57 .BI " fd_set *" exceptfds ", struct timeval *" timeout );
59 .BI "void FD_CLR(int " fd ", fd_set *" set );
61 .BI "int FD_ISSET(int " fd ", fd_set *" set );
63 .BI "void FD_SET(int " fd ", fd_set *" set );
65 .BI "void FD_ZERO(fd_set *" set );
67 .B #include <sys/select.h>
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 );
75 Feature Test Macro Requirements for glibc (see
76 .BR feature_test_macros (7)):
80 _POSIX_C_SOURCE\ >=\ 200112L || _XOPEN_SOURCE\ >=\ 600
85 allow a program to monitor multiple file descriptors,
86 waiting until one or more of the file descriptors become "ready"
87 for some class of I/O operation (e.g., input possible).
88 A file descriptor is considered ready if it is possible to
89 perform the corresponding I/O operation (e.g.,
97 is identical, with three differences:
101 uses a timeout that is a
103 (with seconds and microseconds), while
107 (with seconds and nanoseconds).
113 argument to indicate how much time was left.
115 does not change this argument.
121 argument, and behaves as
126 Three independent sets of file descriptors are watched.
129 will be watched to see if characters become
130 available for reading (more precisely, to see if a read will not
131 block; in particular, a file descriptor is also ready on end-of-file),
134 will be watched to see if a write will not block, and
137 will be watched for exceptions.
138 On exit, the sets are modified in place
139 to indicate which file descriptors actually changed status.
140 Each of the three file descriptor sets may be specified as NULL
141 if no file descriptors are to be watched for the corresponding class
144 Four macros are provided to manipulate the sets.
150 respectively add and remove a given file descriptor from a set.
152 tests to see if a file descriptor is part of the set;
158 is the highest-numbered file descriptor in any of the three sets, plus 1.
161 is an upper bound on the amount of time elapsed before
164 If both fields of the
166 structure are zero, then
169 (This is useful for polling.)
172 is NULL (no timeout),
174 can block indefinitely.
177 is a pointer to a signal mask (see
178 .BR sigprocmask (2));
179 if it is not NULL, then
181 first replaces the current signal mask by the one pointed to by
183 then does the "select" function, and then restores the original
186 Other than the difference in the precision of the
188 argument, the following
193 ready = pselect(nfds, &readfds, &writefds, &exceptfds,
199 executing the following calls:
204 sigprocmask(SIG_SETMASK, &sigmask, &origmask);
205 ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
206 sigprocmask(SIG_SETMASK, &origmask, NULL);
211 is needed is that if one wants to wait for either a signal
212 or for a file descriptor to become ready, then
213 an atomic test is needed to prevent race conditions.
214 (Suppose the signal handler sets a global flag and
216 Then a test of this global flag followed by a call of
218 could hang indefinitely if the signal arrived just after the test
219 but just before the call.
222 allows one to first block signals, handle the signals that have come in,
229 The time structures involved are defined in
236 long tv_sec; /* seconds */
237 long tv_usec; /* microseconds */
247 long tv_sec; /* seconds */
248 long tv_nsec; /* nanoseconds */
253 (However, see below on the POSIX.1-2001 versions.)
257 with all three sets empty,
261 as a fairly portable way to sleep with subsecond precision.
267 to reflect the amount of time not slept; most other implementations
269 (POSIX.1-2001 permits either behavior.)
270 This causes problems both when Linux code which reads
272 is ported to other operating systems, and when code is ported to Linux
273 that reuses a \fIstruct timeval\fP for multiple
275 in a loop without reinitializing it.
278 to be undefined after
281 .\" .PP - it is rumored that:
282 .\" On BSD, when a timeout occurs, the file descriptor bits are not changed.
283 .\" - it is certainly true that:
284 .\" Linux follows SUSv2 and sets the bit masks to zero upon a timeout.
290 return the number of file descriptors contained in the three returned
291 descriptor sets (that is, the total number of bits that are set in
295 which may be zero if the timeout expires before anything interesting happens.
296 On error, \-1 is returned, and
298 is set appropriately; the sets and
300 become undefined, so do not
301 rely on their contents after an error.
305 An invalid file descriptor was given in one of the sets.
306 (Perhaps a file descriptor that was already closed,
307 or one on which an error has occurred.)
310 A signal was caught; see
315 is negative or the value contained within
320 unable to allocate memory for internal tables.
323 was added to Linux in kernel 2.6.16.
326 was emulated in glibc (but see BUGS).
329 conforms to POSIX.1-2001 and
332 first appeared in 4.2BSD).
333 Generally portable to/from
334 non-BSD systems supporting clones of the BSD socket layer (including
336 However, note that the System V variant typically
337 sets the timeout variable before exit, but the BSD variant does not.
340 is defined in POSIX.1g, and in
345 is a fixed size buffer.
352 that is negative or is equal to or larger than
355 in undefined behavior.
356 Moreover, POSIX requires
358 to be a valid file descriptor.
360 Concerning the types involved, the classical situation is that
363 structure are typed as
365 (as shown above), and the structure is defined in
367 The POSIX.1-2001 situation is
372 time_t tv_sec; /* seconds */
373 suseconds_t tv_usec; /* microseconds */
378 where the structure is defined in
387 Concerning prototypes, the classical situation is that one should
392 The POSIX.1-2001 situation is that one should include
399 Libc4 and libc5 do not have a
401 header; under glibc 2.0 and later this header exists.
402 Under glibc 2.0 it unconditionally gives the wrong prototype for
404 Under glibc 2.1 to 2.2.1 it gives
409 Since glibc 2.2.2 the requirements are as shown in the SYNOPSIS.
413 system call modifies its
416 However, the glibc wrapper function hides this behavior
417 by using a local variable for the timeout argument that
418 is passed to the system call.
421 function does not modify its
424 this is the behavior required by POSIX.1-2001.
426 Glibc 2.0 provided a version of
432 Starting with version 2.1, glibc provided an emulation of
434 that was implemented using
438 This implementation remained vulnerable to the very race condition that
440 was designed to prevent.
441 Modern versions of glibc use the (race-free)
443 system call on kernels where it is provided.
447 reliable (and more portable) signal trapping can be achieved
448 using the self-pipe trick
449 (where a signal handler writes a byte to a pipe whose other end
452 in the main program.)
456 may report a socket file descriptor as "ready for reading", while
457 nevertheless a subsequent read blocks.
458 This could for example
459 happen when data has arrived but upon examination has wrong
460 checksum and is discarded.
461 There may be other circumstances
462 in which a file descriptor is spuriously reported as ready.
463 .\" Stevens discusses a case where accept can block after select
464 .\" returns successfully because of an intervening RST from the client.
465 Thus it may be safer to use
467 on sockets that should not block.
468 .\" Maybe the kernel should have returned EIO in such a situation?
474 if the call is interrupted by a signal handler (i.e., the
477 This is not permitted by POSIX.1-2001.
480 system call has the same behavior,
481 but the glibc wrapper hides this behavior by internally copying the
483 to a local variable and passing that variable to the system call.
488 #include <sys/time.h>
489 #include <sys/types.h>
499 /* Watch stdin (fd 0) to see when it has input. */
503 /* Wait up to five seconds. */
507 retval = select(1, &rfds, NULL, NULL, &tv);
508 /* Don't rely on the value of tv now! */
513 printf("Data is available now.\\n");
514 /* FD_ISSET(0, &rfds) will be true. */
516 printf("No data within five seconds.\\n");
522 For a tutorial with discussion and examples, see
525 For vaguely related stuff, see