1 .\" This manpage is copyright (C) 1992 Drew Eckhardt,
2 .\" copyright (C) 1995 Michael Shields.
4 .\" %%%LICENSE_START(VERBATIM)
5 .\" Permission is granted to make and distribute verbatim copies of this
6 .\" manual provided the copyright notice and this permission notice are
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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.
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15 .\" manual page may be incorrect or out-of-date.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
22 .\" Formatted or processed versions of this manual, if unaccompanied by
23 .\" 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 2014-01-31 "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, other than these 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.
162 argument specifies the interval that
164 should block waiting for a file descriptor to become ready.
165 The call will block until either:
167 a file descriptor becomes ready;
169 the call is interrupted by a signal handler; or
175 interval will be rounded up to the system clock granularity,
176 and kernel scheduling delays mean that the blocking interval
177 may overrun by a small amount.
178 If both fields of the
180 structure are zero, then
183 (This is useful for polling.)
186 is NULL (no timeout),
188 can block indefinitely.
191 is a pointer to a signal mask (see
192 .BR sigprocmask (2));
193 if it is not NULL, then
195 first replaces the current signal mask by the one pointed to by
197 then does the "select" function, and then restores the original
200 Other than the difference in the precision of the
202 argument, the following
207 ready = pselect(nfds, &readfds, &writefds, &exceptfds,
213 executing the following calls:
218 pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
219 ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
220 pthread_sigmask(SIG_SETMASK, &origmask, NULL);
225 is needed is that if one wants to wait for either a signal
226 or for a file descriptor to become ready, then
227 an atomic test is needed to prevent race conditions.
228 (Suppose the signal handler sets a global flag and
230 Then a test of this global flag followed by a call of
232 could hang indefinitely if the signal arrived just after the test
233 but just before the call.
236 allows one to first block signals, handle the signals that have come in,
243 The time structures involved are defined in
250 long tv_sec; /* seconds */
251 long tv_usec; /* microseconds */
261 long tv_sec; /* seconds */
262 long tv_nsec; /* nanoseconds */
267 (However, see below on the POSIX.1-2001 versions.)
271 with all three sets empty,
275 as a fairly portable way to sleep with subsecond precision.
281 to reflect the amount of time not slept; most other implementations
283 (POSIX.1-2001 permits either behavior.)
284 This causes problems both when Linux code which reads
286 is ported to other operating systems, and when code is ported to Linux
287 that reuses a \fIstruct timeval\fP for multiple
289 in a loop without reinitializing it.
292 to be undefined after
295 .\" .PP - it is rumored that:
296 .\" On BSD, when a timeout occurs, the file descriptor bits are not changed.
297 .\" - it is certainly true that:
298 .\" Linux follows SUSv2 and sets the bit masks to zero upon a timeout.
304 return the number of file descriptors contained in the three returned
305 descriptor sets (that is, the total number of bits that are set in
309 which may be zero if the timeout expires before anything interesting happens.
310 On error, \-1 is returned, and
312 is set to indicate the error;
313 the file descriptor sets are unmodified,
320 An invalid file descriptor was given in one of the sets.
321 (Perhaps a file descriptor that was already closed,
322 or one on which an error has occurred.)
325 A signal was caught; see
330 is negative or the value contained within
335 unable to allocate memory for internal tables.
338 was added to Linux in kernel 2.6.16.
341 was emulated in glibc (but see BUGS).
344 conforms to POSIX.1-2001 and
347 first appeared in 4.2BSD).
348 Generally portable to/from
349 non-BSD systems supporting clones of the BSD socket layer (including
351 However, note that the System\ V variant typically
352 sets the timeout variable before exit, but the BSD variant does not.
355 is defined in POSIX.1g, and in
360 is a fixed size buffer.
367 that is negative or is equal to or larger than
370 in undefined behavior.
371 Moreover, POSIX requires
373 to be a valid file descriptor.
375 Concerning the types involved, the classical situation is that
378 structure are typed as
380 (as shown above), and the structure is defined in
382 The POSIX.1-2001 situation is
387 time_t tv_sec; /* seconds */
388 suseconds_t tv_usec; /* microseconds */
393 where the structure is defined in
402 Concerning prototypes, the classical situation is that one should
407 The POSIX.1-2001 situation is that one should include
414 Libc4 and libc5 do not have a
416 header; under glibc 2.0 and later this header exists.
417 Under glibc 2.0 it unconditionally gives the wrong prototype for
419 Under glibc 2.1 to 2.2.1 it gives
424 Since glibc 2.2.2 the requirements are as shown in the SYNOPSIS.
425 .SS Multithreaded applications
426 If a file descriptor being monitored by
428 is closed in another thread, the result is unspecified.
429 On some UNIX systems,
431 unblocks and returns, with an indication that the file descriptor is ready
432 (a subsequent I/O operation will likely fail with an error,
433 unless another the file descriptor reopened between the time
435 returned and the I/O operations was performed).
436 On Linux (and some other systems),
437 closing the file descriptor in another thread has no effect on
439 In summary, any application that relies on a particular behavior
440 in this scenario must be considered buggy.
444 interface described in this page is implemented by glibc.
445 The underlying Linux system call is named
447 This system call has somewhat different behavior from the glibc
452 system call modifies its
455 However, the glibc wrapper function hides this behavior
456 by using a local variable for the timeout argument that
457 is passed to the system call.
460 function does not modify its
463 this is the behavior required by POSIX.1-2001.
465 The final argument of the
469 pointer, but is instead a structure of the form:
474 const sigset_t *ss; /* Pointer to signal set */
475 size_t ss_len; /* Size (in bytes) of object pointed
481 This allows the system call to obtain both
482 a pointer to the signal set and its size,
483 while allowing for the fact that most architectures
484 support a maximum of 6 arguments to a system call.
486 Glibc 2.0 provided a version of
492 Starting with version 2.1, glibc provided an emulation of
494 that was implemented using
498 This implementation remained vulnerable to the very race condition that
500 was designed to prevent.
501 Modern versions of glibc use the (race-free)
503 system call on kernels where it is provided.
507 reliable (and more portable) signal trapping can be achieved
508 using the self-pipe trick.
510 a signal handler writes a byte to a pipe whose other end
514 (To avoid possibly blocking when writing to a pipe that may be full
515 or reading from a pipe that may be empty,
516 nonblocking I/O is used when reading from and writing to the pipe.)
520 may report a socket file descriptor as "ready for reading", while
521 nevertheless a subsequent read blocks.
522 This could for example
523 happen when data has arrived but upon examination has wrong
524 checksum and is discarded.
525 There may be other circumstances
526 in which a file descriptor is spuriously reported as ready.
527 .\" Stevens discusses a case where accept can block after select
528 .\" returns successfully because of an intervening RST from the client.
529 Thus it may be safer to use
531 on sockets that should not block.
532 .\" Maybe the kernel should have returned EIO in such a situation?
538 if the call is interrupted by a signal handler (i.e., the
541 This is not permitted by POSIX.1-2001.
544 system call has the same behavior,
545 but the glibc wrapper hides this behavior by internally copying the
547 to a local variable and passing that variable to the system call.
552 #include <sys/time.h>
553 #include <sys/types.h>
563 /* Watch stdin (fd 0) to see when it has input. */
567 /* Wait up to five seconds. */
571 retval = select(1, &rfds, NULL, NULL, &tv);
572 /* Don't rely on the value of tv now! */
577 printf("Data is available now.\\n");
578 /* FD_ISSET(0, &rfds) will be true. */
580 printf("No data within five seconds.\\n");
597 For a tutorial with discussion and examples, see
600 This page is part of release 3.67 of the Linux
603 A description of the project,
604 information about reporting bugs,
605 and the latest version of this page,
607 \%http://www.kernel.org/doc/man\-pages/.