1 .\" This manpage is copyright (C) 1992 Drew Eckhardt,
2 .\" copyright (C) 1995 Michael Shields.
4 .\" %%%LICENSE_START(VERBATIM)
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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15 .\" manual page may be incorrect or out-of-date.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
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17 .\" the use of the information contained herein. The author(s) may not
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19 .\" which is licensed free of charge, as they might when working
22 .\" Formatted or processed versions of this manual, if unaccompanied by
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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 2016-03-15 "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, POSIX.1-2008 */
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
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 a corresponding I/O operation (e.g.,
91 without blocking, or a sufficiently small
95 can monitor only file descriptors numbers that are less than
98 does not have this limitation.
105 is identical, other than these three differences:
109 uses a timeout that is a
111 (with seconds and microseconds), while
115 (with seconds and nanoseconds).
121 argument to indicate how much time was left.
123 does not change this argument.
129 argument, and behaves as
134 Three independent sets of file descriptors are watched.
137 will be watched to see if characters become
138 available for reading (more precisely, to see if a read will not
139 block; in particular, a file descriptor is also ready on end-of-file),
142 will be watched to see if space is available for write (though a large
143 write may still block), and those in
145 will be watched for exceptions.
146 On exit, the sets are modified in place
147 to indicate which file descriptors actually changed status.
148 Each of the three file descriptor sets may be specified as NULL
149 if no file descriptors are to be watched for the corresponding class
152 Four macros are provided to manipulate the sets.
158 respectively add and remove a given file descriptor from a set.
160 tests to see if a file descriptor is part of the set;
166 is the highest-numbered file descriptor in any of the three sets, plus 1.
170 argument specifies the interval that
172 should block waiting for a file descriptor to become ready.
173 The call will block until either:
175 a file descriptor becomes ready;
177 the call is interrupted by a signal handler; or
183 interval will be rounded up to the system clock granularity,
184 and kernel scheduling delays mean that the blocking interval
185 may overrun by a small amount.
186 If both fields of the
188 structure are zero, then
191 (This is useful for polling.)
194 is NULL (no timeout),
196 can block indefinitely.
199 is a pointer to a signal mask (see
200 .BR sigprocmask (2));
201 if it is not NULL, then
203 first replaces the current signal mask by the one pointed to by
205 then does the "select" function, and then restores the original
208 Other than the difference in the precision of the
210 argument, the following
215 ready = pselect(nfds, &readfds, &writefds, &exceptfds,
221 executing the following calls:
226 pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
227 ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
228 pthread_sigmask(SIG_SETMASK, &origmask, NULL);
233 is needed is that if one wants to wait for either a signal
234 or for a file descriptor to become ready, then
235 an atomic test is needed to prevent race conditions.
236 (Suppose the signal handler sets a global flag and
238 Then a test of this global flag followed by a call of
240 could hang indefinitely if the signal arrived just after the test
241 but just before the call.
244 allows one to first block signals, handle the signals that have come in,
251 The time structures involved are defined in
258 long tv_sec; /* seconds */
259 long tv_usec; /* microseconds */
269 long tv_sec; /* seconds */
270 long tv_nsec; /* nanoseconds */
275 (However, see below on the POSIX.1 versions.)
279 with all three sets empty,
283 as a fairly portable way to sleep with subsecond precision.
289 to reflect the amount of time not slept; most other implementations
291 (POSIX.1 permits either behavior.)
292 This causes problems both when Linux code which reads
294 is ported to other operating systems, and when code is ported to Linux
295 that reuses a \fIstruct timeval\fP for multiple
297 in a loop without reinitializing it.
300 to be undefined after
303 .\" .PP - it is rumored that:
304 .\" On BSD, when a timeout occurs, the file descriptor bits are not changed.
305 .\" - it is certainly true that:
306 .\" Linux follows SUSv2 and sets the bit masks to zero upon a timeout.
312 return the number of file descriptors contained in the three returned
313 descriptor sets (that is, the total number of bits that are set in
317 which may be zero if the timeout expires before anything interesting happens.
318 On error, \-1 is returned, and
320 is set to indicate the error;
321 the file descriptor sets are unmodified,
328 An invalid file descriptor was given in one of the sets.
329 (Perhaps a file descriptor that was already closed,
330 or one on which an error has occurred.)
333 A signal was caught; see
338 is negative or exceeds the
344 The value contained within
349 Unable to allocate memory for internal tables.
352 was added to Linux in kernel 2.6.16.
355 was emulated in glibc (but see BUGS).
358 conforms to POSIX.1-2001, POSIX.1-2008, and
361 first appeared in 4.2BSD).
362 Generally portable to/from
363 non-BSD systems supporting clones of the BSD socket layer (including
365 However, note that the System\ V variant typically
366 sets the timeout variable before exit, but the BSD variant does not.
369 is defined in POSIX.1g, and in
370 POSIX.1-2001 and POSIX.1-2008.
374 is a fixed size buffer.
381 that is negative or is equal to or larger than
384 in undefined behavior.
385 Moreover, POSIX requires
387 to be a valid file descriptor.
389 On some other UNIX systems,
390 .\" Darwin, according to a report by Jeremy Sequoia, relayed by Josh Triplett
392 can fail with the error
394 if the system fails to allocate kernel-internal resources, rather than
397 POSIX specifies this error for
401 Portable programs may wish to check for
403 and loop, just as with
406 Concerning the types involved, the classical situation is that
409 structure are typed as
411 (as shown above), and the structure is defined in
413 The POSIX.1 situation is
418 time_t tv_sec; /* seconds */
419 suseconds_t tv_usec; /* microseconds */
424 where the structure is defined in
433 Concerning prototypes, the classical situation is that one should
438 The POSIX.1 situation is that one should include
447 gives the wrong prototype for
449 Under glibc 2.1 to 2.2.1, it gives
454 Since glibc 2.2.2, the requirements are as shown in the SYNOPSIS.
455 .SS Multithreaded applications
456 If a file descriptor being monitored by
458 is closed in another thread, the result is unspecified.
459 On some UNIX systems,
461 unblocks and returns, with an indication that the file descriptor is ready
462 (a subsequent I/O operation will likely fail with an error,
463 unless another the file descriptor reopened between the time
465 returned and the I/O operations was performed).
466 On Linux (and some other systems),
467 closing the file descriptor in another thread has no effect on
469 In summary, any application that relies on a particular behavior
470 in this scenario must be considered buggy.
472 .SS C library/kernel differences
473 The Linux kernel allows file descriptor sets of arbitrary size,
474 determining the length of the sets to be checked from the value of
476 However, in the glibc implementation, the
478 type is fixed in size.
483 interface described in this page is implemented by glibc.
484 The underlying Linux system call is named
486 This system call has somewhat different behavior from the glibc
491 system call modifies its
494 However, the glibc wrapper function hides this behavior
495 by using a local variable for the timeout argument that
496 is passed to the system call.
499 function does not modify its
502 this is the behavior required by POSIX.1-2001.
504 The final argument of the
508 pointer, but is instead a structure of the form:
513 const sigset_t *ss; /* Pointer to signal set */
514 size_t ss_len; /* Size (in bytes) of object pointed
520 This allows the system call to obtain both
521 a pointer to the signal set and its size,
522 while allowing for the fact that most architectures
523 support a maximum of 6 arguments to a system call.
525 POSIX allows an implementation to define an upper limit,
526 advertised via the constant
528 on the range of file descriptors that can be specified
529 in a file descriptor set.
530 The Linux kernel imposes no fixed limit, but the glibc implementation makes
532 a fixed-size type, with
534 defined as 1024, and the
536 macros operating according to that limit.
537 To monitor file descriptors greater than 1023, use
541 Glibc 2.0 provided a version of
547 Starting with version 2.1, glibc provided an emulation of
549 that was implemented using
553 This implementation remained vulnerable to the very race condition that
555 was designed to prevent.
556 Modern versions of glibc use the (race-free)
558 system call on kernels where it is provided.
562 reliable (and more portable) signal trapping can be achieved
563 using the self-pipe trick.
565 a signal handler writes a byte to a pipe whose other end
569 (To avoid possibly blocking when writing to a pipe that may be full
570 or reading from a pipe that may be empty,
571 nonblocking I/O is used when reading from and writing to the pipe.)
575 may report a socket file descriptor as "ready for reading", while
576 nevertheless a subsequent read blocks.
577 This could for example
578 happen when data has arrived but upon examination has wrong
579 checksum and is discarded.
580 There may be other circumstances
581 in which a file descriptor is spuriously reported as ready.
582 .\" Stevens discusses a case where accept can block after select
583 .\" returns successfully because of an intervening RST from the client.
584 Thus it may be safer to use
586 on sockets that should not block.
587 .\" Maybe the kernel should have returned EIO in such a situation?
593 if the call is interrupted by a signal handler (i.e., the
596 This is not permitted by POSIX.1.
599 system call has the same behavior,
600 but the glibc wrapper hides this behavior by internally copying the
602 to a local variable and passing that variable to the system call.
607 #include <sys/time.h>
608 #include <sys/types.h>
618 /* Watch stdin (fd 0) to see when it has input. */
623 /* Wait up to five seconds. */
628 retval = select(1, &rfds, NULL, NULL, &tv);
629 /* Don't rely on the value of tv now! */
634 printf("Data is available now.\\n");
635 /* FD_ISSET(0, &rfds) will be true. */
637 printf("No data within five seconds.\\n");
648 .BR restart_syscall (2),
655 For a tutorial with discussion and examples, see