NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON
SELECT(2) Linux Programmer's Manual SELECT(2)
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O multi-
plexing
/* According to POSIX.1-2001 */
#include <sys/select.h>
/* According to earlier standards */
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
int select(int nfds, fd_set *readfds, fd_set *writefds,
fd_set *exceptfds, struct timeval *timeout);
void FD_CLR(int fd, fd_set *set);
int FD_ISSET(int fd, fd_set *set);
void FD_SET(int fd, fd_set *set);
void FD_ZERO(fd_set *set);
#include <sys/select.h>
int pselect(int nfds, fd_set *readfds, fd_set *writefds,
fd_set *exceptfds, const struct timespec *timeout,
const sigset_t *sigmask);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
pselect(): _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600
select() and pselect() allow a program to monitor multiple file descriptors,
waiting until one or more of the file descriptors become "ready" for some
class of I/O operation (e.g., input possible). A file descriptor is
considered ready if it is possible to perform the corresponding I/O operation
(e.g., read(2)) without blocking.
The operation of select() and pselect() is identical, with three differences:
(i) select() uses a timeout that is a struct timeval (with seconds and
microseconds), while pselect() uses a struct timespec (with seconds and
nanoseconds).
(ii) select() may update the timeout argument to indicate how much time was
left. pselect() does not change this argument.
(iii) select() has no sigmask argument, and behaves as pselect() called with
NULL sigmask.
Three independent sets of file descriptors are watched. Those listed in
readfds will be watched to see if characters become available for reading
(more precisely, to see if a read will not block; in particular, a file
descriptor is also ready on end-of-file), those in writefds will be watched to
see if a write will not block, and those in exceptfds will be watched for
exceptions. On exit, the sets are modified in place to indicate which file
descriptors actually changed status. Each of the three file descriptor sets
may be specified as NULL if no file descriptors are to be watched for the
corresponding class of events.
Four macros are provided to manipulate the sets. FD_ZERO() clears a set.
FD_SET() and FD_CLR() respectively add and remove a given file descriptor from
a set. FD_ISSET() tests to see if a file descriptor is part of the set; this
is useful after select() returns.
nfds is the highest-numbered file descriptor in any of the three sets, plus 1.
timeout is an upper bound on the amount of time elapsed before select()
returns. If both fields of the timeval stucture are zero, then select()
returns immediately. (This is useful for polling.) If timeout is NULL (no
timeout), select() can block indefinitely.
sigmask is a pointer to a signal mask (see sigprocmask(2)); if it is not NULL,
then pselect() first replaces the current signal mask by the one pointed to by
sigmask, then does the "select" function, and then restores the original
signal mask.
Other than the difference in the precision of the timeout argument, the
following pselect() call:
ready = pselect(nfds, &readfds, &writefds, &exceptfds,
timeout, &sigmask);
is equivalent to atomically executing the following calls:
sigset_t origmask;
sigprocmask(SIG_SETMASK, &sigmask, &origmask);
ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
sigprocmask(SIG_SETMASK, &origmask, NULL);
The reason that pselect() is needed is that if one wants to wait for either a
signal or for a file descriptor to become ready, then an atomic test is needed
to prevent race conditions. (Suppose the signal handler sets a global flag
and returns. Then a test of this global flag followed by a call of select()
could hang indefinitely if the signal arrived just after the test but just
before the call. By contrast, pselect() allows one to first block signals,
handle the signals that have come in, then call pselect() with the desired
sigmask, avoiding the race.)
The time structures involved are defined in <sys/time.h> and look like
struct timeval {
long tv_sec; /* seconds */
long tv_usec; /* microseconds */
};
and
struct timespec {
long tv_sec; /* seconds */
long tv_nsec; /* nanoseconds */
};
(However, see below on the POSIX.1-2001 versions.)
Some code calls select() with all three sets empty, nfds zero, and a non-NULL
timeout as a fairly portable way to sleep with subsecond precision.
On Linux, select() modifies timeout to reflect the amount of time not slept;
most other implementations do not do this. (POSIX.1-2001 permits either
behavior.) This causes problems both when Linux code which reads timeout is
ported to other operating systems, and when code is ported to Linux that
reuses a struct timeval for multiple select()s in a loop without
reinitializing it. Consider timeout to be undefined after select() returns.
On success, select() and pselect() return the number of file descriptors
contained in the three returned descriptor sets (that is, the total number of
bits that are set in readfds, writefds, exceptfds) which may be zero if the
timeout expires before anything interesting happens. On error, -1 is
returned, and errno is set appropriately; the sets and timeout become
undefined, so do not rely on their contents after an error.
EBADF An invalid file descriptor was given in one of the sets. (Perhaps a
file descriptor that was already closed, or one on which an error has
occurred.)
EINTR A signal was caught; see signal(7).
EINVAL nfds is negative or the value contained within timeout is invalid.
ENOMEM unable to allocate memory for internal tables.
pselect() was added to Linux in kernel 2.6.16. Prior to this, pselect() was
emulated in glibc (but see BUGS).
select() conforms to POSIX.1-2001 and 4.4BSD (select() first appeared in
4.2BSD). Generally portable to/from non-BSD systems supporting clones of the
BSD socket layer (including System V variants). However, note that the System
V variant typically sets the timeout variable before exit, but the BSD variant
does not.
pselect() is defined in POSIX.1g, and in POSIX.1-2001.
An fd_set is a fixed size buffer. Executing FD_CLR() or FD_SET() with a value
of fd that is negative or is equal to or larger than FD_SETSIZE will result in
undefined behavior. Moreover, POSIX requires fd to be a valid file
descriptor.
Concerning the types involved, the classical situation is that the two fields
of a timeval structure are typed as long (as shown above), and the structure
is defined in <sys/time.h>. The POSIX.1-2001 situation is
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
where the structure is defined in <sys/select.h> and the data types time_t and
suseconds_t are defined in <sys/types.h>.
Concerning prototypes, the classical situation is that one should include
<time.h> for select(). The POSIX.1-2001 situation is that one should include
<sys/select.h> for select() and pselect().
Libc4 and libc5 do not have a <sys/select.h> header; under glibc 2.0 and later
this header exists. Under glibc 2.0 it unconditionally gives the wrong
prototype for pselect(). Under glibc 2.1 to 2.2.1 it gives pselect() when
_GNU_SOURCE is defined. Since glibc 2.2.2 the requirements are as shown in
the SYNOPSIS.
The Linux pselect() system call modifies its timeout argument. However, the
glibc wrapper function hides this behavior by using a local variable for the
timeout argument that is passed to the system call. Thus, the glibc pselect()
function does not modify its timeout argument; this is the behavior required
by POSIX.1-2001.
Glibc 2.0 provided a version of pselect() that did not take a sigmask
argument.
Since version 2.1, glibc has provided an emulation of pselect() that is
implemented using sigprocmask(2) and select(). This implementation remains
vulnerable to the very race condition that pselect() was designed to prevent.
On systems that lack pselect(), reliable (and more portable) signal trapping
can be achieved using the self-pipe trick (where a signal handler writes a
byte to a pipe whose other end is monitored by select() in the main program.)
Under Linux, select() may report a socket file descriptor as "ready for
reading", while nevertheless a subsequent read blocks. This could for example
happen when data has arrived but upon examination has wrong checksum and is
discarded. There may be other circumstances in which a file descriptor is
spuriously reported as ready. Thus it may be safer to use O_NONBLOCK on
sockets that should not block.
On Linux, select() also modifies timeout if the call is interrupted by a
signal handler (i.e., the EINTR error return). This is not permitted by
POSIX.1-2001. The Linux pselect() system call has the same behavior, but the
glibc wrapper hides this behavior by internally copying the timeout to a local
variable and passing that variable to the system call.
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
int
main(void)
{
fd_set rfds;
struct timeval tv;
int retval;
/* Watch stdin (fd 0) to see when it has input. */
FD_ZERO(&rfds);
FD_SET(0, &rfds);
/* Wait up to five seconds. */
tv.tv_sec = 5;
tv.tv_usec = 0;
retval = select(1, &rfds, NULL, NULL, &tv);
/* Don't rely on the value of tv now! */
if (retval == -1)
perror("select()");
else if (retval)
printf("Data is available now.\n");
/* FD_ISSET(0, &rfds) will be true. */
else
printf("No data within five seconds.\n");
exit(EXIT_SUCCESS);
}
For a tutorial with discussion and examples, see select_tut(2).
For vaguely related stuff, see accept(2), connect(2), poll(2), read(2),
recv(2), send(2), sigprocmask(2), write(2), epoll(7), time(7)
This page is part of release 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2008-12-05 SELECT(2)