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NAME
select, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous
I/O multiplexing
SYNOPSIS
#include <network.h>
int select(int n, fd_set *readfds, fd_set *writefds,
fd_set *exceptfds, struct timeval *timeout);
FD_CLR(int fd, fd_set *set);
FD_ISSET(int fd, fd_set *set);
FD_SET(int fd, fd_set *set);
FD_ZERO(fd_set *set);
DESCRIPTION
select waits for a number of file descriptors to change
status.
Three independent sets of descriptors are watched. Those
listed in readfds will be watched to see if characters
become available for reading, those in writefds will be
watched to see if it is ok to immediately write on them,
and those in exceptfds will be watched for exceptions. On
exit, the sets are modified in place to indicate which
descriptors actually changed status.
Four macros are provided to manipulate the sets. FD_ZERO
will clear a set. FD_SET and FD_CLR add or remove a given
descriptor from a set. FD_ISSET tests to see if a
descriptor is part of the set; this is useful after select
returns.
n is the highest-numbered descriptor in any of the three
sets, plus 1.
timeout is an upper bound on the amount of time elapsed
before select returns. It may be zero, causing select to
return immediately. If timeout is NULL (no timeout),
select can block indefinitely.
RETURN VALUE
On success, select returns the number of descriptors con-
tained in the descriptor sets, 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.
ERRORS
EBADF An invalid file descriptor was given in one of the
sets.
EINTR A non blocked signal was caught.
EINVAL n is negative.
ENOMEM select was unable to allocate memory for internal
tables.
NOTES
Some code calls select with all three sets empty, n zero,
and a non-null timeout as a fairly portable way to sleep
with subsecond precision.
EXAMPLE
#include <stdio.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)
printf("Data is available now.\n");
/* FD_ISSET(0, &rfds) will be true. */
else
printf("No data within five seconds.\n");
exit(0);
}
Generally portable to/from non-BSD systems supporting
clones of the BSD socket layer (including System V vari-
ants). However, note that the System V variant typically
sets the timeout variable before exit, but the BSD variant
does not.
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