select, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous
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);
select waits for a number of file descriptors to change
Three independent sets of 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 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
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.
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.
EBADF An invalid file descriptor was given in one of the
EINTR A non blocked signal was caught.
EINVAL n is negative.
ENOMEM select was unable to allocate memory for internal
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.
On Linux, timeout is modified to reflect the amount of
time not slept; most other implementations do not do this.
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 selects in a loop without reinitializing it.
Consider timeout to be undefined after select returns.
struct timeval tv;
/* Watch stdin (fd 0) to see when it has input. */
/* 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! */
printf("Data is available now.\n");
/* FD_ISSET(0, &rfds) will be true. */
printf("No data within five seconds.\n");
4.4BSD (the select function first appeared in 4.2BSD).
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
accept(2), connect(2), poll(2), read(2), recv(2), send(2),