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);
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.
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.
#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); }