#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
int main(void) {
    // open and set up a bunch of sockets (not shown)
    // main loop
    while (1) {
        // initialize the fd_set to all zero
        fd_set readFDs;
        // now set the bits for the descriptors
        // this server is interested in
        // (for simplicity, all of them from min to max)
        int fd;
        for (fd = minFD; fd < maxFD; fd++)
            FD_SET(fd, &readFDs);
        // do the select
        int rc = select(maxFD+1, &readFDs, NULL, NULL, NULL);
        // check which actually have data using FD_ISSET()
        int fd;
        for (fd = minFD; fd < maxFD; fd++)
            if (FD_ISSET(fd, &readFDs))

Return Value

select() returns the number of ready descriptors that are contained in the descriptor sets, or -1 if an error occurred.