select_test.c

#include <stdio.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>

int main(void) {
    fd_set rfds;
    struct timeval tv;
    int retval;

    int fd1[2];

    pid_t p;

    if (pipe(fd1)==-1) {
        perror("pipe");
        return 1;
    }

    p = fork();

    if (p < 0) {
        perror("fork");
        return 1;
    }

    // Parent process
    if (p > 0) {
        FD_ZERO(&rfds);
        FD_SET(fd1[0], &rfds);
        /* Wait up to three seconds. */
        tv.tv_sec = 3;
        tv.tv_usec = 0;
        printf("tv before: %ld.%06ld\n", tv.tv_sec, tv.tv_usec);
        retval = select(fd1[0]+1, &rfds, NULL, NULL, &tv);
        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 three seconds.\n");
        printf("tv after : %ld.%06ld\n", tv.tv_sec, tv.tv_usec);
        return 0;
    } else { // child process
        sleep(1);
        write(fd1[1], "Hello", strlen("Hello"));
        close(fd1[1]);
        exit(0);
    }
}