/*

    This program illustrates the use of fork() to run
    an external command in the background.
    
*/    

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

int main()
{

    pid_t id;
    pid_t pid;
    char seconds[5];

    while (1)
    {

        printf("Sleep for how many seconds?  ");
        scanf("%s", seconds);

        /* Create our external sleep command. */

        char* argv[3] = {"sleep", seconds, 0};

        /* Create a child process for the sleep command. */
   
        pid = fork();

        /* If it failed, exit program. */

        if (pid < 0)
        {
    
            fprintf(stderr, "\nFailed to fork!\n");

            return 0;

        }

        /* Child process executes the sleep command. */

        if (pid == 0)
        {

            printf("The child is running the command \"sleep %s\".\n", seconds);
            execv("/bin/sleep", argv);
            fprintf(stderr, "\nsleep failed!\n");

            exit(EXIT_FAILURE);
    
        }
    
        /*

            Parent process still uses waitpid(), but doesn't use
            a blocking wait.  So, think of it as a periodic check
            on each loop iteration.  If id > 0, we know a child has
            finished, so print a statement.

        */

        do
        {

            id = waitpid(-1, NULL, WNOHANG);

            if (id > 0)
                printf("The child with ID %d has terminated.\n", id);

        } while (id > 0);

    }

    return 0;

}