程序代码：

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

void sig_chld(int); /* prototype for our SIGCHLD handler */

int main(void) {
    struct sigaction act;
    pid_t pid;
    
    /* Assign sig_chld as our SIGCHLD handler */
    act.sa_handler = sig_chld;

    /* We don't want to block any other signals in this example*/
    sigemptyset(&act.sa_mask);

    /* 
     * We're only interested in children that have terminated,
     * not ones which have been stopped(eg user pressing 
     * control-Z at terminal)
     */
    act.sa_flags= SA_NOCLDSTOP;

    /*
     * Make these values effective. If we were writng
     * a real application, we would probably save
     * the old value instead of passing NULL.
     */
    if( sigaction(SIGCHLD, &act, NULL) < 0) {
        fprintf(stderr, "sigaction failed\n");
        return 1;
    }
    
    /* Fork */
    switch(pid = fork()) {
        case -1:
            fprintf(stderr, "ford failed\n");
            return 1;
        case 0:         /* child-- finish straight away */
            _exit(7)    /* exit status = 7 */
        default:        /* parent */
            sleep(10);        /* give child time to finish */
    }
    return 0;
}

/*
 * The signal handler function -- only gets called when
 * a SIGCHLD is received, ie when a child terminate
 */

void sig_chld(int sigo) {
    int status, child_val;
    
    /* Wait for any child without blocking */
    if(waitpid(-1, &status, WNOHANG) < 0) {
        /*
         * calling standard I/O functions like
         * fprintf() in a signal handler is not 
         * recommended, but probably OK in 
         * toy programs like this one.
         */
        fprintf(stderr, "waipid failed\n");
        return;
    }
    
    /*
     * We now have the info in 'status' and 
     * and manipulate it using the macros in 
     * wait.h.
     */
    if(WIFEXITED(status)) {
        child_val = WEXITSTATUS(status);
        printf("child's exited normally with status %d\n", child_val);
    }
}