子进程的使用，父进程根据signal监听子进程的退出。预防僵尸进程的两种方法。

fork_fork.c

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

int main(void)
{
   pid_t pid;

    if ((pid = fork()) < 0)
    {
        fprintf(stderr,"Fork error!/n");
        exit(-1);
    }
    else if (pid == 0) /* first child */
    { 
        if ((pid = fork()) < 0)
        { 
            fprintf(stderr,"Fork error!/n");
            exit(-1);
        }
        else if (pid > 0)
            exit(0); /* parent from second fork == first child */
        /*
         * We're the second child; our parent becomes init as soon
         * as our real parent calls exit() in the statement above.
         * Here's where we'd continue executing, knowing that when
         * we're done, init will reap our status.
         */
        sleep(2);
        printf("Second child, parent pid = %d/n", getppid());
        exit(0);
    }
    
    if (waitpid(pid, NULL, 0) != pid) /* wait for first child */
    {
        fprintf(stderr,"Waitpid error!/n");
        exit(-1);
    }

    /*
     * We're the parent (the original process); we continue executing,
     * knowing that we're not the parent of the second child.
     */
    exit(0);
}

fork_waitby_signal.c

#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <stdlib.h>
#include <signal.h>

static void sig_child(int signo);

int main()
{
    pid_t pid;
    //创建捕捉子进程退出信号
    signal(SIGCHLD,sig_child);
    //循环创建子进程
    while(1)
    {
        pid = fork();
        if (pid < 0)
        {
            perror("fork error:");
            exit(1);
        }
        else if (pid == 0)
        {
            printf("I am a child process.\nI am exiting.\n");
            //子进程退出，成为僵尸进程
            exit(0);
        }
        else
        {
            //父进程休眠20s继续创建子进程
            usleep(5000);
            continue;
        }
    }
    return 0;
}

static void sig_child(int signo)
{
     pid_t        pid;
     int        stat;
     //处理僵尸进程
     while ((pid = waitpid(-1, &stat, WNOHANG)) >0)
            printf("child %d terminated.\n", pid);
}