程序代码：

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

void fun_parent(int );
void fun_child(int );
int pid,pid_f,pid_c;

int main()
{    
    while((pid = fork())<0);
    while(1)
    {
        if(pid == 0)
        {
            pid_c = getpid();
            pid_f = getppid();
            signal(SIGINT,fun_child);
            signal(SIGQUIT,fun_child);
            signal(SIGUSR1,fun_child);
        }
        else 
        {
            signal(SIGINT,SIG_IGN);
            signal(SIGQUIT,SIG_IGN);
            pid_f = getpid();
            pid_c = pid;
            signal(SIGUSR1,fun_parent);
            signal(SIGUSR2,fun_parent);
            signal(SIGTSTP,fun_parent);
        }
    }
    return 0;
}

void fun_parent(int sign)
{
    if(sign == SIGUSR1)
    {
        printf("the bus is going!\n");
    }
    else if(sign == SIGUSR2)
    {
        printf("stop the bus\n");
    }
    else if(sign == SIGTSTP)
    {
        printf("the end!\n");
        kill(pid_c,SIGUSR1);
        wait(NULL);
        exit(0);
    }
}

void fun_child(int sign)
{
    if(sign == SIGINT)
    {
        kill(pid_f,SIGUSR1);
    }
    else if(sign == SIGQUIT)
    {
        kill(pid_f,SIGUSR2);
    }
    else if(sign == SIGUSR1)
    {
        printf("please get off the bus!\n");
        exit(0);
    }
}