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


int g_child_stop = 0;
int g_parent_run = 0;

void sig_child(int signum)
{
    if( SIGUSR1 == signum )
    {
        g_child_stop = 1;
    }
}

void sig_parent(int signum)
{
    if( SIGUSR2 == signum )
    {
        g_parent_run = 1;
    }
}

int main(int argc, char **argv)
{
    int             pid;
    int             wstatus;

    signal(SIGUSR1, sig_child);
    signal(SIGUSR2, sig_parent);

    if( (pid=fork()) < 0 )
    {
        printf("Create child process failure: %s\n", strerror(errno));
        return -2;
    }
    else if(pid == 0)
    {
        /* child process can do something first here, then tell parent process to start running */

        printf("Child process start running and send parent a signal\n");
        kill(getppid(), SIGUSR2);

        while( !g_child_stop )
        {
            sleep(1);
        }
        
        printf("Child process receive signal from parent and exit now\n");
        return 0;
    }

    printf("Parent hangs up untill receive signal from child!\n");
    while( !g_parent_run )
    {
        sleep(1);
    }

    /* parent process can do something here, then tell child process to exit */

    printf("Parent start running now and send child a signal to exit\n");
    kill(pid, SIGUSR1);

    /* parent wait child process exit */
    wait(&wstatus);
    printf("Parent wait child process die and exit now\n");

    return 0;
}