signal을 이용한 pingpong 게임인데요 소스좀 봐주세요..

#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
 
#define BUF_SIZE    512
 
void handler_parent();
void handler_child();
 
 
char *string;
 
int
main()
{
 
    //char *string;
    pid_t pid;
 
    if ( sigset(SIGUSR1, handler_parent) == SIG_ERR ) {
        fprintf(stderr, "cannot set handler_parent for SIGUSR1\n");
        exit(1);
    }
 
    if ( sigset(SIGUSR2, handler_child) == SIG_ERR ) {
        fprintf(stderr, "cannot set handler_child for SIGUSR2\n");
        exit(1);
    }
 
    pid = fork();
 
    switch ( pid ) {
        case -1 :
        {
            printf( "fork faild \n" );
            return -1;
        }
        //child
        case 0 :
        {
            for (;;) {
                kill( getppid(), SIGUSR1);
                pause();
            }
        }
        //parent
        default :
        {
            for (;;) {
                pause();
            for (;;) {
                pause();
                kill( pid, SIGUSR2);
            }
        }
 
    }
 
    exit(0);
}
 
void handler_parent()
{
    printf("Input message: ");
    fgets(string, BUF_SIZE, stdin);
    string[ strlen(string - 1) ] = '\0';
 
    if ( strcmp(string, "ping") == 0 ) {
        strcpy(string, "pong");
    }
    else if ( strcmp(string, "pong") == 0 ) {
        strcpy(string, "ping");
    }
}
 
void handler_child()
{
    fputs(string, stdout);
}