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

void  err_sys(char * msg) {
    printf("%s \n", msg);
    exit(-1); 
}

int main(void)
{
    pid_t pid;

    if ( (pid = fork()) < 0)
        err_sys("fork error");
    else if (pid == 0){		/* child */
        system("ps");
        exit(0);
    }

      /* parent */
    sleep(4);

    system("top");

    exit(0);
}