The line kill(atoi(argv[1]), SIGKILL); sends a SIGKILL signal to the process with the specified ID; this should cause it to terminate. The atoi(argv[1]) part converts the string from the command-line parameters into an integer.

For the rest, I recommend picking up a beginner's guide to C.

There are multiple issues with your code:

• system(3) does NOT return PID of child process, but instead waits for it to exit, and returns its exit code. You need to use the traditional fork+exec approach.
• With child PID available, it's easier to call kill(2) than kill(1)
• Better add sleep in a dead loop waiting for input. This reduces CPU load and your electricity bill.

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

pid_t child_id;

void ctrl_C(int sig){
    kill(child_id, 9);
    printf("\n You pressed Ctrl + C,Good Bye \n");
    exit(0);    
}



int main(){
    printf("I am Programmer \n");
    pid_t child_id = fork();
    if (child_id == 0) {
        execlp("gnome-terminal", "gnome-terminal", "--", "vi", "abcd.txt", NULL);
        return 255;
    }
    signal(SIGINT,ctrl_C);
    while (1) {
        sleep(1);
    }
}

Since main process never calls waitpid for other children all of them become zombies after getting killed.

Update: you should also close pipe ends descriptors in other child processes prior to putting them to sleep, otherwise killer child process will get stuck at waiting for more data to come from the pipe.

} else{
    close(fd[0]);
    close(fd[1]);
    break;
}

Update: sleep takes unsigned int number of seconds, so sleep(0.5) will be equivalent to sleep(0).