Question

If I know a pid of a certain process that doesn’t run the code(say firefox)
how do I assign a signal handler(say SIGINT) to it?

I have now :

    pid = fork();
    printf("forked and my pid is %d\n",pid);
    //check for errors
    if (pid<0){         
        printf("Error: invoking fork to start ss has failed, Exiting\n ");
        exit(1);
    }
    //the child process runs the gulp
    if (pid==0){
        printf("STARTING THE FIREFOX\n");           
                    //calling signal(somehandler,SIGINT); here will bind the child, which is replaced by the firefox new process,hence won't invoke the "somehandler"
        if (execv(args[0],args)<0){
            perror("Error: running s with execvp has failed, Exiting\n");
        }
                    //invoking signal(somehandler,SIGINT); will obviously not do anything
        printf("IVE BEEN KILLED\n");            
    }
    //dad is here
    printf("DAD IS GOING TO KILL\n");
    if (pid>0){
        sleep(6);
                    //how do I bind a handler to that signal????
        kill(get_pidof(string("firefox")),SIGINT);
    }

Answer 1

You can only establish a signal handler from within the process. Put another way, you can’t make firefox call your signal handler when it gets a SIGINT.

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EDIT

As you noticed, indeed signal handlers are not kept after an exec – the image of the process is replaced so it wouldn’t make sense. So, like I said before: you can’t make firefox call your handler even if you control its parent.

I need my program to run another program(say firefox), and to know
when the firefox died or crashed

In that case you want to establish a signal handler for SIGCHLD: your process will jump to it when the child dies.