assign signal handler by pid - c++

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.
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.

Answer 2

As Cnucitar answered here, you can only change signal handler from inside the process.
If you wanted to make a signal handler inside firefox, you could patch it, perhaps thru a plugin. But I am sure that would be a bad idea.