Signal handler example

Question

I am trying to understand how signal handler work. I saw an example on geeksforgeek:

int val = 10;
void handler(int sig){
    val += 5;
}

int main(){
    pid_t pid;
    signal(SIGCHLD, handler);
    if((pid = fork()) == 0){
        val -= 3;
        exit(0);
    }
    waitpid(pid, NULL, 0);
    printf("val = %d\n", val);
    exit(0);
}

I am confused about why the output value is 15. Initially, I guessed it is because the child process has variable val in different address. However, when I tried to print out the address of val in both child process and its parent process, they both display the same memory address.

Answer 1

The address that you print is a virtual address, it's the address of the global variable in the process's memory space. The child process and the parent process have memory spaces that look the same but they're not actually the same memory space (meaning they don't occupy the same physical memory).

this is a very high level (and somewhat inaccurate) answer, I suggest that you read about virtual memory in order to understand this properly.