How is a process signal sent to control the operating system?

In Unix/Linux-based operating systems, sending a signal to a process is done using the system call kill(). This call allows sending a specific signal (e.g., SIGINT, SIGTERM, SIGKILL) to a process by its process ID (PID).

Example usage in C:

#include <signal.h>
#include <unistd.h>

int main() {
    pid_t pid = 1234; // PID of the process to send the signal to
    int sig = SIGTERM; // termination signal
    int result = kill(pid, sig);
    if (result == 0) {
        // signal successfully sent
    } else {
        // error sending signal
    }
    return 0;
}

Besides kill(), there are other methods, such as using the kill command in the terminal or the raise() function to send a signal to oneself. Signals are used for process control: interruption, stopping, resuming, termination, etc.