Signals

The kill() Function

To send signals to other processes, we use the kill() function. Despite the name, it isn’t always used to stop another process, it is simply the available funtion to send signals to other processes.

An example follows.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h> // contains kill() to send signals
#include <signal.h> // contains some useful macros, like SIGKILL and SIGTERM
#include <sys/wait.h>

int main(void) {

    pid_t pid = fork();

    if (pid < 0) {
        perror("fork failed");
        exit(1);
    }

    if (pid == 0) {
        // Child process
        printf("Child: PID = %d\n", getpid());

        // a loop that wont stop
        while (1) {
            printf("Child is running...\n");
            sleep(2);
        }
    } else {
        // Parent process
        printf("Parent: PID = %d, Child PID = %d\n", getpid(), pid);

        sleep(5);  // Let the child run for a bit

        printf("Parent: sending SIGTERM to child\n");
        // recall in the parent process pid = child process id
        // use kill() to send signals
        // Using SIGTERM here, but can use SIGKILL also. 
        // SIGKILL is like "Force Quit" (no recovering if needed)
        kill(pid, SIGTERM); 

        // Wait for the child to terminate
        wait(NULL);
        printf("Parent: child has terminated\n");
    }

    return 0;
}

Handling Signals

Two files are shown below, one designed to handle a received signal named receive.c and another designed to send a signal to the receiver named send.c. Be sure to run each file in separate terminals.

// receive.c
// run with ./receive
// It will print a pid that you will use to run send.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <string.h>

void handle_signal(int sig){
    // this is our handler function
    // be sure you have \n inside or fflush() after printf() to end the line/buffer
    printf("Received %d: %s\n", sig, strsignal(sig));
    
}

int main(){
    printf("Receiver PID: %d\n", getpid());

    // register the handler for a SIGUSR1 signal
    signal(SIGUSR1, handle_signal);

    printf("Receiver: waiting...\n");

    while(1){
        pause();
    }

    return 0;
}

// send.c
// Run receive.c first to get a process id
// Run ./send <receiver-pid> to send the signal

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

int main(int argc, char *argv[]) {
   
    // check that the target pid was provided
    if (argc != 2) {
        fprintf(stderr, "Usage: %s <receiver-pid>\n", argv[0]);
        exit(1);
    }

    // converts the target pid to an int
    pid_t targetProcess = atoi(argv[1]);

    printf("Sender: sending SIGUSR1 to PID %d\n", targetProcess);

    // send the signal (siguser1)
    kill(targetProcess, SIGUSR1);

    return 0;
}