Communication between two process using signals in C

Prerequisite : C signal handling

In this post, the communication between child and parent processes is done using kill() and signal(), fork() system call.

• fork() creates the child process from the parent. The pid can be checked to decide whether it is the child (if pid == 0) or the parent (pid = child process id).
• The parent can then send messages to child using the pid and kill().
• The child picks up these signals with signal() and calls appropriate functions.

Example of how 2 processes can talk to each other using kill() and signal():

// C program to implement sighup(), sigint()
// and sigquit() signal functions
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>

// function declaration
void sighup();
void sigint();
void sigquit();

// driver code
void main()
{
    int pid;

    /* get child process */
    if ((pid = fork()) < 0) {
        perror("fork");
        exit(1);
    }

    if (pid == 0) { /* child */
        signal(SIGHUP, sighup);
        signal(SIGINT, sigint);
        signal(SIGQUIT, sigquit);
        for (;;)
            ; /* loop for ever */
    }

    else /* parent */
    { /* pid hold id of child */
        printf("\nPARENT: sending SIGHUP\n\n");
        kill(pid, SIGHUP);

        sleep(3); /* pause for 3 secs */
        printf("\nPARENT: sending SIGINT\n\n");
        kill(pid, SIGINT);

        sleep(3); /* pause for 3 secs */
        printf("\nPARENT: sending SIGQUIT\n\n");
        kill(pid, SIGQUIT);
        sleep(3);
    }
}

// sighup() function definition
void sighup()

{
    signal(SIGHUP, sighup); /* reset signal */
    printf("CHILD: I have received a SIGHUP\n");
}

// sigint() function definition
void sigint()

{
    signal(SIGINT, sigint); /* reset signal */
    printf("CHILD: I have received a SIGINT\n");
}

// sigquit() function definition
void sigquit()
{
    printf("My DADDY has Killed me!!!\n");
    exit(0);
}

Output: