Chat application between two processes using signals and shared memory

Prerequisite: C signal handling, IPC through shared memory

A signal is used in the UNIX system to notify a process that a particular event has occurred. A signal may be received either synchronously or asynchronously depending on the source and the reason for the event being signalled. A signal must follow the following pattern –

1. A signal is generated by the occurrence of a particular event.

2. A generated signal is delivered to a particular process.

3. The signal must be handled after receiving at the process.

In this problem, the message is sent from one user to another user using kill function. kill function takes two inputs – process id of the receiver process and signal type. For this purpose, we use a shared memory where we store the process id(s) of two processes. We use a handler function which will print the message received from another process. User2 will start to send message to User1 and then they will continue chatting.

User 1

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/types.h>
#include <unistd.h>

#define FILLED 0
#define Ready 1
#define NotReady -1

struct memory {
    char buff[100];
    int status, pid1, pid2;
};

struct memory* shmptr;

// handler function to print message received from user2

void handler(int signum)
{
    // if signum is SIGUSR1, then user 1 is receiving a message from user2

    if (signum == SIGUSR1) {
        printf("Received User2: ");
        puts(shmptr->buff);
    }
}

int main()
{
    // process id of user1

    int pid = getpid();

    int shmid;

    // key value of shared memory
    int key = 12345;

    // shared memory create
    shmid = shmget(key, sizeof(struct memory), IPC_CREAT | 0666);

    // attaching the shared memory

    shmptr = (struct memory*)shmat(shmid, NULL, 0);

    // store the process id of user1 in shared memory
    shmptr->pid1 = pid;
    shmptr->status = NotReady;

    // calling the signal function using signal type SIGUSER1
    signal(SIGUSR1, handler);

    while (1) {
        while (shmptr->status != Ready)
            continue;
        sleep(1);

        // taking input from user1

        printf("User1: ");
        fgets(shmptr->buff, 100, stdin);

        shmptr->status = FILLED;

        // sending the message to user2 using kill function

        kill(shmptr->pid2, SIGUSR2);
    }

    shmdt((void*)shmptr);
    shmctl(shmid, IPC_RMID, NULL);
    return 0;
}

User 2

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/types.h>
#include <unistd.h>

#define FILLED 0
#define Ready 1
#define NotReady -1

struct memory {
    char buff[100];
    int status, pid1, pid2;
};

struct memory* shmptr;

// handler function to print message received from user1

void handler(int signum)
{
    // if signum is SIGUSR2, then user 2 is receiving a message from user1

    if (signum == SIGUSR2) {
        printf("Received From User1: ");
        puts(shmptr->buff);
    }
}

// main function

int main()
{
    // process id of user2
    int pid = getpid();

    int shmid;

    // key value of shared memory
    int key = 12345;

    // shared memory create

    shmid = shmget(key, sizeof(struct memory), IPC_CREAT | 0666);

    // attaching the shared memory

    shmptr = (struct memory*)shmat(shmid, NULL, 0);

    // store the process id of user2 in shared memory
    shmptr->pid2 = pid;

    shmptr->status = NotReady;

    // calling the signal function using signal type SIGUSR2
    signal(SIGUSR2, handler);

    while (1) {
        sleep(1);

        // taking input from user2

        printf("User2: ");
        fgets(shmptr->buff, 100, stdin);
        shmptr->status = Ready;

        // sending the message to user1 using kill function

        kill(shmptr->pid1, SIGUSR1);

        while (shmptr->status == Ready)
            continue;
    }

    shmdt((void*)shmptr);
    return 0;
}

Output:

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