Chatting Program Using Linux System Programming

user1.c


/* Our first program is to be run in one terminal. After the headers the shared memory segment
 (the size of our shared memory structure) is created with a call to shmget,
 with the IPC_CREAT bit specified. */

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

#include <sys/shm.h>

#include "shm_com.h"

int main()
{
    int running = 1;
    void *shared_memory = (void *)0;
    struct shared_use_st *shared_stuff;
    int shmid;
    char buffer[BUFSIZ];

    srand((unsigned int)getpid());    

    shmid = shmget((key_t)1234, sizeof(struct shared_use_st), 0666 | IPC_CREAT);

    if (shmid == -1)
      {
        fprintf(stderr, "shmget failed\n");
        exit(EXIT_FAILURE);
     }

/* We now make the shared memory accessible to the program. */

    shared_memory = shmat(shmid, (void *)0, 0);
    if (shared_memory == (void *)-1)
      {
        fprintf(stderr, "shmat failed\n");
        exit(EXIT_FAILURE);
      }

    printf("Memory attached at %X\n", (int)shared_memory);

/* The next portion of the program assigns the shared_memory segment to shared_stuff,
 which then prints out any text in written_by_user2. The loop continues until end is found
 in written_by_user1. The call to sleep forces the consumer to sit in its critical section,
 which makes the producer wait. */

    shared_stuff = (struct shared_use_st *)shared_memory;
    shared_stuff->written_by_user2 = 0;
    while(running)
     {

       if (!(shared_stuff->written_by_user2))
        {  
        printf("Enter some text: ");
        fgets(buffer, BUFSIZ, stdin);
        
        strncpy(shared_stuff->user_1, buffer, TEXT_SZ);
        shared_stuff->written_by_user1 = 1;
        }

        if (shared_stuff->written_by_user2)
       {
            printf("user2 wrote: %s", shared_stuff->user_2);
            sleep( rand() % 4 ); /* make the other process wait for us ! */
            shared_stuff->written_by_user2 = 0;
            if (strncmp(shared_stuff->user_1, "end", 3) == 0)
            {
                running = 0;
            }


        }


        if (strncmp(buffer, "end", 3) == 0)
       {
                running = 0;
        }

    }
 
/* Lastly, the shared memory is detached and then deleted. */

    if (shmdt(shared_memory) == -1)
   {
        fprintf(stderr, "shmdt failed\n");
        exit(EXIT_FAILURE);
    }

    if (shmctl(shmid, IPC_RMID, 0) == -1)
      {
        fprintf(stderr, "shmctl(IPC_RMID) failed\n");
        exit(EXIT_FAILURE);
      }

    exit(EXIT_SUCCESS);
}




user2.c 

/* The second program is the producer and allows us to enter data for consumers.
 It's very similar to shm1.c and looks like this. */

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

#include <sys/shm.h>

#include "shm_com.h"

int main()
{
    int running = 1;
    void *shared_memory = (void *)0;
    struct shared_use_st *shared_stuff;
    char buffer[BUFSIZ];
    int shmid;

    shmid = shmget((key_t)1234, sizeof(struct shared_use_st), 0666 | IPC_CREAT);

    if (shmid == -1)
      {
        fprintf(stderr, "shmget failed\n");
        exit(EXIT_FAILURE);
       }

    shared_memory = shmat(shmid, (void *)0, 0);
    if (shared_memory == (void *)-1)
      {
        fprintf(stderr, "shmat failed\n");
        exit(EXIT_FAILURE);
      }

    printf("Memory attached at %X\n", (int)shared_memory);

    shared_stuff = (struct shared_use_st *)shared_memory;
    shared_stuff->written_by_user1 = 0;

    while(running) {
    if (!(shared_stuff->written_by_user1))
      {
        printf("Enter some text: ");
        fgets(buffer, BUFSIZ, stdin);
        
        strncpy(shared_stuff->user_2, buffer, TEXT_SZ);
        shared_stuff->written_by_user2 = 1;
       }


       if (shared_stuff->written_by_user1)
       {
            printf("user1 wrote: %s", shared_stuff->user_1);
            sleep( rand() % 4 ); /* make the other process wait for us ! */
            shared_stuff->written_by_user1 = 0;
            if (strncmp(shared_stuff->user_2, "end", 3) == 0)
            {
                running = 0;
            }

         }
                if (strncmp(buffer, "end", 3) == 0)
               {
                running = 0;
               }
    }

        if (shmdt(shared_memory) == -1)
           {
        fprintf(stderr, "shmdt failed\n");
        exit(EXIT_FAILURE);
           }
    exit(EXIT_SUCCESS);
}


 shm_com.h


/* A common header file to describe the shared memory we wish to pass about. */

#define TEXT_SZ 208

struct shared_use_st {
    int written_by_user1;
    int written_by_user2; 
    char user_1[TEXT_SZ];
    char user_2[TEXT_SZ];
};