IPC using shared memory in C

This is a discussion on IPC using shared memory in C within the C Programming forums, part of the General Programming Boards category; I am implementing IPC using shared memory in c linux. But my client and server are generating different addresses. Why ...

  1. #1
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    IPC using shared memory in C

    I am implementing IPC using shared memory in c linux. But my client and server are generating different addresses. Why so?? or how to make them point to same memory location?
    Code:
    //server.c
    #include "/home/user/msgbuf.h"
    #define SHMSZ    127
    int main()
    {
    key_t key;
    message_buf *m;
    char ans='y';
    key = 9876;
    if ((shmid = shmget(key, SHMSZ, IPC_CREAT | 0666)) < 0)
            {
                    perror("shmget");
                    exit(1);
            }
            printf("\nShared Memory Id = %d\n",shmid);
            if ((m = shmat(shmid, NULL, 0)) == (message_buf *) -1)
            {       perror("shmat");
                    exit(1);
            }
          printf("\n m= %d\n",m);
          
    while(ans=='y')
          {
                 sleep(1);
                 printf("Enter your choice");
                 scanf("%c",&ans);
                 getchar();
          }
          return 0;
    }
    Code:
    //client.c
    #include "/home/user/msgbuf.h"
    #define SHMSZ    127
    int main()
    {
    key_t key;
    message_buf *rbuf;
    key = 9876;
    if ((shmid = shmget(key, SHMSZ, 0666)) < 0)
            {
                    perror("shmget");
                    exit(1);
            }
            printf("\nShared Memory Id = %d\n",shmid);
            if ((rbuf = shmat(shmid, NULL, 0)) == (message_buf *) -1)
            {       perror("shmat");
                    exit(1);
            }
          printf("\n rbuf= %d\n",rbuf);
          return 0;
    }
    Code:
    //msgbuf.h
    typedef struct msgbuf1
    {
            int     msglen;
            unsigned char *cp;
    }message_buf;
    Thanks
    Last edited by Babita Sandooja; 04-01-2014 at 01:09 AM.

  2. #2
    and the hat of wrongness Salem's Avatar
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    > unsigned char *cp;
    The answer would be to not store virtual addresses inside the shared memory area.


    If you do this, then the whole problem of "different addresses" goes away. All that matters is that the data has the same offset within the shared memory segment. It's mapped address becomes irrelevant.
    Code:
    //msgbuf.h
    typedef struct msgbuf1
    {
            int     msglen;
            char cp[SHMSZ-sizeof(int)];
    }message_buf;
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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