Segmentation fault error as the reader tries to read from the shared memory

I am trying to understand the System V shared memory APIs. I created a small program where one writes into the shared memory and another reads from the shared memory. But for some reason, I am getting an error :

segmentation fault: 11

as I try to read from the shared memory. I could not find the reason for it.

Here is what I have done:

The following program writes into the shared memory.

    #include <stdio.h>
    #include <sys/ipc.h>
    #include <sys/shm.h>
    #include <sys/types.h>
    #include <sys/stat.h>
    #include <stdlib.h>

    struct shm_seg {
        char *buf;
    };

    int main() {
        int shmid = shmget(1, sizeof(struct shm_seg), IPC_CREAT | S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP);

        if(shmid == -1) {
            printf("Failed to fetch the shared memory id");
            exit(-1);
        }

        struct shm_seg *shm_segp = shmat(shmid, NULL, 0);

        if(shm_segp == (void*)-1) {
            printf("Failed to attach shared memory to the shared memory id");
            exit(-1);
        }

        while (1) {
            shm_segp->buf = "hello";
        }

        if(shmdt(shm_segp) == -1) {
            printf("Failed to detach the shared memory");
            exit(-1);
        }

        if(shmctl(shmid, IPC_RMID, 0) == -1) {
            printf("Failed to delete a shared memory object");
            exit(-1);
        }
    }

and the following code attempts to read from the shared memory.

#include <stdio.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdlib.h>

struct shm_seg {
    char *buf;
};

int main() {
    int shmid = shmget(1, 0, 0);

    if(shmid == -1) {
        printf("Failed to fetch the shared memory id");
        exit(-1);
    }

    struct shm_seg *shm_segp = shmat(shmid, NULL, SHM_RDONLY);

    if(shm_segp == (void*)-1) {
        printf("Failed to attach shared memory to the shared memory id");
        exit(-1);
    }

    int i = 0;
    while(i < 100 ) {
        printf("%sn",shm_segp->buf);
        i++;
    }


    if(shmdt(shm_segp) == -1) {
        printf("Failed to detach the shared memory");
        exit(-1);
    }
}

The above reader program results in a Segmentation fault: 11 error. What could be the reason for this? What is it that I am doing wrong?

Answer

        shm_segp->buf = "hello";

That code makes no sense. This puts a pointer in shared memory.

Your code passes a pointer to a string constant from one program to the other. Then it dereferences that pointer in the other program. But what sense does that make? It’s a pointer into the other program’s memory space that means nothing to a different program.

If you want to pass data from one program to another, you need to actually put the data you want to pass in shared memory. Putting a pointer to it in shared memory won’t do any good if the data itself isn’t in shared memory!

Advertisement

Answer