两个进程的共享 memory 中的结构数组

Question

I'm trying to create an array of structs to share between a parent and child process using "shmget". I am following a template from my professor, but his did not include structs and arrays (the shared memory only stored an int). The following code compiles without warnings but returns " 0" as the output, I am expecting to see "a 10". What am I doing wrong?

Also I am having trouble when i try to declare new variables inside the child processes, I have seen other samples where it works but I don't know why I am being forced to declare them before the fork everytime.

typedef struct { 
    char character;
    int number;
} item;

int main(int argc, char *argv[])
{
    int mem_id;

    mem_id = shmget(IPC_PRIVATE, 10*sizeof(item), SHM_R | SHM_W);
    item * x;
    item * y;
    item * list[10];

    switch(fork())
    {
        case -1:
            perror("Bad fork()"); exit(1);
        case 0:
            *list = shmat(mem_id, NULL, 0);
            if ((int *) list == (int *) -1)
            {perror("Child cannot attach"); exit(1);}           

            x->character = 'a';
            x->number = 10;

            list[0] = x;

            shmdt(list);
            exit(0);
        default:
            *list = shmat(mem_id, NULL, 0);
            if ((int *) list == (int *) -1)
            {perror("Child cannot attach"); exit(1);}

            wait((int *)0);
            y = list[0];
            shmdt(list);

            printf("%c %d\n", y->character, y->number);

            if (shmctl(mem_id, IPC_RMID, 0) <0)
                { perror("cannot remove shared memory"); exit(1);}

            return 0;
    }
    return 0;
}

Answer 1

I am surprised that you didn't segfault since:

1. You don't initialize x as Nemo pointed out
2. Your array "list" is actually an array of pointers to Items instead of being an array of Items.
3. Most importantly. You detach shared memory before you print out the value in y.

The code should look like this:

typedef struct { 
    char character;
    int number;
} item;

int main(int argc, char *argv[])
{
    int mem_id;

    mem_id = shmget(IPC_PRIVATE, 10*sizeof(item), SHM_R | SHM_W);
    item *x;
    item *y;
    item *list;

    switch(fork())
    {
    case -1:
        perror("Bad fork()"); exit(1);
    case 0:
        list = (item *)shmat(mem_id, NULL, 0);
        if ((void *) -1 == (void *)list)
        {
            perror("Child cannot attach"); exit(1);
        }
        x = list;
        x->character = 'a';
        x->number = 10;

        shmdt(list); // No need for this
        exit(0);
    default:
        list = (item *)shmat(mem_id, NULL, 0);
        if ((void *) list == (void *) -1)
        {
           perror("Child cannot attach"); exit(1);
        }

        wait((int *)0);
        y = list;
        printf("%c %d\n", y->character, y->number);

        shmdt(list);

        if (shmctl(mem_id, IPC_RMID, 0) <0)
            { perror("cannot remove shared memory"); exit(1);}

        return 0;
    }
    return 0;
}