c - shmat() - invalid argument

Question

I′m having some trouble, trying to read the shmem. The program writing the data is giving me no error so i don't know what to do anymore.

typedef struct
{
    long id;
} data;

data *dados[MAX];

int main()
{
    key_t key = 5678;
    int shmid;

    if (shmid = shmget(key, MAX * sizeof(data),0666) < 0)
    {
        perror("shmget error");
        return 1;
    }

    if((*dados = shmat(shmid,NULL,0)) == (void*) -1){
        perror("shmat");
    }   

    for (int i = 0; i < MAX; i++)
    {
        printf("time:%ld
", dados[i]->id);
        printf("passed %d",i);
    }

    return 0;
}

Answer 1

if (shmid = shmget(key, MAX * sizeof(data),0666) < 0)

According to C operator order of precedence the < operator has higher precedence than the = operator. Thus the above code actually assigns the boolean shmget(key, MAX * sizeof(data),0666) < 0 result to shmid.

Use explicit bracketing to get the desired result:

if ((shmid = shmget(key, MAX * sizeof(data),0666)) < 0)