parse 2d dynamic int array to Shared Memory

Question

Im trying to build a program that parsing a 2d dynamic array to other program by using shared memory.I search a lot but im a bit confused because im not familiar at this one. My code so far:

int main (int argc, char* argv []){
    int rows,columns;
    if( argc < 3 ){
        printf("Need The size of the 2d array\n");
        return 0;
    }
    rows = atoi(argv[1]);
    columns = atoi(argv[2]);

    time_t t;
    srand((unsigned) time(&t));

    key_t key = ftok(".",'a');
    size_t size = sizeof(key_t) + (rows * columns + 2 + rows) * sizeof(int);
    int shmid = shmget(key,size,IPC_CREAT|IPC_EXCL|S_IRWXU);
    int *memory = shmat(shmid, NULL, 0);
    printf("Shared Memory Key: %d\n", key);

    int *argsflag = memory;
    int *resflag= memory + 1;
    int *res  = memory + 2;

    int **array = (int **) memory + (rows*columns);

    for(int i = 0; i < rows ; i++) {
        for(int j = 0; j < columns; j++) {
            array[i][j] = rand() % 100;
        }
    }
    for(int i = 0; i < rows ; i++) {
        for(int j = 0; j < columns; j++) {
            printf("%d ",array[i][j]);
        }
        printf("\n");
    }


    shmctl(shmid,IPC_RMID,NULL);
    shmdt(memory);
    return(0);
}

Im getting a Segmentation fault (core dumped) and i dont know why.Also by searching i find a solution with struct but i dint get how i can build that.

Answer 1

You cannot have a int** point at a 2D array. It can only point to the first element in a 1D array of int* .

Furthermore, what's the logic of memory + (rows*columns) ? You end up setting the pointer to the last item of the array, rather than the first.

Try this instead:

void* memory = shmat( ... 
...
int (*array)[columns] = memory;
...
array[i][j] = ... ;

Where int (*array)[columns] is an array pointer, which ends up point at the first array in the 2D array.

For details, see Correctly allocating multi-dimensional arrays .