2d array indexing fails for static arrays

Question

I have a basic question related to indexing 2d arrays passed to a function in C. When I use a double pointer to pass it:

• Inside the function printMatrix indexing A as A[i][j] works if memory for A in main was allocated using malloc
• Inside the function printMatrix indexing A as A[i][j] doesn't work if memory for A in main was allocated statically (as in the commented code). I believe I should use: *(*(A+i*N)+j) but why doesn't A[i][j] work when it works for the case above?

---

void printMatrix(int** A,int N) {
    int i=0,j=0;
    for(i=0;i<N;i++) {
        for(j=0;j<N;j++) {
            printf("%2d",A[i][j]);
        }
        printf("\n");
    }
    printf("\n");
    return;
}

int main() {
    //int A[6][6] = {{1,1,1,1,1,1},{2,2,2,2,2,2},{3,3,3,3,3,3},
    //{4,4,4,4,4,4},{5,5,5,5,5,5},{6,6,6,6,6,6}};
    int N = 6,i,j;
    int **A = (int**)malloc(sizeof(int*)*N);
    for(i=0;i<N;i++)
        A[i] = (int*)malloc(sizeof(int)*N);

    for(i=0;i<N;i++) 
        for(j=0;j<N;j++)
            A[i][j] = i+j;

    printMatrix((int**)A,N);
    return 0;
}

Answer 1

Taking an analytical viewpoint, if we declare an array with automatic storage duration in main, such as:

int A[6][6];

We can measure the size of this allocation by using sizeof A, and this will be equal to sizeof (int) * 6 * 6.

In the following code, however, you are allocating sizeof (int *) * 6 + sizeof (int) * 6 * 6 bytes:

int N = 6,i,j;
int **A = (int**)malloc(sizeof(int*)*N);
for(i=0;i<N;i++)
    A[i] = (int*)malloc(sizeof(int)*N);

This should indicate a flaw in your understanding of arrays, and you're doing too much work. For a start, an array is a single continuous block of memory, while what you're allocating is not. You're calling malloc too many times, and allocating too many bytes.

If you continue to sharpen your axe by learning more about C, you'll learn neat tricks such as achieving your goal with one allocation, allocating the same number of bytes, and with one line of code:

int (*A)[N] = malloc(N * sizeof *A);

You'll probably want to adjust the type of print_matrix to reflect the change of A as follows:

void printMatrix(size_t N, int (*A)[N])

P.S. Don't cast malloc

P.P.S. Check malloc for NULL return values.

P.P.P.S. There are two meanings for "static" in C, and neither of them are what you think they are...