Is "reformat" of multi-dimensional array by pointer to incomplete array type allowed in C?

Question

Consider following declaration:

int a[M][N]; // M and N are known compile-time

Would it be legal to treat it like as it was declared as:

int a[N][M];

or even:

int a[A][B]; // where A * B = M * N

in C without breaking its rules (badly)?

I found that it can be acomplished without any cast:

#include <stdio.h>

void print_array(int a[][2], int n);

int main(void)
{
    int a[2][3] = {{1, 2, 3}, {4, 5, 6}};

    //int (*p1)[2] = a; // compile error

    int (*ptr_temp)[] = a; // pointer to array of incomplete type
    int (*p2)[2] = ptr_temp; // compiles without any warning

    print_array(p2, 3);
}

void print_array(int a[][2], int n)
{
    for (int i = 0; i < n; i++)
        for (int j = 0; j < 2; j++)
            printf("a[%d][%d] = %d\n", i, j, a[i][j]);
}

Notice that we cannot assign a to p1 pointer directly. However compiler does not complain when p2 is assigned with ptr_temp even if it seems to be potentially dangerous (it does not require any cast for that). Is it really sanitized? If so, then why it disallows the first assignmment?

Answer 1

The behavior you see is because of the way multidimensional arrays are handled in C.

Have a look at this question Pointer address in a C multidimensional array