Can anyone explain what this warning means and how to cure,

Also written a function to multiply 2 matrices but its throwing up half right half wrong answers is there something silly ive missed out or got wrong?

Code:

typedef coord [2}
void MatCalc (coord *p2d_1, coord *p2d_2, coord *p2d_3, int *r, int *c, int *n)
{
int c_i, r_i, N, sum;
for(r_i=0; r_i<*r; r_i++){
for(c_i=0; c_i<*c; c_i++){
sum = 0;
for(N=0; N<*n; N++){
sum += p2d_1 [r_i] [N] * p2d_2 [N] [c_i];
p2d_3 [r_i] [c_i] = sum;
}
}
}
}

Wjere r and c would be i/j and n k in common matrix terms