how to build function that multiply matrices created via struct in C - c

Question

I am supposed to build a matrix_mul function that display the result of a multiplication between two matrices that were created via a struct. My problem is, my function does not multiply the value inside the matrices instead it return an interger.
#include<stdio.h>
#include<stdlib.h>
typedef struct{
int rows;
int cols;
int **data;
} matrix;
matrix matrix_mul(int **a, int **b){
int rowA=(sizeof(a)/sizeof(a[0]));
int colA=(sizeof(a)/sizeof(a[0][0]))/rowA;
int rowB=(sizeof(b)/sizeof(b[0]));
int colB=(sizeof(b)/sizeof(b[0][0]))/rowB;
int i, j, **mulMatrix;
if(colA != rowB)
printf("error: cannot multiply matrices");
else{
int i, j, k;
mulMatrix = (int **)malloc(rowA * sizeof(int *));
for(i=0; i<rowA; i++)
mulMatrix[i] = (int *)malloc(colB * sizeof(int));
for(i=0; i<rowA; ++i){
for(j=0; j<colB; ++j){
for(k=0; k<colA; ++k)
{
mulMatrix[i][j]+= a[i][k] * b[k][j];
}
}
}
}
printf("Matrix: \n\n");
for(i=0; i< rowA; i++){
for(j=0; j< colB; j++){
printf("%d ", mulMatrix[i][j]);
}
printf("\n\n");
}
}
int main()
{
int i, j;
matrix a, b;
printf("matrix1 - enter number of rows and cols: ");
scanf("%d %d", &a.rows, &a.cols);
a.data = (int **)malloc(a.rows * sizeof(int *));
printf("enter matrix values: ");
for(i=0; i<a.rows; i++){
a.data[i] = (int *)malloc(a.cols * sizeof(int));
for(j=0; j<a.cols; j++){
scanf("%d", &a.data[i][j]);
}
}
printf("matrix2 - enter number of rows and cols: ");
scanf("%d %d", &b.rows, &b.cols);
b.data = (int **)malloc(b.rows * sizeof(int *));
printf("enter matrix values: ");
for(i=0; i<b.rows; i++){
b.data[i] = (int *)malloc(b.cols * sizeof(int));
for(j=0; j<b.cols; j++){
scanf("%d", &b.data[i][j]);
}
}
matrix_mul(a.data, b.data);
return 0;
}

Answer 1

These lines
int rowA=(sizeof(a)/sizeof(a[0]));
int colA=(sizeof(a)/sizeof(a[0][0]))/rowA;
won't give you the results you are looking for.
sizeof(a) is sizeof(int**) and sizeof(a[0]) is sizeof(int*). Since pointers are usually of the same size, rowA will be always set to 1. colA will be set to sizeof(int**)/sizeof(int), which is a constant value. Once again, it's not the value you are hoping to get.
You should pass the matrix objects, a and b to matrix_mul. That'll give you the size of the matrices to use for multiplying them.
matrix matrix_mul(matrix a, matrix b){
Of course, you need to modify the implementation accordingly.