How do I multiply two dynamic matrices in C? - c

Question

I'm trying to multiply two dynamic matrices by passing them through a function. I'm getting a segmentation fault during the multiplication.
The matrices are being passed through a function. The items in the arguments are correct because I had to use them for a different operation in this project. I have a feeling that I messed up with the pointers, but i'm pretty new to C and i'm not sure where I messed up.
double** multiplyMatrices(
double** a,
const uint32_t a_rows,
const uint32_t a_cols,
double** b,
const uint32_t b_cols){
uint32_t i = 0;
uint32_t j = 0;
uint32_t k = 0;
double** c;
//allocate memory to matrix c
c = (double **)malloc(sizeof(double *) * a_rows);
for (i = 0; i < a_rows; i++) {
*(c +i) = (double *)malloc(sizeof(double) * b_cols);
}
//clear matrix c
for(i = 0; i < a_rows; i++){
for(j = 0; j < a_cols; j++){
*c[j] = 0;
}
}
i = 0;
//multiplication
while(j = 0, i < a_rows ){
while(k = 0, j < b_cols){
while(k < a_cols){
//following line is where i'm getting the segmentation fault
*(*(c+(i*b_cols))+j) += (*(*(a+(i*a_cols))+k)) * (*(*(b+(k*b_cols))+j));
k++;
}
j++;
}
i++;
}
return c;
}

Answer 1

The obvious mistake is that you dereference c + i * b_cols while c is an array of pointers of size a_rows. So likely c + i * b_cols is outside of the area that you previously allocated with malloc().
I would suggest to simplify the matrix representation using a single array of double with the size equal to the total number of elements, i.e. rows * cols.
For example:
double *c;
c = malloc(sizeof(double) * a_rows * b_cols);
This not only has better overall performance, but simplifies the code. You would then have to "linearise" the offset inside your unidimensional array to convert from bi-dimensional matrix coordinates. For example:
c[i * b_cols + j] = ...
Of course, the other two matrices need to be allocated, filled and accessed in a similar manner.
For code clarity, I would also replace the while statements by for statements with the actual variable that they loop on. For example:
for (i = 0; i < a_rows; i++)
for (j = 0; j < b_cols; j++)
for (k = 0; k < a_cols; k++)
You can (ab)use the C language in many ways, but the trick is to make it more clear for you in the first place.