C - matrix multiplication segmentation fault - c

Question

I get a segmentation fault when running this code. Anyone know why? Thanks.
#include <stdio.h>
int main()
{
double **m1, **m2, **mr;
int m1_rows, m1_cols, m2_rows, m2_cols, mr_rows, mr_cols;
int i, j, k;
printf("Enter number of rows for matrix 1: ");
scanf("%d", &m1_rows);
printf("Enter number of columns for matrix 1: ");
scanf("%d", &m1_cols);
printf("Enter number of rows for matrix 2: ");
scanf("%d", &m2_rows);
printf("Enter number of columns for matrix 2: ");
scanf("%d", &m2_cols);
//allocate memory for matrix 1 m1
m1 = (double **) calloc(m1_rows, sizeof(double *));
for (i = 0; i < m1_rows; i++) {
m1[i] = (double *) calloc(m1_cols, sizeof(double));
}
//allocate memory for matrix 2 m2
m2 = (double **) calloc(m2_rows, sizeof(double *));
for (i = 0; i < m2_rows; i++) {
m2[i] = (double *) calloc(m2_cols, sizeof(double));
}
//allocate memory for sum matrix mr
mr = (double **) calloc(mr_rows, sizeof(double *));
for (i = 0; i < mr_rows; i++) {
mr[i] = (double *) calloc(mr_cols, sizeof(double));
}
//assign mr_rows and mr_cols
mr_rows = m1_rows;
mr_cols = m2_cols;
//initialize product matrix
for (i = 0; i < m1_rows; i++) {
for (j = 0; j < m2_cols; j++) {
mr[i][j] = 0;
}
}
//perform matrix multiplication
for (i = 0; i < m1_rows; i++) {
for (j = 0; j < m2_cols; j++) {
mr[i][j] = 0;
for (k = 0; k < m1_cols; k++) {
mr[i][j] += m1[i][k] * m2[k][j];
}
}
}
//print result
for (i = 0; i < mr_rows; i++) {
for (j = 0; j < mr_cols; j++) {
printf("%f\t", mr[i][j]);
}
}
//free memory m1
for (i = 0; i < m1_rows; i++); {
free(m1[i]);
}
free(m1);
//free memory m2
for (i = 0; i < m2_rows; i++); {
free(m2[i]);
}
free(m2);
//free memory mr
for (i = 0; i < mr_rows; i++); {
free(mr[i]);
}
free(mr);
return 0;
}
I ran using valgrind valgrind --tool=memcheck a.out for more info on the segmentation fault, but the result was over 30000 errors so it didn't print them out.

Answer 1

You are not assigning mr_rows and mr_cols. They need to be set like this:
mr_rows = m1_rows;
mr_cols = m2_cols;
This line is no good:
mr[i][j] += m1[i][k] * m2[k][j];
That will be accessing elements out of bounds, not least because k is not initialized. You need that line of code inside three nested for loops. Indeed, you may as well roll the zeroising code into this too.
for(i=0; i<m1_rows; i++){
for(j=0; j<m2_cols; j++){
mr[i][j] = 0;
for(k=0; k<m1_cols; k++){
mr[i][j] += m1[i][k]*m2[k][j];
}
}
}
Also, all your memory freeing loops are wrong. Instead of
for(i=0; i<m1_rows; i++);{
free(m1[i]);
}
free(m1);
it should read
for(i=0; i<m1_rows; i++){
free(m1[i]);
}
free(m1);
That stray semicolon was killing you.
You also need to perform a check that the number of columns in m1 equals the number of rows in m2, i.e. test that m1_cols == m2_rows.
One final point. You duplicate your code horribly here. Don't have three identical for loops to allocate a matrix and three identical for loops to deallocate. Extract those operations into helper functions which can be called from main.
That's all that I can find!

Answer 2

You are not assigning any values to mr_rows and mr_cols anywhere. So they will have junk values and you use them to to allocate memory by callin calloc().