Segfault 11 when trying to multiply 2 matrices - c

Question

I'm trying to multiply 2 dynamically allocated arrays. I'm having 2 problems:
When I try unequal sized arrays like [2],[3] and [3],[2] I got a segmentation fault 11, and after staring at my allocations I still can't figure out why.
My final array is formatted with the correct rows and columns, but it displays all 0's.
I'm assuming that this is because I didn't allocate the memory correctly.
-
#include <stdio.h>
#include <stdlib.h>
int main()
{
int **a, **b, **c; //pointers to arrays
int m1_r,m1_c,m2_r,m2_c; //declaring arrays
int i,j,k;
printf("\n");
again://repeat if first matrixes are bad
printf("Enter rows and columns for the first matrix.\n");//first matrix
scanf("%d%d" ,&m1_r,&m1_c);
printf("Enter rows and Columns for the second matrix.\n");//second matrix
scanf("%d%d",&m2_r,&m2_c);
if(m1_c!=m2_r) {
printf("You tried to break my code. Nice try.");
goto again;
}
//memory for first matrix
a = malloc(m1_r * sizeof(int *));
for(i=0; i < m1_r; i++) {
a[i] = malloc(m1_c * sizeof(int));
}
//memory for second matrix
b = malloc(m2_r * sizeof(int *));
for(i=0; i < m2_r; i++) {
b[i] = malloc(m2_c * sizeof(int));
}
//memory for 3rd matrix
c = malloc(m1_r * sizeof(int *));
for(i=0; i < m2_r; i++) {
c[i] = malloc(m2_c * sizeof(int));
}
//input 1st matrix
printf("Enter the numbers of the first matrix.\n");
for (i=0; i<m1_r; i++) {
for (j = 0; j<m1_c; j++) {
scanf("%d", &a[i][j]);
}
}
//input 2nd matrix
printf("Enter the second of the first matrix.\n");
for (i=0; i<m1_r; i++) {
for (j = 0; j<m1_c; j++) {
scanf("%d", &b[i][j]);
}
}
printf("\n");
printf("1st matrix looks like this:\n");
//print 1st matrix
for (i=0; i<m1_r; i++) {
for (j = 0; j<m1_c; j++) {
printf("%d\t", a[i][j]);
}
printf("\n");
}
//print 2nd matrix
printf("\n");
printf("2nd matrix looks like this:\n");
//print 2st matrix
for (i=0; i<m2_r; i++) {
for (j = 0; j<m2_c; j++) {
printf("%d\t", b[i][j]);
}
printf("\n");
}
//initialize result matrix to 0
for(i=0; i<m2_r; i++)
for(j=0; j<m2_c; j++) {
c[i][j]=0;
}
//multipication
for(i=0; i<m1_r; i++)
for(j=0; j<m2_c; j++)
for(k=0; k<m1_c; k++) {
c[i][j]+= a[i][k]*b[k][j];
}
//print result
printf("\nThe result of the matrix multiplication is:");
for(i=0; i<m1_r; i++) {
printf("\n");
for(k=0; k<m2_c; k++) {
printf("%d\t", c[i][j]);
}
}
printf("\n");
return 0;
}

Answer 1

You allocate the wrong amount of memory for the third matrix:
c = malloc(m1_r * sizeof(int *));
for(i=0; i < m2_r; i++)
The loop count should be the same as the number of pointers you malloc.
To avoid this sort of error, consider making a function which you pass in the dimensions and it returns the pointer.
Later on, you overwrite its bounds using different indices again:
for(i=0;i<m2_r;i++)
for(j=0;j<m2_c;j++)
{
c[i][j]=0;
}
Then you overwrite the bounds of b (it was m2_r and m2_c):
for (i=0; i<m1_r; i++) {
for (j = 0; j<m1_c; j++) {
scanf("%d", &b[i][j]);
}
}
To avoid this sort of error, you could use a better naming convention for your variables; and also consider using a struct which holds each pointer plus its dimension variables. Then you can have a function that zeroes any matrix and you only need to pass it a pointer to one of your matrix structs.
BTW if you use calloc instead of malloc then you don't need this loop at all (although you might want to have this function anyway so that you can zero a matrix).
Also you should check for success of scanf and malloc.

Answer 2

There are hell lot of bugs in your code:
First
//memory for 3rd matrix
c = malloc(m1_r * sizeof(int *));
for(i=0; i < m2_r; i++) <----- error: used m2_r instead of m1_r
You assigned m1_r and loop till m2_r.
Second
//input 2nd matrix
printf("Enter the second of the first matrix.\n");
for (i=0; i<m1_r; i++) { <----- error: used m1_r instead of m2_r
for (j = 0; j<m1_c; j++) { <----- error: used m1_c instead of m2_c
scanf("%d", &b[i][j]);
}
}
You are using rows and columns of 1st matrix.
Third
//initialize result matrix to 0
for(i=0; i<m2_r; i++) <----- error: used m2_r instead of m1_r
for(j=0; j<m2_c; j++) {
c[i][j]=0;
}
You used row value of second matrix rather than the first matrix