Question

I’m facing segmentation fault problem when trying to run this code:

#include<stdio.h>
#include<stdlib.h>


int mallocMatrix (int ***matrix, int A, int B) {
    int i;
    *matrix=(int**)malloc(sizeof(int*)*B);
        for(i=0;i<A;i++)
            (*matrix)[i]=(int*)malloc(sizeof(int)*A);
};

void fillMatrix (int ***matrix, int A, int B) {
    int i, j;
    for(i=0;i<B;i++)
    {
        for(j=0;j<A;j++)
            {
                 printf("Matrix[%d][%d]: ",i+1,j+1);
                 scanf("%d", &(*matrix)[i][j]);
            }
        }
};

void displayMatrix (int ***matrix, int A, int B) {
    int i, j;
    printf ("\n");
    for(i=0;i<B;i++)
    {
        for(j=0;j<A;j++)
        {
            printf ("%d ", (*matrix)[i][j]);
        }
    printf ("\n");
    }
};

int main(){
    int **matrix;
    int **matrix_two;
    int **matrix_three;
    int a, b, c, d;

    printf ("[1st matrix] number of columns:");
    scanf ("%d", &a);
    printf ("[1st matrix] number of rows:");
    scanf ("%d", &b);

        mallocMatrix (&matrix, a, b);
        fillMatrix (&matrix, a, b);

    printf ("[2nd matrix] number of columns:");
    scanf ("%d", &c);
    printf ("[2nd matrix] number of rows:");
    scanf ("%d", &d);

        if (a==d)
        {
            mallocMatrix (&matrix_two, c, d);
            fillMatrix (&matrix_two, c, d);
        }
        else
        {
            printf ("The number of columns of the 1st. matrix must be equal to the number of rows of the 2nd. matrix");
            return -1;
        }

    displayMatrix (&matrix, a, b);
    displayMatrix (&matrix_two, c, d);

    mallocMatrix (&matrix_three, b, c);

    /* multiplication */

    int i, j, k;
    for(i = 0; i < b; i++)
    {
        for(j = 0; j < c; j++)
        {
            for(k = 0; k < b; k++)
            {
                matrix_three[i][j] = matrix_three[i][j] + matrix[i][k] * matrix_two[k][j];
            }
        }
    }

    /* end of multiplication */

    displayMatrix (&matrix_three, b, c);
}

Problem pops up after code signed /multiplication/. For example:
– first matrix is 2×2,
– second 3×2.
I’m expecting result as 3×2 matrix, BUT code gives me 2×2 matrix and segmentation fault when trying to reach third column. Please, point error

Answer 1

I think i found your mistake in mallocMatrix:

 for(i=0;i<A;i++)
            (*matrix)[i]=(int*)malloc(sizeof(int)*A);

The loop condition is wrong you said that B is row count, so for every rouw you must alloc an array of A elements so, the loop should be:

for(i=0;i<B;i++)
                (*matrix)[i]=(int*)malloc(sizeof(int)*A);

so when you try 3×2 the third row is never initialized because your loop just iterates 2 times, and when you try to fill the last row crashes