Modifying values pointed to by a struct member in a function - c

Question

I'm trying to create a function to initialise a Matrix structure with its n*n dimensions and an array of elements. However the values from the array passed to the function aren't being correctly assigned to the array in the matrix (see output).
What's causing the problem and is there a better way of doing this?
#include <stdio.h>
#include <stdlib.h>
typedef struct
{
int rows;
int cols;
double *data;
} Matrix;
Matrix loadMatrix(int rows, int cols, double *data); //initialise matrix with elements in 'data'
void printMatrix(Matrix m);
int main(void) {
double data[4] = {1,2,3,4};
Matrix m = loadMatrix(2, 2, data);
printMatrix(m);
free(m.data);
return 0;
}
Matrix loadMatrix(int rows, int cols, double * elements)
{
Matrix result;
result.rows = rows;
result.cols = cols;
result.data = (double *)calloc(rows * cols, sizeof(double));
for(int i = 0; i < rows * cols; i++) {
*result.data = *elements; //copy each element to Matrix instance
result.data++;
elements++;
}
return result;
}
void printMatrix(Matrix m)
{
printf("\nRows: %d\tColumns: %d\n", m.rows, m.cols);
for(int i = 0; i < m.rows; i++) {
for(int j = 0; j < m.cols; j++) {
printf("%.0lf ", *m.data);
m.data++;
}
printf("\n");
}
}
Output
Rows: 2 Columns: 2
0 0
0 0
double free or corruption (out)
exit status -1

Answer 1

In this loop:
for(int i = 0; i < rows * cols; i++) {
*result.data = *elements; //copy each element to Matrix instance
result.data++;
elements++;
}
You're changing what result.data points to on each iteration of the loop. When the loop ends, it points to the end of the allocated array.
When you subsequently print the array:
for(int i = 0; i < m.rows; i++) {
for(int j = 0; j < m.cols; j++) {
printf("%.0lf ", *m.data);
m.data++;
}
printf("\n");
}
You read past the end of the array and move m.data further past the end of the array. This invokes undefined behavior. You further invoke undefined behavior when you call free because m.data no longer points to the value returned by malloc.
You can fix this by using array indexing syntax in both places instead of modifying the pointer value.
In loadMatrix:
for(int i = 0; i < rows * cols; i++) {
result.data[i] = elements[i];
}
In printMatrix:
for(int i = 0; i < m.rows; i++) {
for(int j = 0; j < m.cols; j++) {
printf("%.0lf ", m.data[i*m.rows+j);
}
printf("\n");
}

Answer 2

You are breaking your result.data pointer by incrementing it in loadMatrix. It no longer points to start of the allocated memory when you try to print or free it.
Replace pointer arithmetric nonsense with array indexing to avoid messing the pointer and making the code more simple:
for(int i = 0; i < rows * cols; i++) {
result.data[i] = elements[i];
}
Note that you have the same problem in printMatrix function, but since you don't return the modified copy, it has no effect further in the code. Still I would recommend using array indexing there too.