Question

I’m getting the double free or corruption error upon executing my code. Essentially, I am just creating a matrix in C (of any RxC dimension, which is why pointers are used), swapping two rows, printing the results, and then attempting to free the memory. When I do not swap the rows, the freeing works perfectly. When I do, it crashes. I’ve attempted to alter the way the swapping works to no avail. I think it has something to do with the temporary pointer for swapping going out of scope, but I’m not sure if this is the problem and how I would fix it.

MatElement is just a double.

typedef double MatElement;

main:

int main(int argc, char *argv[]) {

    MatElement** matrix = matrixAlloc(3,3);

    int i;
    int j;

    for(i = 0; i < 3; i++) {
        for(j = 0; j < 3; j++) {
            matrix[i][j] = i+j;
        }
    }
    matrixPrint(matrix, "%5.1f", 3, 3);
    swapRows(matrix, 0, 2);
    matrixPrint(matrix, "%5.1f", 3, 3);

    matrixFree(matrix);
    return 0;
}

The way the matrices are allocated:

MatElement **matrixAlloc(int nr, int nc) {
    int i;
    MatElement *ptr;
    MatElement **A;

    A = malloc(nr * sizeof(MatElement *)); /* array of ptrs   */
    ptr = calloc(nr * nc, sizeof(MatElement)); /* matrix elements */
    for (i = 0; i < nr; i++) /* set row pointers properly */
        A[i] = ptr + nc * i;
    return A;
}

The way they are freed:

void matrixFree(MatElement **A) {
    free(A[0]);
    free(A);
}

The way their rows are swapped:

void swapRows(MatElement** G, int pivotRow, int rowExamined) {
    MatElement* temp;

    temp = G[rowExamined];
    G[rowExamined] = G[pivotRow];
    G[pivotRow] = temp;
}

Does anyone have any idea about what would be causing this double free() / invalid free()?

Answer 1

At some point, you are swapping the first row of the matrix into another position, so the free(A[0]) in matrixFree() is attempting to free a pointer into the middle of the array, instead of the pointer returned by calloc(). You will need to save that original pointer somewhere so that you can pass it, unmolested, to free().