Why does my image get cropped in half when applied Sobel Edge Detector?

Question

Let me start by saying I am very newbie at C++ and so I don't really know the best practices or handle very well with the syntax. I'm trying to read a black and white image, and using the sobel algorithm to detect its edges and output the result, but halfway my execution I get an error:

munmap_chunk(): invalid pointer
Aborted (core dumped)

Though the image is outputted, it's only half of it and I can't seem to figure out whats causing this.

I wrote the following code:

#include <iostream>
#include <omp.h>
#include "CImg.h"
using namespace std;
using namespace cimg_library;

int main() {
    const int x_mask [9] = {
        -1, 0, 1,
        -2, 0, 2,
        -1, 0, 1
    };

    const int y_mask [9] = {
        -1, -2, -1,
        0, 0, 0,
        1, 2, 1
    };

    const char* fileName = "test.png";
    CImg<float> img = CImg<float>(fileName);
    
    int cols = img.width();
    int lines = img.height();
    CImg<float> output = CImg<float>(cols, lines, 1, 1, 0.0);
    printf("Loading %d x %d image...\n", cols, lines);
    const int mask_size = 3;

    int gradient_x;
    int gradient_y;
    // Loop through image ignoring borders
    for(int i = 1; i<cols-1; i++) {
        for(int j = 1; j<lines-1; j++){
            output(j,i) = 0;
            gradient_x = 0;
            gradient_y = 0;

            // Find the x_gradient and y_gradient
            for(int m = 0; m < mask_size; m++) {
                for(int n = 0; n < mask_size; n++) {
                    // Neighbourgh pixels
                    int np_x = j + (m - 1);
                    int np_y = i + (n - 1);

                    float v = img(np_x,np_y);

                    int mask_index = (m*3) + n;

                    gradient_x = gradient_x + (x_mask[mask_index] * v);
                    gradient_y = gradient_y + (y_mask[mask_index] * v);
                }
            }
            float gradient_sum = sqrt((gradient_x * gradient_x) + (gradient_y * gradient_y));
            if(gradient_sum >= 255) {
                gradient_sum = 255;
            } else if(gradient_sum <= 0) {
                gradient_sum = 0;
            }
            output(j, i) = gradient_sum;
        }
    }
    printf("Outputed image of size %d x %d\n", output.width(), output.height());
    output.save("test_edges.png");
 return 0;
}

Applied to this image:

I get this ouput: