Segmentation fault , caused by wrong interval (2D array) - c++

Question

I wanted to learn how threads work, and I tried to make a program, which would use 2 threads, to copy a picture (just to test my newly acquired threading skills) . But I bumped into an error, probably because my interval (created by the interval function) is only working ( I believe) with one dimensional arrays.How can I change my program , to correctly create intervals , which work on 2 dimensional arrays, such as pictures ?
#include <iostream>
#include <vector>
#include <time.h>
#include <thread>
#include <mutex>
#include <png++/png.hpp>
std::mutex my_mutex;
std::vector<int> interval(int max, int n_threads)
{
std::vector<int> intervallum;
int ugras = max / n_threads;
int maradek = max % n_threads;
int n1 = 0;
int n2;
intervallum.push_back(n1);
for (int i = 0; i < n_threads; i++)
{
n2 = n1 + ugras;
if (i == n_threads - 1)
n2 += maradek;
intervallum.push_back(n2);
n1 = n2;
}
return intervallum;
}
void create_image(png::image<png::rgb_pixel> image, png::image<png::rgb_pixel> new_image, int start, int end)
{
std::lock_guard<std::mutex> lock(my_mutex);
for (int i = start; i < end; i++)
for (int j = start; j < end; j++)
{
new_image[i][j].red = image[i][j].red;
new_image[i][j].blue = image[i][j].blue;
new_image[i][j].green = image[i][j].green;
}
}
int main()
{
png::image<png::rgb_pixel> png_image("mandel.png");
int image_size = png_image.get_width() * png_image.get_height();
png::image<png::rgb_pixel> new_image(png_image.get_width(), png_image.get_height());
time_t start, end;
time(&start);
int size = 2;
std::vector<std::thread> threads;
std::vector<int> stuff_interval = interval(image_size, size);
for (int i = 0; i < size-1; i++)
threads.push_back(std::thread(create_image, std::ref(png_image), std::ref(new_image), stuff_interval[i], stuff_interval[i + 1]));
for (auto& i : threads)
i.join();
create_image(png_image,new_image,stuff_interval[size-2],stuff_interval[size-1]);
new_image.write("test.png");
time(&end);
std::cout << (start - end) << std::endl;
return 0;
}

Answer 1

Okay , I found a way around it (this way I am not getting segmentation error, but it does not copy the image correctly, the new image is fully black, here is the code :
EDIT : seems like, I was passing wrong the pictures, that is the reason why the picture was black.
#include <iostream>
#include <vector>
#include <time.h>
#include <thread>
#include <mutex>
#include <png++/png.hpp>
std::mutex my_mutex;
std::vector<int> interval(int max, int n_threads)
{
std::vector<int> intervallum;
int ugras = max / n_threads;
int maradek = max % n_threads;
int n1 = 0;
int n2;
intervallum.push_back(n1);
for (int i = 0; i < n_threads; i++)
{
n2 = n1 + ugras;
if (i == n_threads - 1)
n2 += maradek;
intervallum.push_back(n2);
n1 = n2;
}
return intervallum;
}
void create_image(png::image<png::rgb_pixel>& image, png::image<png::rgb_pixel>& new_image, int start, int end)
{
std::lock_guard<std::mutex> lock(my_mutex);
for (int i = start; i < end; i++)
for (int j = 0; j < image.get_height(); j++)
{
new_image[i][j].red = image[i][j].red;
new_image[i][j].blue = image[i][j].blue;
new_image[i][j].green = image[i][j].green;
}
}
int main()
{
png::image<png::rgb_pixel> png_image("mandel.png");
int image_size = png_image.get_width() * png_image.get_height();
png::image<png::rgb_pixel> new_image(png_image.get_width(), png_image.get_height());
time_t start, end;
time(&start);
int size = 2;
std::vector<std::thread> threads;
std::vector<int> stuff_interval = interval(png_image.get_width(), size);
new_image.write("test2.png");
for (int i = 0; i < size - 1; i++)
threads.push_back(std::thread(create_image, std::ref(png_image), std::ref(new_image), stuff_interval[i], stuff_interval[i + 1]));
for (auto &i : threads)
i.join();
create_image(std::ref(png_image), std::ref(new_image), stuff_interval[size - 1], stuff_interval[size]);
new_image.write("test.png");
time(&end);
std::cout << (start - end) << std::endl;
return 0;
}