Segmentation fault at std::thread

Question

typedef struct _ppm_struct* ppm_struct;
typedef unsigned char ppm_subpixel;

typedef ppm_subpixel (*ppm_pixel_func)(ppm_subpixel);
struct ppm_pixel {
   ppm_subpixel red;
   ppm_subpixel green;
   ppm_subpixel blue;
};

struct _ppm_struct {
   unsigned int width;
   unsigned int height;
   unsigned int max_color;
   ppm_pixel *firstpixel;
};

So I have this function call:

void pppm_color_pixel(ppm_struct p, ppm_struct out, ppm_pixel_func func, int threads) {
//ppm_pixel_func func2 = &inverse_pixel;
unsigned int w = p->width;
unsigned int h = p->height;
ppm_pixel *iterator = p->firstpixel;
std::thread th[threads];

long long total_pixels = w*h;
unsigned int count_for_thread = total_pixels/threads;

for(int j = 0; j<threads; ++j) {

   th[j] = std::thread(pppm_color_chunk_pixel,func, *iterator,count_for_thread);
   iterator = &iterator[count_for_thread];
}

for(int j = 0; j<threads; ++j) {
    th[j].join();
}

The next two function: funct is inverse subpixel

ppm_subpixel inverse_subpixel(ppm_subpixel subpixel) {
    subpixel = 255- subpixel;
    return subpixel;
}

void pppm_color_chunk_pixel(ppm_pixel_func func, ppm_pixel start_pixel, unsigned int count) {
    ppm_pixel *iterator = &start_pixel;
    for(unsigned int i = 0; i< count; ++i) {
        iterator[i].red = func(iterator[i].red);
        iterator[i].green = func(iterator[i].green);
        iterator[i].blue = func(iterator[i].blue);
    }
}

The main:

pppm_color_pixel(p,out, &inverse_subpixel,pppm_get_max_cores());

The problem is that when I run this it shows me a segmentation fault. This happen on this line:

iterator[i].green = func(iterator[i].green);

What I really don't understand is that i = 2 or more when this happens. It doesn't crash from the first time. Even if I try to call only a single thread the result is the same.

Answer 1

pppm_color_chunk_pixel accepts a copy of ppm_pixel. Since it is copied by value, passing *iterator to it will cause only one element to be copied. Iterating over anything after &start_pixel will fail. Since std::thread doesn't pass objects by reference, you also need a reference-wrapper for this.

void pppm_color_chunk_pixel(ppm_pixel_func func, ppm_pixel& start_pixel, unsigned int count) {
ppm_pixel* iterator = &start_pixel;
for (unsigned int i = 0; i < count; ++i) {
    iterator[i].red   = func(iterator[i].red);
    iterator[i].green = func(iterator[i].green);
    iterator[i].blue  = func(iterator[i].blue);
}

Calling it:

for (int j = 0; j < threads; ++j) {
   th[j] = std::thread(pppm_color_chunk_pixel, func, std::ref(*iterator), count_for_thread);
   iterator = &iterator[count_for_thread];
}

I have not tested it, but I guess this is your problem.