std：：线程的分割故障

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;
};

所以我有一个函数调用：

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();
}

下两个函数: funct是逆亚像素

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);
    }
}

主要内容：

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

问题是，当我运行这个程序时，它会显示一个分段错误。这种情况发生在这条线上：

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

我真正不明白的是，当这种情况发生时，i = 2或更多。它从第一次开始就不会崩溃。即使我只调用一个线程，结果也是一样的。

Answer 1

pppm_color_chunk_pixel接受ppm_pixel的副本。由于它是由值复制的，将*iterator传递给它只会导致一个元素被复制。在&start_pixel之后迭代任何东西都会失败。由于std::thread不通过引用传递对象，为此还需要一个引用包装器。

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);
}

称之为：

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];
}

我还没有测试过，但我想这是你的问题。