与编译器优化结合的分段错误

Question

我的代码使用联合来表示 RGB 像素。它在调试模式下运行良好，但启用编译器优化后，它会出现段错误。

您可以在下面看到用于重现错误的简化测试代码。我的系统是 Ubuntu GCC4.8.2。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <inttypes.h>

union Pixel {
    uint32_t color;
    uint8_t at[4];
};

typedef struct Screen {
    union Pixel *pixels;
    int width;
    int height;
    int rPosition;
    int gPosition;
    int bPosition;
} *Screen;

Screen ScreenCreate(int width, int height, uint32_t rPosition, uint32_t gPosition, uint32_t bPosition) {
    Screen this = malloc(sizeof *this);
    this->pixels = malloc(width * height * sizeof this->pixels[0]);
    this->width = width;
    this->height = height;
    this->rPosition = rPosition;
    this->gPosition = gPosition;
    this->bPosition = bPosition;
}

void ScreenDelete(Screen this) {
    free(this->pixels);
    free(this);
}

void ScreenFill(Screen this, uint8_t r, uint8_t g, uint8_t b) {
    union Pixel pixel;
    pixel.color = 0;
    pixel.at[this->rPosition] = r;
    pixel.at[this->gPosition] = g;
    pixel.at[this->bPosition] = b;
    for (int i = 0; i < this->width * this->height; i += 1) {
        this->pixels[i].color = pixel.color;
    }
}

int main(void) {
    Screen screen = ScreenCreate(500, 500, 2, 1, 0);
    ScreenFill(screen, 0xff, 0xff, 0xff);
    ScreenDelete(screen);
    return 0;
}

Answer 1

在您的 ScreenCreate() 函数中，您忘记了一个 return 语句。因此，您的代码会产生未定义的行为。

最后一行应该是

 return this;

相关：根据 C11 标准，第 6.9.1 章，函数定义，第 12 段

If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined.

所以，在 main() 中使用 screen 导致 undefined behaviour。

故事的寓意：启用编译器警告并留意它们。

与编译器优化结合的分段错误

segmentation fault in unions with compiler optimization

c

return

segmentation-fault