Strange segfault from SDL_FreeSurface

Question

I have the following simple SDL code:

#include <SDL.h>
#include <stdbool.h>
#include <stdio.h>

// helpers

bool init(SDL_Window **win, SDL_Surface **surf) {
        int const width = 800;
        int const height = 600;
        if (SDL_Init(SDL_INIT_VIDEO) != 0) {
                fprintf(stderr, "Unable to init SDL: %s\n", SDL_GetError());
                return false;
        }
        *win = SDL_CreateWindow("Picture test",
                                SDL_WINDOWPOS_CENTERED,
                                SDL_WINDOWPOS_CENTERED,
                                width, height, 0);
        if (*win == NULL) {
                fprintf(stderr,
                        "Unable to create window: %s\n",
                        SDL_GetError());
                return false;
        }
        *surf = SDL_GetWindowSurface(*win);
        return true;
}

bool load_media(SDL_Surface **surf) {
        *surf = SDL_LoadBMP("./sample.bmp");
        if (*surf == NULL) {
                fprintf(stderr, "Unable to load data: %s\n", SDL_GetError());
                return false;
        }
        return true;
}

void close(SDL_Window **win, SDL_Surface **surf) {
        SDL_FreeSurface(*surf);
        SDL_DestroyWindow(*win);
        SDL_Quit();
}


int main()
{
        SDL_Window *win;
        SDL_Surface *surf;
        SDL_Surface *img;
        if (!init(&win, &surf)) {
                return EXIT_FAILURE;
        }
        if (!load_media(&img)) {
                return EXIT_FAILURE;
        }
        SDL_BlitSurface(img, NULL, surf, NULL);
        SDL_UpdateWindowSurface(win);
        SDL_Delay(2000);
        close(&win, &img);
}

My code always segfaults on close (the origin of the segfault according to GDB is the line SDL_FreeSurface(*surf)). More strangely, if I replace the call to close with its definition, this still segfaults in exactly the same place. Specifically, if I replace close(&win, &img) with:

SDL_FreeSurface(img);
SDL_DestroyWindow(win);
SDL_Quit();

the code still segfaults at exactly the same place, even though that function is not even being called. Only if I remove the entire close function does this work correctly. I am completely confused as to what could be causing this.

You know there is a C library function `int close(int fd);`? Is there any conflict - compiler warnings? — Weather Vane, Nov 08 '16 at 21:00
@WeatherVane I tried renaming ``close`` to something else, and that fixed it! Surprisingly, despite having every imaginable warning on, the compiler still accepted it! — Koz Ross, Nov 08 '16 at 21:18
@KozRoss moreover it may be accepted without any warning, since in glibc `close` (and many others) is a weak symbol. With that kind of simple names (and limited scope) it might be reasonable to use `static` functions. — keltar, Nov 09 '16 at 05:39

score 5 · Accepted Answer · answered Nov 08 '16 at 21:20

5

Please rename your function

void close(SDL_Window **win, SDL_Surface **surf)

since close is a standard C library function.

answered Nov 08 '16 at 21:20

Weather Vane

33,872
7
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score 2 · Answer 2 · edited Jan 15 '17 at 01:07

2

I can confirm this. Same scenario. Even with -Wall and -Wextra the compiler did not spit out a redeclaration warning. Same for open().

I need an experts' opinion if this is a gcc bug.

Solution 1: declare your close() as a static function (e.g. static close()).
Solution 2: rename your close() function to something else (e.g. my_close_foo()).

edited Jan 15 '17 at 01:07

zx485

28,498
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answered Jan 15 '17 at 00:47

markotikvic

131
3
5

I *hope* that this is a bug - it'd be pretty scary if GCC didn't at least warn you about something this obviously silly. Same goes for Clang. – Koz Ross Jan 15 '17 at 22:34

Strange segfault from SDL_FreeSurface

2 Answers2

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