Segmentation Fault

This is a discussion on Segmentation Fault within the C++ Programming forums, part of the General Programming Boards category; Code: Main.cpp #include "CSDL.h" CSDL sdl; int main(int argc, char *argv[]) { SDL_Surface *screen; sdl.InitSDL(); sdl.SetVidMode(screen); sdl.FillRect(screen, 0, 0, _WIDTH, ...

  1. #1
    Registered User
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    May 2002
    Posts
    31

    Segmentation Fault

    Code:
    Main.cpp
    #include "CSDL.h"
    
    CSDL sdl;
    
    int main(int argc, char *argv[])
    {
     SDL_Surface *screen;
     sdl.InitSDL();
     sdl.SetVidMode(screen);
     sdl.FillRect(screen, 0, 0, _WIDTH, _HEIGHT, 45, 45, 45);
     sdl.UpdateScreen(screen);
     return 0;
    }
    Code:
    FillRect function
    //Fills a rect (x, y, w, h) with color (r, g, b) on screen (screen)
    void CSDL::FillRect(SDL_Surface *screen, int x, int y, int w, int h, Uint8 r, Uint8 g, Uint8 b)
    {
     dst.x = x;
     dst.y = y;
     dst.w = w;
     dst.h = h;
     
     SDL_FillRect(screen, &dst, SDL_MapRGB(screen->format, r, g, b));
     
     return;
    }
    I get a segmentation fault on calls to sdl.FillRect();
    Any ideas why?

    Full Source:
    Attached Files Attached Files
    I never lie... except for right now.

  2. #2
    Code Goddess Prelude's Avatar
    Join Date
    Sep 2001
    Posts
    9,796
    A segmentation fault is caused when you try to access an invalid pointer. Unless sdl.SetVidMode(screen) manages to point screen to a valid location in memory with the correct values (which isn't likely unless SetVidMode takes a reference to a pointer so that it can properly modify the pointer passed to it instead of a copy), FillRect is working with an uninitalized pointer. That will most likely cause problems.
    My best code is written with the delete key.

  3. #3
    Registered User Codeplug's Avatar
    Join Date
    Mar 2003
    Posts
    4,680
    This is the problem.
    Code:
    void CSDL::SetVidMode(SDL_Surface *screen)
    {
        screen = SDL_SetVideoMode(_WIDTH, _HEIGHT, _BPP, SDL_SWSURFACE);
        return;
    }
    Have to run, but others will help you out now that the culprit code is posted.

    gg

  4. #4
    Code Goddess Prelude's Avatar
    Join Date
    Sep 2001
    Posts
    9,796
    >This is the problem.
    You actually download files? I pretty much ignore questions that only give an attached file.

    >screen = SDL_SetVideoMode(_WIDTH, _HEIGHT, _BPP, SDL_SWSURFACE);
    This will not work. screen is passed to SetVidMode by pointer, but pointers are passed by value. In other words, the screen you assign to in SetVidMode is a copy of the pointer you passed to SetVidMode in main. You change the copy, but the original remains uninitialized. This can be fixed by passing a pointer to the pointer:
    Code:
    void CSDL::SetVidMode(SDL_Surface **screen)
    {
        *screen = SDL_SetVideoMode(_WIDTH, _HEIGHT, _BPP, SDL_SWSURFACE);
        return;
    }
    
    ...
    
    #include "CSDL.h"
    
    CSDL sdl;
    
    int main(int argc, char *argv[])
    {
     SDL_Surface *screen;
     sdl.InitSDL();
     sdl.SetVidMode(&screen);
     sdl.FillRect(screen, 0, 0, _WIDTH, _HEIGHT, 45, 45, 45);
     sdl.UpdateScreen(screen);
     return 0;
    }
    Or as a reference to a pointer:
    Code:
    void CSDL::SetVidMode(SDL_Surface *&screen)
    {
        screen = SDL_SetVideoMode(_WIDTH, _HEIGHT, _BPP, SDL_SWSURFACE);
        return;
    }
    
    ...
    
    #include "CSDL.h"
    
    CSDL sdl;
    
    int main(int argc, char *argv[])
    {
     SDL_Surface *screen;
     sdl.InitSDL();
     sdl.SetVidMode(screen);
     sdl.FillRect(screen, 0, 0, _WIDTH, _HEIGHT, 45, 45, 45);
     sdl.UpdateScreen(screen);
     return 0;
    }
    My best code is written with the delete key.

  5. #5
    Registered User
    Join Date
    May 2002
    Posts
    31
    Thanks prelude and codeplug, never was too good at pointers... or classes for that matter, or cpp, or....

    Guess i waited to long to learn them.
    I never lie... except for right now.

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