Segfaults

This is a discussion on Segfaults within the C Programming forums, part of the General Programming Boards category; I'm having problems with a function that writes image data to a buffer. Heres where I allocate the memory in ...

  1. #1
    Dr Dipshi++ mike_g's Avatar
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    Segfaults

    I'm having problems with a function that writes image data to a buffer. Heres where I allocate the memory in main():
    Code:
       fb = malloc(sizeof(int)*FONTSET_WIDTH*FONT_WIDTH*FONTSET_HEIGHT*FONT_HEIGHT);
    And heres my function along with struct definition and global pointer declaration:
    Code:
    struct font_buffer
    {
           Uint32 pixels[FONTSET_WIDTH*FONT_WIDTH*FONTSET_HEIGHT*FONT_HEIGHT];
    }; typedef struct font_buffer font_buffer;
    
    struct screen_buffer *sb = NULL; 
    struct font_buffer *fb = NULL;
    
    void LoadBitmapFont()
    {
        SDL_Surface *font=NULL;
        font = SDL_LoadBMP("Font.bmp");           
        unsigned image_width=FONTSET_WIDTH*FONT_WIDTH;
        unsigned image_height=FONTSET_HEIGHT*FONT_HEIGHT;
        unsigned image_size = image_width * image_height * sizeof(int); 
        unsigned i, inc=sizeof(int);
        Uint32 *src=NULL, *dst=NULL;
        
        for(i=0; i<image_size; i+=inc)
        {
            src = font->pixels+i;
            dst = fb->pixels+i;
            *dst = *src;        
        }          
         
        SDL_FreeSurface(font);
        font=NULL;
    }
    My debugger just says I got segfaults If I dont call this function then the program runs, so it seems as if there must be a problem with the function. Can someone please tell me whats wrong with it? Thanks.

  2. #2
    Dr Dipshi++ mike_g's Avatar
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    Sorry, I found out what was wrong just after posting (this allways seems to happen to me). I was incrementing +4 bytes, when I only have to increment by 1. This isn't the first time I have done this either >_<

  3. #3
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by mike_g View Post
    Sorry, I found out what was wrong just after posting (this allways seems to happen to me). I was incrementing +4 bytes, when I only have to increment by 1. This isn't the first time I have done this either >_<
    If you stop worrying about how big stuff is, you'll stop making that mistake. You should probably never use sizeof() except inside a call to malloc() or an I/O function. So when you set "incr = sizeof(int)" that should have set off an alarm bell.

  4. #4
    and the hat of int overfl Salem's Avatar
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    > fb = malloc(sizeof(int)*FONTSET_WIDTH*FONT_WIDTH*FONTSE T_HEIGHT*FONT_HEIGHT);
    Ugh, that's horrible.
    And wrong, since it assumes sizeof(int) == sizeof(Uint32)

    Read the FAQ, then do
    fb = malloc ( sizeof *fb );
    Will always be right, no matter how you change the contents of the struct.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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