Debug Assertion failure problem

This is a discussion on Debug Assertion failure problem within the C++ Programming forums, part of the General Programming Boards category; I am trying to open a 24-bit bitmap file to use as a simple texture in an OpenGL scene, but ...

  1. #1
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    Debug Assertion failure problem

    I am trying to open a 24-bit bitmap file to use as a simple texture in an OpenGL scene, but I cant get past this debug assertion failure that comes up on fseek or fread.

    Code:
    void loadBMP(char *filename, byte* BitmapP, int* owidth, int* oheight, int max) {
    	byte curnt[3];
    	int width, height;
    
    	FILE* file1;
    	file1 = fopen(filename, "rb");
    	fseek(file1, 18, SEEK_SET);
    	fread(&width, sizeof(int), (size_t)1, file1);
    	fseek(file1, 22, SEEK_SET);
    	fread(&height, sizeof(int), (size_t)1, file1);
    	*owidth = width;
    	*oheight = height;
    	fseek(file1, 54, SEEK_SET);
    	for(int y=1; y < height + 1; y++){
    		for(int x=1; x < width + 1; x++){
    			fread((BitmapP + x*(max*3) + y*3), sizeof(byte), (size_t)3, file1);
    		}
    		fseek(file1, width % 4, SEEK_CUR);
    	}
    	fclose(file1);
    }
    I took most of this code from an online tutorial, but I understand what it's doing and can't figure out why its throwing this error. But i think I have a general idea of where its going wrong. There seems to be a bad pointer remaining after fopen thats causing the first fseek to fail.

    I followed the debugger into
    Code:
    file1 = fopen(filename, "rb");
    vs shows the variable file with the proper data, which was "metalt.bmp"
    then I followed it into fopen.c, line 124,
    Code:
    return( _tfsopen(file, mode, _SH_DENYNO) );
    which jumps back up to line 50, the _tfsopen function. After these declarations
    Code:
    REG1 FILE *stream=NULL;
    REG2 FILE *retval=NULL;
    retval has all its data, but stream does not. It's got the scary
    stream 0x00000000 {_ptr=??? _cnt=??? _base=??? ...} _iobuf *
    and all of its data "cannot be evaluated."

    Long story short, it successfully completes fopen and all its internal functions even though its now saying "bad pointer" all over the place, then runs
    Code:
    fseek(file1, 18, SEEK_SET);
    and then it breaks in fseek.c on line 100 which is
    Code:
    _VALIDATE_RETURN( (stream != NULL), EINVAL, -1);
    seemingly because stream, which is a FILE* does not have my bitmap file likes it's supposed to. Any suggestions?

    I'm running VS2005 with a win32 console app if that makes a difference.

    thanks in advance

  2. #2
    and the hat of wrongness Salem's Avatar
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    It means you're not checking for success.
    Code:
    file1 = fopen(filename, "rb");
    if ( file1 != NULL ) {
      // do the stuff which reads the file.
    } else {
      perror("Can't open file");
    }
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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    Well yes I know that. file1 is never NULL, so this just allows it to run and not open the file. I guess my question would be better if I asked, what can I do to correct the bad pointer problem?

  4. #4
    and the hat of wrongness Salem's Avatar
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    The assertion is telling you that it is NULL.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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    So why would fopen return NULL to file1?

  6. #6
    and the Hat of Guessing tabstop's Avatar
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    If a file with that name cannot be found? ETA: That is to say, in the directory from which you are running the file. Is your .bmp in a different directory?

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    its a resource in my project, in my project directory, and I've also tried the full path. Same results

  8. #8
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    Oh wow I just double checked that and I found I accidently dragged the bmp into the Debug folder in my project. Well I guess that explains that. The texture isnt showing up however, haha. But thats more of a hair-pulling opengl problem which will probably be easier to figure out than debug errors.
    Thanks guys

  9. #9
    and the hat of wrongness Salem's Avatar
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    > So why would fopen return NULL to file1?
    Well if you'd had proper error checking to begin with (and not a presumption of success), and a call to perror(), it would have told you right off the bat what was wrong.

    Tedious and laborious it may be, but when your programs get out into the real world, all sorts of things can go wrong. Say some miscreant decided to feed your program a handful of 10MB .bmp files for textures, what would happen then?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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