Question

I am writing a bitmap loader and the byte count is not working correctly. I am currently using Xcode running a 64 bit unix executable.

The code:

typedef unsigned char BYTE;
typedef unsigned short WORD;
typedef unsigned int DWORD;

typedef struct {
    WORD bfType;
    DWORD bfSize;
    WORD bfReserved1;
    WORD bfReserved2;
    DWORD bfOffbits;
} BITMAPFILEHEADER;

The sizeof function says the BITMAPFILEHEADER struct is 16 Bytes when it should be 14 Bytes. Any suggestions?

Answer 1

This might be because of structure alignment. Here take a look http://peeterjoot.wordpress.com/2009/11/11/c-structure-alignment-padding/

You can force the compiler not to align the structure by adding the attribute __attribute__((__packed__)) but as far as I know it only works in gcc.

Here:

struct BITMAPFILEHEADER_S{
  WORD bfType;
  DWORD bfSize;
  WORD bfReserved1;
  WORD bfReserved2;
  DWORD bfOffbits;
} __attribute__((__packed__));

typedef BITMAPFILEHEADER_S BITMAPFILEHEADER;