Fread skipping characters reading into object


I'm trying to read in a bitmap starting with its header, but fread is skipping characters for me.

I'm using this typedef in my header:

#include <windows.h>  // Used for other
#include <cstdio>
typedef struct tagBITMAPHEADER{
    WORD    wFileType;      
    DWORD   dwFileSize;     
    WORD    dwReserved;     
    WORD    dwReserved2;        
    DWORD   dwBmpDataOffset;
    DWORD   dwBmpHeaderSize;
    DWORD   dwWidth;    
    DWORD   dwHeight;   
    WORD    wPlanes;    
    WORD    wBitsPerPixel;  
    DWORD   dwCompression;  
    DWORD   dwBitmapDataSz; 
    DWORD   dwHRes;     
    DWORD   dwVRes;     
    DWORD   dwColors;   
    DWORD   dwImpColors;    

And in my code, I just use a simple fopen and fread with binary.

#include "ImageLoader.h"
FILE *fBitmap = fopen(FileName,"rb"); //Open file in read / binary  
if (fBitmap) //File is now open
{   fread(&pbhFileInfo,sizeof(BITMAPFILEHEADER),1,fBitmap);

Although my bitmap starts with '424DF25A0D' (hex), the first two variables read in seem to skip the 'F25A'

wFileType = 0x4d42 dwFileSize = 0x0000000d

Any idea what might be up?

Thanks in advance.

asked on Stack Overflow Jan 11, 2010 by rpgFANATIC

3 Answers


Your struct is being aligned by the compiler.

You seem to be using Visual C++. Add this line before your struct definition:

#pragma pack(push 1)

And this line after the definition

#pragma pack(pop)

To learn more, see Why isn’t sizeof for a struct equal to the sum of sizeof of each member?

answered on Stack Overflow Jan 11, 2010 by Drew Dormann • edited May 23, 2017 by Community

In my opinion it is highly unwise to be using a struct in this way. Yes, you can get what you want in this case with a compiler specific pragma. I would consider that an acceptable solution if you were writing a Windows device driver or something else that was already very specific to a particular platform.

But this is loading a file in a standard format. It's code that could run in any environment.

Personally, I would write code that lifted the data out of a character array and plopped it into the structure by hand instead of relying on the structure to be layed out in just the right way by the compiler such that fread will magically put all the little bits of data in the right places.

answered on Stack Overflow Jan 11, 2010 by Omnifarious

Are you opening the file in binary mode? If not, then this is your problem as it will convert the '0D' in 424DF25A0D to some other data as it interprets it as a carriage return/line feed character, instead of just raw binary data.

answered on Stack Overflow Jan 11, 2010 by Grant Peters

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