Programming Forums > Application Development > C++

Seif · May 21st, 2007, 11:20 PM

Hi, I'm having the problem when trying to read in the data to the following struct.

struct BMPFileHeader
{
	unsigned char	fileType[2];
	long			fileSize;
	unsigned short	reserved[2];
	long			dataOffset;
};

now, this struct should only be 14 bytes, but when doing a sizeof of this header, i find that it is giving 16 bytes instead, causing some trouble when reading data.

I remember reading why this happens a while back. but I was wondering, is there any work around ? I would like to keep the struct if possible for ease of coding, but I can read em in individually if need be.... sigh.

I can't change the order of which I declare these either.

Mad_guy · May 22nd, 2007, 1:53 AM

iirc, your compiler will generally pad your structures to an 8-byte boundry, i.e. it'll pad it 'up to' 16.

When I manipulated the PE header, the general way I did it was just make a pointer to the structure and just point it at the start of your file map; pointer arithmetic should take care of it from there (the code to manipulate/read PE files I wrote I still have, you can see it here if you want a reference.)

I don't quite know how you're trying this, though. Have you tried it with the existing structure? If so, providing there were issues, it'd be nice to see some example code.

rsnd · May 22nd, 2007, 6:50 AM

I think most compilers these days including vc++ comes with the #pragma pack preprocessor directive.
sample reference: http://publib.boulder.ibm.com/infoce...f/rnpgpack.htm

Set it to 1 to solve all your problems.

Think there is also the "UNALIGNED" type modifier supported by some compilers

DaWei · May 22nd, 2007, 7:34 AM

If you're working with Windows, I'd recommend you skip the packing and use their predefined stuff for bitmap handling. The bitmap file header, for instance, is

(Toggle Plain Text)

typedef struct tagBITMAPFILEHEADER { 
  WORD    bfType; 
  DWORD   bfSize; 
  WORD    bfReserved1; 
  WORD    bfReserved2; 
  DWORD   bfOffBits; 
} BITMAPFILEHEADER, *PBITMAPFILEHEADER;

This would allow you to use some of their predefined bitmap functions.

Seif · May 24th, 2007, 11:36 AM

cheers for the response. I decided to read each part of the struct in seperately as it is only 5 parts in size. Am working with windows, but really wanted to use my own code.... I know its reinventing the wheel.

I currently have both the file and info header reading in ok, but I am now finding difficulty reading in the actual image data. But alas, I believe I should leave that for another thread.

DaWei · May 24th, 2007, 1:49 PM

You're aware that the data may either contain the pixel values or be an index into a color table?

May 21st, 2007, 11:20 PM	#1
Seif Hobbyist Programmer Join Date: Jan 2006 Location: UK Posts: 214 Rep Power: 3	struct gaining 2 bytes ? Hi, I'm having the problem when trying to read in the data to the following struct. (Toggle Plain Text) struct BMPFileHeader { unsigned char fileType[2]; long fileSize; unsigned short reserved[2]; long dataOffset; }; struct BMPFileHeader { unsigned char fileType[2]; long fileSize; unsigned short reserved[2]; long dataOffset; }; now, this struct should only be 14 bytes, but when doing a sizeof of this header, i find that it is giving 16 bytes instead, causing some trouble when reading data. I remember reading why this happens a while back. but I was wondering, is there any work around ? I would like to keep the struct if possible for ease of coding, but I can read em in individually if need be.... sigh. I can't change the order of which I declare these either.

May 22nd, 2007, 1:53 AM	#2
Mad_guy Hobbyist Programmer Join Date: Oct 2004 Location: Sandstorm, Techno Club Posts: 239 Rep Power: 4	iirc, your compiler will generally pad your structures to an 8-byte boundry, i.e. it'll pad it 'up to' 16. When I manipulated the PE header, the general way I did it was just make a pointer to the structure and just point it at the start of your file map; pointer arithmetic should take care of it from there (the code to manipulate/read PE files I wrote I still have, you can see it here if you want a reference.) I don't quite know how you're trying this, though. Have you tried it with the existing structure? If so, providing there were issues, it'd be nice to see some example code. __________________ os: mac os 10.5.4 revision control: git editor: emacs site

May 22nd, 2007, 6:50 AM	#3
rsnd Hobbyist Programmer Join Date: Jun 2005 Location: Helltown Posts: 162 Rep Power: 4	I think most compilers these days including vc++ comes with the #pragma pack preprocessor directive. sample reference: http://publib.boulder.ibm.com/infoce...f/rnpgpack.htm Set it to 1 to solve all your problems. Think there is also the "UNALIGNED" type modifier supported by some compilers __________________ Spread your wings and fly! Chicken!

May 22nd, 2007, 7:34 AM	#4
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	If you're working with Windows, I'd recommend you skip the packing and use their predefined stuff for bitmap handling. The bitmap file header, for instance, is (Toggle Plain Text) typedef struct tagBITMAPFILEHEADER { WORD bfType; DWORD bfSize; WORD bfReserved1; WORD bfReserved2; DWORD bfOffBits; } BITMAPFILEHEADER, PBITMAPFILEHEADER; typedef struct tagBITMAPFILEHEADER { WORD bfType; DWORD bfSize; WORD bfReserved1; WORD bfReserved2; DWORD bfOffBits; } BITMAPFILEHEADER, PBITMAPFILEHEADER; This would allow you to use some of their predefined bitmap functions. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

May 24th, 2007, 1:49 PM	#6
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	You're aware that the data may either contain the pixel values or be an index into a color table? __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

May 24th, 2007, 11:36 AM	#5
Seif Hobbyist Programmer Join Date: Jan 2006 Location: UK Posts: 214 Rep Power: 3	cheers for the response. I decided to read each part of the struct in seperately as it is only 5 parts in size. Am working with windows, but really wanted to use my own code.... I know its reinventing the wheel. I currently have both the file and info header reading in ok, but I am now finding difficulty reading in the actual image data. But alas, I believe I should leave that for another thread.

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Programming Forums > Application Development > C++

struct gaining 2 bytes ?