Programming Forums > Application Development > C++

dr.p · Apr 20th, 2006, 7:21 PM

Code below defines WORD and DWORD types, and a struct BITMAPFILEHEADER that uses only those types. WORD should be 2 bytes, DWORD 4.

If I do a sizeof(BITMAPFILEHEADER), output is ALWAYS 16 if I use a DWORD for 1 or 2 members in the struct. It should be 10 for 1 member with DWORD, and 14 for 2 members. If I use WORD for all 5 members, sizeof = 10, which is correct. But I always get 16 if 1 or 2 members are DWORD.

Anyone have any idea why this would happen? I've been staring at it for hours and can't seem to see the problem.

I have 4 other structs that are all the correct sizes, and they use WORD and DWORD, as well.

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typedef unsigned short WORD;
typedef unsigned long DWORD;

// see "BITMAPFILEHEADER" in Platform SDK docs
typedef struct tagBITMAPFILEHEADER {
  WORD    bfType;
  DWORD   bfSize;
  WORD    bfReserved1;
  WORD    bfReserved2;
  DWORD   bfOffBits;
} BITMAPFILEHEADER, *PBITMAPFILEHEADER;

Thanks for looking

This is driving me crazy!

The Dark · Apr 20th, 2006, 8:08 PM

It is probably because of structure packing. The compiler will try to put your DWORDS on 4 byte boundaries, so that they are faster for the CPU to process.

How you fix it depends on what compiler you are using. Under visual C you can use

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#pragma pack (1)

... define structures

#pragma pack ()

Which will pack everything as tightly as possible. The second pragma sets the value back to the default.

dr.p · Apr 20th, 2006, 8:14 PM

Thank you, The Dark. You rock.

Apr 20th, 2006, 7:21 PM	#1
dr.p Programmer Join Date: Feb 2006 Location: Ohio Posts: 93 Rep Power: 3	Help With Struct Size Issue Code below defines WORD and DWORD types, and a struct BITMAPFILEHEADER that uses only those types. WORD should be 2 bytes, DWORD 4. If I do a sizeof(BITMAPFILEHEADER), output is ALWAYS 16 if I use a DWORD for 1 or 2 members in the struct. It should be 10 for 1 member with DWORD, and 14 for 2 members. If I use WORD for all 5 members, sizeof = 10, which is correct. But I always get 16 if 1 or 2 members are DWORD. Anyone have any idea why this would happen? I've been staring at it for hours and can't seem to see the problem. I have 4 other structs that are all the correct sizes, and they use WORD and DWORD, as well. (Toggle Plain Text) typedef unsigned short WORD; typedef unsigned long DWORD; // see "BITMAPFILEHEADER" in Platform SDK docs typedef struct tagBITMAPFILEHEADER { WORD bfType; DWORD bfSize; WORD bfReserved1; WORD bfReserved2; DWORD bfOffBits; } BITMAPFILEHEADER, PBITMAPFILEHEADER; typedef unsigned short WORD; typedef unsigned long DWORD; // see "BITMAPFILEHEADER" in Platform SDK docs typedef struct tagBITMAPFILEHEADER { WORD bfType; DWORD bfSize; WORD bfReserved1; WORD bfReserved2; DWORD bfOffBits; } BITMAPFILEHEADER, PBITMAPFILEHEADER; Thanks for looking This is driving me crazy! __________________ Neeley.org

Apr 20th, 2006, 8:08 PM	#2
The Dark Expert Programmer Join Date: Jun 2005 Posts: 893 Rep Power: 4	It is probably because of structure packing. The compiler will try to put your DWORDS on 4 byte boundaries, so that they are faster for the CPU to process. How you fix it depends on what compiler you are using. Under visual C you can use (Toggle Plain Text) #pragma pack (1) ... define structures #pragma pack () #pragma pack (1) ... define structures #pragma pack () Which will pack everything as tightly as possible. The second pragma sets the value back to the default.

Apr 20th, 2006, 8:14 PM	#3
dr.p Programmer Join Date: Feb 2006 Location: Ohio Posts: 93 Rep Power: 3	Thank you, The Dark. You rock. __________________ Neeley.org

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Programming Forums > Application Development > C++

Help With Struct Size Issue