From: eplmst AT lu DOT erisoft DOT se (Martin Stromberg)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: struct problem
Date: 8 Jan 2003 11:03:14 GMT
Organization: Ericsson Erisoft AB, Sweden
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To: djgpp AT delorie DOT com
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Joel Saunders (jbs30000 AT aol DOT com) wrote:
: In writting a simple bmp loader, I'm having problems with the struct for
: the bmp header.  Even with __attribute__((__packed__)) after each item:
: struct BMP_Header {
:         unsigned int BM __attribute__((__packed__));

4 bytes.
:         unsigned long File_Size __attribute__((__packed__));

4 bytes.

:         unsigned int R_1 __attribute__((__packed__)); // Reserved 1

4 bytes.

:         unsigned int R_2 __attribute__((__packed__)); // Reserved 2

4 bytes.

:         unsigned long BMP_Offset __attribute__((__packed__));

4 bytes.

...

: }image;

: I had to do this to use the header information:
: unsigned char image[54];
: #define image_BM ((int *)&image[0])[0]

2 bytes.

: #define image_File_Size ((long *)&image[2])[0]

4 bytes.

: #define image_R_1 ((int *)&image[6])[0]

2 bytes.

: #define image_R_2 ((int *)&image[8])[0]

2 bytes.

: #define image_BMP_Offset ((long *)&image[10])[0]

4 bytes.

: #define image_Header_Info ((long *)&image[14])[0]


Perhaps you see the problem now?


You forgot to check what sizeof(int) equals.

Then a question of my own; is there any advantage using 
"((long *)&image[2])[0]" vs. "*((long *)&image[2])"?


Right,

						MartinS