C / C++ / MFC Discussion Boards

Class/struct declarations and bit patterns

Anthony Appleyard20-Dec-07 10:43

20-Dec-07 10:43

In a class or struct declaration in Visual C++ 2008, for example

class BMPheader{public: char BM[2]; long size; short u,v; long imagestart;};

or

struct BMPheader{char BM[2]; long size; short u,v; long imagestart;};

can I trust on that being an accurate description of the resulting byte pattern inside the computer, to match the format of a binary file that is read in? My experience here is:

Gnu C++ for DOS: no, because after each element was inserted 0 or 1 or 2 or 3 unused bytes, to get each byte to start at an address which was a multiple of 4, unless I inserted a special keyword to tell the compiler to pack the elements close.

Borland C++ 4.5 for Windows: yes.

Re: Class/struct declarations and bit patterns

CPallini20-Dec-07 11:19

CPallini

20-Dec-07 11:19

You have not to worry about alignment, as far you are consistent on writing to and reading from file, for instance (error handling left to the reader)

struct BMPHeader bmphout;
// assign struct members
...
FILE * fpout = fopen("foo.bin", "wb");
fwrite( &bmphout, sizeof(bmphout),1, fpout);
fclose(fpout);

and then

struct BMPHeader bmphin;
FILE * fpin = fopen("foo.bin", "rb");
fread( &bmphin, sizeof(bmphin),1, fpin);
// Now bmphin contains exactly the same data of original bmphout
fclose(fpin);

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

[my articles]

Re: Class/struct declarations and bit patterns

Anthony Appleyard20-Dec-07 11:56

Anthony Appleyard

20-Dec-07 11:56

But here I am reading from a file format which someone else (probably at Microsoft many years ago) invented. So I need to know how to make Visual C++ 2008 compile a struct or class with the elements in the same order as declared with no extra bytes inserted between.

Good Luck!

CPallini20-Dec-07 12:45

CPallini

20-Dec-07 12:45

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

[my articles]

Re: Class/struct declarations and bit patterns [modified]

Mark Salsbery20-Dec-07 13:22

Mark Salsbery

20-Dec-07 13:22

Anthony Appleyard wrote:
But here I am reading from a file format which someone else (probably at Microsoft many years ago) invented.

Why are you not using the Microsoft structure definitions
that have been around just as long?

BITMAPFILEHEADER is packed with 2-byte alignment
BITMAPINFO and BITMAPINFOHEADER are packed with 1-byte alignment.

To see how that affects struct padding and alignment, see

align (C++)[^]
/Zp (Struct Member Alignment)[^]
etc...

If you're going to redefine these structs and expect them to match
everyone else, then you'll probably want to set your structure packing
the same as Microsoft's (see #pragma pack).

*edit*  Also, using platform-specific types like "long" and "short" is a bad<br />
idea if you expect to match the Microsoft definitions.

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

modified on Thursday, December 20, 2007 7:40:28 PM

After a picture is tranformed by GDI+, mouse cursor point problem

followait20-Dec-07 5:57

followait

20-Dec-07 5:57

Like DPtoLP, how to do it with GDI+,
use TransformPoints?

Re: After a picture is tranformed by GDI+, mouse cursor point problem

CPallini20-Dec-07 10:16

CPallini

20-Dec-07 10:16

Yes, it looks promising.
Smile | :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

[my articles]

plzz hlep

coderwizard20-Dec-07 3:45

coderwizard

20-Dec-07 3:45

i has need code for windoz service when starting os.plz send politly.