From: "Mike Collins" <Mike AT NO_SPAM DOT e-col DOT freeserve DOT co DOT uk>
Newsgroups: comp.os.msdos.djgpp
Subject: Somebody please explain : Length of bit fields
Date: Tue, 21 Dec 1999 23:07:57 -0000
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

Folks,

I came across an interesting thing: In the program below, I would have
expected a structure length of 6 bytes, but in fact, when I compile it and
single step it, I read the values for a, b, c, d printed below. Of
particular interest is the value of a ( the sizeof the entire structure) =
8.


#include <stdio.h>

struct CATALOG
{ unsigned unit:8,   // 4 bytes
           category:8,
           refresh :1;
  short num;           // 2 bytes
};


main()
{ int a, b, c, d, e;
  struct CATALOG ctg;
  a = sizeof ctg;
  b = sizeof(ctg.num);
  c = sizeof(ctg.unit);
  d = sizeof(ctg.category);
//  e = sizeof(ctg.refresh);   // DJGPP doesn't like sizeof(a_bit_field);
}

 a: 8
 b: 2
 c: 4
 d: 4

If I move the short in front of the unsigned to change the word boundaries,
like this :

struct CATALOG
{ short num;
  unsigned unit:8,
           category:8,
           refresh :1;
};

... I still get exactly the same results.

If I now reduce the length of catalog.category to a bit field of 7, and
remove the line that reads
"d = ... "  (because DJGPP doesn't like sizeof(a_bit_field))
I get the results printed below.

struct CATALOG
{ short num;
  unsigned unit:8,
           category:7,
           refresh :1;
};

 a: 4
 b: 2
 c: 4

Why does the length of the structure change? The length of all bit fields in
a single unsigned integer should always be 4 bytes (sizeof(unsigned) == 4)
as long as the total number of bits declared is less than 32.

Again, putting the "short num" at the head of the structure or at its tail
makes no difference to the results.
Another thing that interests me is that sizeof(a_bit_field) compiles ok if
the bit field is 8 bits long, but if it is a fraction of a byte, the compile
throws up an error.

Your learned opinions will be most appreciated.
--
Mike Collins

PS. If replying by e-mail, please make the obvious correction to my address.