From: "Anthony.Appleyard" <MCLSSAA2 AT fs2 DOT mt DOT umist DOT ac DOT uk>
Organization:  Materials Science Centre
To: nrotem AT johnbryce DOT co DOT il, djgpp AT delorie DOT com
Date:          Wed, 14 Jan 1998 08:36:31 GMT
Subject:       Re: Bit fields in djgpp: +1 turned into -1
Reply-to: Anthony DOT Appleyard AT umist DOT ac DOT uk
Message-ID: <27055884105@fs2.mt.umist.ac.uk>
Precedence: bulk

  Noam Rotem <nrotem AT johnbryce DOT co DOT il> wrote:-
> When I use bit fields within a struct, initialize them and print the values,
> I get undefined non zero values. For example:
> struct {int x : 1; int y : 1; int z : 1; }
> Test={0,0,0};
> main() {
>   ....
>   Test.x=1; printf("%d",Test.x);
>   ... }
> I usually get -1 as an output, or other non zero values. ...

  A field declared as `int x:1;' has only one bit. As that field is in a
signed mode, that bit is the sign bit, and if that bit is set, the value is
treated as -1. If you want a field `int x:N;' to contain values from 0 to
pow(2,N)-1, you must declare the field as `unsigned int x:N;'.
  If a program contains the words `unsigned' and `register' a lot, I find it
useful to put these lines at the top of the program file:-
#define uns unsigned
#define reg register