From:	Erik Max Francis <max AT alcyone DOT com>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Bit fields in djgpp
Date:	Thu, 15 Jan 1998 00:31:55 -0800
Organization:	Alcyone Systems
Lines:	37
Message-ID:	<34BDC97B.5453B8F8@alcyone.com>
References:	<Chameleon DOT 980113164922 DOT nrotem AT netvision DOT netvision>
NNTP-Posting-Host:	newton.alcyone.com
Mime-Version:	1.0
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

Noam Rotem wrote:

> 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.
> 
> I can live with non zero undefined values, but I would like
> to know the reason for the results I get.

The results you get a correct.  You have declared the bitfields in
question as signed, and for a 1 bit signed field, the only possible
values are 0 and -1.  Assigning 1 to a 1 bit signed field results in
automatic conversion, and so Test.x gets the value -1.

If you wanted them to really be "bits" (0 or 1) in the traditional
sense, you should have declared the bitfields unsigned.

-- 
         Erik Max Francis, &tSftDotIotE / mailto:max AT alcyone DOT com
                       Alcyone Systems / http://www.alcyone.com/max/
  San Jose, California, United States / icbm://+37.20.07/-121.53.38
                                     \
                       "Life may be / the product of imperfections."
                                   / (Marclo Gleiser)

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