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From: cottrell@nbs-vms.arpa (COTTRELL, JAMES)
Newsgroups: net.lang.c
Subject: Bit Fields in Unions
Message-ID: <3352@brl-tgr.ARPA>
Date: Mon, 18-Nov-85 15:36:34 EST
Article-I.D.: brl-tgr.3352
Posted: Mon Nov 18 15:36:34 1985
Date-Received: Wed, 20-Nov-85 00:58:40 EST
Sender: news@brl-tgr.ARPA
Lines: 48

/*
> 	I recently was writing a declaration for a LISP form which looks
> something like this:
> 
>                   --------------------------------- 
> 		   |      14-bit integer             |
>         ---------------------------------------------
>        | gc | tag1 |    either ^ or v                |
>         ---------------------------------------------
> 		   | tag2 |   13-bit pointer         |
> 		    ---------------------------------
> 
> 	The form is 16 bits wide, with tag1 selecting between the
> 14-bit integer or 13-bit pointer with another tag. Attempting to
> write a C declaration for this:
> 
> struct form {
> 	unsigned int gc : 1;
> 	unsigned int tag1 : 1;
> 	union {
> 		unsigned int number : 14;
> 		struct {
> 			unsigned int tag2 : 1;
> 			unsigned int pointer : 13;
> 			} pval;
> 		} val;
> 	};
> 
> 	resulted in a "bit-field outside of struct" error, where
> number :14 was declared in the union.  I could not find any explicit
> mention of bit-fields not being allowed in unions in K & R, but all
> C compilers I have tried have not allowed them.  Does anyone know
> of a C compiler that allows this?  Or does anyone know why this is not
> allowed?

The proper declaration is `short thing'. Bit fields and unions are both
stupid. In many years I have always managed to avoid them. Fiddle with
the bits directly as all real programmers do. These should help:

	#define	GC	(1<<15)
	#define TAG1	(1<<14)
	#define TAG2	(1<<13)
	#define MASK13	((1<<13)-1)
	#define MASK14	((1<<14)-1)

	jim		cottrell@nbs
*/
------