Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!ames!mailrus!cornell!rochester!rutgers!hjuxa!kay
From: kay@hjuxa.UUCP (KAY)
Newsgroups: comp.lang.c
Subject: Re: Unions
Summary: End runs, quite often
Keywords: What can they be used for?
Message-ID: <1038@hjuxa.UUCP>
Date: 20 Oct 88 15:00:50 GMT
References: <322@hrc.UUCP>
Organization: Digital Equipment Corp. Holmdel, NJ
Lines: 30

In article <322@hrc.UUCP>, dan@hrc.UUCP (Dan Troxel VP) writes:
> 
> Except for the memory savings, what are Unions suited for?
> -- 
> Dan Troxel VP of Computer Operations @ 

Probably one of hundreds answering, but....

"Unions provide a way to manipulate different kinds of data in a single area
of storage, without embedding machine-dependent information in the program"
		-K&R, first edition, p138

Actually I usually use them _to_ embed machine _dependant_ information.
I have most often used unions for byte and word swapping when going from one
processor type to another; e.g. PDP to VAX, VAX to 8086, IBM (Serries 1) to
VAX, et cetera:
	union swap {
		short s_val;
		long l_val;
		char bytes[4];	/* see, very machine specific! */
	}

I've also used them to do initial processing on data that always had a
common first element to say what type of data the message contained. Most
recently to put varied information into a constant sized area as in the AT&T
COFF symbol table auxillary (debug) entries. Until you've tried this in
BLISS, you don't appreciate how complex this is.

Got to go.

	Paul S. Kay