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From: jss@sfjec.UUCP (J.S.Schwarz)
Newsgroups: net.lang.c
Subject: Re: Functions with union parameters
Message-ID: <120@sfjec.UUCP>
Date: Thu, 5-Apr-84 17:19:50 EST
Article-I.D.: sfjec.120
Posted: Thu Apr  5 17:19:50 1984
Date-Received: Sat, 7-Apr-84 04:07:12 EST
References: <4107@edai.UUCP>
Organization: Bell Labs, Murray Hill
Lines: 34

To recap: The question concerns a function with an argument declared
to be
        union { ushort ch; char *p; }

edai!ok claims that this is not portable. But what is nonportable in
his(her?) examples is the suggested calls. The portable way to call a
function with such an argument is to declare a variable in the caller
of the same type and use it.

Thus:

    typedef union { ushort ch ; char *p; } UNION;

    callee( arg ) UNION arg ; { ... }

    caller() {
        UNION v ; v.ch = ... ; callee( v ) ; v.p = ... ; callee( v )
        ; .... }

This is fully portable provided your C compiler supports struct and
union arguments. (I realize that some don't.)

This points out the absence in C of any way to take a value and turn
it into a union except by assigning it to a variable of union type.

Edai!ok also asks if there is any guarantee that members of a union
all begin at offset 0.  K&R 8.5 says: "A union may be thought of as a
structure all of whose members begin at offset 0, ..."

Jerry Schwarz -- BTL, Summit -- sfjec!jss

P.S.: Since edai!ok hasn't seen the calls in the SIII kernel, I think
it is unreasonable to accuse the authors of writing non portable
code.