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From: dillon@CORY.BERKELEY.EDU (Matt Dillon)
Newsgroups: comp.sys.atari.st
Subject: Re: Type coercion
Message-ID: <8704130334.AA19440@cory.Berkeley.EDU>
Date: Sun, 12-Apr-87 22:34:30 EST
Article-I.D.: cory.8704130334.AA19440
Posted: Sun Apr 12 22:34:30 1987
Date-Received: Sat, 18-Apr-87 17:40:45 EST
Sender: daemon@ucbvax.BERKELEY.EDU
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>I haven't used unions much, but isn't it true that C may attempt to
>re-align the items within a structure (or union) to lie on proper
>boundaries (e.g. word boundaries, longword boundaries, etc) where
>necessary? This means that there may be padding bytes within the
>structure. Could this cause errors where the elements of the structures
>within the union don't lie "on top of one another" as one would
>expect?

	C will always align entries in a structure or union properly for
that machine, but you should never count on it.  If alignment is absolutely
required for your application, you can assume that C will not re-align
anything already on sizef(char *) (usually longword) boundries.  chars are
not usually re-aligned.  In anycase, this is portable over most machines.

	In terms of having two data items conflicting in a union, here is
an example (this is not neccessarily how your compiler may do things):

	struct {
		char c;
		union {
			char d;
			char *x;
		} u;
	};

	&c = +0
	&u.d = +4
	&u.x = +4

	[c][unused][unused][unused][d][][][]
				   [x..4byt]

	You should *never* rely on unions aligning things in some known manner
if the data items inside the union are different sizes.

				-Matt