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From: kremer@cs.odu.edu (Lloyd Kremer)
Newsgroups: comp.std.c,comp.lang.c
Subject: Re: Common malloc/free practice violates ANSI standard ?
Message-ID: <10282@xanth.cs.odu.edu>
Date: 26 Oct 89 19:50:56 GMT
References: <1989Oct14.043811.669@anucsd.oz> <1989Oct19.101306.16791@twwells.com> <423@cpsolv.UUCP> <1989Oct24.194920.7250@twwells.com>
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I will try to shed some additional light on this, although I may be a fool
to attempt it.  :-)

There is a fundamental difference between
	void *
and
	void *  returned by malloc (or any allocation routine).

In one sense they are opposites.  A void* in general has the most lenient
alignment requirements of any data pointer.  A void* returned by malloc has
the most stringent.

	OBJ* -> void* -> OBJ*
is true for any void*.

	void* -> OBJ* -> void*
is not true for general void*, but is guaranteed true for void* returned
by malloc.

void* is required to be physically identical to char*.  In some
implementations (word oriented architectures), a char* is physically bigger
(has more bits) than pointers to some other objects.  A char* contains a
word address and then a byte offset within the word.  Hence a cast from
a general void* to a smaller pointer type can irreversibly lose data.
A void* returned from malloc is guaranteed to have these additional bits
zero, so the truncation is harmless.

					Lloyd Kremer
					kremer@cs.odu.edu
					Have terminal...will hack!