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Message-ID: <16572.41541.94498.145167@alkaid.it.uu.se>
Date: Tue, 1 Jun 2004 17:35:33 +0200
From: Mikael Pettersson <mikpe@csd.uu.se>
To: William Lee Irwin III <wli@holomorphy.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH][2.6.6-rc3] gcc-3.4.0 fixes
In-Reply-To: <20040601150913.GU2093@holomorphy.com>
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William Lee Irwin III writes:
 > On Tue, Jun 01, 2004 at 04:52:59PM +0200, Mikael Pettersson wrote:
 > > You're assuming pointers have uniform representation.
 > > C makes no such guarantees, and machines _have_ had
 > > different types of representations in the past.
 > > Some not-so-obsolete 64-bit machines in effect use fat
 > > representations for pointers to functions (descriptors),
 > > but they usually cheat and use pointers to the descriptors
 > > instead. However, a C implementation could legally
 > > represent a function pointer as a 128-bit value, while
 > > data pointers remain 64 bits.
 > 
 > IIRC for all types foo, sizeof(foo *) <= sizeof(void *), no?
 > If so, 128-bit function pointers implies >= 128-bit void pointers.

No, sizeof(foo*) <= sizeof(void*) only holds for data pointers.
The C standard is very explicit about not guaranteeing any
relationship between function pointers and void*. However,
a function pointer can be converted to a pointer to a different
function type and back again, without loss of information.

/Mikael
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