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From: throopw@xyzzy.UUCP
Newsgroups: comp.lang.c,comp.unix.wizards
Subject: Re: pointer alignment when int != char *
Message-ID: <243@xyzzy.UUCP>
Date: Tue, 1-Sep-87 14:57:09 EDT
Article-I.D.: xyzzy.243
Posted: Tue Sep  1 14:57:09 1987
Date-Received: Thu, 3-Sep-87 00:41:09 EDT
References: <493@its63b.ed.ac.uk> <6061@brl-smoke.ARPA> <3812@spool.WISC.EDU> <7939@think.UUCP> <588@murphy.UUCP>
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Xref: utgpu comp.lang.c:3916 comp.unix.wizards:3726

> dave@murphy.UUCP (Dave Cornutt)
> However, I will say this, because I don't
> think the standard has really addressed it: there should be a way to take
> any pointer and generate a byte offset from byte 0 in whatever address
> space the code is running in.  The reason is that you need such a beast
> to feed to lseek if you want to access something through one of the
> /dev/mem devices (or maybe /proc).

By "the standard", I presume draft X3J11 is meant.  First, the C
language standard had better say nothing that requires there to even
*BE* a single, linear address space in which "code is running".  There
are many machines where this isn't a well-founded presumption.  Thus the
whole idea of a process-unique "byte 0", or "a byte offset" from there
may not be present in the hardware for which the C source is being
compiled.  To say nothing of whether a C language standard should be
talking about "lseek" and "/dev/mem" in its rationale for a general
feature.

In fact, "the standard" says just about what it ought to say.  It gives
liscence to developers for whom it is natural to supply "byte offsets
from byte 0" to supply them, but does not require it from those
developers for whom it is an impossibility.

--
1+1=3, for sufficently large values of 1.
-- 
Wayne Throop      <the-known-world>!mcnc!rti!xyzzy!throopw