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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Generic read/write function - casting vs union
Date: Thu, 22 Sep 2016 11:48:51 -0700
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Richard Heathfield <rjh@cpax.org.uk> writes:
> On 22/09/16 10:33, Noob wrote:
[...]
>> Something I didn't mention is that the access function actually
>> updates buf in the EPILOG.
>>
>>   buf->in += len;
>>
>> Is it *100%* guaranteed that incrementing buf->in is strictly
>> equivalent to incrementing buf->out ?
>
> You can't do pointer arithmetic on void *. :-)
>
> But you can assign to an unsigned char *, and increment that instead.

Note that gcc permits pointer arithmetic on void* as an extension.
I don't recommend using that extension, but be aware that gcc will
not warn about such pointer arithmetic by default.  You can use
-pedantic or -pedantic-errors to disable the warning and get the
required diagnostics.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"