linux-kernel - RE: [PATCH v2 1/2] system_data

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Message-ID: <a2c3ceecca7648b3bb33f1c8bf8e7893@AcuMS.aculab.com>
Date:   Fri, 2 Oct 2020 09:10:28 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Alejandro Colomar' <colomar.6.4.3@...il.com>,
        Paul Eggert <eggert@...ucla.edu>
CC:     "mtk.manpages@...il.com" <mtk.manpages@...il.com>,
        "linux-man@...r.kernel.org" <linux-man@...r.kernel.org>,
        "libc-alpha@...rceware.org" <libc-alpha@...rceware.org>,
        "gcc@....gnu.org" <gcc@....gnu.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 1/2] system_data_types.7: Add 'void *'

From: Alejandro Colomar
> Sent: 02 October 2020 09:25
>  > For 'void *' you should also mention that one cannot use arithmetic on
>  > void * pointers, so they're special in that way too.
> 
> Good suggestion!

Except that is a gcc extension that is allowed in the kernel.

>  > Also, you should
>  > warn that because one can convert from any pointer type to void * and
>  > then to any other pointer type, it's a deliberate hole in C's
>  > type-checking.
> 
> Also good.  I'll talk about generic function parameters for this.

That isn't what the C standard says at all.
What is says is that you can cast any data pointer to 'void *'
and then cast it back to the same type.

This matters because the compiler will 'remember' structure
alignment through 'void *' casts.
So you can't use memcpy() to copy from a potentially misaligned
(typed) pointer.

'void *' should only be used for structures that are 'a sequence of bytes'.
(eg things that look a bit like read() or write()).

	David

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