Re: [PATCH] fast-export: avoid NULL pointer arithmetic

From: Johannes Schindelin <Johannes.Schindelin@gmx.de>
To: "René Scharfe" <l.s.r@web.de>
Cc: Git List <git@vger.kernel.org>, Junio C Hamano <gitster@pobox.com>
Subject: Re: [PATCH] fast-export: avoid NULL pointer arithmetic
Date: Wed, 9 May 2018 17:43:30 -0400 (DST)	[thread overview]
Message-ID: <nycvar.QRO.7.76.6.1805091743080.77@tvgsbejvaqbjf.bet> (raw)
In-Reply-To: <d50a4d5d-3b99-453e-1b52-4e733453fb78@web.de>

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Hi René,

On Wed, 9 May 2018, René Scharfe wrote:

> Clang 6 reports the following warning, which is turned into an error in a
> DEVELOPER build:
> 
> 	builtin/fast-export.c:162:28: error: performing pointer arithmetic on a null pointer has undefined behavior [-Werror,-Wnull-pointer-arithmetic]
> 		return ((uint32_t *)NULL) + mark;
> 		       ~~~~~~~~~~~~~~~~~~ ^
> 	1 error generated.
> 
> The compiler is correct, and the error message speaks for itself.  There
> is no need for any undefined operation -- just cast mark to void * or
> uint32_t after an intermediate cast to uintptr_t.  That encodes the
> integer value into a pointer and later decodes it as intended.
> 
> While at it remove an outdated comment -- intptr_t has been used since
> ffe659f94d (parse-options: make some arguments optional, add callbacks),
> committed in October 2007.

ACK.

Thanks,
Dscho