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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43bd42e6bcfsm28935975e9.32.2025.03.05.13.35.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 05 Mar 2025 13:35:02 -0800 (PST) Date: Wed, 5 Mar 2025 21:35:01 +0000 From: David Laight To: Dan Carpenter Cc: Markus Elfring , Dmitry Baryshkov , kernel-janitors@vger.kernel.org, freedreno@lists.freedesktop.org, dri-devel@lists.freedesktop.org, linux-arm-msm@vger.kernel.org, Abhinav Kumar , Archit Taneja , Daniel Vetter , David Airlie , Jeykumar Sankaran , Jordan Crouse , Marijn Suijten , Rob Clark , Sean Paul , Simona Vetter , Vinod Koul , cocci@inria.fr, LKML Subject: Re: [RFC] Clarification for =?UTF-8?B?4oCcdW5kZWZpbmVkIGJlaGF2?= =?UTF-8?B?aW91cuKAnT8=?= Message-ID: <20250305213501.496ea4bf@pumpkin> In-Reply-To: References: <40c60719-4bfe-b1a4-ead7-724b84637f55@web.de> <1a11455f-ab57-dce0-1677-6beb8492a257@web.de> <13566308-9a80-e4aa-f64e-978c02b1406d@web.de> <54c30a69-71cf-4582-9086-50eb0d39f273@web.de> <29b32b0d-312d-4848-9e26-9e5e76e527a7@stanley.mountain> <4c489b64-4c25-43e3-90d3-37f2f335d665@stanley.mountain> <20250305141732.26b91742@pumpkin> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Wed, 5 Mar 2025 17:30:28 +0300 Dan Carpenter wrote: > On Wed, Mar 05, 2025 at 02:17:32PM +0000, David Laight wrote: ... > > And the 'fun' starts because NULL isn't required to use the all-zero > > bit pattern. > > Regardless of the bit-pattern, things like (void *)(1 - 1) are valid > > NULL pointers. > > > > Of course, while C allows this, I doubt NULL has ever been other than 0. > > (It was 0 on a system I used many years ago where the O/S invalid pointer > > was ~0.) > > Kernel style guidelines don't even allow if (p == NULL) so we would be > screwed. :P Doesn't matter: if (!p) ... if (p == 0) ... if (p == (void *)0) ... if (p == NULL) ... if (p == (void *)(constant integer expression with value 0)) ... and the equivalent assignments all behave the same regardless of the bit-pattern use for NULL. So: union { long l; void *p; } lpu; lpu.p = 0; return lpu.l; Returns ABI (implementation) defined constant value. I think the only requirement is that it can never be the address of a valid variable. David