From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
	id 81A7A3858C98; Tue,  6 Feb 2024 10:07:19 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 81A7A3858C98
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1707214039;
	bh=mK7OmEp/M7+lCR8B9fE7SuSLF7lYAq0vLS1LxvKgXtw=;
	h=From:To:Subject:Date:In-Reply-To:References:From;
	b=MI9DBQsIIHpHYNZBFC/zJ+XG6L/Z5wLRJCq8GLU6vDb2/4H5bZOfmVJR3Y8fnugfu
	 0N+LlSpG9Yvmjd3+7wiZQ4eBGMFOvVAyiwsEneh311YYxX5DB76szNdwtOpUvRpAqY
	 lWOqAwH+lE9vX83eE/KTXmUziHFcDxE2akoIej74=
From: "paul at crapouillou dot net" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/86879] G++ should warn about redundant tests for null
 pointers returned from functions with __attribute__((returns_nonnull))
Date: Tue, 06 Feb 2024 10:07:18 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 9.0
X-Bugzilla-Keywords: diagnostic
X-Bugzilla-Severity: enhancement
X-Bugzilla-Who: paul at crapouillou dot net
X-Bugzilla-Status: NEW
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: cc
Message-ID: <bug-86879-4-HAn04Rc2h1@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-86879-4@http.gcc.gnu.org/bugzilla/>
References: <bug-86879-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
List-Id: <gcc-bugs.sourceware.org>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D86879

Paul Cercueil <paul at crapouillou dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |paul at crapouillou dot net

--- Comment #3 from Paul Cercueil <paul at crapouillou dot net> ---
I would also like to see a warning, and also in C code, for a different rea=
son.

I have functions that return pointers to opaque structures. In case of an
error, instead of returning NULL and setting errno, it encodes the error co=
de
into the pointer value.

The error code can then be retrieved with the following inline function:

static inline int is_err(const void *ptr)
{
        return (uintptr_t) ptr >=3D (uintptr_t) -4095 ? (int)(intptr_t) ptr=
 : 0;
}

if is_err(ptr) returns 0, then the pointer is valid - otherwise it returns =
the
error code.

Note that this was inspired on the Linux kernel, which has the exact same
mechanism.

What I want to prevent (and warn on), is incorrect error-checking of the
functions using this mechanism. Most often than not, callers will do this:

struct foo *ptr =3D maybe_return_errptr(arg);
if (!ptr) {
   printf("Error!\n");
   return NULL;
}

To avoid this mistake, I could tag my "maybe_return_errptr()" function with
__attribute__((returns_nonnull)). However, even with that, GCC does not warn
about the NULL-check; and it'd be great if it would.=