From patchwork Thu Jul 6 09:50:55 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Florian Weimer X-Patchwork-Id: 21455 Received: (qmail 111856 invoked by alias); 6 Jul 2017 09:51:03 -0000 Mailing-List: contact libc-alpha-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Unsubscribe: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: libc-alpha-owner@sourceware.org Delivered-To: mailing list libc-alpha@sourceware.org Received: (qmail 111817 invoked by uid 89); 6 Jul 2017 09:51:01 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-26.9 required=5.0 tests=BAYES_00, GIT_PATCH_0, GIT_PATCH_1, GIT_PATCH_2, GIT_PATCH_3, RP_MATCHES_RCVD, SPF_HELO_PASS autolearn=ham version=3.3.2 spammy= X-HELO: mx1.redhat.com DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com BA8F3C0587E4 Authentication-Results: ext-mx08.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx08.extmail.prod.ext.phx2.redhat.com; spf=pass smtp.mailfrom=fweimer@redhat.com DKIM-Filter: OpenDKIM Filter v2.11.0 mx1.redhat.com BA8F3C0587E4 Subject: Re: [PATCH] assert: Suppress pedantic warning caused by statement expression [BZ# 21242] To: Zack Weinberg Cc: GNU C Library , Siddhesh Poyarekar References: <6619e7e2-248e-08bc-6dc5-7b4d11bea3df@auburn.edu> <925e8a81-86f9-78a6-1578-660c04e39e3c@redhat.com> From: Florian Weimer Message-ID: <7635632a-e375-ea34-fe4f-19cd08262ce1@redhat.com> Date: Thu, 6 Jul 2017 11:50:55 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: On 07/05/2017 10:15 PM, Zack Weinberg wrote: > On Wed, Jul 5, 2017 at 11:51 AM, Florian Weimer wrote: >> On 07/05/2017 05:46 PM, Zack Weinberg wrote: >>> A problem occurs to me: expressions involving VLAs _are_ evaluated >>> inside sizeof. >> >> The type of the sizeof argument would still be int (due to the >> comparison against 0), so this doesn't actually occur. > > I rechecked what C99 says about sizeof and VLAs, and you're right - > the operand of sizeof is only evaluated when sizeof is _directly_ > applied to a VLA. So this is indeed safe, but I think this wrinkle > should be mentioned in the comment. Perhaps > > /* The first occurrence of EXPR is not evaluated due to the sizeof, > but will trigger any pedantic warnings masked by the __extension__ > for the second occurrence. The explicit comparison against zero > ensures that sizeof is not directly applied to a function pointer or > bit-field (which would be ill-formed) or VLA (which would be evaluated). */ > > zw What about the attached patch? Siddhesh, is this okay during the freeze? I'd like to backport it to 2.25 as well. Thanks, Florian assert: Suppress pedantic warning caused by statement expression 2017-07-06 Florian Weimer [BZ #21242] * assert/assert.h [__GNUC__ && !__STRICT_ANSI__] (assert): Suppress pedantic warning resulting from statement expression. (__ASSERT_FUNCTION): Add missing __extendsion__. diff --git a/assert/assert.h b/assert/assert.h index 22f0195..6801cfe 100644 --- a/assert/assert.h +++ b/assert/assert.h @@ -91,13 +91,19 @@ __END_DECLS ? __ASSERT_VOID_CAST (0) \ : __assert_fail (#expr, __FILE__, __LINE__, __ASSERT_FUNCTION)) # else +/* The first occurrence of EXPR is not evaluated due to the sizeof, + but will trigger any pedantic warnings masked by the __extension__ + for the second occurrence. The explicit comparison against zero is + required to support function pointers and bit fields in this + context, and to suppress the evaluation of variable length + arrays. */ # define assert(expr) \ - ({ \ + ((void) sizeof ((expr) == 0), __extension__ ({ \ if (expr) \ ; /* empty */ \ else \ __assert_fail (#expr, __FILE__, __LINE__, __ASSERT_FUNCTION); \ - }) + })) # endif # ifdef __USE_GNU @@ -113,7 +119,7 @@ __END_DECLS C9x has a similar variable called __func__, but prefer the GCC one since it demangles C++ function names. */ # if defined __cplusplus ? __GNUC_PREREQ (2, 6) : __GNUC_PREREQ (2, 4) -# define __ASSERT_FUNCTION __PRETTY_FUNCTION__ +# define __ASSERT_FUNCTION __extension__ __PRETTY_FUNCTION__ # else # if defined __STDC_VERSION__ && __STDC_VERSION__ >= 199901L # define __ASSERT_FUNCTION __func__