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Wed, 21 May 2025 00:30:06 GMT (envelope-from git) Date: Wed, 21 May 2025 00:30:06 GMT Message-Id: <202505210030.54L0U6tF025794@gitrepo.freebsd.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-branches@FreeBSD.org From: Lexi Winter Subject: git: 513df87288d0 - stable/13 - sys/cdefs.h: add __nodiscard annotation List-Id: Commit messages for all branches of the src repository List-Archive: https://lists.freebsd.org/archives/dev-commits-src-all List-Help: List-Post: List-Subscribe: List-Unsubscribe: X-BeenThere: dev-commits-src-all@freebsd.org Sender: owner-dev-commits-src-all@FreeBSD.org MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit X-Git-Committer: ivy X-Git-Repository: src X-Git-Refname: refs/heads/stable/13 X-Git-Reftype: branch X-Git-Commit: 513df87288d0ac09cb378b40eeb1abd3ee79bee9 Auto-Submitted: auto-generated The branch stable/13 has been updated by ivy: URL: https://cgit.FreeBSD.org/src/commit/?id=513df87288d0ac09cb378b40eeb1abd3ee79bee9 commit 513df87288d0ac09cb378b40eeb1abd3ee79bee9 Author: Lexi Winter AuthorDate: 2025-05-09 21:28:14 +0000 Commit: Lexi Winter CommitDate: 2025-05-20 21:03:29 +0000 sys/cdefs.h: add __nodiscard annotation __nodiscard adds the [[nodiscard]] attribute to a function, type or constructor in C or C++, causing a value so marked to issue a compiler warning if it is discarded (i.e., not used or assigned) other than by casting it to void. this replaces the existing __result_use_or_ignore_check, which has a similar purpose but different semantics. since __nodiscard provides more functionality (at least in GCC) and __result_use_or_ignore_check only had a single user, remove __result_use_or_ignore_check. [[nodiscard]] has been supported in C++ since C++17, but only in C since C23; however, both LLVM and GCC implement it even in older language versions, so it should always be available with a relatively modern compiler. for Clang, [[nodiscard]] in C is only available since LLVM 17, but we can fall back to __attribute__((__warn_unused_result__)) which has the same semantics and provides support back to (at least) LLVM 11. GCC supports [[nodiscard]] in both C and C++ since at least GCC 11. for GCC, we can't provide a fallback as the semantics of its warn_unused_result are different, but since __result_use_or_ignore_check isn't defined for GCC anyway, we don't lose anything here. MFC after: 2 weeks Reviewed by: des, emaste Approved by: des (mentor) Differential Revision: https://reviews.freebsd.org/D50217 (cherry picked from commit 7e0c5e0128c43bbae78190911aead8e1d9475ba5) --- sys/sys/cdefs.h | 38 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 38 insertions(+) diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h index 080ae0912210..c549bfc3c2f3 100644 --- a/sys/sys/cdefs.h +++ b/sys/sys/cdefs.h @@ -394,6 +394,44 @@ #endif #endif +/* + * nodiscard attribute added in C++17 and C23, but supported by both LLVM and + * GCC in earlier language versions, so we use __has_c{,pp}_attribute to test + * for it. + * + * __nodiscard may be used on a function: + * __nodiscard int f(); + * + * or on a struct, union or enum: + * struct __nodiscard S{}; + * struct S f(); + * + * or in C++, on an object constructor. + */ + +#if defined(__cplusplus) && defined(__has_cpp_attribute) +#if __has_cpp_attribute(nodiscard) +#define __nodiscard [[nodiscard]] +#endif +#elif defined(__STDC_VERSION__) && defined(__has_c_attribute) +#if __has_c_attribute(__nodiscard__) +#define __nodiscard [[__nodiscard__]] +#endif +#endif + +#ifndef __nodiscard +/* + * LLVM 16 and earlier don't support [[nodiscard]] in C, but they do support + * __warn_unused_result__ with the same semantics, so fall back to that. + * We can't do this for GCC because the semantics are different. + */ +#ifdef __clang__ +#define __nodiscard __attribute__((__warn_unused_result__)) +#else +#define __nodiscard +#endif +#endif + /* * We use `__restrict' as a way to define the `restrict' type qualifier * without disturbing older software that is unaware of C99 keywords.