https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114870
--- Comment #2 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:8d22474af76a386eed488b3c66124134f0e41363 commit r15-7718-g8d22474af76a386eed488b3c66124134f0e41363 Author: Jakub Jelinek <ja...@redhat.com> Date: Wed Feb 26 19:29:12 2025 +0100 c: stddef.h C23 fixes [PR114870] The stddef.h header for C23 defines __STDC_VERSION_STDDEF_H__ and unreachable macros multiple times in some cases. The header doesn't have normal multiple inclusion guard, because it supports for glibc inclusion with __need_{size_t,wchar_t,ptrdiff_t,wint_t,NULL}. While the definition of __STDC_VERSION_STDDEF_H__ and unreachable is done solely in the #ifdef _STDDEF_H part, so they are defined only if stddef.h is included without those __need_* macros defined. But actually once stddef.h is included without the __need_* macros, _STDDEF_H is then defined and while further stddef.h includes without __need_* macros don't do anything: #if (!defined(_STDDEF_H) && !defined(_STDDEF_H_) && !defined(_ANSI_STDDEF_H) \ && !defined(__STDDEF_H__)) \ || defined(__need_wchar_t) || defined(__need_size_t) \ || defined(__need_ptrdiff_t) || defined(__need_NULL) \ || defined(__need_wint_t) if one includes whole stddef.h first and then stddef.h with some of the __need_* macros defined, the #ifdef _STDDEF_H part is used again. It isn't that big deal for most cases, as it uses extra guarding macros like: #ifndef _GCC_MAX_ALIGN_T #define _GCC_MAX_ALIGN_T ... #endif etc., but for __STDC_VERSION_STDDEF_H__/unreachable nothing like that is used. So, either we do what the following patch does and just don't define __STDC_VERSION_STDDEF_H__/unreachable second time, or use #ifndef unreachable separately for the #define unreachable() case, or use new _GCC_STDC_VERSION_STDDEF_H macro to guard this (or two, one for __STDC_VERSION_STDDEF_H__ and one for unreachable), or rework the initial condition to be just #if !defined(_STDDEF_H) && !defined(_STDDEF_H_) && !defined(_ANSI_STDDEF_H) \ && !defined(__STDDEF_H__) - I really don't understand why the header should do anything at all after it has been included once without __need_* macros. But changing how this behaves after 35 years might be risky for various OS/libc combinations. 2025-02-26 Jakub Jelinek <ja...@redhat.com> PR c/114870 * ginclude/stddef.h (__STDC_VERSION_STDDEF_H__, unreachable): Don't redefine multiple times if stddef.h is first included without __need_* defines and later with them. Move nullptr_t and unreachable and __STDC_VERSION_STDDEF_H__ definitions into the same defined (__STDC_VERSION__) && __STDC_VERSION__ > 201710L #if block. * gcc.dg/c23-stddef-2.c: New test.
