[libcxx-commits] [PATCH] D134591: [libcxx] Make stdatomic.h work when included from a C source file

[Gergely Nagy via Phabricator via libcxx-commits]

ngg created this revision.
ngg added reviewers: libc++, philnik, ldionne.
ngg added a project: libc++.
Herald added a project: All.
ngg requested review of this revision.
Herald added 1 blocking reviewer(s): libc++.

If a C source file includes the libc++ stdatomic.h, compilation will
break because (a) the C++ standard check will fail (which is expected),
and (b) `_LIBCPP_COMPILER_CLANG_BASED` won't be defined because the
logic defining it in `__config` is guarded by a `__cplusplus` check, so
we'll end up with a blank header. Move the detection logic outside of
the `__cplusplus` check to make the second check pass even in a C context
when you're using Clang. Note that `_LIBCPP_STD_VER` is not defined when
in C mode, hence stdatomic.h needs to check if in C++ mode before using
that macro to avoid a warning.

In an ideal world, a C source file wouldn't be including the libc++
header directory in its search path, so we'd never have this issue.
Unfortunately, certain build environments make this hard to guarantee,
and in this case it's easy to tweak this header to make it work in a C
context, so I'm hoping this is acceptable.

Fixes https://github.com/llvm/llvm-project/issues/57710.

Note: I couldn't update the https://reviews.llvm.org/D131435 revision, that's why I've created a new one.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D134591

Files:
  libcxx/include/__config
  libcxx/include/stdatomic.h
  libcxx/test/libcxx/include_as_c.sh.cpp


Index: libcxx/test/libcxx/include_as_c.sh.cpp
===================================================================
--- libcxx/test/libcxx/include_as_c.sh.cpp
+++ libcxx/test/libcxx/include_as_c.sh.cpp
@@ -34,6 +34,7 @@
 #endif
 #include <math.h>
 #include <setjmp.h>
+#include <stdatomic.h>
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
Index: libcxx/include/stdatomic.h
===================================================================
--- libcxx/include/stdatomic.h
+++ libcxx/include/stdatomic.h
@@ -121,7 +121,7 @@
 #  pragma GCC system_header
 #endif
 
-#if _LIBCPP_STD_VER > 20
+#if defined(__cplusplus) && _LIBCPP_STD_VER > 20
 
 #include <atomic>
 #include <version>
@@ -230,6 +230,6 @@
 #   include_next <stdatomic.h>
 # endif
 
-#endif // _LIBCPP_STD_VER > 20
+#endif // defined(__cplusplus) && _LIBCPP_STD_VER > 20
 
 #endif // _LIBCPP_STDATOMIC_H
Index: libcxx/include/__config
===================================================================
--- libcxx/include/__config
+++ libcxx/include/__config
@@ -22,6 +22,16 @@
 #  pragma GCC system_header
 #endif
 
+#if defined(__apple_build_version__)
+#  define _LIBCPP_COMPILER_CLANG_BASED
+#  define _LIBCPP_APPLE_CLANG_VER (__apple_build_version__ / 10000)
+#elif defined(__clang__)
+#  define _LIBCPP_COMPILER_CLANG_BASED
+#  define _LIBCPP_CLANG_VER (__clang_major__ * 100 + __clang_minor__)
+#elif defined(__GNUC__)
+#  define _LIBCPP_COMPILER_GCC
+#endif
+
 #ifdef __cplusplus
 
 // _LIBCPP_VERSION represents the version of libc++, which matches the version of LLVM.
@@ -202,16 +212,6 @@
 #    define __has_include(...) 0
 #  endif
 
-#  if defined(__apple_build_version__)
-#    define _LIBCPP_COMPILER_CLANG_BASED
-#    define _LIBCPP_APPLE_CLANG_VER (__apple_build_version__ / 10000)
-#  elif defined(__clang__)
-#    define _LIBCPP_COMPILER_CLANG_BASED
-#    define _LIBCPP_CLANG_VER (__clang_major__ * 100 + __clang_minor__)
-#  elif defined(__GNUC__)
-#    define _LIBCPP_COMPILER_GCC
-#  endif
-
 #  if !defined(_LIBCPP_COMPILER_CLANG_BASED) && __cplusplus < 201103L
 #    error "libc++ only supports C++03 with Clang-based compilers. Please enable C++11"
 #  endif


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