[libcxx-commits] [libcxx] afec0f0 - [libcxx] Make stdatomic.h work when included from a C source file

[Louis Dionne via libcxx-commits]

Author: Gergely Nagy
Date: 2022-09-27T13:13:11-04:00
New Revision: afec0f0ec38a72bcc6a697c1cefb1dac0bbd02fb

URL: https://github.com/llvm/llvm-project/commit/afec0f0ec38a72bcc6a697c1cefb1dac0bbd02fb
DIFF: https://github.com/llvm/llvm-project/commit/afec0f0ec38a72bcc6a697c1cefb1dac0bbd02fb.diff

LOG: [libcxx] Make stdatomic.h work when included from a C source file

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.

Differential Revision: https://reviews.llvm.org/D134591

Added: 
    

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

Removed: 
    


################################################################################
diff  --git a/libcxx/include/__config b/libcxx/include/__config
index e83b579b094bd..460c4754f1436 100644
--- a/libcxx/include/__config
+++ b/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

diff  --git a/libcxx/include/stdatomic.h b/libcxx/include/stdatomic.h
index d9550c44061c5..ff2a5682f5bb6 100644
--- a/libcxx/include/stdatomic.h
+++ b/libcxx/include/stdatomic.h
@@ -121,7 +121,7 @@ using std::atomic_signal_fence                         // see below
 #  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 @@ using std::atomic_thread_fence _LIBCPP_USING_IF_EXISTS;
 #   include_next <stdatomic.h>
 # endif
 
-#endif // _LIBCPP_STD_VER > 20
+#endif // defined(__cplusplus) && _LIBCPP_STD_VER > 20
 
 #endif // _LIBCPP_STDATOMIC_H

diff  --git a/libcxx/test/libcxx/include_as_c.sh.cpp b/libcxx/test/libcxx/include_as_c.sh.cpp
index a3e5d533040d4..f649c9c54f1be 100644
--- a/libcxx/test/libcxx/include_as_c.sh.cpp
+++ b/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>