Bug 245176 – [CMake] Clang warnings about C code being built in C++ mode

• Component: Tools / Tests
• Status: RESOLVED FIXED

[Adrian Perez]

I haven't researched the root cause yet, but it looks like either the build is using “clang++”
to compile plain C files, or passing “-x c++” to the “clang” driver (less likely): 

  [3/30/6151|2.0/s|  0%] Building CXX object Source/bmalloc/CMakeFiles/bmalloc.dir/libpas/src/libpas/bmalloc_heap.c.o
  clang-14: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]
  [3/31/6151|2.0/s|  0%] Building CXX object Source/bmalloc/CMakeFiles/bmalloc.dir/libpas/src/libpas/bmalloc_heap_config.c.o
  clang-14: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]
  [3/39/6151|2.5/s|  0%] Building CXX object Source/bmalloc/CMakeFiles/bmalloc.dir/libpas/src/libpas/jit_heap.c.o
  clang-14: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]
  [3/40/6151|2.5/s|  0%] Building CXX object Source/bmalloc/CMakeFiles/bmalloc.dir/libpas/src/libpas/jit_heap_config.c.o
  clang-14: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]
  [3/57/6151|3.4/s|  0%] Building CXX object Source/bmalloc/CMakeFiles/bmalloc.dir/libpas/src/libpas/pas_bitfit_page_config_kind.c.o
  clang-14: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]
  [3/95/6151|5.3/s|  1%] Building CXX object Source/bmalloc/CMakeFiles/bmalloc.dir/libpas/src/libpas/pas_fast_megapage_cache.c.o
  clang-14: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]
  [3/101/6151|5.5/s|  1%] Building CXX object Source/bmalloc/CMakeFiles/bmalloc.dir/libpas/src/libpas/pas_heap_config_kind.c.o
  clang-14: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]
  [3/152/6151|7.7/s|  2%] Building CXX object Source/bmalloc/CMakeFiles/bmalloc.dir/libpas/src/libpas/pas_segregated_page_config_kind.c.o
  clang-14: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]
  [3/167/6151|8.3/s|  2%] Building CXX object Source/bmalloc/CMakeFiles/bmalloc.dir/libpas/src/libpas/pas_thread_local_cache.c.o
  clang-14: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]
  [3/172/6151|8.5/s|  2%] Building CXX object Source/bmalloc/CMakeFiles/bmalloc.dir/libpas/src/libpas/pas_utility_heap_config.c.o
  clang-14: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]

...unless this is intentional and some of the libpas files are expected to be built with a C++
compiler for some reason even if their suffix is “.c” (not very likely, I suppose).

[Adrian Perez]

This is easily reproducible with:

  CC=clang CXX=clang++ cmake -S. -Bbuild -DPORT=GTK -GNinja
  ninja -Cbuild

[Fujii Hironori]

What about adding enable_language(C)?

[Yusuke Suzuki]

This is intentional since these files cannot be compiled as C in GCC probably because GCC lacks some constant propagation feature, which requires this explicit C++ handling.

[Yusuke Suzuki]

So, I think, if we are using clang, then we do not need to mark them as C++.

[Stephan Szabo]

We'd noticed something similar in the past, but we thought it was something specific to our downstream compilers, so in our case we ended up forcing a -xc++ into the build in our toolchain.

It feels like this should work from the CMake side (setting the language to CXX for a file that has .c extension), so if it's more general and happens in a test project, I wonder if it's something that should get handled inside the CMake code (well the modules for the compiler).

[Stephan Szabo]

From discussion on CMake discourse, with a minimum 3.20 or policy CMP0119 set to NEW, we'd get the -xc++ automatically when setting the language, but that won't help us for older versions.

[Lauro Moura]

Pull request: https://github.com/WebKit/WebKit/pull/5814

[EWS]

Committed 256626@main (e071c895f5fc): <https://commits.webkit.org/256626@main>

Reviewed commits have been landed. Closing PR #5814 and removing active labels.

[Radar WebKit Bug Importer]

<rdar://problem/102302395>