Best practices for mixed C and C++

[lb90]

Hello,

I'm working on adding DirectWrite support to Pango 2.0. Pango is written in C, but in order to use DirectWrite as of today we are forced to use C++, because DirectWrite headers are incomplete with regards to C interfacing ([1], [2], [3]).

I'd like to ask two questions in particular:

1. Is it completely supported by Meson to specify mixed .c and .cpp files when creating a target? (static libraries, shared libraries or executables)
2. Pango currently calls cc = meson.get_compiler('c') at the top of the meson.build script. Does the usage of cc also affect the C++ compiler? Should I instead add cxx = meson.get_compiler('cpp') and mirror all usages of cc to cxx?

With kind regards,
Thank you!

1. https://devblogs.microsoft.com/oldnewthing/20220113-00/?p=106152
2. https://sourceforge.net/p/mingw-w64/mailman/mingw-w64-public/thread/CAMq1ado5MOQN%3D0xXsHY9f%3DKBHpTHa4W%2Bt-kVkwys5s8k1V5nBQ%40mail.gmail.com/#msg37417555
3. https://sourceforge.net/p/mingw-w64/mailman/mingw-w64-public/thread/aff7dfc0-c5cb-0fd8-5c80-9887a49ebb34%40ycbcr.xyz/#msg37616115

[eli-schwartz]

• meson supports adding both C and C++ sources to the same library or executable, and will by default use the C++ driver to link with. There is a link_language kwarg to override this.
• both the cc and cxx compilers will exist (be searched for, declared as languages for files each needs to compile). The get_compiler() function merely gives you a handle to that compiler. In many cases, it doesn't matter which one you use since e.g. find_library() finds the same libraries in either case and C++ shared libraries are technically linkable by the C compiler because they might provide a C ABI (but you can't really know until you try). Depending on your use case, though, you absolutely do need to use the right compiler depending on what you are doing. For example, cc.compiles() will check if the C compiler can compile some code, and cxx.compiles() will check if the C++ compiler can compile some code, and obviously these are different because that's why Meson defines two languages for them to begin with. Since you can use C headers from C++, e.g. has_function or has_header(_symbol)? will probably work with either one, as long as you're only checking C stuff.

[lb90]

Thanks for the great answer, @eli-schwartz!

I have checked all of the cc usages in Pango. For most of them everything is really obvious, nothing to say. Instead I have doubts regarding:

• add_project_arguments ('-foo ...', language: 'c'). I should change the language arg to ['c', 'cpp'], right?
• When specifying the c_flags argument in a build target, I should also set the cpp_flags, otherwise the cpp compiler won't pick up the flags, is that correct?

By the way, it'd be really great to have a dedicated FAQ section on mesonbuild.com 😉

[xclaesse]

Yes if you mix C/C++ sources that need the same flags you have to define them in both c_flags and cpp_flags and add_project_arguments().

By the way, it'd be really great to have a dedicated FAQ section on mesonbuild.com wink

https://mesonbuild.com/FAQ.html

Feel free to open PR to extend the doc :)

[xclaesse]

Related: https://mesonbuild.com/FAQ.html#why-dont-the-arguments-passed-to-add_project_link_arguments-affect-anything

[eli-schwartz]

In theory you might have flags which are only used by the C sources and not the C++ sources, but... probably not.

So yeah, extend the language to apply it for both.

[afud]

@eli-schwartz
Hello.

I have a mixed 'c' and 'c++' project, and add all c++ files and c files in the same sources to generate a executable file. Can I
use 'link_language' to avoid error of 'No host machine compiler for xxx.c'? What should I do?

Best wishes
Thanks

[eli-schwartz]

No, the link language only controls which compiler is used to link the object files together.

The object files themselves are still created by the correct compiler for each language. Any files with a C file extension require a C compiler, any files with a C++ file extension require a C++ compiler.

If you are getting that error, it sounds like in the project('myproject', 'c', 'cpp') you actually don't have the C language declared in the list of languages your project uses. So meson doesn't look for a C compiler, and thus doesn't have it available when it finds sources named *.c.

So if you fix this teeny oversight things should start working, most likely.

[afud]

Yes, I'm missing the discription of this. When adding the 'c' language specifiction, it working. thanks a lot.

@eli-schwartz
anther tiny question about the output name of a library or module, which starts with 'lib' automaticly, is this design for standardization?

Best wishes

[eli-schwartz]

On Windows it doesn't much matter, but it's required on Unix in order to work correctly with compilers. For consistency, it starts with "lib" on Windows as well.

[afud]

I see, thank you.