modules/pkg-config: Add support for Cflags.private

[lb90]

Fixes #14749

[xclaesse]

How does it work to get those private cflags from pkgconf? Should we also make Meson use them somewhere? I would like to see a unit test for that. Sorry if it's there and I missed it.

[xclaesse]

Note that usually projects do not build both shared and static libraries on windows, because it requires compiling everything twice and that capability has only been added recently in Meson. That means that in the case you build only the static library, it is fine to add e.g. MY_LIB_STATIC_COMPILE into Cflags field, or include that into some generated header file. Most projects do that already.

[lb90]

Hi @xclaesse!

How does it work to get those private cflags from pkgconf? Should we also make Meson use them somewhere?

Well, Meson doesn't read .pc files by itself, it invokes a pkg-config binary. All it takes is using pkg-conf with Meson.

Note that usually projects do not build both shared and static libraries on windows

MSYS2 distributes both the shared library and the static library in a single package (and it usually builds the library twice for that). However the package ships one .pc file named after the library. So one can simply do:

dependency('foo') # follow default_library
dependency('foo', static: false) # shared library
dependency('foo', static: true) # static library

See for example the file list in https://packages.msys2.org/packages/mingw-w64-ucrt-x86_64-libepoxy. Fedora mingw packages do the same:

• https://packages.fedoraproject.org/pkgs/mingw-glib2/mingw64-glib2-static/fedora-rawhide.html#files
• https://packages.fedoraproject.org/pkgs/mingw-glib2/mingw64-glib2/fedora-rawhide.html#files

[xclaesse]

All it takes is using pkg-conf with Meson.

You still need to tell pkgconf if you want static or not. We usually pass --static arg for libs but I'm not sure we do for cflags. In either case, we should have a unit test for it

[xclaesse]

Fedora mingw packages do the same

I'm pretty sure they do that by configuring meson twice and keeping only one of the .pc file generated. For glib I would be surprised if that even works because the macro is not in the .pc file but in generated header file.

[moi15moi]

We usually pass --static arg for libs but I'm not sure we do for cflags.

You're right, meson doesn't pass --static for cflags like it does for libs.
See:

meson/mesonbuild/dependencies/pkgconfig.py

Lines 159 to 189 in 3429a2b

	@lru_cache(maxsize=None)
	def cflags(self, name: str, allow_system: bool = False,
	define_variable: PkgConfigDefineType = None) -> ImmutableListProtocol[str]:
	env = None
	if allow_system:
	env = os.environ.copy()
	env['PKG_CONFIG_ALLOW_SYSTEM_CFLAGS'] = '1'
	args: T.List[str] = []
	args += self._define_variable_args(define_variable)
	args += ['--cflags', name]
	ret, out, err = self._call_pkgbin(args, env=env)
	if ret != 0:
	raise DependencyException(f'Could not generate cflags for {name}:\n{err}\n')
	return self._split_args(out)
	@lru_cache(maxsize=None)
	def libs(self, name: str, static: bool = False, allow_system: bool = False,
	define_variable: PkgConfigDefineType = None) -> ImmutableListProtocol[str]:
	env = None
	if allow_system:
	env = os.environ.copy()
	env['PKG_CONFIG_ALLOW_SYSTEM_LIBS'] = '1'
	args: T.List[str] = []
	args += self._define_variable_args(define_variable)
	if static:
	args.append('--static')
	args += ['--libs', name]
	ret, out, err = self._call_pkgbin(args, env=env)
	if ret != 0:
	raise DependencyException(f'Could not generate libs for {name}:\n{err}\n')
	return self._split_args(out)

[dcbaker]

AFAICT there's no reason we don't other than no one's written the code. pkg-config and pkgconf both accept that (and those are the only two pkg-config implementations we really claim to support)

[moi15moi]

What is the point of adding the flag? It's a shared library, so it doesnt have any purpose, no?

[lb90]

Yeah, it's just a check that setting cflags_private doesn't crash Meson. Not much, I know 🙂 The test could be extended but we have to detect pkconf vs pkg-config