Impossible to use CMake's find_package to find C++ library if C library is installed as well

[traversaro]

After the split discussed in #872, it seems that if both the C and the C++ library are installed, only the C library is found. This happens as both library install a configuration file called msgpack-config.cmake, even if there is not direct conflict as the files are installed in different directories.

Detailed example

To give you an example, I have a sample program like the following:

project(HelloWorld)
cmake_minimum_required(VERSION 3.16)

find_package(msgpack REQUIRED)

message(STATUS "msgpack_DIR: ${msgpack_DIR}")

if(TARGET msgpackc)
  message(STATUS "TARGET msgpackc is found")
else()
  message(STATUS "TARGET msgpackc is NOT found")
endif()

if(TARGET msgpackc-cxx)
  message(STATUS "TARGET msgpackc-cxx is found")
else()
  message(STATUS "TARGET msgpackc-cxx is NOT found")
endif()

If I run CMake when only the C++ library is installed in the /home/traversaro/msgpack_ws/install prefix, I obtained the following output:

traversaro@IITICUBLAP257:~/msgpack_ws/example/build$ cmake .
-- msgpack_DIR: /home/traversaro/msgpack_ws/install/lib/cmake/msgpackc-cxx
-- TARGET msgpackc is NOT found
-- TARGET msgpackc-cxx is found
-- Configuring done
-- Generating done
-- Build files have been written to: /home/traversaro/msgpack_ws/example/build

However, if I also install the C library in the /home/traversaro/msgpack_ws/install prefix, the following happens, i.e. the C library is found instead, making it impossible to use the msgpackc-cxx target:

-- msgpack_DIR: /home/traversaro/msgpack_ws/install/lib/cmake/msgpack
-- TARGET msgpackc is found
-- TARGET msgpackc-cxx is NOT found
-- Configuring done
-- Generating done
-- Build files have been written to: /home/traversaro/msgpack_ws/example/build

[traversaro]

The most straightforward way to fix this is to modify either the C or C++ library to change their "CMake package name", i.e. the name that you pass to find_package to find it. However, this would mean breaking compatibility with existing downstream libraries that use find_package(msgpack REQUIRED) to find the library that changed its "CMake package name".

[traversaro]

See also #974 that was kind of related to this.

[redboltz]

I'm not an expert of cmake.
Where the package name decided? Just a file name of msgpack-config.cmake.in ?
Now, both c_master and cpp_master has the same file msgpack-config.cmake.in.
Am I understanding correctly?

I didn't know user writes find_package as follows:

find_package(msgpack REQUIRED)

It should be

find_package(msgpackc REQUIRED)

for C and

find_package(msgpackc-cxx REQUIRED)

for C++.

ADD_LIBRARY (msgpackc-cxx INTERFACE)

It is corresponding to

ADD_LIBRARY (msgpackc-cxx INTERFACE)

But for C

    ADD_LIBRARY (msgpackc
        ${msgpackc_SOURCES}
        ${msgpackc_HEADERS}
    )

and

    ADD_LIBRARY (msgpackc-static STATIC
        ${msgpackc_SOURCES}
        ${msgpackc_HEADERS}
    )

are defined conditionally.

I think that msgpackc is acceptable name for C package.

After this update, user can write

find_package(msgpackc REQUIRED)  # for C
find_package(msgpackc-cxx REQUIRED) # for C++

I'm not sure how user can get cmake variables such as C include directory and C++ include directory. I guess that cmake automatically defines the variables that has msgpackc msgpackc-cxx prefix. Is that right?

[redboltz]

I mean the current HEAD doesn't support

find_package(msgpackc REQUIRED)  # for C
find_package(msgpackc-cxx REQUIRED) # for C++

Some modification is required. Reanme msgpack-config.cmake.in file and using code? I guess.
Then it should work. As you mentioned, it is a breaking change but user needs to accept the change.

Could you write PR for that ?

[traversaro]

I'm not an expert of cmake.
Where the package name decided? Just a file name of msgpack-config.cmake.in ?
Now, both c_master and cpp_master has the same file msgpack-config.cmake.in.
Am I understanding correctly?

Yes, that is correct, the find_package(msgpack REQUIRED) looks for a file named msgpack-config.cmake or msgpackConfig.cmake in many locations, and returns the first that it finds. If only one is found in the install prefix, there is no problem, but if both are found, only the first one found is returned.

[traversaro]

I guess that cmake automatically defines the variables that has msgpackc msgpackc-cxx prefix. Is that right?

Actually all this informations are contained in the imported targets msgpackc and msgpackc-cxx, i.e. to compile a simple program you do:

project(msgpackcexample)

cmake_minimum_required(VERSION 3.16)

find_package(msgpack REQUIRED)

add_executable(msgpackcexample msgpackcexample.c)
target_link_libraries(msgpackcexample PRIVATE msgpackc)

or for C++:

project(msgpacxxkcexample)

cmake_minimum_required(VERSION 3.16)

find_package(msgpack REQUIRED)

add_executable(msgpackcxxexample msgpackcxxexample.c)
target_link_libraries(msgpackcxxexample PRIVATE msgpackc-cxx)

CMake automatically handles that the include directories, the definition and the libraries to link are correctly propagated.

Could you write PR for that ?

Sure!

[traversaro]

Could you write PR for that ?

Sure!

Done in:

• Change CMake package name of C library to msgpackc #1044
• Change CMake package name of C++ library to msgpackc-cxx #1045

[traversaro]

I think that we can close as this was fixed in releases 5 of both the C and C++ library, thanks a lot!

[eli-schwartz]

I'm pretty confused why it was renamed to msgpackc-cxx, instead of, say, msgpack-cxx. It confused me twice when trying to figure out the new dependency name.

I guess the bigger issue, though, is that the C library is now called both msgpack (pkg-config), and msgpackc (cmake). I wonder why that was necessary at all. It should definitely be forced to be consistent, though.

[redboltz]

@traversaro could you answer the question?

I guess that C-library should be called msgpackc on pkg-config. But I don't have much knowledge about pkg-config.
https://github.com/msgpack/msgpack-c/tree/c_master has pkg-config related code?

[eli-schwartz]

Indeed:

msgpack-c/CMakeLists.txt

Lines 117 to 121 in 551308a

	CONFIGURE_FILE (
	msgpack.pc.in
	msgpack.pc
	@ONLY
	)

msgpack-c/CMakeLists.txt

Lines 270 to 272 in 551308a

	IF (NOT MSVC)
	INSTALL (FILES ${CMAKE_CURRENT_BINARY_DIR}/msgpack.pc DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig)
	ENDIF ()

This can be found with Meson using:

dependency('msgpack')

And the cmake files can also be found with meson:

dependency('msgpackc')

If they had the same name, then you could just look that up, and Meson would try both pkg-config and cmake.

...

By the way, maybe a pkg-config file for msgpackc-cxx could be provided?