CMake dependencies add absolute paths to pkgconfig Libs

[kone-tlammi]

Describe the bug
When using pkgconfig module to generate .pc files for a project that depend on cmake projects the resulting .pc file has absolute paths to libraries.

This behavior causes issues with Yocto builds where resulting .pc files have absolute paths pointing to a different sysroot which might not exist when some other component is consuming the said .pc.

To Reproduce
meson.build

project('sample', 'cpp')

dep = dependency('OpenSSL', modules: ['OpenSSL::SSL'], method: 'cmake')

lib = static_library('lib', 'lib.cpp', dependencies: dep)
pkgconf = import('pkgconfig')
pkgconf.generate(name: 'lib', filebase: 'lib', description: 'lib', libraries: lib)

Resulting lib.pc

prefix=/usr/local
includedir=${prefix}/include
libdir=${prefix}/lib

Name: lib
Description: lib
Version: undefined
Libs: -L${libdir} -llib /usr/lib/libcrypto.so /usr/lib/libssl.so
Cflags: -I${includedir} -I/usr/include

Expected behavior
Libs field should have -lcrypto -lssl or there should be some more manual way to avoid the current behavior.

system parameters

• Is this a cross build or just a plain native build (for the same computer)?
  • Sample is for native builds but this also happens for cross builds
• what operating system (e.g. MacOS Catalina, Windows 10, CentOS 8.0, Ubuntu 18.04, etc.)
  • Manjaro Linux (Ubuntu 22.04 for Yocto)
• what Python version are you using e.g. 3.8.0
  • Python 3.12.6 (3.10.14 in Yocto)
• what meson --version
  • Native 1.6.0 (1.3.1 in Yocto)
• what ninja --version if it's a Ninja build
  • Native 1.12.1 (1.10.2 in Yocto)

[eli-schwartz]

IIRC the problem here is that cmake dependency lookup only exposes the absolute path to a library. Meson tries to avoid this whenever it can, but "whenever it can" means when using find_library or dependency.

For OpenSSL in particular meson has a method: 'system' that will probably in most cases yield a superior result rather than using cmake. The example only works because it hardcodes an (unlikely) method. Other dependencies especially where the upstream project doesn't provide pkgconfig files at all and the only dependency finder is cmake, would make a more useful example.

[kone-tlammi]

Yeah I thought this might be a cmake issue. In my actual use case the OpenSSL is not a direct dependency but acquired via paho-mqtt-c. I avoided the issue by replacing the dependencies with bunch of calls to cpp.find_library. I guess there is no better approach here?

[eli-schwartz]

Well, eclipse-paho/paho.mqtt.c#756 might help.