[question] "cpp_info.system_libs" and static linking of some system libs

[amachado-pie]

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Whe are porting some custom makefiles with Conan v1 to Cmake + Conan 2.

Some recipes, such as libpng, use this approach:

def package_info(self):
    if self.settings.os in ["Linux", "Android", "FreeBSD", "SunOS", "AIX"]:
        self.cpp_info.system_libs.append("m")

This raises an issue when you want to mix static linking of just some system libraries, such as the case of libm using -Wl,-Bstatic -lstdc++ -lm -Wl,-Bdynamic

The issue is that system libraries of the conan recipes that added it as a dependency are repeated in the linking command:

[100%] Linking CXX executable
/usr/local/bin/cmake -E cmake_link_script CMakeFiles/project.dir/link.txt --verbose=1
/usr/bin/g++  -m32
...
-lpthread
-static-libstdc++
-static-libgcc
-Bstatic -lstdc++ -lm -Wl,-Bdynamic
/root/.conan2/p/b/freetafaa4833c1df1/p/lib/libfreetype.a
/root/.conan2/p/b/libpnb01f5f21fd40f/p/lib/libpng.a
...
/root/.conan2/p/b/opens5b1ba6b148939/p/lib/libcrypto.a
-lrt
/root/.conan2/p/b/xmlsead5ac171be143/p/lib/libxmlsec1.a
/root/.conan2/p/b/libxmab7a8911f883b/p/lib/libxml2.a
...
/root/.conan2/p/b/zlib8d00874752b7e/p/lib/libz.a
/root/.conan2/p/b/libic52d0797df088e/p/lib/libiconv.a
/root/.conan2/p/b/libic52d0797df088e/p/lib/libcharset.a
-ldl
-lpthread
-lm            <---
/root/.conan2/p/b/libjp57a4193473e2a/p/lib/libjpeg.a

So the executable is linked dynamically to libm:

ldd build/Release/project 
        linux-gate.so.1 (0xed5bd000)
        libdl.so.2 => /lib32/libdl.so.2 (0xec1dc000)
        librt.so.1 => /lib32/librt.so.1 (0xec1d3000)
        libpthread.so.0 => /lib32/libpthread.so.0 (0xec1b6000)
        libm.so.6 => /lib32/libm.so.6 (0xec161000)  <---
        libc.so.6 => /lib32/libc.so.6 (0xebfa9000)
        /lib/ld-linux.so.2 (0xed5bf000)

We need to still statically link some system libraries like libm because we need to support old distros that have outdated libs.

I tried to hack the dependencies in CMake to remove -lm like this but without success:

function(remove_transitive_dependencies target)
  if(TARGET ${target})
    message(STATUS "Target: ${target}")
    get_target_property(libs ${target} INTERFACE_LINK_LIBRARIES)
    if(libs)
      foreach(lib ${libs})
      message(STATUS "  lib ${lib}")
        if(lib STREQUAL "-lm")
          message(STATUS "    Removing transitive dependency '-lm' from target '${target}'")
          list(REMOVE_ITEM libs ${lib})
        endif()
      endforeach()
      set_target_properties(${target} PROPERTIES INTERFACE_LINK_LIBRARIES "${libs}")
    endif()
  endif()
endfunction()

# Remove transitive dependencies from common libraries
foreach(lib ${project_libraries})
  remove_transitive_dependencies(${lib})
endforeach()

Any idea about how can we deal with this issue?
Thanks

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