CMake 2.6 Notes

Linking

CMake 2.6 implements a new approach to generating link lines for targets.

Consider these libraries:

  /path/to/libfoo.a
  /path/to/libfoo.so

Previously if someone wrote

  target_link_libraries(myexe /path/to/libfoo.a)

CMake would generate this code to link it:

  ... -L/path/to -Wl,-Bstatic -lfoo -Wl,-Bdynamic ...

This worked most of the time, but some platforms (such as OS X) do not support the -Bstatic or equivalent flag. This made it impossible to link to the static version of a library without creating a symlink in another directory and using that one instead.

Now CMake will generate this code:

  ... /path/to/libfoo.a ...

This guarantees that the correct library is chosen.

There is a side-effect of this fix. Projects used to be able to write this (wrong) code and it would work by accident:

  add_executable(myexe myexe.c)
  target_link_libraries(myexe /path/to/libA.so B)

where "B" is meant to link "/path/to/libB.so". This code is incorrect because it asks CMake to link to B but does not provide the proper linker search path for it. It used to work by accident because the -L/path/to would get added as part of the implementation of linking to A. The correct code would be

  link_directories(/path/to)
  add_executable(myexe myexe.c)
  target_link_libraries(myexe /path/to/libA.so B)

or even better

  add_executable(myexe myexe.c)
  target_link_libraries(myexe /path/to/libA.so /path/to/libB.so)

In order to support projects that have this bug, we've added a compatibility feature that adds the "-L/path/to" paths for all libraries linked with full paths even though the linker will not need those paths to find the main libraries. The compatibility mode is enabled when a link line contains a non-full-path library (like B) and either CMAKE_BACKWARDS_COMPATIBILITY is set to 2.4 or lower or CMAKE_LINK_OLD_PATHS is set to true.

If you are trying to build a project and run into this problem, a quick-fix is to run

 cmake -DCMAKE_LINK_OLD_PATHS:BOOL=ON .

in the top of the build tree.