CMP0060

Link libraries by full path even in implicit directories.

Policy CMP0003 was introduced with the intention of always linking library files by full path when a full path is given to the target_link_libraries() command. However, on some platforms (e.g. HP-UX) the compiler front-end adds alternative library search paths for the current architecture (e.g. /usr/lib/<arch> has alternatives to libraries in /usr/lib for the current architecture). On such platforms the find_library() may find a library such as /usr/lib/libfoo.so that does not belong to the current architecture.

Prior to policy CMP0003 projects would still build in such cases because the incorrect library path would be converted to -lfoo on the link line and the linker would find the proper library in the arch-specific search path provided by the compiler front-end implicitly. At the time we chose to remain compatible with such projects by always converting library files found in implicit link directories to -lfoo flags to ask the linker to search for them. This approach allowed existing projects to continue to build while still linking to libraries outside implicit link directories via full path (such as those in the build tree).

CMake does allow projects to override this behavior by using an IMPORTED library target with its IMPORTED_LOCATION property set to the desired full path to a library file. In fact, many Find Modules are learning to provide Imported Targets instead of just the traditional Foo_LIBRARIES variable listing library files. However, this makes the link line generated for a library found by a Find Module depend on whether it is linked through an imported target or not, which is inconsistent. Furthermore, this behavior has been a source of confusion because the generated link line for a library file depends on its location. It is also problematic for projects trying to link statically because flags like -Wl,-Bstatic -lfoo -Wl,-Bdynamic may be used to help the linker select libfoo.a instead of libfoo.so but then leak dynamic linking to following libraries. (See the LINK_SEARCH_END_STATIC target property for a solution typically used for that problem.)

When the special case for libraries in implicit link directories was first introduced the list of implicit link directories was simply hard-coded (e.g. /lib, /usr/lib, and a few others). Since that time, CMake has learned to detect the implicit link directories used by the compiler front-end. If necessary, the find_library() command could be taught to use this information to help find libraries of the proper architecture.

For these reasons, CMake 3.3 and above prefer to drop the special case and link libraries by full path even when they are in implicit link directories. Policy CMP0060 provides compatibility for existing projects.

The OLD behavior for this policy is to ask the linker to search for libraries whose full paths are known to be in implicit link directories. The NEW behavior for this policy is to link libraries by full path even if they are in implicit link directories.

This policy was introduced in CMake version 3.3. Unlike most policies, CMake version 3.19.0-rc3 does not warn by default when this policy is not set and simply uses OLD behavior. See documentation of the CMAKE_POLICY_WARNING_CMP0060 variable to control the warning.