Help/policy/CMP0060.rst

CMP0060
-------

Link libraries by full path even in implicit directories.

Policy :policy:`CMP0003` was introduced with the intention of always
linking library files by full path when a full path is given to the
:command:`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 :command:`find_library` may find a library such as
``/usr/lib/libfoo.so`` that does not belong to the current architecture.

Prior to policy :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
:ref:`IMPORTED library target <Imported Targets>` with its
:prop_tgt:`IMPORTED_LOCATION` property set to the desired full path to
a library file.  In fact, many :ref:`Find Modules` are learning to provide
:ref:`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 :prop_tgt:`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 :command:`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 |release| does *not* warn by default when this policy
is not set and simply uses ``OLD`` behavior.  See documentation of the
:variable:`CMAKE_POLICY_WARNING_CMP0060 <CMAKE_POLICY_WARNING_CMP<NNNN>>`
variable to control the warning.

.. include:: DEPRECATED.txt