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66 lines
3.5 KiB
66 lines
3.5 KiB
9 years ago
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CMP0060
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-------
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Link libraries by full path even in implicit directories.
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Policy :policy:`CMP0003` was introduced with the intention of always
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linking library files by full path when a full path is given to the
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:command:`target_link_libraries` command. However, on some platforms
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(e.g. HP-UX) the compiler front-end adds alternative library search paths
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for the current architecture (e.g. ``/usr/lib/<arch>`` has alternatives
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to libraries in ``/usr/lib`` for the current architecture).
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On such platforms the :command:`find_library` may find a library such as
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``/usr/lib/libfoo.so`` that does not belong to the current architecture.
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Prior to policy :policy:`CMP0003` projects would still build in such
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cases because the incorrect library path would be converted to ``-lfoo``
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on the link line and the linker would find the proper library in the
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arch-specific search path provided by the compiler front-end implicitly.
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At the time we chose to remain compatible with such projects by always
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converting library files found in implicit link directories to ``-lfoo``
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flags to ask the linker to search for them. This approach allowed existing
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projects to continue to build while still linking to libraries outside
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implicit link directories via full path (such as those in the build tree).
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CMake does allow projects to override this behavior by using an
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:ref:`IMPORTED library target <Imported Targets>` with its
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:prop_tgt:`IMPORTED_LOCATION` property set to the desired full path to
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a library file. In fact, many :ref:`Find Modules` are learning to provide
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:ref:`Imported Targets` instead of just the traditional ``Foo_LIBRARIES``
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variable listing library files. However, this makes the link line
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generated for a library found by a Find Module depend on whether it
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is linked through an imported target or not, which is inconsistent.
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Furthermore, this behavior has been a source of confusion because the
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generated link line for a library file depends on its location. It is
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also problematic for projects trying to link statically because flags
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like ``-Wl,-Bstatic -lfoo -Wl,-Bdynamic`` may be used to help the linker
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select ``libfoo.a`` instead of ``libfoo.so`` but then leak dynamic linking
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to following libraries. (See the :prop_tgt:`LINK_SEARCH_END_STATIC`
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target property for a solution typically used for that problem.)
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When the special case for libraries in implicit link directories was first
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introduced the list of implicit link directories was simply hard-coded
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(e.g. ``/lib``, ``/usr/lib``, and a few others). Since that time, CMake
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has learned to detect the implicit link directories used by the compiler
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front-end. If necessary, the :command:`find_library` command could be
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taught to use this information to help find libraries of the proper
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architecture.
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For these reasons, CMake 3.3 and above prefer to drop the special case
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and link libraries by full path even when they are in implicit link
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directories. Policy ``CMP0060`` provides compatibility for existing
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projects.
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The OLD behavior for this policy is to ask the linker to search for
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libraries whose full paths are known to be in implicit link directories.
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The NEW behavior for this policy is to link libraries by full path even
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if they are in implicit link directories.
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This policy was introduced in CMake version 3.3. Unlike most policies,
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CMake version |release| does *not* warn by default when this policy
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is not set and simply uses OLD behavior. See documentation of the
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:variable:`CMAKE_POLICY_WARNING_CMP0060 <CMAKE_POLICY_WARNING_CMP<NNNN>>`
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variable to control the warning.
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.. include:: DEPRECATED.txt
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