ad3c2c0b03
Every policy's documentation has a paragraph on what version of CMake introduced it, how to set the policy, and whether CMake warns if the policy is not set. The wording of this paragraph has diverged across policies over time. Factor the paragraph out into a standard advice document included by every policy.
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70 lines
3.5 KiB
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CMP0060
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-------
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.. versionadded:: 3.3
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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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.. |INTRODUCED_IN_CMAKE_VERSION| replace:: 3.3
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.. |WARNS_OR_DOES_NOT_WARN| replace:: does *not* warn by default
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.. include:: STANDARD_ADVICE.txt
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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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