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109 lines
4.5 KiB
109 lines
4.5 KiB
cmake_policy
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------------
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Manage CMake Policy settings. See the :manual:`cmake-policies(7)`
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manual for defined policies.
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As CMake evolves it is sometimes necessary to change existing behavior
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in order to fix bugs or improve implementations of existing features.
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The CMake Policy mechanism is designed to help keep existing projects
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building as new versions of CMake introduce changes in behavior. Each
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new policy (behavioral change) is given an identifier of the form
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``CMP<NNNN>`` where ``<NNNN>`` is an integer index. Documentation
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associated with each policy describes the ``OLD`` and ``NEW`` behavior
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and the reason the policy was introduced. Projects may set each policy
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to select the desired behavior. When CMake needs to know which behavior
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to use it checks for a setting specified by the project. If no
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setting is available the ``OLD`` behavior is assumed and a warning is
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produced requesting that the policy be set.
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Setting Policies by CMake Version
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The ``cmake_policy`` command is used to set policies to ``OLD`` or ``NEW``
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behavior. While setting policies individually is supported, we
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encourage projects to set policies based on CMake versions:
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.. code-block:: cmake
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cmake_policy(VERSION <min>[...<max>])
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``<min>`` and the optional ``<max>`` are each CMake versions of the form
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``major.minor[.patch[.tweak]]``, and the ``...`` is literal. The ``<min>``
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version must be at least ``2.4`` and at most the running version of CMake.
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The ``<max>`` version, if specified, must be at least the ``<min>`` version
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but may exceed the running version of CMake. If the running version of
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CMake is older than 3.12, the extra ``...`` dots will be seen as version
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component separators, resulting in the ``...<max>`` part being ignored and
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preserving the pre-3.12 behavior of basing policies on ``<min>``.
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This specifies that the current CMake code is written for the given
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range of CMake versions. All policies known to the running version of CMake
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and introduced in the ``<min>`` (or ``<max>``, if specified) version
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or earlier will be set to use ``NEW`` behavior. All policies
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introduced in later versions will be unset (unless the
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:variable:`CMAKE_POLICY_DEFAULT_CMP<NNNN>` variable sets a default).
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This effectively requests behavior preferred as of a given CMake
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version and tells newer CMake versions to warn about their new policies.
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Note that the :command:`cmake_minimum_required(VERSION)`
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command implicitly calls ``cmake_policy(VERSION)`` too.
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Setting Policies Explicitly
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^^^^^^^^^^^^^^^^^^^^^^^^^^^
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.. code-block:: cmake
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cmake_policy(SET CMP<NNNN> NEW)
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cmake_policy(SET CMP<NNNN> OLD)
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Tell CMake to use the ``OLD`` or ``NEW`` behavior for a given policy.
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Projects depending on the old behavior of a given policy may silence a
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policy warning by setting the policy state to ``OLD``. Alternatively
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one may fix the project to work with the new behavior and set the
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policy state to ``NEW``.
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.. include:: ../policy/DEPRECATED.txt
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Checking Policy Settings
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^^^^^^^^^^^^^^^^^^^^^^^^
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.. code-block:: cmake
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cmake_policy(GET CMP<NNNN> <variable>)
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Check whether a given policy is set to ``OLD`` or ``NEW`` behavior.
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The output ``<variable>`` value will be ``OLD`` or ``NEW`` if the
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policy is set, and empty otherwise.
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CMake Policy Stack
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^^^^^^^^^^^^^^^^^^
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CMake keeps policy settings on a stack, so changes made by the
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``cmake_policy`` command affect only the top of the stack. A new entry on
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the policy stack is managed automatically for each subdirectory to
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protect its parents and siblings. CMake also manages a new entry for
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scripts loaded by :command:`include` and :command:`find_package` commands
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except when invoked with the ``NO_POLICY_SCOPE`` option
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(see also policy :policy:`CMP0011`).
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The ``cmake_policy`` command provides an interface to manage custom
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entries on the policy stack:
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.. code-block:: cmake
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cmake_policy(PUSH)
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cmake_policy(POP)
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Each ``PUSH`` must have a matching ``POP`` to erase any changes.
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This is useful to make temporary changes to policy settings.
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Calls to the :command:`cmake_minimum_required(VERSION)`,
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``cmake_policy(VERSION)``, or ``cmake_policy(SET)`` commands
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influence only the current top of the policy stack.
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Commands created by the :command:`function` and :command:`macro`
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commands record policy settings when they are created and
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use the pre-record policies when they are invoked. If the function or
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macro implementation sets policies, the changes automatically
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propagate up through callers until they reach the closest nested
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policy stack entry.
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