share/cmake-3.6/Help/command/cmake_minimum_required.rst

cmake_minimum_required
----------------------

Set the minimum required version of cmake for a project and
update `Policy Settings`_ to match the version given::

  cmake_minimum_required(VERSION major.minor[.patch[.tweak]]
                         [FATAL_ERROR])

If the current version of CMake is lower than that required it will
stop processing the project and report an error.

The ``FATAL_ERROR`` option is accepted but ignored by CMake 2.6 and
higher.  It should be specified so CMake versions 2.4 and lower fail
with an error instead of just a warning.

.. note::
  Call the ``cmake_minimum_required()`` command at the beginning of
  the top-level ``CMakeLists.txt`` file even before calling the
  :command:`project` command.  It is important to establish version
  and policy settings before invoking other commands whose behavior
  they may affect.  See also policy :policy:`CMP0000`.

  Calling ``cmake_minimum_required()`` inside a :command:`function`
  limits some effects to the function scope when invoked.  Such calls
  should not be made with the intention of having global effects.

Policy Settings
^^^^^^^^^^^^^^^

The ``cmake_minimum_required(VERSION)`` command implicitly invokes the
:command:`cmake_policy(VERSION)` command to specify that the current
project code is written for the given version of CMake.
All policies introduced in the specified version or earlier will be
set to use NEW behavior.  All policies introduced after the specified
version will be unset.  This effectively requests behavior preferred
as of a given CMake version and tells newer CMake versions to warn
about their new policies.

When a version higher than 2.4 is specified the command implicitly
invokes::

  cmake_policy(VERSION major[.minor[.patch[.tweak]]])

which sets the cmake policy version level to the version specified.
When version 2.4 or lower is given the command implicitly invokes::

  cmake_policy(VERSION 2.4)

which enables compatibility features for CMake 2.4 and lower.