# CMP0000

Warning

The `OLD` behavior of this policy was removed in CMake version 4.0. This policy must be set to `NEW` by a call to `cmake_minimum_required()` or `cmake_policy()`.

A minimum required CMake version must be specified.

CMake requires that projects specify the version of CMake to which they have been written. This policy has been put in place so users trying to build the project may be told when they need to update their CMake. Specifying a version also helps the project build with CMake versions newer than that specified. Use the `cmake_minimum_required()` command at the top of your main `CMakeLists.txt` file:

    cmake_minimum_required(VERSION <major>.<minor>)

where `<major>.<minor>` is the version of CMake you want to support (such as `3.14`). The command will ensure that at least the given version of CMake is running and help newer versions be compatible with the project. See documentation of `cmake_minimum_required()` for details.

Note that the command invocation must appear in the `CMakeLists.txt` file itself; a call in an included file is not sufficient. The `OLD` behavior was to silently ignore the missing invocation. The `NEW` behavior is to issue an error instead of a warning.

This policy was introduced in CMake version 2.6.0. Prior to removal in CMake version 4.0, it could be set by `cmake_policy()` or `cmake_minimum_required()`. If it was not set, CMake warned, and used `OLD` behavior.