cmake_policy

Manage CMake Policy settings. See the cmake-policies(7) manual for defined policies.

As CMake evolves it is sometimes necessary to change existing behavior in order to fix bugs or improve implementations of existing features. The CMake Policy mechanism is designed to help keep existing projects building as new versions of CMake introduce changes in behavior. Each new policy (behavioral change) is given an identifier of the form CMP<NNNN> where <NNNN> is an integer index. Documentation associated with each policy describes the OLD and NEW behavior and the reason the policy was introduced. Projects may set each policy to select the desired behavior. When CMake needs to know which behavior to use it checks for a setting specified by the project. If no setting is available the OLD behavior is assumed and a warning is produced requesting that the policy be set.

Setting Policies by CMake Version

The cmake_policy command is used to set policies to OLD or NEW behavior. While setting policies individually is supported, we encourage projects to set policies based on CMake versions:

cmake_policy(VERSION <min>[...<max>])

<min> and the optional <max> are each CMake versions of the form major.minor[.patch[.tweak]], and the ... is literal. The <min> version must be at least 2.4 and at most the running version of CMake. The <max> version, if specified, must be at least the <min> version but may exceed the running version of CMake. If the running version of CMake is older than 3.12, the extra ... dots will be seen as version component separators, resulting in the ...<max> part being ignored and preserving the pre-3.12 behavior of basing policies on <min>.

This specifies that the current CMake code is written for the given range of CMake versions. All policies known to the running version of CMake and introduced in the <min> (or <max>, if specified) version or earlier will be set to use NEW behavior. All policies introduced in later versions will be unset (unless the CMAKE_POLICY_DEFAULT_CMP<NNNN> variable sets a default). This effectively requests behavior preferred as of a given CMake version and tells newer CMake versions to warn about their new policies.

Note that the cmake_minimum_required(VERSION) command implicitly calls cmake_policy(VERSION) too.

Setting Policies Explicitly

cmake_policy(SET CMP<NNNN> NEW)
cmake_policy(SET CMP<NNNN> OLD)

Tell CMake to use the OLD or NEW behavior for a given policy. Projects depending on the old behavior of a given policy may silence a policy warning by setting the policy state to OLD. Alternatively one may fix the project to work with the new behavior and set the policy state to NEW.

Checking Policy Settings

cmake_policy(GET CMP<NNNN> <variable>)

Check whether a given policy is set to OLD or NEW behavior. The output <variable> value will be OLD or NEW if the policy is set, and empty otherwise.

CMake Policy Stack

CMake keeps policy settings on a stack, so changes made by the cmake_policy command affect only the top of the stack. A new entry on the policy stack is managed automatically for each subdirectory to protect its parents and siblings. CMake also manages a new entry for scripts loaded by include() and find_package() commands except when invoked with the NO_POLICY_SCOPE option (see also policy CMP0011). The cmake_policy command provides an interface to manage custom entries on the policy stack:

cmake_policy(PUSH)
cmake_policy(POP)

Each PUSH must have a matching POP to erase any changes. This is useful to make temporary changes to policy settings. Calls to the cmake_minimum_required(VERSION), cmake_policy(VERSION), or cmake_policy(SET) commands influence only the current top of the policy stack.

Commands created by the function() and macro() commands record policy settings when they are created and use the pre-record policies when they are invoked. If the function or macro implementation sets policies, the changes automatically propagate up through callers until they reach the closest nested policy stack entry.