# CMAKE_SYSTEM_VERSION

The version of the operating system for which CMake is to build. See the `CMAKE_SYSTEM_NAME` variable for the OS name.

## System Version for Host Builds

When the `CMAKE_SYSTEM_NAME` variable takes its default value then `CMAKE_SYSTEM_VERSION` is by default set to the same value as the `CMAKE_HOST_SYSTEM_VERSION` variable so that the build targets the host system version.

In the case of a host build then `CMAKE_SYSTEM_VERSION` may be set explicitly when first configuring a new build tree in order to enable targeting the build for a different version of the host operating system than is actually running on the host. This is allowed and not considered cross compiling so long as the binaries built for the specified OS version can still run on the host.

## System Version for Cross Compiling

When the `CMAKE_SYSTEM_NAME` variable is set explicitly to enable cross compiling then the value of `CMAKE_SYSTEM_VERSION` must also be set explicitly to specify the target system version.