# CMAKE_TOOLCHAIN_FILE

Added in version 3.21.

This is a CMake Environment Variable. Its initial value is taken from the calling process environment.

The `CMAKE_TOOLCHAIN_FILE` environment variable specifies a default value for the `CMAKE_TOOLCHAIN_FILE` variable when there is no explicit configuration given on the first run while creating a new build tree. On later runs in an existing build tree the value persists in the cache as `CMAKE_TOOLCHAIN_FILE`.