CMAKECACHEMAJORVERSION

CMAKE_CACHE_MAJOR_VERSION, more commonly seen as CMAKE_CACHE_MAJOR_VERSION in CMakeCache.txt, is an internal cache metadata variable used by CMake to identify the major version of the cache format. It is generated automatically and stored in the build directory's CMakeCache.txt when a project is configured.

The major version works in concert with CMAKE_CACHE_MINOR_VERSION to indicate cache format compatibility. CMake compares the cache’s major version with the major version of the running CMake executable; if they do not match, the existing cache is considered incompatible. In that scenario, CMake will typically regenerate the build system rather than attempt to reuse an incompatible cache.

The variable is not designed to be manually edited by users. It appears in the cache file as a simple string value (for example, CMAKE_CACHE_MAJOR_VERSION:STRING=3). The exact numbers reflect changes in the cache format used by CMake between releases and are not intended to convey the version of the CMake software itself.

Handling cache incompatibility usually involves removing or regenerating the cache. If upgrading or downgrading CMake across major versions, the recommended action is to delete the CMakeCache.txt file (and often the CMakeFiles directory) or run a fresh configure to produce a compatible cache.

In summary, CMAKE_CACHE_MAJOR_VERSION is an internal mechanism that helps ensure cache compatibility across CMake major-version boundaries, preventing misinterpretation of cached variables and reducing the risk of incorrect builds.