CMAKE_BUILD_PARALLEL_LEVEL

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

Specifies the maximum number of concurrent processes to use when building using the cmake --build command line Build Tool Mode.

If this variable is defined empty the native build tool’s default number is used.

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Licensed under the BSD 3-clause License.
https://cmake.org/cmake/help/v3.17/envvar/CMAKE_BUILD_PARALLEL_LEVEL.html