IIRC, this is documented in the CPP part of the GCC manual. Whenever a header file is completely enclosed in a ifndef guard, and that guard is still defined, it

IIRC, this is documented in the CPP part of the GCC manual. Whenever a header file is completely enclosed in a ifndef guard, and that guard is still defined, it will skip the whole file. Another option is "#pragma once"; it's non-standard, but every popular C compiler (even MSVC) understands it, and has the advantage that there's no risk of accidentally using the same include guard for more than one header file.