C doesn't impose a specific threading model. So a C threading benchmark is specific to the library you are using. And if you choose a heavyweight thread library to compare to a language which is not using heavyweight threads, it is not an apples to apples comparison. Arguably it could be, if C imposed a specific threading model, which it does not.
Yes and that was my point. The benchmark is a comparison of implementations - compilers, run times and libraries. I am not sure how you would benchmark a language.
is there a nice lightweight threading library in c that distributes _many_ small threads on _few_ (not one) posix threads? if so: please tell me. i am genuinely interested.
You are comparing implementations, not languages