That's horrible. I read it the first time as "third success." At least "mtx" is a little easier to see (though still quite silly).

Can anyone defend this?

The Cxx committees try really had to make everything backwards compatible. This includes trying to not step on existing namespaces. These names, along with mtx, etc., are just going overboard though.

If the C committees want to avoid namespace collisions, why not use _Keyword style names like _Mutex_init and _Mutex_destroy?