POSIX pre-2008 had <ucontext.h> with proper stack switching functions, used as a fallback in a number of coroutine libraries. The fallback status is due to a self-inflicted flaw: they save and restore the signal mask, thus still need to go through the kernel (why e.g. Linux does not put signal mask manipulation in the vDSO, I don’t know). POSIX yanked them and now recommends that “applications be rewritten to use POSIX threads instead”, which is asinine.