Why was ucontext added to and then removed from POSIX?

Question

As far as I know, ucontext offers something better then setjmp. But it's deprecated and now removed from the POSIX spec. So why did it appear and why was it removed?

Answer 1

The signature of makecontext from ucontext.h is:

void makecontext(ucontext_t *ucp, void (*func)(),
       int argc, ...);

Note that func uses an empty parenthesis as argument, but this is a deprecated feature in standard C:

C11(ISO/IEC 9899:201x) §6.11.6 Function declarators

The use of function declarators with empty parentheses (not prototype-format parameter type declarators) is an obsolescent feature.

This is the reason it's deprecated (and somehow unfortunate).