int lua_yield  (lua_State *L, int nresults);

Yields a coroutine.

This function should only be called as the return expression of a C function, as follows: 

     return lua_yield (L, nresults);

When a C function calls lua_yield in that way, the running coroutine suspends its execution, and the call to lua_resume that started this coroutine returns. The parameter nresults is the number of values from the stack that are passed as results to lua_resume.