A pointer to the memory used for the function’s return value.
If the function is declared as returning void, then this value
is garbage and should not be used.
Otherwise, fun must fill the object to which this points,
following the same special promotion behavior as ffi_call.
That is, in most cases, ret points to an object of exactly the
size of the type specified when cif was constructed. However,
integral types narrower than the system register size are widened. In
these cases your program may assume that ret points to an
ffi_arg object.