ios – Why Do libdispatch Macros Seem Empty in `_dispatch_client_callout`?

I’m studying the supply code of libdispatch and got here throughout the implementation of the _dispatch_client_callout perform as follows:

#outline _dispatch_get_tsd_base()
#outline _dispatch_get_unwind_tsd() (NULL)
#outline _dispatch_set_unwind_tsd(u) do {(void)(u);} whereas (0)
#outline _dispatch_free_unwind_tsd()

#outline possible(x) __builtin_expect(!!(x), 1)
#outline unlikely(x) __builtin_expect(!!(x), 0)

void
_dispatch_client_callout(void *ctxt, dispatch_function_t f)
{
    _dispatch_get_tsd_base();
    void *u = _dispatch_get_unwind_tsd();
    if (possible(!u)) return f(ctxt);
    _dispatch_set_unwind_tsd(NULL);
    f(ctxt);
    _dispatch_free_unwind_tsd();
    _dispatch_set_unwind_tsd(u);
}

Within the _dispatch_client_callout perform, a sequence of macros are invoked, however these macros appear to have empty implementations, equivalent to:

#outline _dispatch_get_tsd_base()
#outline _dispatch_get_unwind_tsd() (NULL)
#outline _dispatch_set_unwind_tsd(u) do {(void)(u);} whereas (0)
#outline _dispatch_free_unwind_tsd()

Why is that this the case? What ought to the precise implementations of those macros appear like?