Have a section of callback pointers and check against the bounds of the section? G++ uses this mechanism for static constructors, but it's general purpose (Linux uses it to collect lists of drivers to initialize, for instance, with macros like IRQCHIP_DECLARE that populate a section for each type of entity, which then gets scanned at boot time).

You would need a linker script to collect the callbacks into a section and provide a symbol for the end, and define variables something like

int (my_sort_callback_ptr)(void , void *) __attribute__((section, "sort_callbacks"))=my_sort_comparator;

You would, of course, use a macro for that.