11.4.3 mark_object

The first thing that is checked while marking an object is whether the object is a real Lisp object Lisp_Type_Record or just an integer or a character. Integers and characters are the only two types that are stored directly - without another level of indirection, and therefore they don’t have to be marked and collected. See How Lisp Objects Are Represented in C.

The second case is the one we have to handle. It is the one when we are dealing with a pointer to a Lisp object. But, there exist also three possibilities, that prevent us from doing anything while marking: The object is read only which prevents it from being garbage collected, i.e. marked (C_READONLY_RECORD_HEADER). The object in question is already marked, and need not be marked for the second time (checked by MARKED_RECORD_HEADER_P). If it is a special, unmarkable object (UNMARKABLE_RECORD_HEADER_P, apparently, these are objects that sit in some const space, and can therefore not be marked, see this_one_is_unmarkable in alloc.c).

Now, the actual marking is feasible. We do so by once using the macro MARK_RECORD_HEADER to mark the object itself (actually the special flag in the lrecord header), and calling its special marker "method" marker if available. The marker method marks every other object that is in reach from our current object. Note, that these marker methods should not call mark_object recursively, but instead should return the next object from where further marking has to be performed.

In case another object was returned, as mentioned before, we reiterate the whole mark_object process beginning with this next object.