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The first step in garbage collection is to mark all heap objects in use. Each heap cell has a bit reserved for this purpose. For pairs (cons cells) the lowest order bit (0) of the CDR is used. For other types, bit 8 of the CAR is used. The GC bits are never set except during garbage collection. Special C macros are defined in scm.h to allow easy manipulation when GC bits are possibly set. CAR, TYP3, and TYP7 can be used on GC marked cells as they are.

— Macro: GCCDR x

Returns the CDR of a cons cell, even if that cell has been GC marked.

— Macro: GCTYP16 x

Returns the 16 bit type code of a cell.

We need to (recursively) mark only a few objects in order to assure that all accessible objects are marked. Those objects are sys_protects[] (for example, dynwinds), the current C-stack and the hash table for symbols, symhash.

— Function: void gc_mark (SCM obj)

The function gc_mark() is used for marking SCM cells. If obj is marked, gc_mark() returns. If obj is unmarked, gc_mark sets the mark bit in obj, then calls gc_mark() on any SCM components of obj. The last call to gc_mark() is tail-called (looped).

— Function: void mark_locations (STACKITEM x[], sizet len)

The function mark_locations is used for marking segments of C-stack or saved segments of C-stack (marked continuations). The argument len is the size of the stack in units of size (STACKITEM).

Each longword in the stack is tried to see if it is a valid cell pointer into the heap. If it is, the object itself and any objects it points to are marked using gc_mark. If the stack is word rather than longword aligned (#define WORD_ALIGN), both alignments are tried. This arrangement will occasionally mark an object which is no longer used. This has not been a problem in practice and the advantage of using the c-stack far outweighs it.