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18.3.6 Garbage Collecting Simple Smobs

It is often useful to define very simple smob types — smobs which have no data to mark, other than the cell itself, or smobs whose first data word is simply an ordinary Scheme object, to be marked recursively. Guile provides some functions to handle these common cases; you can use this function as your smob type's mark function, if your smob's structure is simple enough.

If the smob refers to no other Scheme objects, then no action is necessary; the garbage collector has already marked the smob cell itself. In that case, you can use zero as your mark function.

— C Function: SCM scm_markcdr (SCM x)

Mark the references in the smob x, assuming that x's first data word contains an ordinary Scheme object, and x refers to no other objects. This function simply returns x's first data word.

This is only useful for simple smobs created by SCM_NEWSMOB or SCM_RETURN_NEWSMOB, not for smobs allocated as double cells.

— C Function: size_t scm_free0 (SCM x)

Do nothing; return zero. This function is appropriate for smobs that use either zero or scm_markcdr as their marking functions, and refer to no heap storage, including memory managed by malloc, other than the smob's header cell.

This function should not be needed anymore, because simply passing NULL as the free function does the same.