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A Scheme object of type SCM that does not fulfill the
SCM_IMP predicate holds an encoded reference to a heap cell.
This reference can be decoded to a C pointer to a heap cell using the
SCM2PTR macro. The encoding of a pointer to a heap cell into a
SCM value is done using the PTR2SCM macro.
Extract and return the heap cell pointer from a non-immediate
SCMobject x.
Return a
SCMvalue that encodes a reference to the heap cell pointer x.
Note that it is also possible to transform a non-immediate SCM
value by using SCM_UNPACK into a scm_t_bits variable.
However, the result of SCM_UNPACK may not be used as a pointer to
a scm_t_cell: only SCM2PTR is guaranteed to transform a
SCM object into a valid pointer to a heap cell. Also, it is not
allowed to apply PTR2SCM to anything that is not a valid pointer
to a heap cell.
Summary:
SCM2PTR on SCM values for which SCM_IMP is
false!
(scm_t_cell *) SCM_UNPACK (x)! Use SCM2PTR
(x) instead!
PTR2SCM for anything but a cell pointer!