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18.2.7.3 Non-immediate objects

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.

(scm_t_cell *) SCM2PTR (SCM x)

Macro

Extract and return the heap cell pointer from a non-immediate SCM object x.

SCM PTR2SCM (scm_t_cell * x)

Macro

Return a SCM value 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:

• Only use SCM2PTR on SCM values for which SCM_IMP is false!
• Don't use (scm_t_cell *) SCM_UNPACK (x)! Use SCM2PTR (x) instead!
• Don't use PTR2SCM for anything but a cell pointer!