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Smobs are called smob because they are small: they normally have only
room for one void* or SCM value plus 16 bits. The
reason for this is that smobs are directly implemented by using the
low-level, two-word cells of Guile that are also used to implement
pairs, for example. (see Data Representation for the details.)
One word of the two-word cells is used for SCM_SMOB_DATA (or
SCM_SMOB_OBJECT), the other contains the 16-bit type tag and
the 16 extra bits.
In addition to the fundamental two-word cells, Guile also has
four-word cells, which are appropriately called double cells.
You can use them for double smobs and get two more immediate
words of type scm_t_bits.
A double smob is created with SCM_NEWSMOB2 or
SCM_NEWSMOB3 instead of SCM_NEWSMOB. Its immediate
words can be retrieved as scm_t_bits with
SCM_SMOB_DATA_2 and SCM_SMOB_DATA_3 in addition to
SCM_SMOB_DATA. Unsurprisingly, the words can be set to
scm_t_bits values with SCM_SET_SMOB_DATA_2 and
SCM_SET_SMOB_DATA_3.
Of course there are also SCM_SMOB_OBJECT_2,
SCM_SMOB_OBJECT_3, SCM_SET_SMOB_OBJECT_2, and
SCM_SET_SMOB_OBJECT_3.