Warning: This is the manual of the legacy Guile 2.0 series. You may want to read the manual of the current stable series instead.
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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_new_double_smob. 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.