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For each cell type it is generally up to the implementation of that type which of the corresponding cell entries hold Scheme objects and which hold raw C values. However, there is one basic rule that has to be followed: Scheme pairs consist of exactly two cell entries, which both contain Scheme objects. Further, a cell which contains a Scheme object in it first entry has to be a Scheme pair. In other words, it is not allowed to store a Scheme object in the first cell entry and a non Scheme object in the second cell entry.
Determine, whether the Scheme object x is a Scheme pair, i.e. whether x references a heap cell consisting of exactly two entries, where both entries contain a Scheme object. In this case, both entries will have to be accessed using the
SCM_CELL_OBJECTmacros. On the contrary, if theSCM_CONSPpredicate is not fulfilled, the first entry of the Scheme cell is guaranteed not to be a Scheme value and thus the first cell entry must be accessed using theSCM_CELL_WORD_0macro.