Each argument of a foreign function (except for the control argument) is of type term_t, an opaque handle to a Prolog term. Three groups of functions are available for the analysis of terms. The first just validates the type, like the Prolog predicates var1, atom1, etc and are called PL_is_*(). The second group attempts to translate the argument into a C primitive type. These predicates take a term_t and a pointer to the appropriate C-type and return TRUE or FALSE depending on successful or unsuccessful translation. If the translation fails, the pointed-to data is never modified.