SWI-Prolog -- API-function PL

Documentation
- Reference manual
  - Foreign Language Interface
    - The Foreign Include File
      - Calling Prolog from C
        
        Predicate references
        
        PL_pred()
        
        PL_predicate()
        
        PL_predicate_info()
- Packages

Availability:C-language interface function

predicate_t PL_predicate(const char *name, int arity, const char* module)

Same as PL_pred(), but provides a more convenient interface to the C programmer. If the module argument module is NULL, the current context module is used. The predicate_t handle may be stored as global data and reused for future queries^{227PL_predicate()
involves 5 hash lookups (two to get the atoms, one to get the module,
one to get the functor and the final one to get the predicate associated
with the functor in the module)} as illustrated below.

static predicate_t p = 0;

  ...
  if ( !p )
    p = PL_predicate("is_a", 2, "database");

Note that PL_cleanup() invalidates the predicate handle. Foreign libraries that use the above mechanism must implement the module uninstall() function to clear the predicate_t global variable.