SWI-Prolog -- Manual

Documentation
- Reference manual
  - Foreign Language Interface
    - The Foreign Include File
      - Analysing Terms via the Foreign Interface
        
        Testing the type of a term
        
        Reading data from a term
        
        Exchanging text using length and string
        
        Wide-character versions
        
        PL_new_atom_wchars()
        
        PL_atom_wchars()
        
        PL_get_wchars()
        
        PL_unify_wchars()
        
        PL_unify_wchars_diff()
        
        Reading a list
        
        An example: defining write/1 in C
- Packages

h4 id="sec:foreign-unicode">12.4.3.4 Wide-character versions

Support for exchange of wide-character strings is still under consideration. The functions dealing with 8-bit character strings return failure when operating on a wide-character atom or Prolog string object. The functions below can extract and unify both 8-bit and wide atoms and string objects. Wide character strings are represented as C arrays of objects of the type pl_wchar_t, which is guaranteed to be the same as wchar_t on platforms supporting this type. For example, on MS-Windows, this represents 16-bit UCS2 characters, while using the GNU C library (glibc) this represents 32-bit UCS4 characters.

atom_t PL_new_atom_wchars(size_t len, const pl_wchar_t *s): Create atom from wide-character string as PL_new_atom_nchars() does for ISO-Latin-1 strings. If s only contains ISO-Latin-1 characters a normal byte-array atom is created. If len is (size_t)-1, it is computed from s using wcslen().
pl_wchar_t* PL_atom_wchars(atom_t atom, int *len): Extract characters from a wide-character atom. Succeeds on any atom marked as‘text'. If the underlying atom is a wide-character atom, the returned pointer is a pointer into the atom structure. If it is an ISO-Latin-1 character, the returned pointer comes from Prolog's‘buffer ring' (see PL_get_chars()).
int PL_get_wchars(term_t t, size_t *len, pl_wchar_t **s, unsigned flags): Wide-character version of PL_get_chars(). The flags argument is the same as for PL_get_chars().
int PL_unify_wchars(term_t t, int type, size_t len, const pl_wchar_t *s): Unify t with a textual representation of the C wide-character array s. The type argument defines the Prolog representation and is one of PL_ATOM, PL_STRING, PL_CODE_LIST or PL_CHAR_LIST.
int PL_unify_wchars_diff(term_t +t, term_t -tail, int type, size_t len, const pl_wchar_t *s): Difference list version of PL_unify_wchars(), only supporting the types PL_CODE_LIST and PL_CHAR_LIST. It serves two purposes. It allows for returning very long lists from data read from a stream without the need for a resizing buffer in C. Also, the use of difference lists is often practical for further processing in Prolog. Examples can be found in packages/clib/readutil.c from the source distribution.