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b_external(++PredSpec, +CName)

Defines PredSpec to be a nondeterministic external predicate linked to the C function whose system name is CName.
Of the form Atom/Integer (predicate name/arity).
Atom or a string.

This built-in predicate is obsolete!


Declares the PredSpec to be a non-deterministic Prolog predicate (in the caller module) linked to the ``C'' function whose system name is CName.

If the visibility of PredSpec is not declared, it is set to local.

If necessary, an underscore is prepended to CName to get its form as used by the C compiler.

If a call to PredSpec has already been compiled as a deterministic external call, error 62 is raised (``inconsistent procedure redefinition''). This can be prevented by defining the external before compiling any call to it or by using the declaration predicate b_external/1.

Modes and Determinism


(4) instantiation fault
Either PredSpec or CName is not instantiated.
(5) type error
PredSpec is not of the form Atom/Integer.
(5) type error
CName is not an atom or a string.
(62) inconsistent procedure redefinition
A call to PredSpec has already been compiled as a deterministic external call.
(211) External function does not exist
External function does not exist.


   % file to create an external predicate.
      % cat sin.c

   % external.h contains the macros for the external interface.
      #include        "external.h"
      #include        <math.h>

      p_sines(vel, tel, vlist, tlist)
      value           vel, vlist;
      type            tel, tlist;
              pword *p;

              if (IsNil(tlist))
              p = vlist.ptr + 1;
              Remember(2, p->val, p->tag);
              Return_Unify_Float(vel, tel,

   % compile with ECLiPSe include files.
      % cc -c -I/usr/local/ECLIPSE/include sin.c
      % eclipse

   % load the .o file dynamically into the system with math option.
      [eclipse]: load('sin.o',"-lm").

   % link the object file with a predicate definition.
      [eclipse]: b_external(sines/2,p_sines).

   % check on existence and flags of sines/2.
      [eclipse]: get_flag(sines/2, type, T),
              get_flag(sines/2, call_type, C_type),
              get_flag(sines/2, visibility, Vis).
      T = user
      C_type = b_external
      Vis = local     More? (;)

   % use sines/2.
      [eclipse]: sines(E,[0.0,45.0,90.0,270.0]).
      E = 0.0     More? (;)
      E = 0.707107     More? (;)
      E = 1.0     More? (;)
      E = -1.0     More? (;)
      no (more) solution.

      b_external(p/0, S).             (Error 4).
      b_external(PredSpec, p_pred).   (Error 4).
      b_external("p/0", p_p0).        (Error 5).
      b_external(p/0, 123).           (Error 5).

      [eclipse]: [user].
       :- external(a/0, c_a). % should use b_external/1.
       p :- a.
       user   compiled 60 bytes in 0.00 seconds
      [eclipse]: b_external(a/0, c_a).  (Error 62).

      b_external(mess/1,"p_messg").   (Error 211).
% call load/1,2 first.

See Also

external / 1, external / 2, b_external / 1, load / 1