Exceptions from C

Consider, for example, a function which returns the square root of its argument after checking that the argument is valid. If the argument is invalid, the function should raise an exception instead.

     /* math.c */
     
     #include <math.h>
     #include <stdio.h>
     #include <sicstus/sicstus.h>
     
     extern double sqrt_check(double d);
     double sqrt_check(double d)
     {
       if (d < 0.0)
         {    /* build a domain_error/4 exception term */
           SP_term_ref culprit=SP_new_term_ref();
           SP_term_ref argno=SP_new_term_ref();
           SP_term_ref expdomain=SP_new_term_ref();
           SP_term_ref t1=SP_new_term_ref();
     
           SP_put_float(culprit, d);
           SP_put_integer(argno, 1);
           SP_put_string(expdomain, ">=0.0");
           SP_cons_functor(t1, SP_atom_from_string("sqrt"), 1, culprit);
           SP_cons_functor(t1, SP_atom_from_string("domain_error"), 4,
                           t1, argno, expdomain, culprit);
           SP_raise_exception(t1);    /* raise the exception */
           return 0.0;
         }
       return sqrt(d);
     }
     

The Prolog interface to this function is defined in a file math.pl. The function uses the sqrt() library function, and so the math library -lm has to be included:

     /* math.pl */
     
     foreign_resource(math, [sqrt_check]).
     
     foreign(sqrt_check, c, sqrt(+float, [-float])).
     
     :- load_foreign_resource(math).
     

A linked foreign resource is created:

     % splfr math.pl math.c -lm
     

A simple session using this function could be:

     % sicstus
     SICStus 3.9 (sparc-solaris-5.7): Thu Aug 19 16:25:28 MET DST 1999
     Licensed to SICS
     | ?- [math].
     % consulting /home/san/pl/math.pl...
     % /home/san/pl/math.pl consulted, 10 msec 816 bytes
     
     yes
     | ?- sqrt(5.0,X).
     
     X = 2.23606797749979 ?
     
     yes
     | ?- sqrt(a,X).
     ! Type error in argument 1 of user:sqrt/2
     ! number expected, but a found
     ! goal:  sqrt(a,_143)
     
     | ?- sqrt(-5,X).
     ! Domain error in argument 1 of user:sqrt/1
     ! expected '>=0.0', found -5.0
     ! goal:  sqrt(-5.0)
     

The above example used the foreign language interface with dynamic linking. To statically link math.s.o with the Prolog emulator, the following steps would have been taken:

     % splfr -S math.pl math.c -lm
     SICStus 3.9 (sparc-solaris-5.7): Thu Aug 19 16:25:28 MET DST 1999
     Licensed to SICS
     % spXxQwsr.c generated, 0 msec
     
     yes
     % spld -D -o mathsp --resources=./math.s.o
     SICStus 3.9 (sparc-solaris-5.7): Thu Aug 19 16:25:28 MET DST 1999
     Licensed to SICS
     % spYdLTgi1.c generated, 0 msec
     
     yes
     Created "mathsp"
     % ./mathsp
     SICStus 3.9 (sparc-solaris-5.7): Thu Aug 19 16:25:28 MET DST 1999
     Licensed to SICS
     | ?- [math].
     % consulting /a/filur/export/labs/isl/sicstus/jojo/sicstus38p/math.pl...
     % consulted /a/filur/export/labs/isl/sicstus/jojo/sicstus38p/math.pl in module user, 0 msec 960 bytes
     
     yes
     | ?- sqrt(5.0,X).
     
     X = 2.23606797749979 ?
     
     yes