Consider, for example, a function returning 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.plforeign_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 | ?- sqrt(5.0,X). X = 2.23606797749979 | ?- 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 % 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 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 | ?- sqrt(5.0,X). X = 2.23606797749979