Variables can be modified through the foreign C interface

[Vésy Jordan]

Hello,

on GNU Prolog 1.5.0 tested on Arch Linux with kernel 5.17.8,

with

---------- bugged.pl ----------

:- foreign(magic(term)).

go(X) :-

    X = value(0),

    magic(X).

-------------------------------

and

---------- bugged.c ----------#include "gprolog.h"
PlBool magic(PlTerm term) {
    int functor, arity;
    PlTerm *args = Pl_Rd_Compound_Check(term, &functor, &arity);
    args[0] = Pl_Mk_Number(1);
    return PL_TRUE;
}
------------------------------

The value of the X variable is modified like this:

-----------------------------​| ?- go(X).

X = value(1)
-----------------------------

Shouldn't it either fail or leave X at "value(0)" instead of modifying its value ?

Thanks,

Jordan Vésy