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Re: Segfault on X = [a,X].


From: Daniel Diaz
Subject: Re: Segfault on X = [a,X].
Date: Sat, 24 Mar 2012 20:04:46 +0100

This is not a bug.

X = [a,X] creates a cyclic term during the unification (this happens when a variables is unified with a compound term which contains this variable as subterm). Gnu Prolog does not handle cyclic terms. The standard unification does not perform the occurs check for efficiency reasons. There is a a predicate doing this test: unify_with_occurs_check.

unify_with_occurs_check(X, [a,X]) will fail.

Daniel


Le 23 mars 2012 à 15:24, Konstantin Weitz a écrit :

gprolog segfaults on X = [a,X].
  • The type of machine you are using (use uname -a under Unix).
    Linux open-client 3.0.0-16-generic-pae #29-Ubuntu SMP Tue Feb 14 13:56:31 UTC 2012 i686 i686 i386 GNU/Linux

  • GNU Prolog version (use gprolog --version).
    Prolog top-Level (GNU Prolog) 1.4.1
    By Daniel Diaz
    Copyright (C) 1999-2012 Daniel Diaz

  • The precise commands we need to type to reproduce the bug.
    address@hidden:~/gprolog-1.4.1/src/TopComp$ ./gprolog
    GNU Prolog 1.4.1
    By Daniel Diaz
    Copyright (C) 1999-2012 Daniel Diaz
    | ?- X = [a,X].
    Segmentation fault


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