Fatal Error: Segmentation Violation

Hi all,

I'm trying to call my prolog program from C, but am having trouble.
The program itself takes a sentence in English and parses it out into
something like:

sentence(subject(np(det(the),noun(boy))),predicate(vp(intrans_v(gives))))

To do this, in my C program, I call read_line/1 which reads the line
and puts each word in a list that is given as the argument:
[the,boy,gives,'.']

(I found read_line/1 at
http://www.csupomona.edu/~jrfisher/www/prolog_tutorial/read_linepl.txt
and modified it to work with gprolog)

My C code does the following to get that input and copy it to another
PlTerm* called sentence to use as the argument in a later call:
  Pl_Query_Begin(TRUE);
  input[0] = Mk_Variable();
  Pl_Query_Call(Find_Atom("read_line"), 1, input);
  Copy_Term(&sentence[1],input);
  Pl_Query_End(PL_CUT);
  Pl_Query_Begin(TRUE);
  sentence[0] = Mk_String(str); /* sentence[0] = Parse_form */

The prolog predicate c_parse/2 looks like this:
c_parse(Parse_form,Input) :-
         trim_period(Input,I),
         (s(Parse_form,I,[]);comp_s(Parse_form,I,[])).

Parse_form is the result which would look like the output I gave
above. Input is the list. trim_period/2 removes the period at the end
of the list and leaves the result in the term I. s/3 and comp_s/3
parses I and leaves the result in Parse_form.

At this point in the C program, i call
  parsed = Pl_Query_Call(Find_Atom("c_parse"), 2, sentence);
and i get
Fatal Error: Segmentation Violation_#219430624
and the program crashes.

I have no idea what is going on. Can anyone help?

thanks in advance,

CB