How to not lose all your money in the 'slot machine'

Section 21.6 in the YUM states (sic) ...

long int YAP_NewSlots(int NumberOfSlots)
    Allocate NumberOfSlots from the stack and return an handle to the 
last one.
    Th eother hand can be obtained by decrementing the handle.

... so I implemented ...

    long int sl = YAP_NewSlots(2);
    YAP_PutInSlot(sl,YAP_ARG5);
    YAP_PutInSlot(sl-1,YAP_ARG4); // handle decremented as documented
    ...do prolog callback...
    YAP_Term x4 = YAP_GetFromSlot(sl-1); // handle decremented as documented
    YAP_Term x5 = YAP_GetFromSlot(sl);
    YAP_RecoverSlots(2);

... and all hell broke loose ...

The 'sl-1' slot never had a valid value and after swapping the arguments 
I had an 'aha moment' when it was still the 'sl-1' slot that was wrong. 
Changed 'sl-1' to 'sl+1' and it's now ok.

Regards,
Tony,
Durban, South Africa

PS I will soon be releasing 'tinka' (initially x86 only) - an open 
source glue framework for using Yap (and later others) with any shared 
library api, including callbacks. Hope this whets a few appetites and 
nobody considers it spam.

Example of GTK+2 Tutorial 'Hello World' example in tinka_for_yap 
(original C at end):

destroy(_,_) :-
    gtk_main_quit.

delete_event(_,_,_,1) :-
    g_print('delete event occurred\n').

hello(_,_) :-
    g_print('Hello World\n').

:-  % GTK tut Hello World
    argCVAddress(AC,AV),
    gtk_init(AC,AV),
    gtk_window_new( topLevel,W),
    g_signal_connect(W,delete_event,delete_event/3+1,null),
%
% for example the individually loadable definitions for the above API 
call are:
%
%     api(g_signal_connect_data,'libgobject-2.0.so',
%        
[gPointer,pStr,cDecl(gCallback),gPointer,gClosureNotify,in=gConnect],
%        gULong,[]/*return code check - default to fail on null*/).
%     g_signal_connect(A,B,C,D) :-
%        g_signal_connect_data(A,B,C,D,0,0).
%     gConnect( after, 1).
%     gConnect( swapped, 2).
%     ... various gXxx type definitions
%
% all the rest is dynamic at execution time as traced below
%     (Note that 'git' refers to 'gist in time'; not git the best 
revision control system)
%
%     % git - g_signal_connect/4
%     %   found gist - g_signal_connect/4
%     g_signal_connect(A,B,C,D) :-
%        g_signal_connect_data(A,B,C,D,0,0).
%     % git - g_signal_connect_data/6
%     %   found api - g_signal_connect_data
%     :- 
assertz(dll_(g_signal_connect_data,'libgobject-2.0.so',[iaciiivz,iaiiiivz])).
%     g_signal_connect_data(A,B,C,D,E,F) :-
%        g_signal_connect_data(A,B,C,D,E,F,_).
%     g_signal_connect_data(A,B,C,D,E,F,G) :-
%        lookup(gConnect,F,H),
%        dllCall(g_signal_connect_data,(A,B,C,D,E,H),G).
%     % git - gConnect/2
%     %   found gist - gConnect/2
%     gConnect(after,1).
%     gConnect(swapped,2).
%     % 
api(g_signal_connect_data,(135690240,delete_event,delete_event/3+1,null,0,0),_2906).
%     %    apiIn(iaciiivz).
%     %    
apiOut(iaciiivz,(135690240,delete_event,delete_event/3+1,null,0,0),6).
%
    g_signal_connect(W,destroy,destroy/2,null),
    gtk_container_set_border_width(W,10),
    gtk_button_new_with_label('Hello World',B),
    g_signal_connect(B,clicked,hello/2,null),
    g_signal_connect_swapped(B,clicked,gtk_widget_destroy/1,W),
    gtk_container_add(W,B),
    gtk_widget_show(B),
    gtk_widget_show(W),
    gtk_main.

/*
#include <gtk/gtk.h>

static void hello( GtkWidget *widget, gpointer   data )
{
    g_print ("Hello World\n");
}

static gboolean delete_event( GtkWidget *widget, GdkEvent  *event, 
gpointer   data )
{
    g_print ("delete event occurred\n");
    return TRUE;
}

static void destroy( GtkWidget *widget, gpointer   data )
{
    gtk_main_quit ();
}

int main( int   argc, char *argv[] )
{
    GtkWidget *window;
    GtkWidget *button;

    gtk_init (&argc, &argv);
    window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
    g_signal_connect (G_OBJECT (window), "delete_event", G_CALLBACK 
(delete_event), NULL);
    g_signal_connect (G_OBJECT (window), "destroy", G_CALLBACK 
(destroy), NULL);
    gtk_container_set_border_width (GTK_CONTAINER (window), 10);
    button = gtk_button_new_with_label ("Hello World");
    g_signal_connect (G_OBJECT (button), "clicked", G_CALLBACK (hello), 
NULL);
    g_signal_connect_swapped (G_OBJECT (button), "clicked", G_CALLBACK 
(gtk_widget_destroy), G_OBJECT (window));
    gtk_container_add (GTK_CONTAINER (window), button);
    gtk_widget_show (button);
    gtk_widget_show (window);
    gtk_main ();
    return 0;
}
*/

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