See also: [Adding Tk to an Existing Xt Program]
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[George Peter Staplin]: The code that follows creates an Xt button and a Tk button, in the same application.

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  #include <stdio.h>
  #include <stdlib.h>
  #include <X11/Intrinsic.h> 
  #include <X11/StringDefs.h> 
  #include <X11/Xaw/Command.h>  
  #include <tcl.h>
  #include <tk.h>

  Tcl_Interp *interp;
		
  void Action (Widget w, XtPointer client_data, XtPointer call_data) { 
  fprintf (stderr, "You pressed me.\n");
  exit (0); 
  }


  int main (int argc, char *argv[]) {
  XtAppContext appContext;
  Widget win, button;
  XEvent event;
  int i = 1;

	char sAppInit[] = "pack [button .b -text \"Hello\"]";
	
	win = XtVaAppInitialize(
		&appContext,          
		"TclTkXt",           
		NULL, 
		0,                
		&argc, 
		argv,            
		NULL,                   
		NULL);                  

	button = XtVaCreateManagedWidget (
		"XtButton",              
		commandWidgetClass,     
		win,               
		NULL);                  
		
  interp = Tcl_CreateInterp ();

  Tcl_FindExecutable (argv[0]); /* Does Tcl_FindExecutable behave better *before* Tcl_CreateInterp()? */

	if (Tcl_Init (interp) != TCL_OK) {
	fprintf (stderr, "Tcl_Init error"); 
	exit (1);
	}

	if (Tk_Init (interp) != TCL_OK) {
	fprintf (stderr, "Tk_Init error");
	exit (1);
	}

  

  XtAddCallback (button, XtNcallback, Action, 0);
  XtRealizeWidget (win);
	
	if (Tcl_Eval (interp, sAppInit) != TCL_OK) {
	fprintf (stderr, "Tcl_Eval error %s", Tcl_GetStringResult (interp));
	}

	for (;;) {

		if (XtAppPending (appContext) > 0) {
		XtAppNextEvent (appContext, &event);
		XtDispatchEvent (&event);	
		} else {
		Tcl_DoOneEvent (TCL_DONT_WAIT);
		usleep (200);
		}
		
	}
	
  return (0);
  }
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Here's a package that contains a Makefile and the code above: http://www.xmission.com/~georgeps/TclTkXt.tgz  You may need to change the Makefile string -ltcl83 to -ltcl8.3 and Linux users need to add -ldl.

Have fun!
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"[Event-oriented programming]"
has background information.