Updated 2012-01-16 05:00:02 by RLE

First understand the difference between embedding vs. extending. If you decide that embedding is the approach you want, this page is meant to record advice about best practices.

In this example we will create a shell program that embeds Tcl and an extension by the name foo. This type of program is sometimes known as a big wish.

A Tcl interpreter (literally, a Tcl_Interp*) gets extended by a Tcl extension by being passed to one of two initialization routines provided by the extension library. For the foo extension, the names of these initialization routines are Foo_Init and Foo_SafeInit. Normal, or trusted interpreters are passed to the Foo_Init routine, while safe interpreters are passed to the Foo_SafeInit routine.

Since we have chosen embedding in this example, we will directly call the Foo_Init routine on the master interpreter of our big wish (foosh?) that contains foo.

Here is a sketch of what the C code for our big wish program should look like:
  #include <tcl.h>
  #include "foo.h"

  static int
  Foosh_AppInit(Tcl_Interp *interp) {
      if (Tcl_Init(interp) == TCL_ERROR) {
          return TCL_ERROR;
      if (Foo_Init(interp) == TCL_ERROR) {
          return TCL_ERROR;
      Tcl_StaticPackage(interp, "Foo", Foo_Init, Foo_SafeInit);
      Tcl_SetVar(interp, "tcl_rcFileName", "~/.fooshrc", TCL_GLOBAL_ONLY);
      return TCL_OK;

  main(int argc, char **argv) {
      Tcl_Main(argc, argv, Foosh_AppInit);

See also embedding vs. extending