Chapter 22. Expect As Just Another Tcl Extension

In this chapter, I will describe how to use Expect as just another extension to Tcl. You can wrap it together with popular Tcl extensions such as Tcl-DP, TkSteal, and others. There are two basic approaches you can take to doing this. You can add extensions to Expect, or you can add Expect as an extension to another Tcl-based program.

While most of the material in this chapter will be of interest only to C and C++ programmers, I will also mention a couple of differences in how Expect behaves and can be used when it is combined with other Tcl extensions.

Adding Expect To Another Tcl-based Program

This section describes how to add Expect to another Tcl-based program. I will use the tclsh program as an example. tclsh is the “Tcl shell” that comes with Tcl. tclsh comes with no other extensions, but you can use it as a template for creating a Tcl-based program with other extensions.

The Tcl source directory contains the template in tclAppInit.c. Copy this file to a new directory and look inside it at the function Tcl_AppInit. You will find the following lines:

if (Tcl_Init(interp) == TCL_ERROR) {
    return TCL_ERROR;
}

After this line, add code to initialize Expect:

if (Exp_Init(interp) == TCL_ERROR) {
    return TCL_ERROR;
}

You may want to add other extensions as well. Generally, you can add the extension initializations in any order unless they attempt to use the same command names. In that case, the later extensions “win”. The basic Tcl commands ...

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