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Practical Programming in Tcl & Tk, Third Edition
By Brent B. Welch

Table of Contents
Part VI.  C Programming


Chapter 44. C Programming and Tcl

This chapter explains how to extend a Tcl application with new built-in commands. Tcl 8.0 replaces the original string-based command interface with a more efficient dual-ported object interface. This chapter describes both interfaces.

Tcl is implemented in a C library that is easy to integrate into an existing application. By adding the Tcl interpreter to your application, you can configure and control it with Tcl scripts, and with Tk you can provide a nice graphical interface to it. This was the original model for Tcl. Applications would be largely application-specific C code and include a small amount of Tcl for configuration and the graphical interface. However, the basic Tcl shells proved so useful by themselves that relatively few Tcl programmers need to worry about programming in C or C++.

Tcl is designed to be easily extensible by writing new command implementations in C. A command implemented in C is more efficient than an equivalent Tcl procedure. A more pressing reason to write C code is that it may not be possible to provide the same functionality purely in Tcl. Suppose you have a new device, perhaps a color scanner or a unique input device. The programming interface to that device is through a set of C procedures that initialize and manipulate the state of the device. Without some work on your part, that interface is not accessible to your Tcl scripts. You are in the same situation if you have a C or C++ library that implements some specialized function such as a database. Fortunately, it is rather straightforward to provide a Tcl interface that corresponds to the C or C++ interface.

Note: Where this chapter says "C", you can always think "C or C++". There is also a package called TclBlend that lets you extend Tcl by writing Java instead of C, and to evaluate Tcl scripts from Java. Find out more about TclBlend at:

http://www.scriptics.com/java/


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