Simple Tcl Example

This example illustrates how you can hook Tcl to a very simple C program containing a function and a global variable.

The C Code

/* File : example.c */

/* A global variable */
double Foo = 3.0;

/* Compute the greatest common divisor of positive integers */
int gcd(int x, int y) {
  int g;
  g = y;
  while (x > 0) {
    g = x;
    x = y % x;
    y = g;
  }
  return g;
}

The SWIG interface

/* File: example.i */
%module example

extern int gcd(int x, int y);
extern double Foo;

Compilation

1. swig -tcl example.i

2. Compile example_wrap.c and example.c to create the extension example.so.

Using the extension

Key points

• Use the load statement to load your extension module into Tcl. For example:

  load ./example.so
  

• C functions work just like Tcl functions. For example:

  set g [gcd 42 105]
  

• C global variables are accessed as Tcl variables. For example:

  set a $Foo
  set Foo $newvalue