swigweb/tutorial.ht

SWIG Tutorial

<p>
<img src="images/tutorial.png">

<p>
So you want to get going in a hurry?  To illustrate the use of SWIG,
suppose you have some C functions you want added to Tcl, Perl, and Python.
Specifically, let's say you have them in a file 'example.c'

<ul><tt><pre>
/* File : example.c */

#include &lt;time.h&gt
double My_variable = 3.0;

int fact(int n) {
    if (n &lt;= 1) return 1;
    else return n*fact(n-1);
}

int my_mod(int x, int y) {
    return (x%y);
}
	
char *get_time()
{
    time_t ltime;
    time(&amp;ltime);
    return ctime(&amp;ltime);
}

</pre></tt></ul>

<h3> Interface file </h3>

Now, in order to add these files to your favorite scripting language, you need to write an
"interface file" which is the input to SWIG.  An interface file for these
C functions might look like this :

<ul><tt><pre>
/* example.i */
%module example
%{
/* Put header files here (optional) */
%}

extern double My_variable;
extern int fact(int n);
extern int my_mod(int x, int y);
extern char *get_time();
</pre></tt></ul>

<h3> Building a Tcl module </h3>

At the UNIX prompt, type the following (shown for Linux):

<blockquote>
<pre><tt>
unix % swig -tcl example.i
Making wrappers for Tcl
unix % gcc -fpic -c example.c example_wrap.c \
       -I/usr/local/include 
unix % gcc -shared example.o example_wrap.o -o example.so
unix % tclsh
% load ./example.so example
% puts $My_variable
3.0
% fact 5
120
% my_mod 7 3
1
% get_time
Sun Feb 11 23:01:07 1996

% 
</tt></pre></blockquote>

The <tt> swig </tt> command produces a file <a href = "tutorial/example_wrap.html">
<tt> example_wrap.c </tt> </a> that should be compiled and linked with
the rest of the program.  In this case, we have built a dynamically 
loadable extension that can be loaded into the Tcl interpreter using
the 'load' command.   

<p>
If your machine does not support dynamic loading, it is also easy to 
build a new version of the tclsh interpreter as follows :

<blockquote>
<pre><tt>
unix % swig -tcl -ltclsh.i example.i
unix % gcc example.c example_wrap.c -I/usr/local/include \
       -L/usr/local/lib -ltcl -lsocket -ldl -lm -o my_tclsh
unix % my_tclsh
% puts $My_variable
3.0
% fact 5
120
%
</tt></pre></blockquote>

In this case, the new version of
tclsh is functionally identical to the original, but has new functions
added to it.

<h3> Building a Python module </h3>

Turning C code into a Python module is also easy.  Simply do the following (shown for Irix):

<blockquote> <tt> <pre>

unix % swig -python example.i
Making wrappers for Python
unix % gcc -c example.c example_wrap.c \
       -I/usr/local/include/python1.4 \
       -I/usr/local/lib/python1.4/config
unix % ld -shared example.o example_wrap.o -o examplemodule.so 
</pre> </tt> </blockquote>

We can now use the Python module as follows : 
<blockquote> <tt> <pre>
>>> import example
>>> example.fact(5)
120
>>> example.my_mod(7,3)
1
>>> example.get_time()
'Sun Feb 11 23:01:07 1996'
>>>
</pre>
</tt> </blockquote>

<h3> Building a Perl module </h3>
Finally, you can build a Perl5 module as follows (shown for Solaris):

<blockquote><tt><pre>
unix % swig -perl5 example.i
Making wrappers for Perl5
unix % gcc -c example.c example_wrap.c \
       -I/usr/lib/perl/solaris/5.003/CORE 
unix % ld -G example.o example_wrap.o -o example.so
unix % perl
use example;
print $example::My_variable,"\n";
print example::fact(5),"\n";
print example.get_time(),"\n";
&lt;ctrl-d&gt;
3.0
120
Sun Feb 11 23:01:07 1996
unix % 
</pre></tt></blockquote>


<h3> SWIG for the truly lazy </h3>

As it turns out, it is not always necessary to write a special interface
file.  If your C code is relatively clean, you can just run SWIG directly
on the source like this :

<blockquote>
<tt> <pre>
unix % swig -tcl -module example example.c
unix % gcc -c example.c example_wrap.c -I/usr/local/include
unix % ld -shared example.o example_wrap.o -o example.so
</pre></tt></blockquote>

<h3> Adding Documentation </h3>

Documentation can now be added using C/C++ comments.  For example :

<ul><tt><pre>
/* example.i */

%title "Simple Example"
%{
/* Put header files here */
%}

%section "My Commands"
extern double My_variable; // This is an interesting variable
extern int fact(int n);    // Computes n factorial
extern int my_mod(int x, int y); // Calculates x % y
extern char *get_time();
/* Returns the current time as a string */
</pre></tt></ul>

We can now run SWIG using : <br>
<ul> <tt> % swig -tcl -dhtml example.i </tt> </ul>

This will produce a documentation file <a href="tutorial/example_wrap_tcl.html"> <tt> example_wrap.html </tt> </a>. 

<br><br>
The documentation system produces documentation using the syntax of the
target language.  Thus, if you used Perl5 instead of Tcl, the documentation
file might look like <a href="tutorial/example_wrap_perl5.html"> this </a>.  <br> <br>

Documentation can also be produced in ASCII and LaTeX formats.  
The C/C++ comments containing documentation can span multiple lines and
can include embedded LaTeX or HTML if desired.

<h3> Running SWIG under Windows NT </h3>

SWIG also works perfectly well under Windows NT/95 systems.  SWIG
is typically invoked from the command prompt and can be used with
NMAKE.   Modules are typically compiled in the form of a DLL that
can be dynamically loaded into Tcl,Python, or Perl.   With a little
work, SWIG can also be used as a custom build option within 
MS Developer Studio.  

<h3> That's it (well, mostly) </h3>

That's about everything you need to know. Here's the short checklist :

<ul>
<li> Make sure you specify a module name.
<li> Use ANSI C/C++ syntax
<li> Figure out how to compile a shared library module (may require reading a few man 
pages for your compiler).
<li> Relax.
</ul>