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<H1><a name="SWIG"></a>5 SWIG Basics</H1>
<!-- INDEX -->
<div class="sectiontoc">
<ul>
<li><a href="#SWIG_nn2">Running SWIG</a>
<ul>
<li><a href="#SWIG_nn3">Input format</a>
<li><a href="#output">SWIG Output</a>
<li><a href="#SWIG_nn5">Comments</a>
<li><a href="#SWIG_nn6">C Preprocessor</a>
<li><a href="#SWIG_nn7">SWIG Directives</a>
<li><a href="#SWIG_nn8">Parser Limitations</a>
</ul>
<li><a href="#SWIG_nn9">Wrapping Simple C Declarations</a>
<ul>
<li><a href="#SWIG_nn10">Basic Type Handling</a>
<li><a href="#SWIG_nn11">Global Variables</a>
<li><a href="#SWIG_nn12">Constants</a>
<li><a href="#SWIG_nn13">A brief word about <tt>const</tt></a>
<li><a href="#SWIG_nn14">A cautionary tale of <tt>char *</tt></a>
</ul>
<li><a href="#SWIG_nn15">Pointers and complex objects</a>
<ul>
<li><a href="#SWIG_nn16">Simple pointers</a>
<li><a href="#SWIG_nn17">Run time pointer type checking</a>
<li><a href="#SWIG_nn18">Derived types, structs, and classes</a>
<li><a href="#SWIG_nn19">Undefined datatypes</a>
<li><a href="#SWIG_nn20">Typedef</a>
</ul>
<li><a href="#SWIG_nn21">Other Practicalities</a>
<ul>
<li><a href="#SWIG_nn22">Passing structures by value</a>
<li><a href="#SWIG_nn23">Return by value</a>
<li><a href="#SWIG_nn24">Linking to structure variables</a>
<li><a href="#SWIG_nn25">Linking to <tt>char *</tt></a>
<li><a href="#SWIG_nn26">Arrays</a>
<li><a href="#SWIG_readonly_variables">Creating read-only variables</a>
<li><a href="#SWIG_nn28">Renaming and ignoring declarations</a>
<li><a href="#SWIG_default_args">Default/optional arguments</a>
<li><a href="#SWIG_nn30">Pointers to functions and callbacks</a>
</ul>
<li><a href="#SWIG_nn31">Structures and unions</a>
<ul>
<li><a href="#SWIG_nn32">Typedef and structures</a>
<li><a href="#SWIG_nn33">Character strings and structures</a>
<li><a href="#SWIG_nn34">Array members</a>
<li><a href="#SWIG_nn35">Structure data members</a>
<li><a href="#SWIG_nn36">C constructors and destructors </a>
<li><a href="#SWIG_adding_member_functions">Adding member functions to C structures</a>
<li><a href="#SWIG_nn38">Nested structures</a>
<li><a href="#SWIG_nn39">Other things to note about structure wrapping</a>
</ul>
<li><a href="#SWIG_nn40">Code Insertion</a>
<ul>
<li><a href="#SWIG_nn41">The output of SWIG</a>
<li><a href="#SWIG_nn42">Code insertion blocks</a>
<li><a href="#SWIG_nn43">Inlined code blocks</a>
<li><a href="#SWIG_nn44">Initialization blocks</a>
</ul>
<li><a href="#SWIG_nn45">An Interface Building Strategy</a>
<ul>
<li><a href="#SWIG_nn46">Preparing a C program for SWIG</a>
<li><a href="#SWIG_nn47">The SWIG interface file</a>
<li><a href="#SWIG_nn48">Why use separate interface files?</a>
<li><a href="#SWIG_nn49">Getting the right header files</a>
<li><a href="#SWIG_nn50">What to do with main()</a>
</ul>
</ul>
</div>
<!-- INDEX -->


<p>
This chapter describes the basic operation of SWIG, the structure of its
input files, and how it handles standard ANSI C declarations.  C++ support is
described in the next chapter.  However, C++ programmers should still read this
chapter to understand the basics.
Specific details about each target language are described in later
chapters. 
</p>

<H2><a name="SWIG_nn2"></a>5.1 Running SWIG</H2>


<p>
To run SWIG, use the <tt>swig</tt> command with one or more of the
following options and a filename like this:
</p>

<div class="shell"><pre>
swig [ <em>options</em> ] filename

-chicken              Generate CHICKEN wrappers
-csharp               Generate C# wrappers
-guile                Generate Guile wrappers
-java                 Generate Java wrappers
-mzscheme             Generate Mzscheme wrappers
-ocaml                Generate Ocaml wrappers
-perl                 Generate Perl wrappers
-php                  Generate PHP wrappers
-pike                 Generate Pike wrappers
-python               Generate Python wrappers
-ruby                 Generate Ruby wrappers
-sexp                 Generate Lisp S-Expressions wrappers
-tcl                  Generate Tcl wrappers
-xml                  Generate XML wrappers
-c++                  Enable C++ parsing
-D<em>symbol</em>              Define a preprocessor symbol
-Fstandard            Display error/warning messages in commonly used format
-Fmicrosoft           Display error/warning messages in Microsoft format
-help                 Display all options
-I<em>dir</em>                 Add a directory to the file include path
-l<em>file</em>                Include a SWIG library file.
-module <em>name</em>          Set the name of the SWIG module
-o <em>outfile</em>            Name of output file
-outdir <em>dir</em>           Set language specific files output directory
-swiglib              Show location of SWIG library
-version              Show SWIG version number

</pre></div>
<p>
This is a subset of commandline options.
Additional options are also defined for each target language.  A full list
can be obtained by typing <tt>swig -help</tt> or <tt>swig
-<em>lang</em> -help</tt>.
</p>

<H3><a name="SWIG_nn3"></a>5.1.1 Input format</H3>


<p>
As input, SWIG expects a file containing ANSI C/C++ declarations and
special SWIG directives.  More often than not, this is a special SWIG
interface file which is usually denoted with a special <tt>.i</tt> or
<tt>.swg</tt> suffix.  In certain cases, SWIG can be used directly on
raw header files or source files.  However, this is not the most
typical case and there are several reasons why you might not want to
do this (described later).
</p>

<p>
The most common format of a SWIG interface is as follows:
</p>

<div class="code"><pre>
%module mymodule 
%{
#include "myheader.h"
%}
// Now list ANSI C/C++ declarations
int foo;
int bar(int x);
...
</pre></div>
<p>
The name of the module is supplied using the special <tt>%module</tt>
directive (or the <tt>-module</tt> command line option).  This
directive must appear at the beginning of the file and is used to name
the resulting extension module (in addition, this name often defines
a namespace in the target language).  If the module name is supplied on the
command line, it overrides the name specified with the
<tt>%module</tt> directive.
</p>

<p>
Everything in the <tt>%{ ... %}</tt> block is simply copied verbatim
to the resulting wrapper file created by SWIG.  This section is almost
always used to include header files and other declarations that are
required to make the generated wrapper code compile.  It is important
to emphasize that just because you include a declaration in a SWIG
input file, that declaration does <em>not</em> automatically appear in
the generated wrapper code---therefore you need to make sure you
include the proper header files in the <tt>%{ ... %}</tt> section.  It
should be noted that the text enclosed in <tt>%{ ... %}</tt> is not
parsed or interpreted by SWIG.  The <tt>%{...%}</tt> syntax and
semantics in SWIG is analogous to that of the declarations section
used in input files to parser generation tools such as yacc or bison.
</p>

<H3><a name="output"></a>5.1.2 SWIG Output</H3>


<p>
The output of SWIG is a C/C++ file that contains all of the wrapper
code needed to build an extension module.  SWIG may generate some
additional files depending on the target language. By default, an input file
with the name <tt>file.i</tt> is transformed into a file
<tt>file_wrap.c</tt> or <tt>file_wrap.cxx</tt> (depending on whether
or not the <tt>-c++</tt> option has been used).  The name of the
output file can be changed using the <tt>-o</tt> option.  In certain
cases, file suffixes are used by the compiler to determine the source
language (C, C++, etc.). Therefore, you have to use the
<tt>-o</tt> option to change the suffix of the SWIG-generated wrapper
file if you want something different than the default. For example:
</p>

<div class="shell"><pre>
$ swig -c++ -python -o example_wrap.cpp example.i
</pre></div>

<p>
The C/C++ output file created by SWIG often
contains everything that is needed to construct a extension module
for the target scripting language. SWIG is not a stub compiler nor is it
usually necessary to edit the output file (and if you look at the output,
you probably won't want to).      To build the final extension module, the
SWIG output file is compiled and linked with the rest of your C/C++
program to create a shared library. 
</p>

<p>
Many target languages will also generate proxy class files in the
target language. The default output directory for these language 
specific files is the same directory as the generated C/C++ file. This can
can be modified using the <tt>-outdir</tt> option. For example:
</p>

<div class="shell"><pre>
$ swig -c++ -python -outdir pyfiles -o cppfiles/example_wrap.cpp example.i
</pre></div>
<p>
If the directories <tt>cppfiles</tt> and <tt>pyfiles</tt> exist, the following
will be generated:</p>
<div class="shell"><pre>
cppfiles/example_wrap.cpp
pyfiles/example.py
</pre></div>

<H3><a name="SWIG_nn5"></a>5.1.3 Comments</H3>


<p>
C and C++ style comments may appear anywhere in interface files.  In
previous versions of SWIG, comments were used to generate
documentation files. However, this feature is currently under repair
and will reappear in a later SWIG release.
</p>

<H3><a name="SWIG_nn6"></a>5.1.4 C Preprocessor</H3>


<p>
Like C, SWIG preprocesses all input files through an enhanced version
of the C preprocessor.  All standard preprocessor features are
supported including file inclusion, conditional compilation and
macros. However, <tt>#include</tt> statements are ignored unless the
<tt>-includeall</tt> command line option has been supplied.  The
reason for disabling includes is that SWIG is sometimes used to
process raw C header files.  In this case, you usually only want the
extension module to include functions in the supplied header file
rather than everything that might be included by that header file
(i.e., system headers, C library functions, etc.).
</p>

<p>
It should also be noted that the SWIG preprocessor skips all text
enclosed inside a <tt>%{...%}</tt> block.  In addition, the
preprocessor includes a number of macro handling enhancements that
make it more powerful than the normal C preprocessor.  These
extensions are described in the "<a href="Preprocessor.html#Preprocessor">Preprocessor</a>" chapter.
</p>

<H3><a name="SWIG_nn7"></a>5.1.5 SWIG Directives</H3>


<p>
Most of SWIG's operation is controlled by special directives that are
always preceded by a "<tt>%</tt>" to distinguish them from normal C
declarations. These directives are used to give SWIG hints or to alter
SWIG's parsing behavior in some manner.   
</p>

<p>
Since SWIG directives are not legal C syntax, it is generally not
possible to include them in header files.  However, SWIG directives can be
included in C header files using conditional compilation like this:
</p>

<div class="code"><pre>
/* header.h  --- Some header file */

/* SWIG directives -- only seen if SWIG is running */ 
#ifdef SWIG
%module foo
#endif
</pre>
</div>

<p>
<tt>SWIG</tt> is a special preprocessing symbol defined by SWIG when
it is parsing an input file.
</p>

<H3><a name="SWIG_nn8"></a>5.1.6 Parser Limitations</H3>


<p>
Although SWIG can parse most C/C++ declarations, it does not
provide a complete C/C++ parser implementation.  Most of these
limitations pertain to very complicated type declarations and certain
advanced C++ features.  Specifically, the following features are not
currently supported:
</p>

<ul>
<li>Non-conventional type declarations.
For example, SWIG does not support declarations such as the following
(even though this is legal C):

<div class="code">
<pre>
/* Non-conventional placement of storage specifier (extern) */
const int extern Number;

/* Extra declarator grouping */
Matrix (foo);    // A global variable

/* Extra declarator grouping in parameters */
void bar(Spam (Grok)(Doh));

</pre>
</div>

<p>
In practice, few (if any) C programmers actually write code like
this since this style is never featured in programming books.  However,
if you're feeling particularly obfuscated, you can certainly break SWIG (although why would you want to?).
</p>
</li>

<li>Running SWIG on C++ source files (what would appear in a .C or .cxx file) 
is not recommended.  Even though SWIG can parse C++ class declarations,
it ignores declarations that are decoupled from their
original class definition (the declarations are parsed, but a lot of warning
messages may be generated).  For example:

<div class="code">
<pre>
/* Not supported by SWIG */
int foo::bar(int) {
    ... whatever ...
}
</pre>
</div>
</li>

<li>Certain advanced features of C++ such as nested classes 
are not yet supported. Please see the section on using SWIG
with C++ for more information.
</ul>

<p>
In the event of a parsing error, conditional compilation can be used to skip
offending code.  For example:
</p>

<div class="code">
<pre>
#ifndef SWIG
... some bad declarations ...
#endif
</pre>
</div>
<p>
Alternatively, you can just delete the offending code from the interface file.
</p>

<p>
One of the reasons why SWIG does not provide a full C++ parser
implementation is that it has been designed to work with incomplete
specifications and to be very permissive in its handling of C/C++
datatypes (e.g., SWIG can generate interfaces even when there are
missing class declarations or opaque datatypes).  Unfortunately, this
approach makes it extremely difficult to implement certain parts of a
C/C++ parser as most compilers use type information to assist in the
parsing of more complex declarations (for the truly curious, the
primary complication in the implementation is that the SWIG parser
does not utilize a separate <em>typedef-name</em> terminal symbol as
described on p. 234 of K&amp;R).
</p>

<H2><a name="SWIG_nn9"></a>5.2 Wrapping Simple C Declarations</H2>


<p>
SWIG wraps simple C declarations by creating an interface that closely matches
the way in which the declarations would be used in a C program.
For example, consider the following interface file:
</p>

<div class="code"><pre>
%module example

%inline %{
extern double sin(double x);
extern int strcmp(const char *, const char *);
extern int Foo;
%}
#define STATUS 50
#define VERSION "1.1"
</pre></div>
<p>
In this file, there are two functions <tt>sin()</tt> and <tt>strcmp()</tt>,
a global variable <tt>Foo</tt>, and two constants <tt>STATUS</tt> and
<tt>VERSION</tt>.  When SWIG creates an extension module, these
declarations are accessible as scripting language functions, variables, and
constants respectively.  For example, in Tcl:
</p>

<div class="targetlang"><pre>
% sin 3
5.2335956
% strcmp Dave Mike
-1
% puts $Foo
42
% puts $STATUS
50
% puts $VERSION
1.1
</pre></div>
<p>
Or in Python:
</p>

<div class="targetlang"><pre>
&gt;&gt;&gt; example.sin(3)
5.2335956
&gt;&gt;&gt; example.strcmp('Dave','Mike')
-1
&gt;&gt;&gt; print example.cvar.Foo
42
&gt;&gt;&gt; print example.STATUS
50
&gt;&gt;&gt; print example.VERSION
1.1
</pre></div>
<p>
Whenever possible, SWIG creates an interface that closely matches the underlying C/C++
code. However, due to subtle differences between languages, run-time
environments, and semantics, it is not always possible to do so.   The
next few sections describes various aspects of this mapping.
</p>

<H3><a name="SWIG_nn10"></a>5.2.1 Basic Type Handling</H3>


<p>
In order to build an interface, SWIG has to convert C/C++ datatypes to
equivalent types in the target language.  Generally,
scripting languages provide a more limited set of primitive types than C.
Therefore, this conversion process involves a certain amount of type
coercion.
</p>

<p>
Most scripting languages provide a single integer type that is implemented using
the <tt>int</tt> or <tt>long</tt> datatype in C.   The following list shows
all of the C datatypes that SWIG will convert to and from integers in the target language:
</p>

<div class="code"><pre>
int
short
long
unsigned
signed
unsigned short
unsigned long
unsigned char
signed char
bool
</pre></div>

<p>
When an integral value is converted from C, a cast is used to convert it to
the representation in the target language.
Thus, a 16 bit short in C may be promoted to a 32 bit integer.  When integers are 
converted in the other direction, the value is cast back into the original C type.  
If the value is too large to fit, it is silently truncated.
<!-- Dave: Maybe we should fix this -->
</p>

<p>
<tt>unsigned char</tt> and <tt>signed char</tt> are special cases that
are handled as small 8-bit integers. Normally, the <tt>char</tt>
datatype is mapped as a one-character ASCII string. </p>

<p>
The <tt>bool</tt> datatype is cast to and from an integer value of 0
and 1 unless the target language provides a special boolean type.</p>

<p>
Some care is required when working with large integer values. Most
scripting languages use 32-bit integers so mapping a 64-bit long
integer may lead to truncation errors. Similar problems may arise with
32 bit unsigned integers (which may appear as large negative
numbers). As a rule of thumb, the <tt>int</tt> datatype and all
variations of <tt>char</tt> and <tt>short</tt> datatypes are safe to
use. For <tt>unsigned int</tt> and <tt>long</tt> datatypes, you will
need to carefully check the correct operation of your program after
it has been wrapped with SWIG.
</p>

<p>
Although the SWIG parser supports the <tt>long long</tt> datatype, not
all language modules support it.  This is because <tt>long long</tt>
usually exceeds the integer precision available in the target
language.  In certain modules such as Tcl and Perl5, <tt>long
long</tt> integers are encoded as strings. This allows the full range
of these numbers to be represented.  However, it does not allow
<tt>long long</tt> values to be used in arithmetic expressions.  It
should also be noted that although <tt>long long</tt> is part
of the ISO C99 standard, it is not universally supported by all C
compilers.  Make sure you are using a compiler that supports <tt>long
long</tt> before trying to use this type with SWIG.
</p>

<p>
SWIG recognizes the following floating point types :</p>

<div class="code"><pre>
float
double
</pre></div>

<p>
Floating point numbers are mapped to and from the natural
representation of floats in the target language. This is almost always
a C <tt>double</tt>. The rarely used datatype of <tt>long double</tt>
is not supported by SWIG.</p>

<p>
The <tt>char</tt> datatype is mapped into a NULL terminated ASCII
string with a single character. When used in a scripting language it
shows up as a tiny string containing the character value. When
converting the value back into C, SWIG takes a character string
from the scripting language and strips off the first character as the
char value. Thus if the value "foo" is assigned to a
<tt>char</tt> datatype, it gets the value `f'.</p>

<p>
The <tt>char *</tt> datatype is handled as a NULL-terminated ASCII
string. SWIG maps this into a 8-bit character string in the target
scripting language. SWIG converts character strings in the target
language to NULL terminated strings before passing them into
C/C++. The default handling of these strings does not allow them to
have embedded NULL bytes.  Therefore, the <tt>char *</tt> datatype is
not generally suitable for passing binary data.  However, it is
possible to change this behavior by defining a SWIG typemap.  See the chapter
on <a href="Typemaps.html#Typemaps">Typemaps</a> for details about this.
</p>

<p>
At this time, SWIG does not provide any special support for Unicode or
wide-character strings (the C <tt>wchar_t</tt> type).  This is a
delicate topic that is poorly understood by many programmers and not
implemented in a consistent manner across languages.  For those
scripting languages that provide Unicode support, Unicode strings are
often available in an 8-bit representation such as UTF-8 that can be
mapped to the <tt>char *</tt> type (in which case the SWIG interface
will probably work).  If the program you are wrapping uses Unicode,
there is no guarantee that Unicode characters in the target language
will use the same internal representation (e.g., UCS-2 vs. UCS-4).
You may need to write some special conversion functions.
</p>

<H3><a name="SWIG_nn11"></a>5.2.2 Global Variables</H3>


<p>
Whenever possible, SWIG maps C/C++ global variables into scripting language
variables.  For example,
</p>

<div class="code"><pre>
%module example
double foo;

</pre></div>
<p>
results in a scripting language variable like this:
</p>

<div class="code"><pre>
# Tcl
set foo [3.5]                   ;# Set foo to 3.5
puts $foo                       ;# Print the value of foo

# Python
cvar.foo = 3.5                  # Set foo to 3.5
print cvar.foo                  # Print value of foo

# Perl
$foo = 3.5;                     # Set foo to 3.5
print $foo,"\n";                # Print value of foo

# Ruby
Module.foo = 3.5               # Set foo to 3.5
print Module.foo, "\n"         # Print value of foo
</pre></div>
<p>
Whenever the scripting language variable is used, the underlying C
global variable is accessed.  Although SWIG makes every
attempt to make global variables work like scripting language
variables, it is not always possible to do so.  For instance, in
Python, all global variables must be accessed through a special
variable object known as <tt>cvar</tt> (shown above).  In Ruby, variables are
accessed as attributes of the module. Other languages may
convert variables to a pair of accessor functions.  For example, the
Java module generates a pair of functions <tt>double get_foo()</tt>
and <tt>set_foo(double val)</tt> that are used to manipulate the
value.
</p>

<p>
Finally, if a global variable has been declared as <tt>const</tt>, it
only supports read-only access.  Note: this behavior is new to SWIG-1.3.
Earlier versions of SWIG incorrectly handled <tt>const</tt> and created
constants instead.
</p>

<H3><a name="SWIG_nn12"></a>5.2.3 Constants</H3>


<p>
Constants can be created using <tt>#define</tt>, enumerations,
or a special <tt>%constant</tt> directive.  The following
interface file shows a few valid constant declarations :</p>

<div class="code"><pre>
#define I_CONST       5               // An integer constant
#define PI            3.14159         // A Floating point constant
#define S_CONST       "hello world"   // A string constant
#define NEWLINE       '\n'            // Character constant

enum boolean {NO=0, YES=1};
enum months {JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG,
             SEP, OCT, NOV, DEC};
%constant double BLAH = 42.37;
#define F_CONST (double) 5            // A floating pointer constant with cast
#define PI_4 PI/4
#define FLAGS 0x04 | 0x08 | 0x40

</pre></div>
<p>
In <tt>#define</tt> declarations, the type of a constant is inferred
by syntax. For example, a number with a decimal point is assumed to be
floating point.  In addition, SWIG must be able to fully resolve all
of the symbols used in a <tt>#define</tt> in order for a constant to
actually be created.  This restriction is necessary because
<tt>#define</tt> is also used to define preprocessor macros that are
definitely not meant to be part of the scripting language interface.
For example:
</p>

<div class="code">
<pre>
#define EXTERN extern

EXTERN void foo();
</pre>
</div>
<p>
In this case, you probably don't want to create a constant called 
<tt>EXTERN</tt> (what would the value be?).  In general,
SWIG will not create constants for macros unless the value can
be completely determined by the preprocessor.  For instance, in the above example,
the declaration
</p>

<div class="code">
<pre>
#define PI_4  PI/4
</pre>
</div>
<p>
defines a constant because <tt>PI</tt> was already defined as a
constant and the value is known.
</p>

<p>
The use of constant expressions is allowed, but SWIG does not evaluate
them. Rather, it passes them through to the output file and lets the C
compiler perform the final evaluation (SWIG does perform a limited
form of type-checking however).</p>

<p>
For enumerations, it is critical that the original enum definition be
included somewhere in the interface file (either in a header file or
in the <tt>%{,%}</tt> block). SWIG only translates the enumeration
into code needed to add the constants to a scripting language. It
needs the original enumeration declaration in order to get the correct
enum values as assigned by the C compiler.
</p>

<p>
The <tt>%constant</tt> directive is used to more precisely create
constants corresponding to different C datatypes.  Although it is not
usually not needed for simple values, it is more useful when working
with pointers and other more complex datatypes.  Typically, <tt>%constant</tt>
is only used when you want to add constants to the scripting language
interface that are not defined in the original header file. 
</p>

<H3><a name="SWIG_nn13"></a>5.2.4 A brief word about <tt>const</tt></H3>


<p>
A common confusion with C programming is the semantic meaning of the
<tt>const</tt> qualifier in declarations--especially when it is mixed
with pointers and other type modifiers. In fact, previous versions of SWIG
handled <tt>const</tt> incorrectly--a situation that SWIG-1.3.7 and newer
releases have fixed.
</p>

<p>
Starting with SWIG-1.3, all variable declarations, regardless of any
use of <tt>const</tt>, are wrapped as global variables.  If a
declaration happens to be declared as <tt>const</tt>, it is wrapped as
a read-only variable. To tell if a variable is <tt>const</tt> or not,
you need to look at the right-most occurrence of the <tt>const</tt>
qualifier (that appears before the variable name).  If the right-most
<tt>const</tt> occurs after all other type modifiers (such as
pointers), then the variable is <tt>const</tt>.  Otherwise, it is not.
</p>

<p>
Here are some examples of <tt>const</tt> declarations.
</p>

<div class="code">
<pre>
const char a;           // A constant character
char const b;           // A constant character (the same)
char *const c;          // A constant pointer to a character
const char *const d;    // A constant pointer to a constant character
</pre>
</div>
<p>
Here is an example of a declaration that is not <tt>const</tt>:
</p>

<div class="code">
<pre>
const char *e;          // A pointer to a constant character.  The pointer
                        // may be modified.
</pre>
</div>
<p>
In this case, the pointer <tt>e</tt> can change---it's only the value
being pointed to that is read-only.
</p>

<p>
<b>Compatibility Note:</b> One reason for changing SWIG to handle
<tt>const</tt> declarations as read-only variables is that there are
many situations where the value of a <tt>const</tt> variable might
change.  For example, a library might export a symbol as
<tt>const</tt> in its public API to discourage modification, but still
allow the value to change through some other kind of internal
mechanism.  Furthermore, programmers often overlook the fact that with
a constant declaration like <tt>char *const</tt>, the underlying data
being pointed to can be modified--it's only the pointer itself that is
constant.  In an embedded system, a <tt>const</tt> declaration might
refer to a read-only memory address such as the location of a
memory-mapped I/O device port (where the value changes, but writing to
the port is not supported by the hardware).  Rather than trying to
build a bunch of special cases into the <tt>const</tt> qualifier, the
new interpretation of <tt>const</tt> as "read-only" is simple and
exactly matches the actual semantics of <tt>const</tt> in C/C++.  If
you really want to create a constant as in older versions of SWIG, use
the <tt>%constant</tt> directive instead.  For example:
</p>

<div class="code">
<pre>
%constant double PI = 3.14159;
</pre>
</div>

<p>
or
</p>

<div class="code">
<pre>
#ifdef SWIG
#define const %constant
#endif
const double foo = 3.4;
const double bar = 23.4;
const int    spam = 42;
#ifdef SWIG
#undef const
#endif
...

</pre>
</div>

<H3><a name="SWIG_nn14"></a>5.2.5 A cautionary tale of <tt>char *</tt></H3>


<p>
Before going any further, there is one bit of caution involving
<tt>char *</tt> that must now be mentioned.  When strings are passed
from a scripting language to a C <tt>char *</tt>, the pointer usually
points to string data stored inside the interpreter.  It is almost
always a really bad idea to modify this data.  Furthermore, some
languages may explicitly disallow it.  For instance, in Python,
strings are supposed be immutable.   If you violate this, you will probably
receive a vast amount of wrath when you unleash your module on the world.
</p>

<p>
The primary source of problems are functions that might modify string data in place.
A classic example would be a function like this:
</p>

<div class="code">
<pre>
char *strcat(char *s, const char *t)
</pre>
</div>

<p>
Although SWIG will certainly generate a wrapper for this, its behavior
will be undefined.  In fact, it will probably cause your application
to crash with a segmentation fault or other memory related problem.
This is because <tt>s</tt> refers to some internal data in the target
language---data that you shouldn't be touching.
</p>

<p>
The bottom line: don't rely on <tt>char *</tt> for anything other than read-only 
input values.   However, it must be noted that you could change the behavior of SWIG
using <a href="Typemaps.html#Typemaps">typemaps</a>.  
</p>

<H2><a name="SWIG_nn15"></a>5.3 Pointers and complex objects</H2>


<p>
Most C programs manipulate arrays, structures, and other types of objects.  This section
discusses the handling of these datatypes.
</p>

<H3><a name="SWIG_nn16"></a>5.3.1 Simple pointers</H3>


<p>
Pointers to primitive C datatypes such as </p>

<div class="code"><pre>
int *
double ***
char **
</pre></div>
<p>
are fully supported by SWIG.  Rather than trying to convert the data being pointed to into a scripting
representation, SWIG simply encodes the pointer itself into a
representation that contains the actual value of the pointer and a type-tag.
Thus, the SWIG representation of the above
pointers (in Tcl), might look like this:</p>

<div class="targetlang"><pre>
_10081012_p_int
_1008e124_ppp_double
_f8ac_pp_char
</pre></div>

<p>
A NULL pointer is represented by the string "NULL" or the value 0
encoded with type information.</p>

<p>
All pointers are treated as opaque objects by SWIG. Thus, a pointer
may be returned by a function and passed around to other C functions
as needed.  For all practical purposes, the scripting language
interface works in exactly the same way as you would use the
pointer in a C program.  The only difference is that there is no mechanism for
dereferencing the pointer since this would require the target language
to understand the memory layout of the underlying object.
</p>

<p>
The scripting language representation of a pointer value should never be
manipulated directly.   Even though the values shown look like hexadecimal
addresses, the numbers used may differ from the actual machine address (e.g.,
on little-endian machines, the digits may appear in reverse order).
Furthermore, SWIG does not
normally map pointers into high-level objects such as associative
arrays or lists (for example, converting an
<tt>int *</tt> into an list of integers). There are several reasons
why SWIG does not do this:</p>

<ul>
<li>There is not enough information in a C declaration to properly map
pointers into higher level constructs. For example, an <tt>int *</tt>
may indeed be an array of integers, but if it contains ten million
elements, converting it into a list object is probably a bad idea.
</li>

<li>The underlying semantics associated with a pointer is not known
by SWIG.   For instance, an <tt>int *</tt> might not be an array at all--perhaps it
is an output value!
</li>

<li>By handling all pointers in a consistent manner, the implementation of SWIG is greatly
simplified and less prone to error.
</li>
</ul>

<H3><a name="SWIG_nn17"></a>5.3.2 Run time pointer type checking</H3>


<p>
By allowing pointers to be manipulated from a scripting language, extension modules
effectively bypass compile-time type checking in the C/C++
compiler.  To prevent errors, a type signature is encoded into all
pointer values and is used to perform run-time type checking.  This
type-checking process is an integral part of SWIG and can not be
disabled or modified without using typemaps (described in later
chapters).
</p>

<p>
Like C, <tt>void *</tt> matches any kind of pointer.  Furthermore,
<tt>NULL</tt> pointers can be passed to any function that expects to
receive a pointer.  Although this has the potential to cause a crash,
<tt>NULL</tt> pointers are also sometimes used
as sentinel values or to denote a missing/empty value.  Therefore,
SWIG leaves NULL pointer checking up to the application.
</p>

<H3><a name="SWIG_nn18"></a>5.3.3 Derived types, structs, and classes</H3>


<p>
For everything else (structs, classes, arrays, etc...) SWIG applies a
very simple rule :</p>

<center>
<b>Everything else is a pointer</b>
</center>

<p>
In other words, SWIG manipulates everything else by reference. This
model makes sense because most C/C++ programs make heavy use of
pointers and SWIG can use the type-checked pointer mechanism already
present for handling pointers to basic datatypes.</p>

<p>
Although this probably sounds complicated, it's really quite
simple. Suppose you have an interface file like this :</p>

<div class="code"><pre>
%module fileio
FILE *fopen(char *, char *);
int fclose(FILE *);
unsigned fread(void *ptr, unsigned size, unsigned nobj, FILE *);
unsigned fwrite(void *ptr, unsigned size, unsigned nobj, FILE *);
void *malloc(int nbytes);
void free(void *);

</pre></div>

<p>
In this file, SWIG doesn't know what a <tt>FILE</tt> is, but since it's used
as a pointer, so it doesn't really matter what it is. If you wrapped
this module into Python, you can use the functions just like you
expect :</p>

<div class="targetlang"><pre>
# Copy a file 
def filecopy(source,target):
	f1 = fopen(source,"r")
	f2 = fopen(target,"w")
	buffer = malloc(8192)
	nbytes = fread(buffer,8192,1,f1)
	while (nbytes &gt; 0):
		fwrite(buffer,8192,1,f2)
		nbytes = fread(buffer,8192,1,f1)
	free(buffer)

</pre></div>

<p>
In this case <tt>f1</tt>,<tt> f2</tt>, and <tt>buffer</tt> are all
opaque objects containing C pointers. It doesn't matter what value
they contain--our program works just fine without this knowledge.</p>

<H3><a name="SWIG_nn19"></a>5.3.4 Undefined datatypes</H3>


<p>
When SWIG encounters an undeclared datatype, it automatically assumes
that it is a structure or class. For example, suppose the following
function appeared in a SWIG input file:</p>

<div class="code"><pre>
void matrix_multiply(Matrix *a, Matrix *b, Matrix *c);
</pre></div>

<p>
SWIG has no idea what a "<tt>Matrix</tt>" is.  However, it is obviously
a pointer to something so SWIG generates a wrapper using its generic pointer
handling code. 
</p>

<p>
Unlike C or C++, SWIG does not actually care whether <tt>Matrix</tt>
has been previously defined in the interface file or not.  This
allows SWIG to generate interfaces from
only partial or limited information. In some cases, you may not care
what a <tt>Matrix</tt> really is as long as you can pass an opaque reference to
one around in the scripting language interface.
</p>

<p>
An important detail to mention is that SWIG will gladly generate
wrappers for an interface when there are unspecified type names.
However, <b>all unspecified types are internally handled as pointers
to structures or classes!</b> For example, consider the following declaration:
</p>

<div class="code">
<pre>
void foo(size_t num);
</pre>
</div>

<p>
If <tt>size_t</tt> is undeclared, SWIG generates wrappers
that expect to receive a type of <tt>size_t *</tt> (this mapping is described shortly).
As a result, the scripting interface might behave strangely.  For example:
</p>

<div class="code">
<pre>
foo(40);
TypeError: expected a _p_size_t.
</pre>
</div>

<p>
The only way to fix this problem is to make sure you properly declare type names using
<tt>typedef</tt>.
</p>

<!-- We might want to add an error reporting flag to swig -->

<H3><a name="SWIG_nn20"></a>5.3.5 Typedef</H3>


<p>
Like C, <tt>typedef</tt> can be used to define new type names in SWIG. For example:
</p>

<div class="code"><pre>
typedef unsigned int size_t;
</pre></div>

<p>
<tt>typedef</tt> definitions appearing in a SWIG interface
are not propagated to the generated wrapper code.  Therefore, they
either need to be defined in an included header file or placed in the
declarations section like this:
</p>

<div class="code">
<pre>
%{
/* Include in the generated wrapper file */
typedef unsigned int size_t;
%}
/* Tell SWIG about it */
typedef unsigned int size_t;
</pre>
</div>

<p>
or
</p>

<div class="code">
<pre>
%inline %{
typedef unsigned int size_t;
%}
</pre>
</div>

<p>
In certain cases, you might be able to include other header files to collect type information.
For example:
</p>

<div class="code">
<pre>
%module example
%import "sys/types.h"
</pre>
</div>

<p>
In this case, you might run SWIG as follows:
</p>

<div class="shell">
<pre>
$ swig -I/usr/include -includeall example.i
</pre>
</div>

<p>
It should be noted that your mileage will vary greatly here.
System headers are notoriously complicated and may rely upon a variety
of non-standard C coding extensions (e.g., such as special directives
to GCC).  Unless you exactly specify the right include directories and
preprocessor symbols, this may not work correctly (you will have to
experiment).
</p>

<p>
SWIG tracks <tt>typedef</tt> declarations and uses this information
for run-time type checking. For instance, if you use the above <tt>typedef</tt> and
had the following function declaration:
</p>

<div class="code">
<pre>
void foo(unsigned int *ptr);
</pre>
</div>

<p>
The corresponding wrapper function will accept arguments of
type <tt>unsigned int *</tt> or <tt>size_t *</tt>.
</p>

<H2><a name="SWIG_nn21"></a>5.4 Other Practicalities</H2>


<p>
So far, this chapter has presented almost everything you need to know to use SWIG
for simple interfaces. However, some C programs use idioms that are somewhat
more difficult to map to a scripting language interface.  This section describes
some of these issues.
</p>

<H3><a name="SWIG_nn22"></a>5.4.1 Passing structures by value</H3>


<p>
Sometimes a C function takes structure parameters that are passed
by value.  For example, consider the following function:
</p>

<div class="code"><pre>
double dot_product(Vector a, Vector b);
</pre></div>

<p>
To deal with this, SWIG transforms the function to use pointers by
creating a wrapper equivalent to the following:
</p>

<div class="code"><pre>
double wrap_dot_product(Vector *a, Vector *b) {
    Vector x = *a;
    Vector y = *b;
    return dot_product(x,y);
}
</pre></div>

<p>
In the target language, the <tt>dot_product()</tt> function now accepts pointers
to Vectors instead of Vectors.  For the most part, this transformation
is transparent so you might not notice.
</p>

<H3><a name="SWIG_nn23"></a>5.4.2 Return by value</H3>


<p>
C functions that return structures or classes datatypes by value are more difficult
to handle. Consider the following function:</p>

<div class="code"><pre>
Vector cross_product(Vector v1, Vector v2);
</pre></div>

<p>
This function wants to return <tt>Vector</tt>, but SWIG only really supports
pointers.  As a result, SWIG creates a wrapper like this:
</p>

<div class="code"><pre>
Vector *wrap_cross_product(Vector *v1, Vector *v2) {
        Vector x = *v1;
        Vector y = *v2;
        Vector *result;
        result = (Vector *) malloc(sizeof(Vector));
        *(result) = cross(x,y);
        return result;
}
</pre></div>

<p>
or if SWIG was run with the <tt>-c++</tt> option:</p>

<div class="code"><pre>
Vector *wrap_cross(Vector *v1, Vector *v2) {
        Vector x = *v1;
        Vector y = *v2;
        Vector *result = new Vector(cross(x,y)); // Uses default copy constructor
        return result;
}
</pre></div>

<p>
In both cases, SWIG allocates a new object and returns a reference to it. It
is up to the user to delete the returned object when it is no longer
in use. Clearly, this will leak memory if you are unaware of the implicit
memory allocation and don't take steps to free the result.  That said, it should be
noted that some language modules can now automatically track newly created objects and
reclaim memory for you.  Consult the documentation for each language module for more details.
</p>

<p>
It should also be noted that the handling of pass/return by value in
C++ has some special cases.  For example, the above code fragments
don't work correctly if <tt>Vector</tt> doesn't define a default
constructor.  The section on SWIG and C++ has more information about this case.
</p>

<H3><a name="SWIG_nn24"></a>5.4.3 Linking to structure variables</H3>


<p>
When global variables or class members involving structures are
encountered, SWIG handles them as pointers. For example, a global
variable like this</p>

<div class="code"><pre>
Vector unit_i;
</pre></div>

<p>
gets mapped to an underlying pair of set/get functions like this :</p>

<div class="code"><pre>
Vector *unit_i_get() {
	return &amp;unit_i;
}
void unit_i_set(Vector *value) {
	unit_i = *value;
}
</pre></div>

<p>
Again some caution is in order. A global variable created in this
manner will show up as a pointer in the target scripting language. It
would be an extremely bad idea to free or destroy such a pointer.    Also,
C++ classes must supply a properly defined copy constructor in order for
assignment to work correctly.
</p>

<H3><a name="SWIG_nn25"></a>5.4.4 Linking to <tt>char *</tt></H3>


<p>
When a global variable of type <tt>char *</tt> appears, SWIG uses <tt>malloc()</tt> or
<tt>new</tt> to allocate memory for the new value.   Specifically, if you have a variable
like this
</p>

<div class="code">
<pre>
char *foo;
</pre>
</div>

<p>
SWIG generates the following code:
</p>

<div class="code">
<pre>
/* C mode */
void foo_set(char *value) {
   if (foo) free(foo);
   foo = (char *) malloc(strlen(value)+1);
   strcpy(foo,value);
}

/* C++ mode.  When -c++ option is used */
void foo_set(char *value) {
   if (foo) delete [] foo;
   foo = new char[strlen(value)+1];
   strcpy(foo,value);
}
</pre>
</div>

<p>
If this is not the behavior that you want, consider making the variable read-only using the
<tt>%immutable</tt> directive.  Alternatively, you might write a short assist-function to set the value
exactly like you want.  For example:
</p>

<div class="code">
<pre>
%inline %{
  void set_foo(char *value) {
       strncpy(foo,value, 50);
   }
%}
</pre>
</div>

<p>
Note: If you write an assist function like this, you will have to call
it as a function from the target scripting language (it does not work
like a variable).  For example, in Python you will have to write:
</p>

<div class="targetlang">
<pre>
&gt;&gt;&gt; set_foo("Hello World")
</pre>
</div>

<p>
A common mistake with <tt>char *</tt> variables is to link to a variable declared like this:
</p>

<div class="code">
<pre>
char *VERSION = "1.0";
</pre>
</div>

<p>
In this case, the variable will be readable, but any attempt to change
the value results in a segmentation or general protection fault.  This
is due to the fact that SWIG is trying to release the old value using
<tt>free</tt> or <tt>delete</tt> when the string literal value currently assigned to the variable wasn't
allocated using <tt>malloc()</tt> or <tt>new</tt>.
To fix this behavior, you can
either mark the variable as read-only, write a typemap (as described in Chapter 6), 
or write a special set function as shown.  Another alternative is to declare the
variable as an array:
</p>

<div class="code">
<pre>
char VERSION[64] = "1.0";
</pre>
</div>

<p>
When variables of type <tt>const char *</tt> are declared, SWIG still generates functions for setting and
getting the value.  However, the default behavior does <em>not</em> release the previous contents (resulting in
a possible memory leak).  In fact, you may get a warning message such as this when wrapping such a variable:
</p>

<div class="shell">
<pre>
example.i:20. Typemap warning. Setting const char * variable may leak memory
</pre>
</div>

<p>
The reason for this behavior is that <tt>const char *</tt> variables are often used to point to string literals.
For example:
</p>

<div class="code">
<pre>
const char *foo = "Hello World\n";
</pre>
</div>

<p>
Therefore, it's a really bad idea to call <tt>free()</tt> on such a
pointer.  On the other hand, it <em>is</em> legal to change the
pointer to point to some other value.  When setting a variable of this
type, SWIG allocates a new string (using malloc or new) and changes
the pointer to point to the new value.  However, repeated
modifications of the value will result in a memory leak since the old
value is not released. 
</p>


<H3><a name="SWIG_nn26"></a>5.4.5 Arrays</H3>


<p>
Arrays are fully supported by SWIG, but they are always handled as pointers instead
of mapping them to a special array object or list in the target language.  Thus, the
following declarations :</p>

<div class="code"><pre>
int foobar(int a[40]);
void grok(char *argv[]);
void transpose(double a[20][20]);
</pre></div>

<p>
are processed as if they were really declared like this:
</p>

<div class="code"><pre>
int foobar(int *a);
void grok(char **argv);
void transpose(double (*a)[20]);
</pre></div>

<p>
Like C, SWIG does not perform array bounds checking.  
It is up to the
user to make sure the pointer points a suitably allocated region of memory. 
</p>

<p>
Multi-dimensional arrays are transformed into a pointer to an array of one less
dimension.  For example:
</p>

<div class="code">
<pre>
int [10];         // Maps to int *
int [10][20];     // Maps to int (*)[20]
int [10][20][30]; // Maps to int (*)[20][30]
</pre>
</div>

<p>
It is important to note that in the C type system, a multidimensional
array <tt>a[][]</tt> is <b>NOT</b> equivalent to a single pointer
<tt>*a</tt> or a double pointer such as <tt>**a</tt>.  Instead, a
pointer to an array is used (as shown above) where the actual value of
the pointer is the starting memory location of the array.  The
reader is strongly advised to dust off their C book and re-read the
section on arrays before using them with SWIG.
</p>

<p>
Array variables are supported, but are read-only by default.  For example:
</p>

<div class="code">
<pre>
int   a[100][200];
</pre>
</div>

<p>
In this case, reading the variable 'a' returns a pointer of type <tt>int (*)[200]</tt>
that points to the first element of the array <tt>&amp;a[0][0]</tt>.  Trying to modify 'a' results
in an error.  This is because SWIG does not know how to copy data from the target
language into the array.   To work around this limitation, you may want to write
a few simple assist functions like this:
</p>

<div class="code">
<pre>
%inline %{
void a_set(int i, int j, int val) {
   a[i][j] = val;
}
int a_get(int i, int j) {
   return a[i][j];
}
%}
</pre>
</div>

<p>
To dynamically create arrays of various sizes and shapes, it may be useful to write
some helper functions in your interface.  For example:
</p>

<div class="code">
<pre>
// Some array helpers
%inline %{
  /* Create any sort of [size] array */
  int *int_array(int size) {
     return (int *) malloc(size*sizeof(int));
  }
  /* Create a two-dimension array [size][10] */
  int (*int_array_10(int size))[10] {
     return (int (*)[10]) malloc(size*10*sizeof(int));
  }
%}
</pre>
</div>

<p>
Arrays of <tt>char</tt> are handled as a special case by SWIG.  In this case, strings in the
target language can be stored in the array.  For example, if you have a declaration like this,
</p>

<div class="code">
<pre>
char pathname[256];
</pre>
</div>

<p>
SWIG generates functions for both getting and setting the value that are equivalent to the following
code:
</p>

<div class="code">
<pre>
char *pathname_get() {
   return pathname;
}
void pathname_set(char *value) {
   strncpy(pathname,value,256);
}
</pre>
</div>

<p>
In the target language, the value can be set like a normal variable.
</p>

<H3><a name="SWIG_readonly_variables"></a>5.4.6 Creating read-only variables</H3>


<p>
A read-only variable can be created by using the <tt>%immutable</tt>
directive as shown :</p>

<div class="code"><pre>
// File : interface.i

int 	a; 			// Can read/write
%immutable;
int	b,c,d			// Read only variables
%mutable;
double	x,y			// read/write
</pre></div>

<p>
The <tt>%immutable</tt> directive enables read-only mode until it is
explicitly disabled using the <tt>%mutable</tt> directive.  As an alternative to turning
read-only mode off and on like this, individual declarations can also be tagged as
immutable.  For example:
</p>

<div class="code"><pre>
%immutable x;                   // Make x read-only
...
double x;                       // Read-only (from earlier %immutable directive)
double y;                       // Read-write
...
</pre></div>

<p>
The <tt>%mutable</tt> and <tt>%immutable</tt> directives are actually 
<a href="Customization.html#features">%feature directives</a> defined like this:
</p>

<div class="code"><pre>
#define %immutable   %feature("immutable")
#define %mutable     %feature("immutable","")
</pre></div>

<p>
If you wanted to make all wrapped variables read-only, barring one or two, it might be easier to take this approach:
</p>

<div class="code"><pre>
%immutable;                     // Make all variables read-only
%feature("immutable","0") x;    // except, make x read/write
...
double x;
double y;
double z;
...
</pre></div>

<p>
Read-only variables are also created when declarations are declared as <tt>const</tt>.
For example:
</p>

<div class="code">
<pre>
const int foo;               /* Read only variable */
char * const version="1.0";  /* Read only variable */
</pre></div>

<p>
<b>Compatibility note:</b> Read-only access used to be controlled by a pair of directives
<tt>%readonly</tt> and <tt>%readwrite</tt>.   Although these directives still work, they
generate a warning message.   Simply change the directives to <tt>%immutable;</tt> and
<tt>%mutable;</tt> to silence the warning.  Don't forget the extra semicolon!
</p>

<H3><a name="SWIG_nn28"></a>5.4.7 Renaming and ignoring declarations</H3>


<p>
Normally, the name of a C declaration is used when that declaration is
wrapped into the target language. However, this may generate a
conflict with a keyword or already existing function in the scripting
language. To resolve a name conflict, you can use the <tt>%rename</tt>
directive as shown :</p>

<div class="code"><pre>
// interface.i

%rename(my_print) print;
extern void print(char *);

%rename(foo) a_really_long_and_annoying_name;
extern int a_really_long_and_annoying_name;

</pre></div>

<p>
SWIG still calls the correct C function, but in this case the
function <tt>print()</tt> will really be called "<tt>my_print()</tt>"
in the target language. </p>

<p>
The placement of the <tt>%rename</tt> directive is arbitrary as long as it appears
before the declarations to be renamed.  A common technique is to write code for
wrapping a header file like this:
</p>

<div class="code"><pre>
// interface.i

%rename(my_print) print;
%rename(foo) a_really_long_and_annoying_name;

%include "header.h"
</pre></div>

<p>
<tt>%rename </tt>applies a renaming operation to all future
occurrences of a name. The renaming applies to functions, variables,
class and structure names, member functions, and member data. For
example, if you had two-dozen C++ classes, all with a member function
named `print' (which is a keyword in Python), you could rename them
all to `output' by specifying :</p>

<div class="code"><pre>
%rename(output) print; // Rename all `print' functions to `output'
</pre></div>

<p>
SWIG does not normally perform any checks to see if the functions it wraps are
already defined in the target scripting language. However, if you are
careful about namespaces and your use of modules, you can usually
avoid these problems.</p>

<p>
Closely related to <tt>%rename</tt> is the <tt>%ignore</tt> directive.  <tt>%ignore</tt> instructs SWIG
to ignore declarations that match a given identifier.  For example:
</p>

<div class="code">
<pre>
%ignore print;         // Ignore all declarations named print
%ignore _HAVE_FOO_H;   // Ignore an include guard constant
...
%include "foo.h"       // Grab a header file
...
</pre>
</div>

<p>
One use of <tt>%ignore</tt> is to selectively remove certain declarations from a header file without having
to add conditional compilation to the header.   However, it should be stressed that this only works for simple
declarations.  If you need to remove a whole section of problematic code, the SWIG preprocessor should be used instead.
</p>

<p>
More powerful variants of <tt>%rename</tt> and <tt>%ignore</tt> directives can be used to help 
wrap C++ overloaded functions and methods or C++ methods which use default arguments. This is described in the 
<a href="SWIGPlus.html#ambiguity_resolution_renaming">Ambiguity resolution and renaming</a> section in the C++ chapter.
</p>

<p>
<b>Compatibility note: </b> Older versions of SWIG provided a special <tt>%name</tt> directive for renaming declarations.
For example:
</p>

<div class="code">
<pre>
%name(output) extern void print(char *);
</pre>
</div>

<p>
This directive is still supported, but it is deprecated and should probably be avoided.  The <tt>%rename</tt>
directive is more powerful and better supports wrapping of raw header file information.
</p>

<H3><a name="SWIG_default_args"></a>5.4.8 Default/optional arguments</H3>


<p>
SWIG supports default arguments in both C and C++ code.  For example:
</p>

<div class="code"><pre>
int plot(double x, double y, int color=WHITE);
</pre></div>

<p>
In this case, SWIG generates wrapper code where the
default arguments are optional in the target language. For example, this function could be
used in Tcl as follows :</p>

<div class="targetlang"><pre>
% plot -3.4 7.5 				# Use default value
% plot -3.4 7.5 10				# set color to 10 instead

</pre></div>

<p>
Although the ANSI C standard does not allow default arguments, default
arguments specified in a SWIG interface work with both C and C++.
</p>

<p>
<b>Note:</b> There is a subtle semantic issue concerning the use 
of default arguments and the SWIG generated wrapper code.  When default
arguments are used in C code, the default values are emitted into the wrappers and the
function is invoked with a full set of arguments. This is different to when wrapping C++
where an overloaded wrapper method is generated for each defaulted argument.
Please refer to the section on <a href="SWIGPlus.html#SWIGPlus_default_args">default arguments</a>
in the C++ chapter for further details.
</p>

<H3><a name="SWIG_nn30"></a>5.4.9 Pointers to functions and callbacks</H3>


<p>
Occasionally, a C library may include functions that expect to receive
pointers to functions--possibly to serve as callbacks. SWIG
provides full support for function pointers provided that the callback
functions are defined in C and not in the target language.  For example,
consider a function like this:
</p>

<div class="code"><pre>
int binary_op(int a, int b, int (*op)(int,int));
</pre></div>

<p>
When you first wrap something like this into an extension module, you
may find the function to be impossible to use.  For instance, in Python:
</p>

<div class="targetlang"><pre>
&gt;&gt;&gt; def add(x,y):
...     return x+y
...
&gt;&gt;&gt; binary_op(3,4,add)
Traceback (most recent call last):
  File "&lt;stdin&gt;", line 1, in ?
TypeError: Type error. Expected _p_f_int_int__int
&gt;&gt;&gt;
</pre>
</div>

<p>
The reason for this error is that SWIG doesn't know how to map a scripting
language function into a C callback.  However, existing C functions can
be used as arguments provided you install them as constants.
One way to do this is to use the <tt>%constant</tt> directive like this:
</p>

<div class="code"><pre>
/* Function with a callback */
int binary_op(int a, int b, int (*op)(int,int));

/* Some callback functions */
%constant int add(int,int);
%constant int sub(int,int);
%constant int mul(int,int);
</pre></div>

<p>
In this case, <tt>add</tt>, <tt>sub</tt>, and <tt>mul</tt> become function pointer
constants in the target scripting language.  This allows you to use them as follows:
</p>

<div class="targetlang">
<pre>
&gt;&gt;&gt; binary_op(3,4,add)
7
&gt;&gt;&gt; binary_op(3,4,mul)
12
&gt;&gt;&gt;
</pre>
</div>

<p>
Unfortunately, by declaring the callback functions as constants, they are no longer accesible
as functions. For example:
</p>

<div class="targetlang">
<pre>
&gt;&gt;&gt; add(3,4)
Traceback (most recent call last):
  File "&lt;stdin&gt;", line 1, in ?
TypeError: object is not callable: '_ff020efc_p_f_int_int__int'
&gt;&gt;&gt;
</pre>
</div>

<p>
If you want to make a function available as both a callback function and a function, you
can use the <tt>%callback</tt> and <tt>%nocallback</tt> directives like this:
</p>

<div class="code">
<pre>
/* Function with a callback */
int binary_op(int a, int b, int (*op)(int,int));

/* Some callback functions */
%callback("%s_cb")
int add(int,int);
int sub(int,int);
int mul(int,int);
%nocallback
</pre></div>

<p>
The argument to <tt>%callback</tt> is a printf-style format string that
specifies the naming convention for the callback constants (<tt>%s</tt> gets replaced
by the function name).  The callback mode remains in effect until it is explicitly
disabled using <tt>%nocallback</tt>.  When you do this, the interface now works as follows:
</p>

<div class="targetlang">
<pre>
&gt;&gt;&gt; binary_op(3,4,add_cb)
7
&gt;&gt;&gt; binary_op(3,4,mul_cb)
12
&gt;&gt;&gt; add(3,4)
7
&gt;&gt;&gt; mul(3,4)
12
</pre>
</div>

<p>
Notice that when the function is used as a callback, special names
such as <tt>add_cb</tt> is used instead.  To call the function
normally, just use the original function name such as <tt>add()</tt>.
</p>

<p>
SWIG provides a number of extensions to standard C printf formatting
that may be useful in this context.  For instance, the following
variation installs the callbacks as all upper-case constants such as
<tt>ADD</tt>, <tt>SUB</tt>, and <tt>MUL</tt>:
</p>

<div class="code"><pre>
/* Some callback functions */
%callback("%(upper)s")
int add(int,int);
int sub(int,int);
int mul(int,int);
%nocallback
</pre></div>

<p>
A format string of <tt>"%(lower)s"</tt> converts all characters to lower-case.
A string of <tt>"%(title)s"</tt> capitalizes the first character and converts the
rest to lower case.
</p>

<p>
And now, a final note about function pointer support.  Although SWIG
does not normally allow callback functions to be written in the target language, this
can be accomplished with the use of typemaps and other advanced SWIG features.
This is described in a later chapter.
</p>

<H2><a name="SWIG_nn31"></a>5.5 Structures and unions</H2>


<p>
This section describes the behavior of SWIG when processing ANSI C structures and union declarations.  Extensions to
handle C++ are described in the next section.
</p>

<p>
If SWIG encounters the definition of a structure or union, it
creates a set of accessor functions. Although SWIG does not need
structure definitions to build an interface, providing definitions
make it possible to access structure members. The accessor functions
generated by SWIG simply take a pointer to an object and allow access
to an individual member. For example, the declaration :</p>

<div class="code"><pre>
struct Vector {
	double x,y,z;
}

</pre></div>

<p>
gets transformed into the following set of accessor functions :</p>

<div class="code"><pre>
double Vector_x_get(struct Vector *obj) {
	return obj-&gt;x;
}
double Vector_y_get(struct Vector *obj) { 
	return obj-&gt;y;
}
double Vector_z_get(struct Vector *obj) { 
	return obj-&gt;z;
}
void Vector_x_set(struct Vector *obj, double value) {
	obj-&gt;x = value;
}
void Vector_y_set(struct Vector *obj, double value) {
	obj-&gt;y = value;
}
void Vector_z_set(struct Vector *obj, double value) {
	obj-&gt;z = value;
}
</pre></div>

<p>
In addition, SWIG creates default constructor and destructor functions if none are
defined in the interface.  For example:
</p>

<div class="code"><pre>
struct Vector *new_Vector() {
    return (Vector *) calloc(1,sizeof(struct Vector));
}
void delete_Vector(struct Vector *obj) {
    free(obj);
}
</pre>
</div>

<p>
Using these low-level accessor functions, an object can be minimally manipulated from the target
language using code like this:
</p>

<div class="code">
<pre>
v = new_Vector()
Vector_x_set(v,2)
Vector_y_set(v,10)
Vector_z_set(v,-5)
...
delete_Vector(v)
</pre>
</div>

<p>
However, most of SWIG's language modules also provide a high-level interface that is more convenient. Keep reading.
</p>

<H3><a name="SWIG_nn32"></a>5.5.1 Typedef and structures</H3>


<p>
SWIG supports the following construct which is quite common in C
programs :</p>

<div class="code"><pre>
typedef struct {
	double x,y,z;
} Vector;

</pre></div>

<p>
When encountered, SWIG assumes that the name of the object is `Vector'
and creates accessor functions like before.  The only difference is
that the use of <tt>typedef</tt> allows SWIG to drop the
<tt>struct</tt> keyword on its generated code. For example:
</p>

<div class="code">
<pre>
double Vector_x_get(Vector *obj) {
	return obj-&gt;x;
}
</pre>
</div>

<p>
If two different names are used like this :</p>

<div class="code"><pre>
typedef struct vector_struct {
	double x,y,z;
} Vector;

</pre></div>

<p>
the name <tt>Vector</tt> is used instead of <tt>vector_struct</tt> since
this is more typical C programming style. If declarations defined later in the interface use the type <tt>struct
vector_struct</tt>, SWIG knows that this is the same as
<tt>Vector</tt> and it generates the appropriate type-checking code.
</p>

<H3><a name="SWIG_nn33"></a>5.5.2 Character strings and structures</H3>


<p>
Structures involving character strings require some care. SWIG assumes
that all members of type <tt>char *</tt> have been dynamically
allocated using <tt>malloc()</tt> and that they are NULL-terminated
ASCII strings. When such a member is modified, the previously contents
will be released, and the new contents allocated. For example :</p>

<div class="code"><pre>
%module mymodule
...
struct Foo {
	char *name;
	...
}

</pre></div>

<p>
This results in the following accessor functions :</p>

<div class="code"><pre>
char *Foo_name_get(Foo *obj) {
	return Foo-&gt;name;
}

char *Foo_name_set(Foo *obj, char *c) {
	if (obj-&gt;name) free(obj-&gt;name);
	obj-&gt;name = (char *) malloc(strlen(c)+1);
	strcpy(obj-&gt;name,c);
	return obj-&gt;name;
}
</pre></div>

<p>
If this behavior differs from what you need in your applications,
the SWIG "memberin" typemap can be used to change it.  See the
typemaps chapter for further details.
</p>

<p>
Note: If the <tt>-c++</tt> option is used, <tt>new</tt> and <tt>delete</tt> are used to
perform memory allocation.
</p>

<H3><a name="SWIG_nn34"></a>5.5.3 Array members</H3>


<p>
Arrays may appear as the members of structures, but they will be
read-only. SWIG will write an accessor function that returns the
pointer to the first element of the array, but will not write a
function to change the contents of the array itself. 
When this
situation is detected, SWIG may generate a warning message such as the
following :</p>

<div class="shell"><pre>
interface.i:116. Warning. Array member will be read-only
</pre></div>

<p>
To eliminate the warning message, typemaps can be used, but this is
discussed in a later chapter.  In many cases, the warning message is
harmless.
</p>

<H3><a name="SWIG_nn35"></a>5.5.4 Structure data members</H3>


<p>
Occasionally, a structure will contain data members that are themselves structures.  For example:
</p>

<div class="code">
<pre>
typedef struct Foo {
   int x;
} Foo;

typedef struct Bar {
   int y;
   Foo f;           /* struct member */
} Bar;
</pre>
</div>

<p>
When a structure member is wrapped, it is always handled as a pointer.
For example:
</p>

<div class="code">
<pre>
Foo *Bar_f_get(Bar *b) {
   return &amp;b-&gt;f;
}
void Bar_f_set(Bar *b, Foo *value) {
   b-&gt;f = *value;
}
</pre>
</div>

<p>
The reasons for this are somewhat subtle but have to do with the
problem of modifying and accessing data inside the data member.  For
example, suppose you wanted to modify the value of <tt>f.x</tt> 
of a <tt>Bar</tt> object like this:
</p>

<div class="code">
<pre>
Bar *b;
b-&gt;f.x = 37;
</pre>
</div>

<p>
Translating this assignment to function calls (as would be used inside the scripting
language interface) results in the following code:
</p>

<div class="code">
<pre>
Bar *b;
Foo_x_set(Bar_f_get(b),37);
</pre>
</div>

<p>
In this code, if the <tt>Bar_f_get()</tt> function were to return a <tt>Foo</tt> instead of a
<tt>Foo *</tt>, then the resulting modification would be applied to a <em>copy</em> of <tt>f</tt> and not
the data member <tt>f</tt> itself.  Clearly that's not what you want!
</p>

<p>
It should be noted that this transformation to pointers only occurs if SWIG knows that a data member
is a structure or class.   For instance, if you had a structure like this,
</p>

<div class="code">
<pre>
struct Foo {
   WORD   w;
};
</pre>
</div>

<p>
and nothing was known about <tt>WORD</tt>, then SWIG will generate more normal accessor functions
like this:
</p>

<div class="code">
<pre>
WORD Foo_w_get(Foo *f) {
    return f-&gt;w;
}
void Foo_w_set(FOO *f, WORD value) {
    f-&gt;w = value;
}
</pre>
</div>

<p>
<b>Compatibility Note: </b>  SWIG-1.3.11 and earlier releases transformed all non-primitive member datatypes
to pointers.  Starting in SWIG-1.3.12,  this transformation <em>only</em> occurs if a datatype is known to be a structure,
class, or union.    This is unlikely to break existing code.  However, if you need to tell SWIG that an undeclared
datatype is really a struct, simply use a forward struct declaration such as <tt>"struct Foo;"</tt>.
</p>

<H3><a name="SWIG_nn36"></a>5.5.5 C constructors and destructors </H3>


<p>
When wrapping structures, it is generally useful to have a mechanism
for creating and destroying objects.  If you don't do anything, SWIG
will automatically generate functions for creating and destroying
objects using <tt>malloc()</tt> and <tt>free()</tt>.  Note: the use of
<tt>malloc()</tt> only applies when SWIG is used on C code (i.e., when the
<tt>-c++</tt> option  is <em>not</em> supplied on the command line).  C++ is handled
differently.
</p>

<p>
If you don't want SWIG to generate constructors and destructors, you
can use the <tt>%nodefault</tt> directive or the <tt>-no_default</tt>
command line option.  For example:
</p>

<div class="shell"><pre>
swig -no_default example.i 
</pre></div>

<p>
or
</p>

<div class="code"><pre>
%module foo
...
%nodefault;        // Don't create default constructors/destructors
... declarations ...
%makedefault;      // Reenable default constructors/destructors
</pre></div>

<p>
If you need more precise control, <tt>%nodefault</tt> can selectively target individual structure 
definitions.  For example:
</p>

<div class="code">
<pre>
%nodefault Foo;          // No default constructor/destructors for Foo
...
struct Foo {             // No default generated.
};

struct Bar {             // Default constructor/destructor generated.
};
</pre>
</div>

<p>
<b>Compatibility note:</b> Prior to SWIG-1.3.7, SWIG did not generate default constructors
or destructors unless you explicitly turned them on using <tt>-make_default</tt>. 
However, it appears that most users want to have constructor and destructor functions so it
has now been enabled as the default behavior.
</p>

<H3><a name="SWIG_adding_member_functions"></a>5.5.6 Adding member functions to C structures</H3>


<p>
Most languages provide a mechanism for creating classes and
supporting object oriented programming. From a C standpoint, object
oriented programming really just boils down to the process of
attaching functions to structures. These functions normally operate
on an instance of the structure (or object).  Although there is a
natural mapping of C++ to such a scheme, there is no direct mechanism
for utilizing it with C code. However, SWIG provides a special
<tt>%extend</tt> directive that makes it possible to attach
methods to C structures for purposes of building an object oriented
interface. Suppose you have a C header file with
the following declaration :</p>

<div class="code"><pre>
/* file : vector.h */
...
typedef struct {
	double x,y,z;
} Vector;

</pre></div>

<p>
You can make a <tt>Vector</tt> look alot like a class by writing a SWIG interface like this:
</p>

<div class="code"><pre>
// file : vector.i
%module mymodule
%{
#include "vector.h"
%}

%include vector.h            // Just grab original C header file
%extend Vector {             // Attach these functions to struct Vector
	Vector(double x, double y, double z) {
		Vector *v;
		v = (Vector *) malloc(sizeof(Vector));
		v-&gt;x = x;
		v-&gt;y = y;
		v-&gt;z = z;
		return v;
	}
	~Vector() {
		free(self);
	}
	double magnitude() {
		return sqrt(self-&gt;x*self-&gt;x+self-&gt;y*self-&gt;y+self-&gt;z*self-&gt;z);
	}
	void print() {
		printf("Vector [%g, %g, %g]\n", self-&gt;x,self-&gt;y,self-&gt;z);
	}
};

</pre></div>

<p>
Now, when used with proxy classes in Python, you can do things like
this :</p>

<div class="targetlang"><pre>
&gt;&gt;&gt; v = Vector(3,4,0)                 # Create a new vector
&gt;&gt;&gt; print v.magnitude()                # Print magnitude
5.0
&gt;&gt;&gt; v.print()                  # Print it out
[ 3, 4, 0 ]
&gt;&gt;&gt; del v                      # Destroy it
</pre></div>

<p>
The <tt>%extend</tt> directive can also be used inside the definition
of the Vector structure. For example:</p>

<div class="code"><pre>
// file : vector.i
%module mymodule
%{
#include "vector.h"
%}

typedef struct {
	double x,y,z;
	%extend {
		Vector(double x, double y, double z) { ... }
		~Vector() { ... }
		...
	}
} Vector;
</pre></div>

<p>
Finally, <tt>%extend</tt> can be used to access externally written
functions provided they follow the naming convention used in this
example :</p>

<div class="code"><pre>
/* File : vector.c */
/* Vector methods */
#include "vector.h"
Vector *new_Vector(double x, double y, double z) {
	Vector *v;
	v = (Vector *) malloc(sizeof(Vector));
	v-&gt;x = x;
	v-&gt;y = y;
	v-&gt;z = z;
	return v;
}
void delete_Vector(Vector *v) {
	free(v);
}

double Vector_magnitude(Vector *v) {
	return sqrt(v-&gt;x*v-&gt;x+v-&gt;y*v-&gt;y+v-&gt;z*v-&gt;z);
}

// File : vector.i
// Interface file
%module mymodule
%{
#include "vector.h"
%}

typedef struct {
	double x,y,z;
	%extend {
                Vector(int,int,int); // This calls new_Vector()
               ~Vector();            // This calls delete_Vector()
		double magnitude();  // This will call Vector_magnitude()
		...
	}
} Vector;
</pre>
</div>

<p>
A little known feature of the <tt>%extend</tt> directive is that
it can also be used to add synthesized attributes or to modify the
behavior of existing data attributes.  For example, suppose you wanted
to make <tt>magnitude</tt> a read-only attribute of <tt>Vector</tt>
instead of a method.  To do this, you might write some code like this:
</p>

<div class="code">
<pre>
// Add a new attribute to Vector
%extend Vector {
    const double magnitude;
}
// Now supply the implementation of the Vector_magnitude_get function
%{
const double Vector_magnitude_get(Vector *v) {
  return (const double) return sqrt(v-&gt;x*v-&gt;x+v-&gt;y*v-&gt;y+v-&gt;z*v-&gt;z);
}
%}

</pre>
</div>

<p>
Now, for all practial purposes, <tt>magnitude</tt> will appear like an attribute
of the object.
</p>

<p>
A similar technique can also be used to work with problematic data members.
For example, consider this interface:
</p>

<div class="code">
<pre>
struct Person {
   char name[50];
   ...
}
</pre>
</div>

<p>
By default, the <tt>name</tt> attribute is read-only because SWIG does not
normally know how to modify arrays.  However, you can rewrite the interface
as follows to change this:
</p>

<div class="code">
<pre>
struct Person {
    %extend {
       char *name;
    }
...
}

// Specific implementation of set/get functions
%{
char *Person_name_get(Person *p) {
   return p-&gt;name;
}
void Person_name_set(Person *p, char *val) {
   strncpy(p-&gt;name,val,50);
}
%}
</pre>
</div>

<p>
Finally, it should be stressed that even though <tt>%extend</tt>
can be used to add new data members, these new members can not require
the allocation of additional storage in the object (e.g., their values must
be entirely synthesized from existing attributes of the structure).
</p>

<p>
<b>Compatibility note:</b>  The <tt>%extend</tt> directive is a new
name for the <tt>%addmethods</tt> directive.  Since <tt>%addmethods</tt> could
be used to extend a structure with more than just methods, a more suitable
directive name has been chosen.
</p>

<H3><a name="SWIG_nn38"></a>5.5.7 Nested structures</H3>


<p>
Occasionally, a C program will involve structures like this :</p>

<div class="code"><pre>
typedef struct Object {
	int objtype;
	union {
		int 	ivalue;
		double	dvalue;
		char	*strvalue;
		void	*ptrvalue;
	} intRep;
} Object;

</pre></div>

<p>
When SWIG encounters this, it performs a structure splitting operation
that transforms the declaration into the equivalent of the
following:</p>

<div class="code"><pre>
typedef union {
	int 		ivalue;
	double		dvalue;
	char		*strvalue;
	void		*ptrvalue;
} Object_intRep;

typedef struct Object {
	int objType;
	Object_intRep intRep;
} Object;

</pre></div>

<p>
SWIG will then create an <tt>Object_intRep</tt> structure for use inside
the interface file. Accessor functions will be created for both
structures. In this case, functions like this would be created :</p>

<div class="code"><pre>
Object_intRep *Object_intRep_get(Object *o) {
	return (Object_intRep *) &amp;o-&gt;intRep;
}
int Object_intRep_ivalue_get(Object_intRep *o) {
	return o-&gt;ivalue;
}
int Object_intRep_ivalue_set(Object_intRep *o, int value) {
	return (o-&gt;ivalue = value);
}
double Object_intRep_dvalue_get(Object_intRep *o) {
	return o-&gt;dvalue;
}
... etc ...

</pre></div>

<p>
Although this process is a little hairy, it works like you would expect in the
target scripting language--especially when proxy classes are used.  For instance, in Perl:
</p>

<div class="targetlang"><pre>
# Perl5 script for accessing nested member
$o = CreateObject();                    # Create an object somehow
$o-&gt;{intRep}-&gt;{ivalue} = 7              # Change value of o.intRep.ivalue
</pre></div>

<p>
If you have a lot nested structure declarations, it is
advisable to double-check them after running SWIG. Although,
there is a good chance that they will work, you may have to
modify the interface file in certain cases.

<H3><a name="SWIG_nn39"></a>5.5.8 Other things to note about structure wrapping</H3>


<p>
SWIG doesn't care if the declaration of a structure in a <tt>.i</tt> file exactly matches
that used in the underlying C code (except in the case of nested
structures). For this reason, there are no problems omitting
problematic members or simply omitting the structure definition
altogether. If you are happy passing pointers around, this can
be done without ever giving SWIG a structure definition.</p>

<p>
Starting with SWIG1.3, a number of improvements have been made to SWIG's
code generator. Specifically, even though structure access has been described
in terms of high-level accessor functions such as this,
</p>

<div class="code">
<pre>
double Vector_x_get(Vector *v) {
   return v-&gt;x;
}
</pre>
</div>

<p>
most of the generated code is actually inlined directly into wrapper
functions.  Therefore, no function <tt>Vector_x_get()</tt> actually
exists in the generated wrapper file.  For example, when creating a Tcl module,
the following function is generated instead:
</p>

<div class="code">
<pre>
static int
_wrap_Vector_x_get(ClientData clientData, Tcl_Interp *interp, 
                   int objc, Tcl_Obj *CONST objv[]) {
    struct Vector *arg1 ;
    double result ;
    
    if (SWIG_GetArgs(interp, objc, objv,"p:Vector_x_get self ",&amp;arg0,
                     SWIGTYPE_p_Vector) == TCL_ERROR)
         return TCL_ERROR;
    result = (double ) (arg1-&gt;x);
    Tcl_SetObjResult(interp,Tcl_NewDoubleObj((double) result));
    return TCL_OK;
}
</pre>
</div>

<p>
The only exception to this rule are methods defined with <tt>%extend</tt>.  In this
case, the added code is contained in a separate function.
</p>

<p>
Finally, it is important to note that most language modules may choose to
build a more advanced interface. Although you may never use the low-level
interface described here, most of SWIG's language modules use it in
some way or another.
</p>

<H2><a name="SWIG_nn40"></a>5.6 Code Insertion</H2>


<p>
Sometimes it is necessary to insert special code into the resulting
wrapper file generated by SWIG.  For example, you may want to include
additional C code to perform initialization or other operations.
There are four common ways to insert code, but it's useful to know how the
output of SWIG is structured first.</p>

<H3><a name="SWIG_nn41"></a>5.6.1 The output of SWIG</H3>


<p>
When SWIG creates its output file, it is broken up into four sections
corresponding to runtime code, headers, wrapper functions, and module
initialization code (in that order).   
</p>

<ul>
<li><b>Runtime code</b>.  <br>
This code is internal to SWIG and is used to include
type-checking and other support functions that are used by the rest of the module.
</li>

<li><b>Header section</b>.  <br>
This is user-defined support code that has been included by 
the <tt>%{ ... %}</tt> directive.  Usually this consists of header files and 
other helper functions.
</li>

<li><b>Wrapper code</b>.  <br>
These are the wrappers generated automatically by SWIG.
</li>

<li><b>Module initialization</b>.  <br>
The function generated by SWIG to initialize
the module upon loading.
</li>
</ul>

<H3><a name="SWIG_nn42"></a>5.6.2 Code insertion blocks</H3>


<p>
Code is inserted into the appropriate code section by using one
of the following code insertion directives:
</p>

<div class="code">
<pre>
%runtime %{
   ... code in runtime section ...
%}

%header %{
   ... code in header section ...
%}

%wrapper %{
   ... code in wrapper section ...
%}

%init %{
   ... code in init section ...
%}
</pre>
</div>

<p>
The bare <tt>%{ ... %}</tt> directive is a shortcut that is the same as
<tt>%header %{ ... %}</tt>.
</p>

<p>
Everything in a code insertion block is copied verbatim into the output file and is 
not parsed by SWIG.   Most SWIG input files have at least one such block to include header
files and support C code.  Additional code blocks may be placed anywhere in a
SWIG file as needed. </p>

<div class="code"><pre>
%module mymodule
%{
#include "my_header.h"
%}
... Declare functions here
%{

void some_extra_function() {
  ...
}
%}
</pre></div>

<p>
A common use for code blocks is to write "helper" functions. These
are functions that are used specifically for the purpose of building
an interface, but which are generally not visible to the normal C
program. For example :</p>

<div class="code"><pre>
%{
/* Create a new vector */
static Vector *new_Vector() {
	return (Vector *) malloc(sizeof(Vector));
}

%}
// Now wrap it 
Vector *new_Vector();
</pre></div>

<H3><a name="SWIG_nn43"></a>5.6.3 Inlined code blocks</H3>


<p>
Since the process of writing helper functions is fairly common,
there is a special inlined form of code block that is used as follows
:</p>

<div class="code"><pre>
%inline %{
/* Create a new vector */
Vector *new_Vector() {
	return (Vector *) malloc(sizeof(Vector));
}
%}

</pre></div>

<p>
The <tt>%inline</tt> directive inserts all of the code that follows
verbatim into the header portion of an interface file. The code is
then parsed by both the SWIG preprocessor and parser.
Thus, the above example creates a new command <tt>new_Vector</tt> using only one
declaration. Since the code inside an <tt>%inline %{ ... %}</tt> block
is given to both the C compiler and SWIG, it is illegal to include any
SWIG directives inside a <tt>%{ ... %}</tt> block.</p>

<H3><a name="SWIG_nn44"></a>5.6.4 Initialization blocks</H3>


<p>
When code is included in the <tt>%init</tt> section, it is copied directly into the 
module initialization function.  For example, if you needed to perform some extra
initialization on module loading, you could write this:
</p>

<div class="code"><pre>
%init %{
	init_variables();
%}
</pre></div>

<H2><a name="SWIG_nn45"></a>5.7 An Interface Building Strategy</H2>


<p>
This section describes the general approach for building interface
with SWIG. The specifics related to a particular scripting language
are found in later chapters.</p>

<H3><a name="SWIG_nn46"></a>5.7.1 Preparing a C program for SWIG</H3>


<p>
SWIG doesn't require modifications to your C code, but if you feed it
a collection of raw C header files or source code, the results might
not be what you expect---in fact, they might be awful. Here's a series
of steps you can follow to make an interface for a C program :</p>

<ul>
<li>Identify the functions that you want to wrap. It's probably not
necessary to access every single function in a C program--thus, a
little forethought can dramatically simplify the resulting scripting
language interface. C header files are particularly good source for
finding things to wrap.

<li>Create a new interface file to describe the scripting language
interface to your program.

<li>Copy the appropriate declarations into the interface file or use
SWIG's <tt>%include</tt> directive to process an entire C
source/header file. 

<li>Make sure everything in the interface file uses ANSI C/C++syntax.

<li>Make sure all necessary `<tt>typedef</tt>' declarations and
type-information is available in the interface file.

<li>If your program has a main() function, you may need to rename it
(read on).

<li>Run SWIG and compile.
</ul>

<p>
Although this may sound complicated, the process turns out to be
fairly easy once you get the hang of it. 
</p>

<p>
In the process of building an interface, SWIG may encounter syntax errors or
other problems.  The best way to deal with this is to simply copy the offending
code into a separate interface file and edit it.   However, the SWIG developers
have worked very hard to improve the SWIG parser--you should report parsing errors
to the <a href="http://www.swig.org/mail.html">swig-dev mailing list</a> or to the 
<a href="http://www.swig.org/bugs.html">SWIG bug tracker</a>.
</p>

<H3><a name="SWIG_nn47"></a>5.7.2 The SWIG interface file</H3>


<p>
The preferred method of using SWIG is to generate separate interface
file. Suppose you have the following C header file :</p>

<div class="code"><pre>
/* File : header.h */

#include &lt;stdio.h&gt;
#include &lt;math.h&gt;

extern int foo(double);
extern double bar(int, int);
extern void dump(FILE *f);

</pre></div>

<p>
A typical SWIG interface file for this header file would look like the
following :</p>

<div class="code"><pre>
/* File : interface.i */
%module mymodule
%{
#include "header.h"
%}
extern int foo(double);
extern double bar(int, int);
extern void dump(FILE *f);

</pre></div>

<p>
Of course, in this case, our header file is pretty simple so we could
have made an interface file like this as well:</p>

<div class="code"><pre>
/* File : interface.i */
%module mymodule
%include header.h
</pre></div>

<p>
Naturally, your mileage may vary.</p>

<H3><a name="SWIG_nn48"></a>5.7.3 Why use separate interface files?</H3>


<p>
Although SWIG can parse many header files, it is more common to write a
special <tt>.i</tt> file defining the interface to a package. There
are several reasons why you might want to do this:
</p>

<ul>
<li>It is rarely necessary to access every single function in a large
package. Many C functions might have little or no use in a scripted
environment. Therfore, why wrap them?

<li>Separate interface files provide an opportunity to provide more
precise rules about how an interface is to be constructed.

<li>Interface files can provide more structure and organization.

<li>SWIG can't parse certain definitions that appear in header
files. Having a separate file allows you to eliminate or work around
these problems.

<li>Interface files provide a more precise definition of what the interface
is. Users wanting to extend the system can go to the interface file
and immediately see what is available without having to dig it out of
header files.
</ul>

<H3><a name="SWIG_nn49"></a>5.7.4 Getting the right header files</H3>


<p>
Sometimes, it is necessary to use certain header files in order for
the code generated by SWIG to compile properly. Make sure you
include certain header files by using a <tt>%{,%}</tt> block like this:
</p>

<div class="code"><pre>
%module graphics
%{
#include &lt;GL/gl.h&gt;
#include &lt;GL/glu.h&gt;
%}

// Put rest of declarations here
...
</pre></div>

<H3><a name="SWIG_nn50"></a>5.7.5 What to do with main()</H3>


<p>
If your program defines a <tt>main()</tt> function, you may need to
get rid of it or rename it in order to use a scripting language. Most
scripting languages define their own <tt>main()</tt> procedure that
is called instead. <tt>main()</tt> also makes no sense when
working with dynamic loading. There are a few approaches to solving
the <tt>main()</tt> conflict :</p>

<ul>
<li>Get rid of <tt>main()</tt> entirely. 

<li>Rename <tt>main()</tt> to something else. You can do this by
compiling your C program with an option like <tt>-Dmain=oldmain</tt>.

<li>Use conditional compilation to only include <tt>main()</tt> when
not using a scripting language.
</ul>

<p>
Getting rid of <tt>main()</tt> may cause potential initialization
problems of a program. To handle this problem, you may consider
writing a special function called <tt>program_init()</tt> that
initializes your program upon startup. This function could then be
called either from the scripting language as the first operation, or
when the SWIG generated module is loaded.</p>

<p>
As a general note, many C programs only use the <tt>main()</tt>
function to parse command line options and to set parameters. However,
by using a scripting language, you are probably trying to create a
program that is more interactive. In many cases, the old
<tt>main()</tt> program can be completely replaced by a Perl, Python,
or Tcl script.</p>

<p>
<b>Note:</b> If some cases, you might be inclined to create a
scripting language wrapper for <tt>main()</tt>.  If you do this, the
compilation will probably work and your module might even load
correctly.  The only trouble is that when you call your
<tt>main()</tt> wrapper, you will find that it actually invokes the
<tt>main()</tt> of the scripting language interpreter itself!    This behavior
is a side effect of the symbol binding mechanism used in the dynamic linker.
The bottom line: don't do this.
</p>

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