# 7 Preprocessing

- File inclusion
- File imports
- Conditional Compilation
- Macro Expansion
- SWIG Macros
- C99 and GNU Extensions
- Preprocessing and %{ ... %} blocks
- Preprocessing and { ... }
- Viewing preprocessor output

SWIG includes its own enhanced version of the C preprocessor. The preprocessor supports the standard preprocessor directives and macro expansion rules. However, a number of modifications and enhancements have been made. This chapter describes some of these modifications.

## 7.1 File inclusion

To include another file into a SWIG interface, use the `%include` directive like this:

>     %include "pointer.i"

Unlike, `#include`, `%include` includes each file once (and will not reload the file on subsequent `%include` declarations). Therefore, it is not necessary to use include-guards in SWIG interfaces.

By default, the `#include` is ignored unless you run SWIG with the `-includeall` option. The reason for ignoring traditional includes is that you often don't want SWIG to try and wrap everything included in standard header system headers and auxilliary files.

## 7.2 File imports

SWIG provides another file inclusion directive with the `%import` directive. For example:

>     %import "foo.i"

The purpose of `%import` is to collect certain information from another SWIG interface file or a header file without actually generating any wrapper code. Such information generally includes type declarations (e.g., `typedef`) as well as C++ classes that might be used as base-classes for class declarations in the interface. The use of `%import` is also important when SWIG is used to generate extensions as a collection of related modules. This is an advanced topic and is described in a later chapter.

The `-importall` directive tells SWIG to follow all `#include` statements as imports. This might be useful if you want to extract type definitions from system header files without generating any wrappers.

## 7.3 Conditional Compilation

SWIG fully supports the use of `#if`, `#ifdef`, `#ifndef`, `#else`, `#endif` to conditionally include parts of an interface. The following symbols are predefined by SWIG when it is parsing the interface:

>     SWIG                            Always defined when SWIG is processing a file
>     SWIGIMPORTED                    Defined when SWIG is importing a file with %import
>     SWIGMAC                         Defined when running SWIG on the Macintosh
>     SWIGWIN                         Defined when running SWIG under Windows
>     SWIG_VERSION                    Hexadecimal number containing SWIG version,
>                                     such as 0x010311 (corresponding to SWIG-1.3.11).

>     SWIGCHICKEN                     Defined when using CHICKEN
>     SWIGCSHARP                      Defined when using C#
>     SWIGGUILE                       Defined when using Guile
>     SWIGJAVA                        Defined when using Java
>     SWIGMZSCHEME                    Defined when using Mzscheme
>     SWIGOCAML                       Defined when using Ocaml
>     SWIGPERL                        Defined when using Perl
>     SWIGPERL5                       Defined when using Perl5
>     SWIGPHP                         Defined when using PHP
>     SWIGPHP4                        Defined when using PHP4
>     SWIGPYTHON                      Defined when using Python
>     SWIGRUBY                        Defined when using Ruby
>     SWIGSEXP                        Defined when using S-expressions
>     SWIGTCL                         Defined when using Tcl
>     SWIGTCL8                        Defined when using Tcl8.0
>     SWIGXML                         Defined when using XML

In addition, SWIG defines the following set of standard C/C++ macros:

>     __LINE__                        Current line number
>     __FILE__                        Current file name
>     __STDC__                        Defined to indicate ANSI C
>     __cplusplus                     Defined when -c++ option used

Interface files can look at these symbols as necessary to change the way in which an interface is generated or to mix SWIG directives with C code. These symbols are also defined within the C code generated by SWIG (except for the symbol ``SWIG`' which is only defined within the SWIG compiler).

## 7.4 Macro Expansion

Traditional preprocessor macros can be used in SWIG interfaces. Be aware that the `#define` statement is also used to try and detect constants. Therefore, if you have something like this in your file,

>     #ifndef _FOO_H 1
>     #define _FOO_H 1
>     ...
>     #endif

you may get some extra constants such as `_FOO_H` showing up in the scripting interface.

More complex macros can be defined in the standard way. For example:

>     #define EXTERN extern
>     #ifdef __STDC__
>     #define _ANSI(args)   (args)
>     #else
>     #define _ANSI(args) ()
>     #endif

The following operators can appear in macro definitions:

- `#x`
  Converts macro argument `x` to a string surrounded by double quotes ("x").
- `x ## y`
  Concatenates x and y together to form `xy`.
- ``x``
  If `x` is a string surrounded by double quotes, do nothing. Otherwise, turn into a string like `#x`. This is a non-standard SWIG extension.

## 7.5 SWIG Macros

SWIG provides an enhanced macro capability with the `%define` and `%enddef` directives. For example:

>     %define ARRAYHELPER(type,name)
>     %inline %{
>     type *new_ ## name (int nitems) {
>        return (type *) malloc(sizeof(type)*nitems);
>     }
>     void delete_ ## name(type *t) {
>        free(t);
>     }
>     type name ## _get(type *t, int index) {
>        return t[index];
>     }
>     void name ## _set(type *t, int index, type val) {
>        t[index] = val;
>     }
>     %}
>     %enddef

>     ARRAYHELPER(int, IntArray)
>     ARRAYHELPER(double, DoubleArray)

The primary purpose of `%define` is to define large macros of code. Unlike normal C preprocessor macros, it is not necessary to terminate each line with a continuation character (\)--the macro definition extends to the first occurrence of `%enddef`. Furthermore, when such macros are expanded, they are reparsed through the C preprocessor. Thus, SWIG macros can contain all other preprocessor directives except for nested `%define` statements.

The SWIG macro capability is a very quick and easy way to generate large amounts of code. In fact, many of SWIG's advanced features and libraries are built using this mechanism (such as C++ template support).

## 7.6 C99 and GNU Extensions

SWIG-1.3.12 and newer releases support variadic preprocessor macros. For example:

>     #define DEBUGF(fmt,...)   fprintf(stderr,fmt,__VA_ARGS__)

When used, any extra arguments to `...` are placed into the special variable `__VA_ARGS__`. This also works with special SWIG macros defined using `%define`.

SWIG allows a variable number of arguments to be empty. However, this often results in an extra comma (,) and syntax error in the resulting expansion. For example:

>     DEBUGF("hello");   --> fprintf(stderr,"hello",);

To get rid of the extra comma, use `##` like this:

>     #define DEBUGF(fmt,...)   fprintf(stderr,fmt, ##__VA_ARGS__)

SWIG also supports GNU-style variadic macros. For example:

>     #define DEBUGF(fmt, args...)  fprintf(stdout,fmt,args)

Comment: It's not entirely clear how variadic macros might be useful to interface building. However, they are used internally to implement a number of SWIG directives and are provided to make SWIG more compatible with C99 code.

## 7.7 Preprocessing and %{ ... %} blocks

The SWIG preprocessor does not process any text enclosed in a code block %{ ... %}. Therefore, if you write code like this,

>     %{
>     #ifdef NEED_BLAH
>     int blah() {
>        ...
>     }
>     #endif
>     %}

the contents of the `%{ ... %}` block are copied without modification to the output (including all preprocessor directives).

## 7.8 Preprocessing and { ... }

SWIG always runs the preprocessor on text appearing inside `{ ... }`. However, sometimes it is desirable to make a preprocessor directive pass through to the output file. For example:

>     %extend Foo {
>        void bar() {
>           #ifdef DEBUG
>            printf("I'm in bar\n");
>           #endif
>        }
>     }

By default, SWIG will interpret the `#ifdef DEBUG` statement. However, if you really wanted that code to actually go into the wrapper file, prefix the preprocessor directives with `%` like this:

>     %extend Foo {
>        void bar() {
>           %#ifdef DEBUG
>            printf("I'm in bar\n");
>           %#endif
>        }
>     }

SWIG will strip the extra `%` and leave the preprocessor directive in the code.

## 7.9 Viewing preprocessor output

Like many compilers, SWIG supports a `-E` command line option to display the output from the preprocessor. When the `-E` switch is used, SWIG will not generate any wrappers. Instead the results after the preprocessor has run are displayed. This might be useful as an aid to debugging and viewing the results of macro expansions.