11 Preprocessing

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.

11.1 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 auxiliary files.

11.2 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 later in the Working with Modules 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.

11.3 Conditional Compilation

SWIG fully supports the use of #if, #ifdef, #ifndef, #else, #endif to conditionally include parts of an interface.

SWIG's preprocessor conditionals support the standard C/C++ preprocessor integer expressions. As a SWIG-specific extension, string equality and inequality tests are also supported, for example:

#if defined __cplusplus && (#__VA_ARGS__ != "" || #TYPE == "void")

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
SWIG_VERSION                    Hexadecimal (binary-coded decimal) number containing SWIG version,
                                such as 0x010311 (corresponding to SWIG-1.3.11).

SWIGCSHARP                      Defined when using C#
SWIGD                           Defined when using D
SWIGGO                          Defined when using Go
SWIGGUILE                       Defined when using Guile
SWIGJAVA                        Defined when using Java
SWIGJAVASCRIPT                  Defined when using Javascript
SWIG_JAVASCRIPT_JSC             Defined when using Javascript with -jsc
SWIG_JAVASCRIPT_V8              Defined when using Javascript with -v8 or -node
SWIGLUA                         Defined when using Lua
SWIGMZSCHEME                    Defined when using Mzscheme
SWIGOCAML                       Defined when using OCaml
SWIGOCTAVE                      Defined when using Octave
SWIGPERL                        Defined when using Perl
SWIGPHP                         Defined when using PHP (any version)
SWIGPHP7                        Defined when using PHP 7 or later (with a compatible C API)
SWIGPYTHON                      Defined when using Python
SWIGR                           Defined when using R
SWIGRUBY                        Defined when using Ruby
SWIGSCILAB                      Defined when using Scilab
SWIGTCL                         Defined when using Tcl
SWIGXML                         Defined when using XML

SWIG also defines SWIG_VERSION and a target language macro in the generated wrapper file (since SWIG 4.1.0 - in older versions these were defined for some target languages but this wasn't consistent). Best practice is to use SWIG-time conditional checks because that results in smaller generated wrapper sources.

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

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

The following are language specific symbols that might be defined:

SWIG_D_VERSION                  Unsigned integer target version when using D
SWIGGO_CGO                      Defined when using Go for cgo
SWIGGO_GCCGO                    Defined when using Go for gccgo
SWIGGO_INTGO_SIZE               Size of the Go type int when using Go (32 or 64)
SWIGPYTHON_BUILTIN              Defined when using Python with -builtin
SWIG_RUBY_AUTORENAME            Defined when using Ruby with -autorename

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.

11.4 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

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 ISOC_(args)   (args)
#define ISOC_(args) ()

The following operators can appear in macro definitions:

11.5 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) {
type name ## _get(type *t, int index) {
  return t[index];
void name ## _set(type *t, int index, type val) {
  t[index] = val;

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).

11.6 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.

11.7 Preprocessing and delimiters

The preprocessor handles { }, " " and %{ %} delimiters differently.

11.7.1 Preprocessing and %{ ... %} & " ... " delimiters

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() {

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

11.7.2 Preprocessing and { ... } delimiters

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");

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");

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

11.8 Preprocessor and Typemaps

Typemaps support a special attribute called noblock where the { ... } delimiters can be used, but the delimiters are not actually generated into the code. The effect is then similar to using "" or %{ %} delimiters but the code is run through the preprocessor. For example:

#define SWIG_macro(CAST) (CAST)$input
%typemap(in) Int {$1= SWIG_macro(int);}

might generate



#define SWIG_macro(CAST) (CAST)$input
%typemap(in, noblock=1) Int {$1= SWIG_macro(int);}

might generate



#define SWIG_macro(CAST) (CAST)$input
%typemap(in) Int %{$1=SWIG_macro(int);%}

would generate


11.9 Viewing preprocessor output

Like many compilers, SWIG supports a -E command line option to display the output from the preprocessor. When the -E option 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.

11.10 The #error and #warning directives

SWIG supports the commonly used #warning and #error preprocessor directives. The #warning directive will cause SWIG to issue a warning then continue processing. The #error directive will cause SWIG to exit with a fatal error. Example usage:

#error "This is a fatal error message"
#warning "This is a warning message"

The #error behaviour can be made to work like #warning if the -cpperraswarn commandline option is used. Alternatively, the #pragma directive can be used to the same effect, for example:

  /* Modified behaviour: #error does not cause SWIG to exit with error */
  #pragma SWIG cpperraswarn=1
  /* Normal behaviour: #error does cause SWIG to exit with error */
  #pragma SWIG cpperraswarn=0