SWIG Basics

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.

5.1 Running SWIG

To run SWIG, use the swig command with options options and a filename like this:

swig [ options ] filename

where filename is a SWIG interface file or a C/C++ header file. Below is a subset of options that can be used. Additional options are also defined for each target language. A full list can be obtained by typing swig -help or swig -lang -help.

-allegrocl            Generate ALLEGROCL wrappers
-chicken              Generate CHICKEN wrappers
-clisp                Generate CLISP wrappers
-cffi                 Generate CFFI wrappers
-csharp               Generate C# wrappers
-guile                Generate Guile wrappers
-java                 Generate Java wrappers
-lua                  Generate Lua wrappers
-modula3              Generate Modula 3 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
-uffi                 Generate Common Lisp / UFFI wrappers
-xml                  Generate XML wrappers

-c++                  Enable C++ parsing
-Dsymbol              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
-Idir                 Add a directory to the file include path
-lfile                Include a SWIG library file.
-module name          Set the name of the SWIG module
-o outfile            Name of output file
-outdir dir           Set language specific files output directory
-swiglib              Show location of SWIG library
-version              Show SWIG version number

5.1.1 Input format

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 .i or .swg 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).

The name of the module is supplied using the special %module directive (or the -module 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 %module directive.

Everything in the %{ ... %} 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 not automatically appear in the generated wrapper code---therefore you need to make sure you include the proper header files in the %{ ... %} section. It should be noted that the text enclosed in %{ ... %} is not parsed or interpreted by SWIG. The %{...%} 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.

5.1.2 SWIG Output

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 file.i is transformed into a file file_wrap.c or file_wrap.cxx (depending on whether or not the -c++ option has been used). The name of the output file can be changed using the -o 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 -o option to change the suffix of the SWIG-generated wrapper file if you want something different than the default. For example:

$ swig -c++ -python -o example_wrap.cpp example.i

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.

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 -outdir option. For example:

$ swig -c++ -python -outdir pyfiles -o cppfiles/example_wrap.cpp example.i

If the directories cppfiles and pyfiles exist, the following will be generated:

cppfiles/example_wrap.cpp
pyfiles/example.py

5.1.3 Comments

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.

5.1.4 C Preprocessor

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, #include statements are ignored unless the -includeall 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.).

It should also be noted that the SWIG preprocessor skips all text enclosed inside a %{...%} 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 "Preprocessor" chapter.

5.1.5 SWIG Directives

Most of SWIG's operation is controlled by special directives that are always preceded by a "%" 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.

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:

SWIG is a special preprocessing symbol defined by SWIG when it is parsing an input file.

5.1.6 Parser Limitations

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:

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

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 typedef-name terminal symbol as described on p. 234 of K&R).

5.2 Wrapping Simple C Declarations

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:

%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"

In this file, there are two functions sin() and strcmp(), a global variable Foo, and two constants STATUS and VERSION. When SWIG creates an extension module, these declarations are accessible as scripting language functions, variables, and constants respectively. For example, in Tcl:

>>> example.sin(3)
5.2335956
>>> example.strcmp('Dave','Mike')
-1
>>> print example.cvar.Foo
42
>>> print example.STATUS
50
>>> print example.VERSION
1.1

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.

5.2.1 Basic Type Handling

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.

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

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.

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

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

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 int datatype and all variations of char and short datatypes are safe to use. For unsigned int and long datatypes, you will need to carefully check the correct operation of your program after it has been wrapped with SWIG.

Although the SWIG parser supports the long long datatype, not all language modules support it. This is because long long usually exceeds the integer precision available in the target language. In certain modules such as Tcl and Perl5, long long integers are encoded as strings. This allows the full range of these numbers to be represented. However, it does not allow long long values to be used in arithmetic expressions. It should also be noted that although long long 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 long long before trying to use this type with SWIG.

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

The char 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 char datatype, it gets the value `f'.

The char * 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 char * 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 Typemaps for details about this.

At this time, SWIG does not provide any special support for Unicode or wide-character strings (the C wchar_t 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 char * 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.

5.2.2 Global Variables

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

# 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

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 cvar (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 double get_foo() and set_foo(double val) that are used to manipulate the value.

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

5.2.3 Constants

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

#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

In #define 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 #define in order for a constant to actually be created. This restriction is necessary because #define is also used to define preprocessor macros that are definitely not meant to be part of the scripting language interface. For example:

In this case, you probably don't want to create a constant called EXTERN (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

defines a constant because PI was already defined as a constant and the value is known.

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).

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 %{,%} 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.

The %constant 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, %constant is only used when you want to add constants to the scripting language interface that are not defined in the original header file.

5.2.4 A brief word about const

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

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

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

In this case, the pointer e can change---it's only the value being pointed to that is read-only.

Compatibility Note: One reason for changing SWIG to handle const declarations as read-only variables is that there are many situations where the value of a const variable might change. For example, a library might export a symbol as const 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 char *const, the underlying data being pointed to can be modified--it's only the pointer itself that is constant. In an embedded system, a const 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 const qualifier, the new interpretation of const as "read-only" is simple and exactly matches the actual semantics of const in C/C++. If you really want to create a constant as in older versions of SWIG, use the %constant directive instead. For example:

5.2.5 A cautionary tale of char *

Before going any further, there is one bit of caution involving char * that must now be mentioned. When strings are passed from a scripting language to a C char *, 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.

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

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 s refers to some internal data in the target language---data that you shouldn't be touching.

The bottom line: don't rely on char * for anything other than read-only input values. However, it must be noted that you could change the behavior of SWIG using typemaps.

5.3 Pointers and complex objects

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

5.3.1 Simple pointers

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:

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

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.

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 int * into an list of integers). There are several reasons why SWIG does not do this:

5.3.2 Run time pointer type checking

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).

Like C, void * matches any kind of pointer. Furthermore, NULL pointers can be passed to any function that expects to receive a pointer. Although this has the potential to cause a crash, NULL 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.

5.3.3 Derived types, structs, and classes

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

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.

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

%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 *);

In this file, SWIG doesn't know what a FILE 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 :

# 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 > 0):
		fwrite(buffer,8192,1,f2)
		nbytes = fread(buffer,8192,1,f1)
	free(buffer)

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

5.3.4 Undefined datatypes

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:

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

Unlike C or C++, SWIG does not actually care whether Matrix 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 Matrix really is as long as you can pass an opaque reference to one around in the scripting language interface.

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

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

The only way to fix this problem is to make sure you properly declare type names using typedef.

5.3.5 Typedef

typedef 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:

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

$ swig -I/usr/include -includeall example.i

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).

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

The corresponding wrapper function will accept arguments of type unsigned int * or size_t *.

5.4 Other Practicalities

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.

5.4.1 Passing structures by value

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

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

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

5.4.2 Return by value

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

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

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;
}

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;
}

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.

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 Vector doesn't define a default constructor. The section on SWIG and C++ has more information about this case.

5.4.3 Linking to structure variables

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

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.

5.4.4 Linking to char *

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

/* 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);
}

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

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:

A common mistake with char * variables is to link to a variable declared like this:

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 free or delete when the string literal value currently assigned to the variable wasn't allocated using malloc() or new. 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:

When variables of type const char * are declared, SWIG still generates functions for setting and getting the value. However, the default behavior does not 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:

example.i:20. Typemap warning. Setting const char * variable may leak memory

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

Therefore, it's a really bad idea to call free() on such a pointer. On the other hand, it is 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.

5.4.5 Arrays

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 :

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.

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

It is important to note that in the C type system, a multidimensional array a[][] is NOT equivalent to a single pointer *a or a double pointer such as **a. 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.

In this case, reading the variable 'a' returns a pointer of type int (*)[200] that points to the first element of the array &a[0][0]. 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:

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

// 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));
  }
%}

Arrays of char 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,

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

5.4.6 Creating read-only variables

A read-only variable can be created by using the %immutable directive as shown :

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

The %mutable and %immutable directives are actually %feature directives defined like this:

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

Read-only variables are also created when declarations are declared as const. For example:

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

5.4.7 Renaming and ignoring declarations

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 %rename directive as shown :

// 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;

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

The placement of the %rename 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:

%rename 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 :

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.

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

%ignore print;         // Ignore all declarations named print
%ignore _HAVE_FOO_H;   // Ignore an include guard constant
...
%include "foo.h"       // Grab a header file
...

Any function, variable etc which matches %ignore will not be wrapped and therefore will not be available from the target language. A common usage of %ignore 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.

More powerful variants of %rename and %ignore directives can be used to help wrap C++ overloaded functions and methods or C++ methods which use default arguments. This is described in the Ambiguity resolution and renaming section in the C++ chapter.

Compatibility note: Older versions of SWIG provided a special %name directive for renaming declarations. For example:

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

5.4.8 Default/optional arguments

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 :

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

Note: 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 default arguments in the C++ chapter for further details.

5.4.9 Pointers to functions and callbacks

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:

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:

>>> def add(x,y):
...     return x+y
...
>>> binary_op(3,4,add)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: Type error. Expected _p_f_int_int__int
>>>

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 %constant directive like this:

/* 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);

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

Unfortunately, by declaring the callback functions as constants, they are no longer accessible as functions. For example:

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

/* 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

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

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

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 ADD, SUB, and MUL:

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

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.

5.5 Structures and unions

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.

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 :

double Vector_x_get(struct Vector *obj) {
	return obj->x;
}
double Vector_y_get(struct Vector *obj) { 
	return obj->y;
}
double Vector_z_get(struct Vector *obj) { 
	return obj->z;
}
void Vector_x_set(struct Vector *obj, double value) {
	obj->x = value;
}
void Vector_y_set(struct Vector *obj, double value) {
	obj->y = value;
}
void Vector_z_set(struct Vector *obj, double value) {
	obj->z = value;
}

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

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

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

5.5.1 Typedef and structures

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 typedef allows SWIG to drop the struct keyword on its generated code. For example:

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

5.5.2 Character strings and structures

Structures involving character strings require some care. SWIG assumes that all members of type char * have been dynamically allocated using malloc() 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 :

char *Foo_name_get(Foo *obj) {
	return Foo->name;
}

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

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.

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

5.5.3 Array members

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 :

interface.i:116. Warning. Array member will be read-only

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.

5.5.4 Structure data members

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

When a structure member is wrapped, it is handled as a pointer, unless the %naturalvar directive is used where it is handled more like a C++ reference (see C++ Member data). The accessors to the member variable as a pointer is effectively wrapped as follows:

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 f.x of a Bar object like this:

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

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

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,

and nothing was known about WORD, then SWIG will generate more normal accessor functions like this:

Compatibility Note: SWIG-1.3.11 and earlier releases transformed all non-primitive member datatypes to pointers. Starting in SWIG-1.3.12, this transformation only 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 "struct Foo;".

5.5.5 C constructors and destructors

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 malloc() and free(). Note: the use of malloc() only applies when SWIG is used on C code (i.e., when the -c++ option is not supplied on the command line). C++ is handled differently.

If you don't want SWIG to generate default constructors for your interfaces, you can use the %nodefaultctor directive or the -nodefaultctor command line option. For example:

swig -nodefaultctor example.i

If you need more precise control, %nodefaultctor can selectively target individual structure definitions. For example:

%nodefaultctor Foo;      // No default constructor for Foo
...
struct Foo {             // No default constructor generated.
};

struct Bar {             // Default constructor generated.
};

Since ignoring the implicit or default destructors most of the times produce memory leaks, SWIG will always try to generate them. If needed, however, you can selectively disable the generation of the default/implicit destructor by using %nodefaultdtor

%nodefaultdtor Foo; // No default/implicit destructor for Foo
...
struct Foo {              // No default destructor is generated.
};

struct Bar {              // Default destructor generated.
};

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

Note: There are also the -nodefault option and %nodefault directive, which disable both the default or implicit destructor generation. This could lead to memory leaks across the target languages, and is highly recommended you don't use them.

5.5.6 Adding member functions to C structures

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 %extend 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 :

You can make a Vector look a lot like a class by writing a SWIG interface like this:

// 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->x = x;
		v->y = y;
		v->z = z;
		return v;
	}
	~Vector() {
		free($self);
	}
	double magnitude() {
		return sqrt($self->x*$self->x+$self->y*$self->y+$self->z*$self->z);
	}
	void print() {
		printf("Vector [%g, %g, %g]\n", $self->x,$self->y,$self->z);
	}
};

Note the usage of the $self special variable. Its usage is identical to a C++ 'this' pointer and should be used whenever access to the struct instance is required.

>>> v = Vector(3,4,0)                 # Create a new vector
>>> print v.magnitude()                # Print magnitude
5.0
>>> v.print()                  # Print it out
[ 3, 4, 0 ]
>>> del v                      # Destroy it

The %extend directive can also be used inside the definition of the Vector structure. For example:

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

typedef struct {
	double x,y,z;
	%extend {
		Vector(double x, double y, double z) { ... }
		~Vector() { ... }
		...
	}
} Vector;

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

/* 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->x = x;
	v->y = y;
	v->z = z;
	return v;
}
void delete_Vector(Vector *v) {
	free(v);
}

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

A little known feature of the %extend 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 magnitude a read-only attribute of Vector instead of a method. To do this, you might write some code like this:

// 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->x*v->x+v->y*v->y+v->z*v->z);
}
%}

Now, for all practical purposes, magnitude will appear like an attribute of the object.

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

By default, the name 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:

struct Person {
    %extend {
       char *name;
    }
...
}

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

Finally, it should be stressed that even though %extend 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).

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

5.5.7 Nested structures

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

typedef union {
	int 		ivalue;
	double		dvalue;
	char		*strvalue;
	void		*ptrvalue;
} Object_intRep;

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

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

Object_intRep *Object_intRep_get(Object *o) {
	return (Object_intRep *) &o->intRep;
}
int Object_intRep_ivalue_get(Object_intRep *o) {
	return o->ivalue;
}
int Object_intRep_ivalue_set(Object_intRep *o, int value) {
	return (o->ivalue = value);
}
double Object_intRep_dvalue_get(Object_intRep *o) {
	return o->dvalue;
}
... etc ...

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:

# Perl5 script for accessing nested member
$o = CreateObject();                    # Create an object somehow
$o->{intRep}->{ivalue} = 7              # Change value of o.intRep.ivalue

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.

5.5.8 Other things to note about structure wrapping

SWIG doesn't care if the declaration of a structure in a .i 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.

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,

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

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 ",&arg0,
                     SWIGTYPE_p_Vector) == TCL_ERROR)
         return TCL_ERROR;
    result = (double ) (arg1->x);
    Tcl_SetObjResult(interp,Tcl_NewDoubleObj((double) result));
    return TCL_OK;
}

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

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.

5.6 Code Insertion

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.

5.6.1 The output of SWIG

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).

5.6.2 Code insertion blocks

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

%runtime %{
   ... code in runtime section ...
%}

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

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

%init %{
   ... code in init section ...
%}

The bare %{ ... %} directive is a shortcut that is the same as %header %{ ... %}.

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.

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 :

5.6.3 Inlined code blocks

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

The %inline 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 new_Vector using only one declaration. Since the code inside an %inline %{ ... %} block is given to both the C compiler and SWIG, it is illegal to include any SWIG directives inside a %{ ... %} block.

5.6.4 Initialization blocks

When code is included in the %init 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:

5.7 An Interface Building Strategy

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

5.7.1 Preparing a C program for SWIG

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 :

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

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 swig-devel mailing list or to the SWIG bug tracker.

5.7.2 The SWIG interface file

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

A typical SWIG interface file for this header file would look like the following :

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

5.7.3 Why use separate interface files?

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

5.7.4 Getting the right header files

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 %{,%} block like this:

5.7.5 What to do with main()

If your program defines a main() 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 main() procedure that is called instead. main() also makes no sense when working with dynamic loading. There are a few approaches to solving the main() conflict :

Getting rid of main() may cause potential initialization problems of a program. To handle this problem, you may consider writing a special function called program_init() 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.

As a general note, many C programs only use the main() 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 main() program can be completely replaced by a Perl, Python, or Tcl script.

Note: If some cases, you might be inclined to create a scripting language wrapper for main(). 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 main() wrapper, you will find that it actually invokes the main() 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.

5 SWIG Basics