13 Argument Handling

In Chapter 5, SWIG's treatment of basic datatypes and pointers was described. In particular, primitive types such as int and double are mapped to corresponding types in the target language. For everything else, pointers are used to refer to structures, classes, arrays, and other user-defined datatypes. However, in certain applications it is desirable to change SWIG's handling of a specific datatype. For example, you might want to return multiple values through the arguments of a function. This chapter describes some of the techniques for doing this.

13.1 The typemaps.i library

This section describes the typemaps.i library file--commonly used to change certain properties of argument conversion.

13.1.1 Introduction

Suppose you had a C function like this:

void add(double a, double b, double *result) {
  *result = a + b;

From reading the source code, it is clear that the function is storing a value in the double *result parameter. However, since SWIG does not examine function bodies, it has no way to know that this is the underlying behavior.

One way to deal with this is to use the typemaps.i library file and write interface code like this:

// Simple example using typemaps
%module example
%include "typemaps.i"

%apply double *OUTPUT { double *result };
%inline %{
extern void add(double a, double b, double *result);

The %apply directive tells SWIG that you are going to apply a special type handling rule to a type. The "double *OUTPUT" specification is the name of a rule that defines how to return an output value from an argument of type double *. This rule gets applied to all of the datatypes listed in curly braces-- in this case "double *result".

When the resulting module is created, you can now use the function like this (shown for Python):

>>> a = add(3, 4)
>>> print a

In this case, you can see how the output value normally returned in the third argument has magically been transformed into a function return value. Clearly this makes the function much easier to use since it is no longer necessary to manufacture a special double * object and pass it to the function somehow.

Once a typemap has been applied to a type, it stays in effect for all future occurrences of the type and name. For example, you could write the following:

%module example
%include "typemaps.i"

%apply double *OUTPUT { double *result };

%inline %{
extern void add(double a, double b, double *result);
extern void sub(double a, double b, double *result);
extern void mul(double a, double b, double *result);
extern void div(double a, double b, double *result);

In this case, the double *OUTPUT rule is applied to all of the functions that follow.

Typemap transformations can even be extended to multiple return values. For example, consider this code:

%include "typemaps.i"
%apply int *OUTPUT { int *width, int *height };

// Returns a pair (width, height)
void getwinsize(int winid, int *width, int *height);

In this case, the function returns multiple values, allowing it to be used like this:

>>> w, h = genwinsize(wid)
>>> print w
>>> print h

It should also be noted that although the %apply directive is used to associate typemap rules to datatypes, you can also use the rule names directly in arguments. For example, you could write this:

// Simple example using typemaps
%module example
%include "typemaps.i"

extern void add(double a, double b, double *OUTPUT);
extern void add(double a, double b, double *OUTPUT);

Typemaps stay in effect until they are explicitly deleted or redefined to something else. To clear a typemap, the %clear directive should be used. For example:

%clear double *result;      // Remove all typemaps for double *result

13.1.2 Input parameters

The following typemaps instruct SWIG that a pointer really only holds a single input value:

int *INPUT
short *INPUT
long *INPUT
unsigned int *INPUT
unsigned short *INPUT
unsigned long *INPUT
double *INPUT
float *INPUT

When used, it allows values to be passed instead of pointers. For example, consider this function:

double add(double *a, double *b) {
  return *a+*b;

Now, consider this SWIG interface:

%module example
%include "typemaps.i"
extern double add(double *, double *);
extern double add(double *INPUT, double *INPUT);

When the function is used in the scripting language interpreter, it will work like this:

result = add(3, 4)

13.1.3 Output parameters

The following typemap rules tell SWIG that pointer is the output value of a function. When used, you do not need to supply the argument when calling the function. Instead, one or more output values are returned.

short *OUTPUT
long *OUTPUT
unsigned int *OUTPUT
unsigned short *OUTPUT
unsigned long *OUTPUT
double *OUTPUT
float *OUTPUT

These methods can be used as shown in an earlier example. For example, if you have this C function :

void add(double a, double b, double *c) {
  *c = a+b;

A SWIG interface file might look like this :

%module example
%include "typemaps.i"
%inline %{
extern void add(double a, double b, double *OUTPUT);

In this case, only a single output value is returned, but this is not a restriction. An arbitrary number of output values can be returned by applying the output rules to more than one argument (as shown previously).

If the function also returns a value, it is returned along with the argument. For example, if you had this:

extern int foo(double a, double b, double *OUTPUT);

The function will return two values like this:

iresult, dresult = foo(3.5, 2)

13.1.4 Input/Output parameters

When a pointer serves as both an input and output value you can use the following typemaps :

int *INOUT
short *INOUT
long *INOUT
unsigned int *INOUT
unsigned short *INOUT
unsigned long *INOUT
double *INOUT
float *INOUT

A C function that uses this might be something like this:

void negate(double *x) {
  *x = -(*x);

To make x function as both and input and output value, declare the function like this in an interface file :

%module example
%include "typemaps.i"
extern void negate(double *);
extern void negate(double *INOUT);

Now within a script, you can simply call the function normally :

a = negate(3);         # a = -3 after calling this

One subtle point of the INOUT rule is that many scripting languages enforce mutability constraints on primitive objects (meaning that simple objects like integers and strings aren't supposed to change). Because of this, you can't just modify the object's value in place as the underlying C function does in this example. Therefore, the INOUT rule returns the modified value as a new object rather than directly overwriting the value of the original input object.

Compatibility note : The INOUT rule used to be known as BOTH in earlier versions of SWIG. Backwards compatibility is preserved, but deprecated.

13.1.5 Using different names

As previously shown, the %apply directive can be used to apply the INPUT, OUTPUT, and INOUT typemaps to different argument names. For example:

// Make double *result an output value
%apply double *OUTPUT { double *result };

// Make Int32 *in an input value
%apply int *INPUT { Int32 *in };

// Make long *x inout
%apply long *INOUT {long *x};

To clear a rule, the %clear directive is used:

%clear double *result;
%clear Int32 *in, long *x;

Typemap declarations are lexically scoped so a typemap takes effect from the point of definition to the end of the file or a matching %clear declaration.

13.2 Applying constraints to input values

In addition to changing the handling of various input values, it is also possible to use typemaps to apply constraints. For example, maybe you want to insure that a value is positive, or that a pointer is non-NULL. This can be accomplished including the constraints.i library file.

13.2.1 Simple constraint example

The constraints library is best illustrated by the following interface file :

// Interface file with constraints
%module example
%include "constraints.i"

double exp(double x);
double log(double POSITIVE);         // Allow only positive values
double sqrt(double NONNEGATIVE);     // Non-negative values only
double inv(double NONZERO);          // Non-zero values
void   free(void *NONNULL);          // Non-NULL pointers only

The behavior of this file is exactly as you would expect. If any of the arguments violate the constraint condition, a scripting language exception will be raised. As a result, it is possible to catch bad values, prevent mysterious program crashes and so on.

13.2.2 Constraint methods

The following constraints are currently available

POSITIVE                     Any number > 0 (not zero)
NEGATIVE                     Any number < 0 (not zero)
NONNEGATIVE                  Any number >= 0
NONPOSITIVE                  Any number <= 0
NONZERO                      Nonzero number
NONNULL                      Non-NULL pointer (pointers only).

13.2.3 Applying constraints to new datatypes

The constraints library only supports the primitive C datatypes, but it is easy to apply it to new datatypes using %apply. For example :

// Apply a constraint to a Real variable
%apply Number POSITIVE { Real in };

// Apply a constraint to a pointer type
%apply Pointer NONNULL { Vector * };

The special types of "Number" and "Pointer" can be applied to any numeric and pointer variable type respectively. To later remove a constraint, the %clear directive can be used :

%clear Real in;
%clear Vector *;