SWIG:Examples:java:pointer

SWIG/Examples/java/pointer/

Simple Pointer Handling

This example illustrates a couple of techniques for handling simple pointers in SWIG. The prototypical example is a C function that operates on pointers such as this:

void add(int *x, int *y, int *r) { 
    *r = *x + *y;
}

By default, SWIG wraps this function exactly as specified and creates an interface that expects pointer objects for arguments. SWIG wraps a C pointer with a type wrapper class, for example, SWIGTYPE_p_int for an int*. The only problem is how does one go about creating these objects from a Java program?

Possible Solutions

Write some helper functions to explicitly create objects. For example:

int *new_int(int ivalue) {
  int *i = (int *) malloc(sizeof(ivalue));
  *i = ivalue;
  return i;
}
int get_int(int *i) {
  return *i;
}

void delete_int(int *i) {
  free(i);
}

The SWIG pointer library provides an easier way.
For example, in the interface file you would do this:

%include cpointer.i
%pointer_functions(int, intp);

and from Java you would use pointers like this:

SWIGTYPE_p_int a = example.new_intp();
SWIGTYPE_p_int b = example.new_intp();
SWIGTYPE_p_int c = example.new_intp();
example.intp_assign(a,37);
example.intp_assign(b,42);

// Note that getCPtr() has package access by default
System.out.println("     a =" + Long.toHexString(SWIGTYPE_p_int.getCPtr(a)));
System.out.println("     b =" + Long.toHexString(SWIGTYPE_p_int.getCPtr(b)));
System.out.println("     c =" + Long.toHexString(SWIGTYPE_p_int.getCPtr(c)));

// Call the add() function with some pointers
example.add(a,b,c);

// Now get the result
int res = example.intp_value(c);
System.out.println("     37 + 42 =" + res);

// Clean up the pointers
example.delete_intp(a);
example.delete_intp(b);
example.delete_intp(c);

Use the SWIG typemap library. This library allows you to completely change the way arguments are processed by SWIG. For example:
```
%include "typemaps.i"
void add(int *INPUT, int *INPUT, int *OUTPUT);
```
And in a Java program:
```
int[] r = {0};
example.sub(37,42,r);
System.out.println("Result =" + r[0]);
```
Needless to say, this is substantially easier although a bit unusual.

A final alternative is to use the typemaps library in combination with the %apply directive. This allows you to change the names of parameters that behave as input or output parameters. For example:

%include "typemaps.i"
%apply int *INPUT {int *x, int *y};
%apply int *OUTPUT {int *r};

void add(int *x, int *y, int *r);
void sub(int *x, int *y, int *r);
void mul(int *x, int *y, int *r);
... etc ...

Example

The following example illustrates the use of these features for pointer extraction.

example.c (C Source)
example.i (Swig interface)
runme.java (Java program)

Notes

Since pointers are used for so many different things (arrays, output values, etc...) the complexity of pointer handling can be as complicated as you want to make it.
More documentation on the typemaps.i and cpointer.i library files can be found in the SWIG user manual. The files also contain documentation.