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;
}

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

• example.c (C Source)
• example.i (Swig interface)
• main.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.