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);
  }
  

  Now, in a script you would do this:

  a = new_int(37)
  b = new_int(42)
  c = new_int(0)
  add(a,b,c)
  r = get_int(c)
  print "Result = #{r}\n"
  delete_int(a)
  delete_int(b)
  delete_int(c)
  

• Use the SWIG pointer library. For example, in the interface file you would do this:

  %include "pointer.i"
  

  and in a script you would do this:

  a = ptrcreate("int",37)
  b = ptrcreate("int",42)
  c = ptrcreate("int")
  add(a,b,c)
  r = ptrvalue(c)
  print "Result = #{r}\n"
  ptrfree(a)
  ptrfree(b)
  ptrfree(c)
  

  The advantage to using the pointer library is that it unifies some of the helper functions behind a common set of names. For example, the same set of functions work with int, double, float, and other fundamental types.

• 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 script:

  r = add(37,42)
  print "Result = #{r}\n"
  

  Needless to say, this is substantially easier.

• 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)
• runme.rb (Ruby Script)

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 pointer.i library files can be found in the SWIG user manual. The files also contain documentation.

• The pointer.i library is designed primarily for convenience. If you are concerned about performance, you probably want to use a different approach.