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"
  

  $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)
• example.pl (Perl 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.