SWIG:Examples:python:pointer

SWIG/Examples/python/pointer/

Simple Pointer Handling

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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. The only problem is how does one go about creating these objects from a script?

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
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
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
```
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

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

example.c (C Source)
example.i (Swig interface)
example.py (Python 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.