SWIG/Examples/go/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. This only works when there is a precise correspondence between the C type and some Go type.

Other approaches

  • The SWIG pointer library provides a different, safer, way to handle pointers. For example, in the interface file you would do this: 
    %include cpointer.i
%pointer_functions(int, intp);
    and from Go you would use pointers like this: 
    a := example.New_intp()
b := example.New_intp()
c := example.New_intp()
Intp_Assign(a, 37)
Intp_Assign(b, 42)

fmt.Println("     a =", a)
fmt.Println("     b =", b)
fmt.Println("     c =", c)

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

// Now get the result
res := example.Intp_value(c)
fmt.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 Go program: 
    r := []int{0}
example.Sub(37,42,r)
fmt.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.

    Notes