SWIG:Examples:go:simple

SWIG/Examples/go/simple/

Simple Go Example

This example illustrates how you can hook Go to a very simple C program containing a function and a global variable.

The C Code

Suppose you have the following C code:

/* File : example.c */

/* A global variable */
double Foo = 3.0;

/* Compute the greatest common divisor of positive integers */
int gcd(int x, int y) {
  int g;
  g = y;
  while (x > 0) {
    g = x;
    x = y % x;
    y = g;
  }
  return g;
}

The SWIG interface

Here is a simple SWIG interface file:

/* File: example.i */
%module example

extern int gcd(int x, int y);
extern double Foo;

Compilation

These are the instructions if you are using 6g/8g rather than gccgo.

Run swig -go example.i. This will create the three files example.go, example_gc.c, and example_wrap.c.
Compile example.go using 6g or 8g; e.g., 6g example.go.
Compile example_gc.c using 6c or 8c; e.g., 6c example_gc.c.
Put the two object files together into an archive named example.a; e.g., gopack grc example.a example.6 example_gc.6.
Compile the example_wrap.c file using your standard C compiler with the -fpic option; e.g., gcc -c -O -fpic example_wrap.c.
Also compile the actual code, not generated by SWIG; e.g., gcc -c -O -fpic example.c.
Put the gcc compiled object files into a shared library; e.g., gcc -shared -o example.so example_wrap.o example.o.
Compile the program which demonstrates how to use the library; e.g., 6g runme.go.
Link the program; e.g., 6l -o runme runme.6.
Now you should have a program runme.

Using the extension

The Go program which demonstrates calling the C functions from Go is runme.go.

Key points

Use the import statement to load your extension module from Go. For example:
```
import "example"
```
C functions work just like Go functions. However, the function names are automatically capitalized in order to make the names visible from other Go packages. For example:
```
g := example.Gcd(42,105)
```
(If there are name conflicts, you can use the %rename directive in the .i file or the -rename option to Go to rename one or the other symbol).
C global variables are accessed using getter and setter functions. The getter function is named Get followed by the capitalized name of the C variable. The Setter function uses Set instead of Get.
```
a = example.GetFoo()
```