hello.go

package main

func Hello() string {
	return "Hello, from a Go extension!"
}

helloc.c

#include <stdio.h>
#include "helloc.h"

void HelloC() {
    printf("Hello, from a C extension!\n");
}

helloc.h

#include <stdio.h>
#include <stdlib.h>

void HelloC();

main.go

package main

import (
	"fmt"
	"plugin"
)

type extension struct {
	path     string
	funcName string
}

func (e *extension) load() (func() string, error) {
	p, err := plugin.Open(e.path)
	if err != nil {
		return nil, err
	}

	f, err := p.Lookup(e.funcName)
	if err != nil {
		return nil, err
	}

	return f.(func() string), nil
}

func main() {
	for _, e := range []extension{
		{"./lib/hello.so", "Hello"},
		{"./lib/helloc.so", "HelloC"},
	} {
		f, err := e.load()
		if err != nil {
			panic(err)
		}
		fmt.Println(fmt.Sprintf("Calling %s() from %s:\n\t%s", e.funcName, e.path, f()))
	}
}

Makefile

run:
	@gcc helloc.c  -fPIC -shared -o lib/helloc.so
	@go build -o lib/hello.so -buildmode=plugin hello.go
	@go run main.go