[dbohdan] 2016-09-19: The following is, to my best knowledge, about as simple as a Tcl extension written in [Go] can get. The wrapper functions `*_cgo` are necessary since you http://stackoverflow.com/a/37641451%|%can't pass a pointer to a Go function%|% to C code. In a real extension those wrappers could be generated automatically via `[https://blog.golang.org/generate%|%go generate]` and a Tcl script. The code has been tested with Go v1.6.3 on Linux x86_64. Build and run with `gmake test`. 

** Makefile **
======none
libtclgoexample.so: *.go
<replace this with a tab>go build -o $@ -buildmode=c-shared
test: libtclgoexample.so
<replace this with a tab>echo 'load libtclgoexample.so; puts [hello]; puts [square 5]' | tclsh
.PHONY: test
======

** example.go **
======c
//example.go
package main

/*
#include <stdlib.h>
#include <tcl.h>
#include <tclDecls.h>
#cgo LDFLAGS: -ltcl8.6

int Hello_Cmd_cgo(ClientData cdata, Tcl_Interp *interp, int objc,
        Tcl_Obj *const objv[]);
int Square_Cmd_cgo(ClientData cdata, Tcl_Interp *interp, int objc,
        Tcl_Obj *const objv[]);
*/
import "C"

import (
        "reflect"
        "strconv"
        "unsafe"
)

const (
        TCL_OK       = 0
        TCL_ERROR    = 1
        TCL_RETURN   = 2
        TCL_BREAK    = 3
        TCL_CONTINUE = 4
)

//export Hello_Cmd
func Hello_Cmd(cdata C.ClientData, interp *C.struct_Tcl_Interp,
        objc C.int, objv **C.Tcl_Obj) C.int {
        if objc != 1 {
                return TCL_ERROR
        }
        result := C.CString("Hello, World!")
        defer C.free(unsafe.Pointer(result))
        C.Tcl_SetObjResult(interp, C.Tcl_NewStringObj(result, -1))
        return TCL_OK
}

//export Square_Cmd
func Square_Cmd(cdata C.ClientData, interp *C.struct_Tcl_Interp,
        objc C.int, objv **C.Tcl_Obj) C.int {
        if objc != 2 {
                return TCL_ERROR
        }

        // http://stackoverflow.com/a/14828189
        var objs []*C.Tcl_Obj
        sliceHeader := (*reflect.SliceHeader)((unsafe.Pointer(&objs)))
        sliceHeader.Cap = (int)(objc)
        sliceHeader.Len = (int)(objc)
        sliceHeader.Data = uintptr(unsafe.Pointer(objv))

        arg := C.GoString(C.Tcl_GetString(objs[1]))
        i, err := strconv.Atoi(arg)
        if err != nil {
                return TCL_ERROR
        }
        C.Tcl_SetObjResult(interp, C.Tcl_NewIntObj((C.int)(i*i)))
        return TCL_OK
}

func (interp *C.struct_Tcl_Interp) createCommand(name string, f unsafe.Pointer) {
        cname := C.CString(name)
        defer C.free(unsafe.Pointer(cname))
        C.Tcl_CreateObjCommand(interp, cname, (*C.Tcl_ObjCmdProc)(f), nil, nil)
}

//export Tclgoexample_Init
func Tclgoexample_Init(interp *C.struct_Tcl_Interp) C.int {
        interp.createCommand("::hello", unsafe.Pointer(C.Hello_Cmd_cgo))
        interp.createCommand("::square", unsafe.Pointer(C.Square_Cmd_cgo))

        return TCL_OK
}

func main() {}
======

** wrappers.go **
======c
//wrappers.go
package main

/*
#include <tcl.h>
#include <tclDecls.h>

int Hello_Cmd_cgo(ClientData cdata, Tcl_Interp *interp, int objc,
        Tcl_Obj *const objv[]) {
        int Hello_Cmd(ClientData cdata, Tcl_Interp *interp, int objc,
                Tcl_Obj *const objv[]);
        return Hello_Cmd(cdata, interp, objc, objv);
}

int Square_Cmd_cgo(ClientData cdata, Tcl_Interp *interp, int objc,
        Tcl_Obj *const objv[]) {
        int Square_Cmd(ClientData cdata, Tcl_Interp *interp, int objc,
                Tcl_Obj *const objv[]);
        return Square_Cmd(cdata, interp, objc, objv);
}
*/
import "C"
======

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