Go Advent Day 18 - Go Outside


Outside is a Go package to dynamically link to and execute functions in Windows Dynamic Link Libraries and Linux Shared Libraries. Its current status is ‘prerelease’ with only 32-bit register size implemented and tested so far. Also, some functionality is very experimental and will probably change a lot before release 0.1.

I only came to explore Go as a viable general purpose or glue language in the 2nd quarter of 2013. Not so much because of its (at the time) fairly low profile and newness but more because I had no real need to change from my home-grown developer-base-of-one Prolog system. So, realising the amount of effort it would take to roadworthy the beast for general consumption (typical WORN code - Write Once Read Never), I decided to rather tag along with Go for future works.

My needs almost always involve interacting with external (hence Outside) functions in proprietary or open-source Windows .dll or Linux .so files. Cgo is a perfectly capable solution but I did find the reliance on gcc a bit detracting from the scripting look and feel of pure Go.

Tackling the (initially) rather bewildering reflect package turned out to be the reward I needed.

syscall on Windows already has [Must]LoadDLL, [Must]FindProc and Call to load libraries, find entry-points and call procedures. Note: Because these functions are Windows-specific, they don’t show up in documentation on golang.org. See godoc and syscall on reading documentation for other systems. Linux provides more limited (6 arguments max) Syscall functions, but neither dynamic loading nor lookup. Also, the neater Call is missing. In outside similar Linux functionality has been implemented using the dl library and some C code.

All that was missing (on Windows) was a method of “black-boxing” the otherwise messy unsafe.Pointer conversions and type coercion necessary to issue a Call (or its underlying Syscall). For this we have (drum roll please) the magical reflect.MakeFunc. Within a MakeFunc’d function we can transform the input arguments, do the call and manipulate the return in any way desired.

Enough already, show me an example

Suppose we want to ‘stylize’ an image using ImageMagick (like William Kennedy did on day 9)

Starting with a rather dental portrayal of our favourite rodent…

we want to ‘weather’ him a bit…

(Ok, I’m from the sunny south of the equator, so sleet is a bit tongue-in-cheek - and I never miss a chance to take a dig at Microsoft for forcing the spelling favorite down the international throat)

Start with the your choice of prototypes

Identify the C prototypes for the functions we will need.

  MagickWand *NewMagickWand(void);
  MagickWand *DestroyMagickWand(MagickWand *wand);
  MagickBooleanType MagickReadImage(MagickWand *wand, const char *filename);
  MagickBooleanType MagickSketchImage(MagickWand *wand,
      const double radius,const double sigma, const double angle);
  MagickBooleanType MagickEqualizeImage(MagickWand *wand)
  MagickBooleanType MagickWriteImage(MagickWand *wand, const char *filename);

Set up the Go function variables and types

Do the usual usual type-name to name-type transposition, name simplification and casing that we all love.

  type Wand struct{}

  var New func() *Wand
  var Destroy func(m *Wand) *Wand
  var Read func(m *Wand, filename string) bool
  var Sketch func(m *Wand,
      radius, sigma, angle float64) bool
  var Equalize func(m *Wand) bool
  var Write func(m *Wand, filename string) bool

Relate the Go functions to the dll entry-points

  var allApis = outside.Apis{
      {"NewMagickWand", &New},
      {"DestroyMagickWand", &Destroy},
      {"MagickReadImage", &Read},
      {"MagickSketchImage", &Sketch},
      {"MagickEqualizeImage", &Equalize},
      {"MagickWriteImage", &Write},

This code does all the setup needed. The DLL or SO is loaded and the entry-points are resolved in a soft fashion - that is, there is no hard failure at this point for any missing names, they are merely logged. The bool 2nd argument to AddDllApis determines whether string types are converted to char string *char (false), or UTF-16 string *uint16 (true) types. The Linux dll name is definitely distribution dependent (I used the current Mint Debian Linux). You can get pre-built binary releases from ImageMagick For a .dll or .so to be loaded dynamically, its directory needs to be in the path when the program is run (not necessary at compile time). After init, functions are immediately executable. A call to an unresolved entry-point will panic.

  func init() {
      var dll string
      if runtime.GOOS == "windows" {
          dll = "CORE_RL_wand_.dll"
      if runtime.GOOS == "linux" {
          dll = "libMagickWand.so.5"
      outside.AddDllApis(dll, false, allApis)

Add your own Go flavour

Although you can use the functions directly, there is no harm in a bit of Go idiomatic embelishment.

  func (m *Wand) Read(filename string) *Wand {
      Read(m, filename)
      return m
  func (m *Wand) Sketch(radius, sigma, angle float64) *Wand {
      Sketch(m, radius, sigma, angle)
      return m
  func (m *Wand) Equalize() *Wand {
      return m
  func (m *Wand) Write(filename string) *Wand {
      Write(m, filename)
      return m

Formulate the main plan of action

  func main() {
      m := New()
      defer Destroy(m)
      m.Read("i.png").Sketch(0, 30, 60).Equalize().Write("o.jpeg")

Wait a minute - what about error handling?

Since we’re treating the bool return sequence as a single entity we could

  type Bool bool
  var ... func(...) (Bool, error)

with the optional Error method on Bool

  func (ok Bool) Error func(e error) (bool, error) {
      if ok {
          return ok, nil
      if e != nil {
           panic("Failed with: " + e.Error())
      panic("Failed with unknown error")

Throw in the Go housekeeping header and run

  package main

  import (

Complete code

Happy holidays and Go safely.

comments powered by Disqus