Go, meet Lua

Go, meet Lua
Using Lua in Go and vice-versa!
André Burgaud @andreburgaud
4/15/2020

You 💖Go... Why would you need Lua?
● NGINX (OpenResty)
● Wireshark
● Nmap
● Pandoc
● HAProxy
● Redis
● Roblox
● Rockbox
● SciTE
● LÖVE

Agenda
● CGO
○ Using C from Go (inline code and separate files)
○ Using Go from C (export)
● Embedding Lua in Go (CGO)
○ Executing a lua script from Go
○ Building a basic Lua REPL in Go
● Extending Lua with Go
○ LuaJIT FFI
● Using a Lua Interpreter in Go

CGO - Inline C code in GO
Go code
C code in Go comments
main.go
I’m using C
code!

CGO - Inline C code in GO (code/demo)
package main
/*
#include <stdio.h>
void hello() {
printf("Hello from C!n");
}
*/
import "C" // No new line between block comment and 'import "C"'
func main() {
C.hello()
}

CGO - Separate *.h and *.c Files
Go code
Go code
header files in Go comments
main.go
hello.h
hello.a
exe
hello.c
①
②

CGO - Separate *.h and *.c Files (code/demo)
package main
// #include <stdlib.h>
// #include <hello.h>
import "C"
import (
"unsafe"
)
func main() {
C.hello()
name := C.CString("GoMN")
defer C.free(unsafe.Pointer(name))
C.print(name)
}

main.c
CGO - Using Go from C (export)
Include golib.h
Go code with
Exported functions
main.go
golib.h
golib.so
a.out
①
②
-buildmode=c-shared
golib.so
③
a.out
No fair! C is
using me!

CGO - Using Go from C (code / demo)
package main
import (
"C"
"os"
)
//export Pwd
func Pwd() *C.char {
d, err := os.Getwd()
if err != nil {
return C.CString("Error")
}
return C.CString(d)
}
func main() {}

Executing a Lua Script in Go
Go code
Go code
Executes hello.lua
header files and
CGO linking
in Go comments
main.go
lua*.h
liblua.a
exe
lauxlib.h
I’m using
Lua C code!
hello.lua

Executing a Lua Script in Go
● CGO ☠ … Again
● Problem with C Macros - #define luaL_loadfile(L,f) luaL_loadfilex(L,f,NULL)
● Unsafe pointers (need to free allocated memory)

Building a Lua REPL in Go
Go code
Go code
Executes REPL loop and
Interpretes Lua instructions
header files and
CGO linking
in Go comments
main.go
lua*.h
liblua.a
exe
lauxlib.h
I’m using
Lua C code!
>>> print “hello”

Building a Lua REPL in Go
● Here we CGO ☠ … Again
● Problem with C Macros - #define luaL_loadfile(L,f) luaL_loadfilex(L,f,NULL)
● Unsafe
● Hard to debug

Extend LuaJIT with Go
Go code with
Exported functions
main.go
golib.so
①
②
-buildmode=c-shared
$ luajit getcwd.lua
/a/b/c/d
$
getcwd.lua
I’m happy to
give a hand
to my friend
Lua

Extend LuaJIT with Go
● LuaJIT FFI
● Extend Lua to include functions not available in the interpreter STL
● Example: current working directory
● In Go: os.Getwd()
○ Export function in Go
○ Use the FFI module available in LuaJIT (calling external C functions)
○ Add C declaration
○ Call the Go function (LuaJIT sees it as a C function)

Export Function in Go (same as C example)
package main
import "C"
import (
"os"
)
//export Pwd
func Pwd() *C.char {
d, err := os.Getwd()
if err != nil {
return C.CString("Error")
}
return C.CString(d)
}
func main() {}

Lua Interpreter in Go 💖
● https://github.com/yuin/gopher-lua
● DoString()
● DoFile()
● Base64 (Bi-directional)

Lua Interpreter in Go 💖
Go code
💘
Import gopher-lua
Go code
Executes and extends lua
code
main.go

Go plugin
● Allows creating modular Go programs
● Allows reusing “plugins” across multiple programs
● https://golang.org/pkg/plugin/
$ go build -buildmode=plugin

Ideas 💡
● Configuration files
● Expose internal APIs
● Extension/Customization (games)
● Templating language (HTML, XML, next ML or JSON…)
● Filtering text processing (pandoc)
● ...

Summary
● Quick review of CGO
● Embed Lua in Go (CGO)
● Extend LuaJIT with Go (FFI)
● Embed and Extend with a Lua interpreter written in Go
● More ideas to explore

Go Online Resources
● https://github.com/andreburgaud/meetup-golang-lua Source code
● https://golang.org/cmd/cgo/ Command cgo
● https://nick.groenen.me/posts/plugins-in-go-18/ Plugins in Go 1.8
● https://medium.com/learning-the-go-programming-language/writing-modular-g
o-programs-with-plugins-ec46381ee1a9 Writing Modular Go Programs with
Plugins
● https://docs.google.com/document/d/1nr-TQHw_er6GOQRsF6T43GGhFDelr
AP0NqSS_00RgZ Go Execution Modes

Lua Interpreters in Go
● https://github.com/Azure/golua A Lua 5.3 engine implemented in Go (MIT)
● https://github.com/Shopify/go-lua A Lua VM in Go (MIT)
● https://github.com/yuin/gopher-lua GopherLua: VM and compiler for Lua in Go
(MIT)
● https://github.com/milochristiansen/lua A Lua 5.3 VM and compiler written in
Go (Zlib)

Go Books
● https://nostarch.com/blackhatgo Black Hat Go
● https://www.gopl.io/ The Go Programming Language
● https://www.packtpub.com/programming/mastering-go-second-edition
Mastering Go

Lua Online Resources
● https://www.lua.org/ Lua Web Site
● https://luajit.org/ The LuaJIT Project
● https://www.oreilly.com/content/learn-lua-from-javascript-part-1-an-introductio
n-to-lua/ Learn Lua from JavaScript, part 1: An introduction to Lua

Lua Books
● https://www.lua.org/pil/ Programming in Lua
● https://www.packtpub.com/application-development/lua-quick-start-guide Lua
Quick Start Guide

Other Resources
● https://love2d.org/ LÖVE
● https://github.com/love2d/love LÖVE

Attributions
● Lua Logo: By Alexandre Nakonechnyj (Grafik-Design) und Lua-Team
(PostScript-Code) - svg from PostScript Source (see below) created from
Lumu, Public Domain,
https://commons.wikimedia.org/w/index.php?curid=37714918
● Gopher: By Takuya Ueda (https://twitter.com/tenntenn). Licensed under the
Creative Commons 3.0 Attributions license,
https://github.com/golang-samples/gopher-vector

Go, meet Lua

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