Function Binding at Runtime

[Serhat Sevki Dincer]

Hi,

I was looking at purego dynamic loading library (without cgo) and loved the idea.

It seems to be working quite well across many OS/ARCH combinations.

You first Dlopen a library, then RegisterLibFunc functions from it.

In sdl3 binding library, it is effectively used like this:

var sdlInit func(InitFlags) bool

func init() {

    libSDL, _ := purego.Dlopen(libraryFileName, purego.RTLD_LAZY)

    purego.RegisterLibFunc(&sdlInit, libSDL, "SDL_Init")

}

func Init(flags InitFlags) bool {
    return sdlInit(flags)
}

So

- sdlInit is an unexported, mutable variable set at runtime (application start)

- Init is bound at link-time, exported and immutable (let's ignore function inlining)

- they have the exact same signature, you can do: sdlInit = Init

Go already allows implementing a function outside the language (in Assembly) like

func Init(flags InitFlags) bool

// implementation in Assembly

But this is still bound at link-time. What I am wondering is the possibility to have just the following and get rid of writable variables like sdlInit:

func Init(InitFlags) bool

func init() {

    libSDL, _ := purego.Dlopen(libraryFileName, purego.RTLD_LAZY)

    purego.RegisterLibFunc(&Init, libSDL, "SDL_Init")

}

So this is like filling a function pointer table at runtime (application startup). This would be doable only from inside init(). What do you think?

Regards..

[Brian Candler]

> So this is like filling a function pointer table at runtime (application startup)

Why not simply:

var Init func(InitFlags) bool

func init() {

    libSDL, _ := purego.Dlopen(libraryFileName, purego.RTLD_LAZY)

    purego.RegisterLibFunc(&Init, libSDL, "SDL_Init")

}

e.g. https://go.dev/play/p/X6LGwo9cAkH

[Brian Candler]

I guess the downside is that consumers can also alter the value of Init.

You want something like a "const" which can be initialized at runtime, which doesn't exist in Go. The solution generally is to make a public accessor function to a hidden variable, and that is the "var sdlInit" in your original code.