Difficulties with C module Structs

stevenkl

Hi there,

I experiment with the new C module and try to write some bindings for Raylib.
The first functions are working great, its really easy.

However, the function ClearBackground(Color color) from Raylib requires a struct:

// Color, 4 components, R8G8B8A8 (32bit)
typedef struct Color {
    unsigned char r;        // Color red value
    unsigned char g;        // Color green value
    unsigned char b;        // Color blue value
    unsigned char a;        // Color alpha value
} Color;

I've defined one like this: local Color_t = c.Struct("CCCC", "r", "g", "b", "a")

If I use the struct only the first field seems to have some effect.

I've put the code here: [Login to see the link]
Maybe someone could have a look at it?
Just want to check if I'm to stupid or maybe there is a problem with the C module beeing still experimental.

Greetings.

Samir

How do you define the ClearBackground() function ?

stevenkl

The function is defined like this:

local raylib = c.Library("raylib.dll")
raylib.ClearBackground = "c(.)"

I'm not sure about the calling convention, but seem to not make any difference using some other.

Edit: If I run my code in VSCode (with the Debugger) its always the same (wrong) color. However, if I run it from the command line the color changes each run, sadly never showing the correct one 🥲

Samir

And how do you call this function ? How do you initialize the struct ?

stevenkl

For the whole code I posted the link to my repo.
raylib.lua

local c = require "c"

local lib = c.Library("raylib.dll")
if not lib then
    error(sys.error)
end
lib.InitWindow        = "c(iiZ)"
lib.CloseWindow       = "c()"
lib.WindowShouldClose = "c()B"
lib.BeginDrawing      = "c()"
lib.EndDrawing        = "c()"
lib.ClearBackground   = "c(.)"
lib.SetTargetFPS      = "c(i)"

---@class Color
---@field r integer 8 bit integer value (0-255)
---@field g integer 8 bit integer value (0-255)
---@field b integer 8 bit integer value (0-255)
---@field a integer 8 bit integer value (0-255)

---Color, 4 components, R8G8B8A8 (32bit)
---@overload fun(r: integer, g: integer, b: integer, a: integer): Color
local Color_t = c.Struct("CCCC", "r", "g", "b", "a")


---@class raylib
---@field LIGHTGRAY Color
---@field GRAY Color
---@field DARKGRAY Color
local raylib = {
    LIGHTGRAY = Color_t(c.uchar(200), c.uchar(200), c.uchar(200), c.uchar(255)),
    GRAY = Color_t(c.uchar(130), c.uchar(130), c.uchar(130), c.uchar(255)),
    DARKGRAY = Color_t(c.uchar(80), c.uchar(80), c.uchar(80), c.uchar(255)),
}

---Creates a Color struct
---@param r integer
---@param g integer
---@param b integer
---@param a integer
---@return Color
function raylib.Color(r, g, b, a)
    return Color_t(
        c.uchar(r), -- r
        c.uchar(g), -- g
        c.uchar(b), -- b
        c.uchar(a)  -- a
    )
end

---Initialize window and OpenGL context
---@param width integer
---@param height integer
---@param title string
function raylib.InitWindow(width, height, title)
    lib.InitWindow(c.int(width), c.int(height), c.string(title))
end

---Close window and unload OpenGL context
function raylib.CloseWindow()
    lib.CloseWindow()
end

---Check if application should close (KEY_ESCAPE pressed or windows close icon clicked)
---@return boolean
function raylib.WindowShouldClose()
    return lib.WindowShouldClose()
end

---Set background color (framebuffer clear color)
---@param color Color
function raylib.ClearBackground(color)
    assert(type(color) == "Struct" and color.type == "Color_t", "Type not supported")
    lib.ClearBackground(color)
end

---Setup canvas (framebuffer) to start drawing
function raylib.BeginDrawing()
    lib.BeginDrawing()
end

---End canvas drawing and swap buffers (double buffering)
function raylib.EndDrawing()
    lib.EndDrawing()
end

---Set target FPS (maximum)
---@param fps integer
function raylib.SetTargetFPS(fps)
    lib.SetTargetFPS(c.int(fps))
end

return raylib

test.lua

local r = require "raylib"

local function main()
    r.InitWindow(800, 600, "Test")
    r.SetTargetFPS(60)

    while not r.WindowShouldClose() do
        r.BeginDrawing()
        r.ClearBackground(r.Color(200, 200, 200, 255))
        r.EndDrawing()
    end

    r.CloseWindow()
    return 0
end

sys.exit(main() or 1)

Samir

There might be a problem when the Struct is passed by value (the case for the ClearBackground() function). I will check this thank you for the report.

Samir

Ok I have some news.

This is due to Struct initialization code that don't work as expected.
For now, if you want your example to work (show a gray colored window), initialize the Color_t struct with a table like :

function raylib.Color(r, g, b, a)
    return Color_t {
        r = r, --red
        g = g, --green
        b = b, --blue
        a = a  --alpha
    }
end

As a side note, you can use directly Lua values as arguments for Struct initialization (no need to use c.uchar() as LuaRT will do the conversion from Lua to C value for you), and in this case no need to wrap the Struct Color with a Color function, you can use it directly to save you a lot of time !

ruby

does c module mean that like luajit we can load other dll by using .h header file?

if not, maybe with some simple pure lua script helper we can achieve this goal: load .dll using .h header to supply definition signature

stevenkl

ruby Not exactly with a header file, but you can define the function signature after loading the DLL.

[Login to see the link] With a table it works as expected, thank you. For now I use the wrapper functions for better LSP supprt 😄

Samir

[Login to see the link] not directly headers file, but you have to define by yourself each C types needed

[Login to see the link] oh ok I understand now 😉

stevenkl

I've found another problem, this time when a c-function would return a struct.
The c-function is defined as: RLAPI Vector2 GetWindowPosition(void);.

My Lua definition:

lib.GetWindowPosition = "c()."
local Vector2_t = c.Struct("ff", "x", "y")

When I call the function the Lua vm dies with: Unknown '.' signature character

Samir

Yes returned by-values Struct is not yet implemented

Samir

Update
I have implemented the "." and "u" (Struct and Union) return values by value for next LuaRT update.

In your case, you will be able to do that :

local v = r.Vector2(r.GetWindowPosition())

stevenkl

Nice, I'm waiting for it 😊