Question: How to handle a **char parameter that was mapped to a ptr cstring?

[tech4rd]

First and foremost, thanks for the library. As a Nim newbie this has helped me wrapping the GLFW and OpenGL libraries.

Second, if this is not the best place to ask questions, please let me know and I will redirect it elsewhere.

I have several small hiccups while doing experiments. This one is for wrapping the openGL function glShaderSource. The string represents an array of strings. I was assuming that a cstringArray would be the equivalent type. However when I compile:

  var vertexShader = glCreateShader(GL_VERTEX_SHADER)
  var vertexShaderTextArr : array[0..0,cstring] = [cstring(vertexShaderText)]
  var xx: cstringArray = cast[cstringArray](vertexShaderTextArr.addr)
  var zero: GLint_typedef = 0 
  glShaderSource(vertexShader, GLsizei(1),
                xx, zero.addr)

I get the compilation error:

Error: type mismatch: got <GLuint_587203219, GLsizei, cstringArray, ptr GLint_typedef>
        ... but expected one of:
        ... PFNGLSHADERSOURCEPROC_587204782 = proc (a0: GLuint_587203219, a1: GLsizei_587203223, a2: ptr cstring, a3: ptr GLint_typedef_587203217){.cdecl.}

So a2 is a ptr cstring, which I assume is compatible to a **char. But why is cstringArraynot used? How should one deal with arrays of C strings in Fuhtark?

TIA

[PMunch]

You're right in that a cstringArray is compatible with char**.

Futhark unfortunately isn't aware of this, so char** turns into ptr cstring instead (because it is aware that char* is cstring).

Since they are byte compatible though you can simply cast it like so:

glShaderSource(vertexShader, GLsizei(1), cast[cstringArray](xx), zero.addr)

Alternatively you can cast xx to ptr cstring instead of cstringArray, or even just cast vertexShaderTextArr to ptr cstring directly.

Futhark performs a very low-level wrap of C code, and since Nim is more strict about types than C you end up with a bit more casting back and forth. However in this case I think it's worthwhile to add detection for the char** case, so I'll leave this issue open until it's fixed.

[PMunch]

This is where the fix could be implemented

futhark/src/futhark/opir.nim

Lines 88 to 92 in 0bdce95

	if underlyingType["kind"].str == "base" and underlyingType["value"].str == "cschar": # in {CXType_Char_S, CXType_SChar}:
	if info.depth == 1:
	%*{"kind": "base", "value": "cstring"}
	else:
	%*{"kind": "pointer", "depth": info.depth - 1, "base": {"kind": "base", "value": "cstring"}}

[tech4rd]

@PMunch Thank you for the feedback. I will go with the cast of vertexShaderTextArr directly to ptr cstring directly.