Re: [LUAJit] ffi and pointer math
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- Subject: Re: [LUAJit] ffi and pointer math
- From: "Robert G. Jakabosky" <bobby@...>
- Date: 2011年5月11日 02:12:12 -0700
On Tuesday 10, Kaj Eijlers wrote:
> Hi,
> God I love LuaJit. Sliced bread can finally retire.
> However.....
>
> I am trying to do some openGL, which works like a charm, but I've ran into
> a problem. I'm sure it's me but I can't figure it out.
>
> I need to pass in a pointer to an array of structs, which poses no problem,
>
> typedef struct
> {
> float u;
> float v;
> } TexCoords;
> typedef struct
> {
> float x;
> float y;
> } Positions;
> typedef unsigned int Colors;
> typedef struct
> {
> Positions Position;
> TexCoords TexCoord;
> Colors Color;
> } PTC;
>
> Creating an array of those with
>
> local verts = ffi.new("PTC[3]")
> local vertOffs = verts
>
> sending the pointer off:
> gl.glVertexPointer(2, gl.GL_FLOAT, ffi.sizeof("PTC"), vertoffs);
> Works like a charm, the sizeof is the stride, vertoffs the pointer
> (although I am not sure if I created a copy when I assigned to 'verts')
I think 'vertOffs' is the same value as 'verts', so no copy.
>
> Now I need to pass a pointer to a member inside PTC, in this case TexCoords
> which would be at offset 8.
> Try as I might I can't create a pointer to it.
>
> I tried
> local tex = verts + 8 but that doesn't work
> I tried
> local ptr = ffi.new("unsigned char*")
> ptr = verts
> ptr = ptr + 8
>
> But it doesn't do what I expect.
It might help if you show an example in C code of what you are trying to do.
Also "verts + 8" in Lua, should be the same as "&verts[8]" in C. So 'tex'
would be a pointer to the 8th element of the verts array (which only has 3
elements?).
You might be able to do what you really need with just this:
verts[0].TexCoord
But if you really need a pointer to the field of a struct, then something like
this might work:
-- get byte pointer to first array element
local ptr = ffi.cast("unsigned char*", verts[0])
-- get field's byte offset.
local field_offset = ffi.offsetof("PTC", "TexCoord");
-- cast pointer to field back to correct type.
local tex = ffi.cast("TexCoords *", ptr + field_offset);
--
Robert G. Jakabosky