Re: do function casts ever disappear?

  • From: Dimiter 'malkia' Stanev <malkia@xxxxxxxxx>
  • To: luajit@xxxxxxxxxxxxx
  • Date: Fri, 20 Jul 2012 11:29:03 -0700

I think that if you keep the value somewhere, it won't get garbage collected.

I chose a different path, and I like it better - <> - it also comes with emulation layer for things missing, like immediate mode for iOS OpenGL ES 2.0, logging, error checking, etc.

this way my luajit/ffi portion would not stayed clutter with casting/getProcAddress things. (And wglGetProcAddress would only work once the context is established, which means it cannot be done early, which is the same for regal - but it it hides it all from you).

On 7/20/2012 10:48 AM, William Adams wrote:
With the recent discussions on callabacks, I begain to doubt my OpenGL 
extension implementation.

What am I trying to achieve:  OpenGL has a number of extension functions.  The 
only way to get at those functions is to get a pointer to the function by way 
of a library call, then cast that to a function prototype, and make the call.

Doubt:  Does the function pointer cast ever become invalid/garbage collected, 



  typedef int (__attribute__((__stdcall__)) *PROC)();
  PROC wglGetProcAddress(LPCSTR lpszProc);

typedef void (* PFNGLSHADERSOURCEPROC) (GLuint shader, GLsizei count, GLchar* 
*string, const GLint *length);

function GetFunctionProtoName(fname)
  local upname = fname:upper();
  local protoname = string.format("PFN%sPROC", upname);
  return protoname;

function CastFunctionPointer(fname, funcptr)
  local protoname = GetFunctionProtoName(fname);
  local castfunc = ffi.cast(protoname, funcptr);
  return castfunc;

function GetWglFunction(fname)
  local funcptr = gl.wglGetProcAddress(fname);
  if funcptr == nil then
   return nil
  local castfunc = CastFunctionPointer(fname, funcptr);
  return castfunc;

OglMan_mt = {
  __index = function(tbl, key)
   local funcptr = GetWglFunction(key)
   -- Set the function into the table of
   -- known functions, so next time around,
   -- it this code will not need to execute
   rawset(tbl, key, funcptr)
   return funcptr;

Which allows for this:

OglMan.glShaderSource(self.ID, 1, src_array, nil);

In GetWglFunction(), I get the pointer to the function using wglGetProcAddress. 
 Looking at it now, I think the return value of PROC should be intptr_t, but 
the real question is:  Will the cast being one in CastFunctionPointer() stick 
around forever, or is there any magic that will make it disappear when I least 
expect it?  This isn't a callback, so I don't think the callback slots are any 
influence, but still, I have a doubt as to how many function pointers can live.

I'm holding onto the cast within the table, along with the key, which is the 
function name.  Is that sufficient to keep the cast around?

-- William

- Shaping clay is easier than digging it out of the ground.                                     

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