Thanks Guillaume! I also found a simple test from a previous post by Thomas
Heller,
 http://aspn.activestate.com/ASPN/Mail/Message/ctypes-users/1687142
 *"NULL pointers are logically false, and non-null pointers are true.... So
passing them to bool() you get this:"*

So a quick fix for me was:
  def isNullPointer(p):
 if bool(pMsg) == 0:
 return True
 else:
 return False
 Marcus
  On 11/16/05, Guillaume Proux <gproux@[...].com>  wrote:
> 
>  I thought the question was not very difficult but after a small experiment
>  it is more involved than it could appear.
> 
>  I made a dummy DLL
> 
>  --- c code ---
>  static const Coord val = { 1,2 };
> 
>  DLLIMPORT Coord* GetXY (int a)
>  {
>  if (a)
>  return &val;
>  else
>  return NULL;
>  }
>  --- end ---
> 
>  Then experimented with it
> 
>  >>> os.chdir(r"c:\dev\dummydll")
>  >>> from ctypes import *
>  >>> class Coord(Structure):
>  ... _fields_ = [ ("x",c_int), ("y", c_int)]
>  ...
>  >>> cdll.ReturnNoneDll.GetXY.restype=POINTER(Coord)
>  >>> a = cdll.ReturnNoneDll.GetXY(0)
>  >>> a
>  <ctypes.LP_Coord object at 0x00C03580>
>  >>> a[0]
>  Traceback (most recent call last):
>  File "<stdin>", line 1, in ?
>  ValueError: NULL pointer access
>  >>> a.contents
>  Traceback (most recent call last):
>  File "<stdin>", line 1, in ?
>  ValueError: NULL pointer access
>  >>> cdll.ReturnNoneDll.GetXY.restype=c_long
>  >>> a = cdll.ReturnNoneDll.GetXY(0)
>  >>> a
>  0
> 
>  Hum, what would be nice would be to have some kind of wrapper that either
>  returns None or return the X,Y coordinates...
>  Great because it is easy to do with ctypes
> 
>  >>> def CheckCoord(c):
>  ... if c==0:
>  ... return None
>  ... res=cast(c,POINTER(Coord)).contents
>  ... return (res.x,res.y)
>  ...
>  >>> cdll.ReturnNoneDll.GetXY.restype = CheckCoord
>  >>> cdll.ReturnNoneDll.GetXY(1)
>  (1, 2)
>  >>> cdll.ReturnNoneDll.GetXY(0)
>  >>>
> 
>  Here you are!
> 
>  Guillaume
> 
> 
>  On 11/17/05, Marcus Goldfish <magoldfish@[...].com> wrote:
>  >
>  > I need some help with a ctypes POINTER question. My python module
>  > calls a C-dll and receives a pointer to a structure. A (c-code) null
>  > pointer
>  > indicates no message. How do I test this in ctypes?
>  >  Thanks!
>  > Marcus
>  >  -- code snippet
>  > pMsg = mydll.GetMessage() # pointer to a struct conatining message info
>  >  # if pMsg == null then no message --> how to test this?
>  > # else
>  > # process message
>  >
> 
>