On Aug 4, 2009, at 11:25 AM, Sparky wrote:
On Aug 3, 3:29 pm, Sparky <samnspa...@gmail.com> wrote:
Hello! I am using cTypes on Windows to interface with a dll and I
keep
getting an error when I execute this method:
def eDigitalIn(self, channel, idNum = None, demo = 0, readD=0):
"""
Name: U12.eAnalogIn(channel, idNum = None, demo = 0, readD=0)
Args: See section 4.4 of the User's Guide
Desc: This is a simplified version of Counter. Reads & resets
the counter (CNT).
"""
if idNum is None:
idNum = self.id
ljid = ctypes.c_long(idNum)
state = ctypes.c_long(999)
ecode = staticLib.ECount(ctypes.byref(ljid), demo, channel,
readD, ctypes.byref(state))
if ecode != 0: raise LabJackException(ecode)
if ljid == -1: raise LabJackException(-1, "LabJack not
found.")
return {"idnum":ljid.value, "state":state.value}
Here is the error message:
Traceback (most recent call last):
File "<pyshell#4>", line 1, in <module>
device.eDigitalIn(0)
File "C:\Documents and Settings\All Users\Documents\Python
\LabJackPython_new\u12.py", line 118, in eDigitalIn
ecode = staticLib.ECount(ctypes.byref(ljid), demo, channel,
readD,
ctypes.byref(state))
WindowsError: exception: access violation writing 0x00000000
Here is the signature of the method (which is known to work with C++
programs):
long _stdcall EDigitalIn(long *idnum,
long demo,
long channel,
long readD,
long *state);
staticLib is declared with staticLib = ctypes.windll.LoadLibrary
("ljackuw").
Any ideas?
Thanks,
Sam
One more thing, I seem to be getting back incorrect values for doubles
passed by reference. Any suggestions? I am on a 64-bit machine but I
should think that should not make a difference.
Hi Sam,
ctypes is pretty straightforward, and I wouldn't expect it to break on
something simple. I looked over your code (warning -- I am a ctypes
novice) and it looks OK to me. If you're getting null pointer writes
I'd suspect that the call signature isn't what you think it is,
perhaps due to a lack of an extern "C" declaration or something like
that. In short, I don't think this is a problem in your Python code or
in ctypes.
But since your Python code is the easiest to change, why not do a
quick test of a minimal example?
def eDigitalIn_test_version(self, channel, idNum = None, demo = 0,
readD=0):
ljid = ctypes.c_long(42)
state = ctypes.c_long(999)
ecode = staticLib.ECount(ctypes.byref(ljid), 0, 0, 0,
ctypes.byref(state))
print ecode
Does that work any better?
You could also try replacing the byref() calls with pointers created
by ctypes.pointer(). But I suspect that all this will do is give you
confidence that it isn't your Python code that's wrong.
You might want to write a minimal C program that invokes ECount. That
would help you to prove whether or not you have a working C (not C++!)
interface for your function.
HTH
Philip
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