Re: ctypes-users Digest, Vol 49, Issue 4

Adam Morris wrote, On 06/12/2010 06:44 AM:
>
>     AFAIK ctypes should handle GIL correctly automatically, also when
>     used from threads.
>
>
> Sure am glad to hear that!
>
>     To me it sounds more like a problem with bad alignment or bad
>     parameter
>
>     profile of the callback function.
>
>
> I don't know about bad alignment, but you may be right around the bad 
> parameter profile. I think I've narrowed down the problem to the 
> context argument that is being passed to the SetCallback function of 
> the dll. If I pass it null or 0 or pointer() or even byref(), the 
> result is a bus error of 0x00 when it actually calls the callback, but 
> if I pass it 10 it gives a bus error with address of 0x0a. I'm 
> confused because the example files of the SDK show the C code passing 
> a "0" for the context argument, so then why would it spell trouble 
> using that from Python? Even weirder, if I don't pass any context 
> parameter at all to the SetCallback function, I get the exact same 
> error (or is this because I don't understand how parameter lists work 
> in C?)
>

It seems like the lib uses and dereferences pContext. You have to give 
it something it likes.

You could try to make a little C application that tests this API and 
calls the callback. Try to do it without using the SDK .h files - create 
your own and simplify them as much as possible. That will help you get 
familiar with the API and take ctypes out of the equation. If you can't 
create such an application then you probably also can't do it with 
ctypes - unless you are lucky.

> Here's all the relevant typedefs as defined in the SDK:
>
>
> #Storage declarations:
>
> typedef signed int      ax_int32_t
>
> typedef unsigned short  ax_char16_t
>
> typedef unsigned int    ax_handle_t
>
> # can't find AX_CALLCONV nor AX_API in the SDK
>
>
> #Callback declaration:
>
> typedef void(AX_CALLCONV * ax_vote_callback_t)(ax_handle_t hubHandle, 
> ax_int32_t deviceSerialNumber, ax_int32_t vote, void *pContext)
>
>
> Here's the SetCallback method I have to use, notice the context argument:
>
>
> AX_API ax_result_t AX_CALLCONV  Ax_SetVoteCallback (ax_vote_callback_t 
> pfnCallback, void *pContext
>
>
> Here's what I do in my own code, python 3.1 on Mac OS X, after "from 
> cytpes import *":
>
>
>     def set_vote_callback(self, python_function):
>
>         self.keep_wrap_function_reference = CFUNCTYPE(c_void_p, 
> c_uint, c_int, c_int, c_void_p)
>

Shouldn't the first parameter (ax_handle_t) be an integer?

>         context = c_int(0)  # no idea what to do here, honestly
>

Try to use None instead.

>         result = 
> self.cdll.Ax_SetVoteCallback(self.keep_wrap_function_reference(python_function), 
> context)
>
>         # Or do I need a pointer to the function??
>
>
> Here's the callback I've defined (at the module-level), which gets 
> passed as "python_function" above:
>
>
> def standard_vote_callback(handle, serialNumber, vote, context):
>
>     print("Inside callback!")
>

For debugging and experiments it might be simpler to use
def standard_vote_callback(*args):
     print "Inside callback!", args

>
> I thank anybody who can help me out understanding what I've done wrong 
> here. If anybody knows how to send the "self" using an instance method 
> using that there same context argument that seems to be giving me 
> trouble, I would also be very grateful. I know about using the 
> @staticmethod decorator but since I don't understand what's going on 
> I'm reluctant to play around!
>

Sorry - that doesn't compile.

/Mads

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