-----Original Message-----
From: eryk sun [mailto:eryk...@gmail.com]
Sent: 03 August 2016 13:16
To: python-list@python.org
Cc: Bill Somerville <bsomervi...@flexerasoftware.com>
Subject: Re: Issue with ctypes and callback functions
On Wed, Aug 3, 2016 at 9:38 AM, Bill Somerville
<bsomervi...@flexerasoftware.com> wrote:
>
> Is this a good place to ask questions about the above?
Discussing ctypes is fine here. There's also a ctypes-users list.
Thanks Eryk,
I am trying to handle Python callback functions in a SWIG interface, SWIG does
not support callbacks where the target is a Python callable but I can do this
with ctypes. My problem is that I need to have the callback receive a SWIG
wrapped type as an argument by reference. I see that if I use a
ctypes.Structure type then it works nicely but I have a huge and changing API
to wrap and do not want to manually describe every C struct with
ctypes.Structure when SWIG has already automatically created its own shadow
classes. The SWIG struct wrapper is just a class wrapping a pointer so can be
easily be passed just like ctypes.Structure can be. How do I tell ctypes that
it needs to reconstitute the type from an address in the callback thunk it
generates?
I have tried defining from_param() and self._as_parameter_ for the types which
works for passing to a function but I don't need that as SWIG does that
already. With callback arguments I get various errors depending on what
methods/attributes I supply like:
Parsing argument 0
TypeError: cannot build parameter
At the point where the callback is being called.
To summarize:
++++++++++++++++++++++++++++++++++++
C API like:
typedef struct {
int member;
/* ... */
} my_type;
typedef int (*pfn) (my_type const *);
int fn (pfn);
Implementation could be:
#include "api.h"
int fn (pfn cb) {
my_type t = {42, /* ... */};
return cb (&t);
}
I want Python code like:
from ctypes import *
import api # SWIG extension module
def my_cb (t):
print ('in callback with:', t.member)
return 0
proto = CFUNCTYPE (c_int, my_type)
cb = proto (my_cb)
res = api.fn (cast (cb, c_void_p).value)
print ('fn returned:', res)
------------------------------------------------
I can extend the SWIG shadow class to do the creation of the prototype and
thunk, the required cast above and any extra attributes or methods like
_as_parameter_ and from_param() but I can't see any way of having the Python
callback (my_cb) magically receive the SWIG wrapped 'my_type' struct.
TIA
Bill.
--
https://mail.python.org/mailman/listinfo/python-list
