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Date: Thu, 10 Oct 2013 11:47:49 -0400
Subject: Passing C pionters to Python for use with cffi
From: Eric Frederich <eric.frederich@gmail.com>
To: python-list@python.org
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Hello,

I'm extending an application that supports customization using the C
language.
I am able to write standalone python applications that use the C API's
using cffi.
This is good, but only a first step.

This application allows me to register code that will run on various events
but it has to be C code.
I'd like to write Python code instead.
So basically, my C code will use the Python C API to get a handle to the
module and function (like how they do it here
http://docs.python.org/2/extending/embedding.html#pure-embedding)
What would be the proper way to pass a pointer to a C structure from C to
Python so that I can use ffi.cast and be able to use it from within Python?

I have got this to work but I'm not certain that it is correct, fool-proof,
or portable.

This is how I got it to work from the C side....

PyObject* pArgs = Py_BuildValue("(k)", &some_structure);
PyObject_CallObject(pFunc, pArgs)

... and from the Python side...

def my_function(struct_ptr):
    struct = ffi.cast("mystruct_t *", struct_ptr)


Like I said, this works fine.  I am able to manipulate the structure from
within Python.
I just want to know the correct way to to this.

Thanks,
~Eric

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<div dir=3D"ltr"><div><div>Hello,<br><br>I&#39;m extending an application t=
hat supports customization using the C language.<br>I am able to write stan=
dalone python applications that use the C API&#39;s using cffi.<br>This is =
good, but only a first step.<br>
<br>This application allows me to register code that will run on various ev=
ents but it has to be C code.<br>I&#39;d like to write Python code instead.=
<br>So
 basically, my C code will use the Python C API to get a handle to the=20
module and function (like how they do it here=20
<a href=3D"http://docs.python.org/2/extending/embedding.html#pure-embedding=
">http://docs.python.org/2/extending/embedding.html#pure-embedding</a>)<br>=
What
 would be the proper way to pass a pointer to a C structure from C to=20
Python so that I can use ffi.cast and be able to use it from within=20
Python?<br><br>I have got this to work but I&#39;m not certain that it is c=
orrect, fool-proof, or portable.<br><br>This is how I got it to work from t=
he C side....<br><br>PyObject* pArgs =3D Py_BuildValue(&quot;(k)&quot;, &am=
p;some_structure);<br>
PyObject_CallObject(pFunc, pArgs)<br><br>... and from the Python side...<br=
><br>def my_function(struct_ptr):<br>=A0=A0=A0 struct =3D ffi.cast(&quot;my=
struct_t *&quot;, struct_ptr)<br><br><br>Like I said, this works fine.=A0 I=
 am able to manipulate the structure from within Python.<br>
I just want to know the correct way to to this.<br><br></div>Thanks,<br></d=
iv>~Eric<br></div>

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