return a ctypes object to C

[Arnaud Loonstra]

On 30-10-2019 09:32, Arnaud Loonstra wrote:
> Hi all,
> 
> I'm trying to wrap my head around the ctypes API. I have a C structure I 
> wish to create in Python and then return from python to C.
> 
> So a python method is called from C and needs to return an object which 
> we then process in C again.
> 
> I have a binding to access and create the C methods and structures so in 
> Python I can call the Zmsg() constructor. I now need to return this.
> 
> My python test method is simply:
> 
> def actor_test( *args, **kwargs):
>      print("test")
>      msg = Zmsg()
>      frame = Zframe(b"Hello", 5)
>      msg.prepend(frame)
>      return msg
> 
> the method is called from C as follows:
> 
> PyObject *pReturn = PyObject_CallObject(pFunc, NULL);
> 
> This correctly calls the method. However the returned object is of 
> course a PyObject*. The debugger says it's
> 
> "<czmq._czmq_ctypes.Zmsg object at 0x7ffff5f18e50>"    PyObject
>              [class]    "<class 'czmq._czmq_ctypes.Zmsg'>"
>              [super class]    "<class 'object'>"
>              [meta type]    "<class 'type'>"
>              ob_refcnt    1    Py_ssize_t
> 
> However how I can I get it back to the original C type (zmsg_t *)
> 
> Any help really appreciated.
> 

What I've found so far is that I can return the address of the ctypes 
object.

msg = Zmsg()
frame = Zframe(b"Hello", 5)
msg.prepend(frame)
return addressof(msg._as_parameter_.contents)

In C I can then cast it back to the original type.

PyObject *pReturn = PyObject_CallObject(pFunc, NULL);
assert(pReturn);
long bla = PyLong_AsLong(pReturn);
zmsg_t* test = (zmsg_t *)bla;
assert(test);
char *hello = zmsg_popstr(test);
assert(hello);
assert(streq(hello, "Hello"));

This works, I'm not sure if this is the right way. It also creates a 
complicated setup with the garbage collector.

Anybody better ideas?

Rg,

Arnaud