return a ctypes object to C

Thu Oct 31 09:13:57 EDT 2019

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