python ctype recursive structures
Solution 1:
You almost certainly want to declare next_command as a pointer. Having a structure that contains itself isn't possible (in any language).
I think this is what you want:
class EthercatDatagram(Structure):
pass
EthercatDatagram._fields_ = [
("header", EthercatDatagramHeader),
("packet_data_length", c_int),
("packet_data", c_char_p),
("work_count", c_ushort),
("next_command", POINTER(EthercatDatagram))]
Solution 2:
The reason why
EthercatDatagram._fields_.append(("next_command", EthercatDatagram))
does not work is that the machinery that creates the descriptor objects (see the source of the PyCStructType_setattro
function) for accessing the next_command
attribute is activated only upon assignment to the _fields_
attribute of the class.
Merely appending the new field to the list goes completely unnoticed.
To avoid this pitfall, use always a tuple (and not a list) as the value of the _fields_
attribute: that will make it clear that you have to assign a new value to the attribute and not modify it in place.