Handling struct returned from c++ called from python

Question

I'd like to use a function that is defined in a DLL from Python. The value returned from the C++ function (get_version) is a struct

typedef struct myStruct {
    size_t size;
    char * buff;
} myStruct ;

The Python code is:

lib = CDLL(myDLL.dll)
lib.get_version

The question is how do I handle the returned value?

I've read Voo's answer and reading other posts, but I'm still struggling with this

I declared the struct class (Foo, from Voo's answer) and set the restype The code now looks

class Foo(Structure):
    _fields_ = [('size', c_size_t), ('buff', c_char_p)]

lib = CDLL(myDLL.dll)
lib.get_version
lib.get_version.restype = Foo._fields_

I get the following error TypeError: restype must be a type, a callable, or None

I read about this and if I set the restype not as a list, e.g.: c_char_p, the error doesn't appear

When I set the restype

lib.restype = Foo.fields

The error doesn't appear but the restype for get_version is not set correctly When looking at the variables in debug:

lib.restype = list: [('size', ), ('buff', )]

lib.get_version.restype = PyCSimpleType:

Any help would be appreciated

ctypes works with C functions, so if using C++ make sure to extern "C" the function. — Mark Tolonen
– Mark Tolonen, Commented Jun 25, 2012 at 2:30
Hi, can you tell me please what is return type of function in dll? I've tried with my myStruct but get error: C linkage function cannot return C++ class 'myStruct ' — Aleksandar
– Aleksandar, Commented Dec 25, 2013 at 9:42

Voo · Accepted Answer · 2012-06-24 21:44:19Z

2

You'll have to use the ctypes module. You'll just have to define the struct in your python code with ctypes.

Something like:

>>> from ctypes import *
>>> class Foo(Structure):
...     _fields_ = [("size", c_size_t), ("buff", c_char_p)]

should do the trick. Then you just set the restype of your get_version method to your struct so that the interpreter knows what it does return and then you can use it as expected.

edited Jun 24, 2012 at 21:44

answered Jun 24, 2012 at 21:31

Voo

30.4k13 gold badges91 silver badges165 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

user1478497 Over a year ago

Thank you for your help, I read other posts on the issue but I'm still struggling with this. I've add more information on the original question

Voo Over a year ago

@user1478497 You have to set the restype value of the method not the whole dll. Also you shouldn't point it to _fields_ but the struct itself, i.e. lib.get_version.restype = Foo. Finally you probably should use a better name than foo ;)

Collectives™ on Stack Overflow

Handling struct returned from c++ called from python

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related