Python Ctypes memory allocation for c function

Question

I currently have a python callback function that calls a c function using ctypes library. The c function requires a pointer to a structure for example animal_info_s. I instantiate the structure and pass it as a pointer to the c function and it works. The issue I'm having is when I have multiple threads calling the callback, I'm finding the information being passed back is mixed up between the threads.

class animal_info_s(ctypes.Structure):
    _fields_ = [('dog_type',          ctypes.c_uint16),
                ('cat_type',          ctypes.c_uint16),
                ('bird_type',         ctypes.c_uint16),
                ('epoch_time',        ctypes.c_uint16),
                ('more_information',  ctypes.c_uint16)]


_mod = ctypes.cdll.LoadLibrary('bbuintflib.dll')
animal_info_s = animal_info_s()
get_animal_data = _mod.get_animal_data
get_animal_data.argtypes = [ctypes.POINTER(animal_info_s)]
get_animal_data.restype =   ctypes.c_int

# Python Callback
def GetAnimalData():
    animal_info_p = animal_info_s
    res = get_animal_data(animal_info_p)
    if (res != 0):
        print("Failed to get animal info")
        return

    print ("Receive Time - %d\nDog: %d\nCat: %d\nBird:%d" %(animal_info_p.epoch_time,
                                                            animal_info_p.dog_type,
                                                            animal_info_p.cat_type,
                                                            animal_info_p.bird_type))

I think what is happening is when I instantiate the structure, it is using the same memory location every time. How do I create a new memory location for each thread calling the callback?

I'm not a ctypes expert, but are you using animal_info_p = animal_info_s in GetAnimalData for each thread? That would be using the same struct instance everywhere (and would explain the strange behavior). Don't you want animal_info_p = animal_info_s() instead? — rkersh
– rkersh, Commented Oct 17, 2016 at 20:11

Mark Tolonen · Accepted Answer · 2016-10-18 03:12:32Z

2

The following line should be removed. It redefines the name animal_info_s as a class animal_info_s instance, which then hides the class.

animal_info_s = animal_info_s()

The following line should be changed from:

animal_info_p = animal_info_s

to:

animal_info_p = animal_info_s()

The original line made another name for the animal_info_s name from the first error, which was the one-and-only instance used in the threads. the recommended line creates a new instance of the animal_info_s class each time the callback is called.

answered Oct 18, 2016 at 3:12

Mark Tolonen

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2 Comments

Sam Over a year ago

When creating an new instance of animal_info_s, do I have to free any memory used or does it get handled by Python? Python normally handles it but I'm worried this is a special case (This object is for C) and it might create a memory leak somewhere.

Mark Tolonen Over a year ago

It is freed by Python when the instance goes out of scope at the end of the function. If a C function kept a reference to the memory, you would have to keep a reference to the object in Python as long as the C function needs it.

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