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Groups > comp.lang.python > #100588

Problem on ctypes arguments in a DLL function

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I am trying to use the libusb-win32 v1.2.6.0 with Win7. I wrote a test program(showing below) but stuck with a strange problem. Here is the result:
----------------------------
D:\Work\Python34>python
Python 3.4.3 (v3.4.3:9b73f1c3e601, Feb 24 2015, 22:43:06) [MSC v.1600 32 bit (In
tel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import libusb_u2aTest as u2a
0x10c4
0x1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "D:\Work\Python34\libusb_u2aTest.py", line 364, in <module>
    pHDL = _lib.usb_open(pDevice)
ValueError: Procedure probably called with too many arguments (4 bytes in excess
)
>>>
-----------------
I replace the troubled line with a "pass" to make the loading success, then to check the type of "pDevice", it's correct.

>>> u2a.pDevice
<libusb_u2aTest.LP__usb_device object at 0x017F0120>

It seems no reason that this call can be in trouble:-( Any hint was appreciated. Thanks ahead.


libusb_u2aTest.py listing:
-----------------------------
from ctypes import *
_lib = windll.LoadLibrary("C:\\Windows\\System32\\libusb0.dll")

_PATH_MAX = 512

### Data structures...
class _usb_device_descriptor(Structure):
    _fields_ = [('bLength', c_uint8),
                ('bDescriptorType', c_uint8),
                ('bcdUSB', c_uint16),
                ('bDeviceClass', c_uint8),
                ('bDeviceSubClass', c_uint8),
                ('bDeviceProtocol', c_uint8),
                ('bMaxPacketSize0', c_uint8),
                ('idVendor', c_uint16),
                ('idProduct', c_uint16),
                ('bcdDevice', c_uint16),
                ('iManufacturer', c_uint8),
                ('iProduct', c_uint8),
                ('iSerialNumber', c_uint8),
                ('bNumConfigurations', c_uint8)]

class _usb_device(Structure): pass
class _usb_bus(Structure): pass
_usb_device._fields_ = [('next', POINTER(_usb_device)),
                        ('prev', POINTER(_usb_device)),
                        ('filename', c_int8 * _PATH_MAX),
                        ('bus', POINTER(_usb_bus)),
                        ('descriptor', _usb_device_descriptor),
                        #('config', POINTER(_usb_config_descriptor)), # not implemented yet
                        ('config', POINTER(c_int)),  # to ease this test
                        ('dev', c_void_p),
                        ('devnum', c_uint8),
                        ('num_children', c_ubyte),
                        ('children', POINTER(POINTER(_usb_device)))]

_usb_bus._fields_ = [('next', POINTER(_usb_bus)),
                    ('prev', POINTER(_usb_bus)),
                    ('dirname', c_char * _PATH_MAX),
                    ('devices', POINTER(_usb_device)),
                    ('location', c_uint32),
                    ('root_dev', POINTER(_usb_device))]

_usb_dev_handle = c_void_p


### Function prototype...
# struct usb_bus *usb_get_busses(void);
_lib.usb_get_busses.restype = POINTER(_usb_bus)

# usb_dev_handle *usb_open(struct usb_device *dev);
_lib.usb_open.argtypes = [POINTER(_usb_device)]
_lib.usb_open.restype = _usb_dev_handle


### Test start...
_lib.usb_init()
_lib.usb_find_busses()
_lib.usb_find_devices()
pBus = _lib.usb_get_busses()
if bool(pBus):
    pDevice = pBus[0].devices
    if bool(pDevice):
        print(hex(pDevice[0].descriptor.idVendor))
        print(hex(pDevice[0].descriptor.idProduct))

if pDevice[0].descriptor.idVendor == 0x10c4 and \
   pDevice[0].descriptor.idProduct == 0x0001:
    pHDL = _lib.usb_open(pDevice)  # <--this line is in trouble

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Problem on ctypes arguments in a DLL function jfong@ms4.hinet.net - 2015-12-18 00:41 -0800
  Re: Problem on ctypes arguments in a DLL function eryk sun <eryksun@gmail.com> - 2015-12-18 04:24 -0600
    Re: Problem on ctypes arguments in a DLL function jfong@ms4.hinet.net - 2015-12-18 03:15 -0800

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