CODEWITHSUNDEEP
pythonctypessignedunsigned-integer
Chris
Chris

Reputation: 10101

Convert a string to an 8-bit signed integer in python

I'm trying to patch together a motor control system using python and ctypes and one of the things I need to do is take an text input and convert it to an 8-bit signed integer.

Below is the documentation for the function I'm trying to call. The text that should be input into the program is 'EPOS2'

enter image description here

The data type definition is as shown below (note that 'char*' equates to an 8-bit signed integer)

enter image description here

So How do I convert 'EPOS2' to a value between -128 and 127?

Ultimately what I'm trying to do is something like this:

import ctypes #import the module

lib=ctypes.WinDLL(example.dll) #load the dll

VCS_OpenDevice=lib['VCS_OpenDevice'] #pull out the function

#per the parameters below, each input is expecting (as i understand it) 
#an 8-bit signed integer (or pointer to an array of 8 bit signed integers, 
#not sure how to implement that)
VCS_OpenDevice.argtypes=[ctypes.c_int8, ctypes.c_int8, ctypes.c_int8, ctypes.c_int8]

#create parameters for my inputs
DeviceName ='EPOS2'
ProtocolStackName = 'MAXON SERIAL V2'
InterfaceName = 'USB'
PortName = 'USB0'


#convert strings to signed 8-bit integers (or pointers to an array of signed 8-bit integers)
#code goes here
#code goes here
#code goes here

#print the function with my new converted input parameters


print VCS_OpenDevice(DeviceName,ProtocolStackName,InterfaceName,PortName)

Upvotes: 0

Views: 4755

Answers (2)

Mark Tolonen
Mark Tolonen

Reputation: 178115

Your interface takes char* which are C strings. The equivalent ctypes type is c_char_p. Use:

import ctypes
lib = ctypes.WinDLL('example.dll')
VCS_OpenDevice = lib.VCS_OpenDevice
VCS_OpenDevice.argtypes = [ctypes.c_char_p,ctypes.c_char_p,ctypes.c_char_p,ctypes.c_char_p]

DeviceName ='EPOS2'
ProtocolStackName = 'MAXON SERIAL V2'
InterfaceName = 'USB'
PortName = 'USB0'

print VCS_OpenDevice(DeviceName,ProtocolStackName,InterfaceName,PortName)

Also, WinDLL is normally only needed for Windows system DLLs. If your interfaces are declared __stdcall in the C header file, WinDLL is correct; otherwise, use CDLL.

Additionally, your return code is documented as a DWORD*, which is a bit strange. Why not DWORD? If DWORD* is correct, to access the value of the DWORD pointed to by the return value, you can use:

VCS_OpenDevice.restype = POINTER(c_uint32)
retval = VCS_OpenDevice(DeviceName,ProtocolStackName,InterfaceName,PortName)
print retval.contents.value

Upvotes: 2

Blender
Blender

Reputation: 298512

You could use ctypes:

>>> from ctypes import cast, pointer, POINTER, c_char, c_int
>>> 
>>> def convert(c):
...     return cast(pointer(c_char(c)), POINTER(c_int)).contents.value
... 
>>> map(convert, 'test string')
[116, 101, 115, 116, 32, 115, 116, 114, 105, 110, 103]

Which (as I just found out) matches the output of ord:

>>> map(ord, 'test string')
[116, 101, 115, 116, 32, 115, 116, 114, 105, 110, 103]

Although your data type definitions list it as a char, not a char*, so I'm not sure how you'd handle that.

Upvotes: 2

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