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

Question

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'

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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)

It's not a char, it's a char*. Which is, technically speaking, a pointer to a char, but is generally used do denote an array of chars terminated with the Null Byte - or a String. How are you trying to call this function? Example code will let us know what exactly is missing. — FrankieTheKneeMan
– FrankieTheKneeMan, Commented Jan 8, 2013 at 23:38
Could you clarify the use of ctypes here? Are you calling another library and want to use it to do that, or can you literally just emulate a structure using the struct module? — Jon Clements
– Jon Clements, Commented Jan 8, 2013 at 23:40
I put what I've got currently. Hopefully this sheds some light on what I'm trying to do. — Chris
– Chris, Commented Jan 9, 2013 at 0:02

Blender · Accepted Answer · 2013-01-08 23:40:42Z

2

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.

answered Jan 8, 2013 at 23:40

Blender

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Comments

Mark Tolonen · Accepted Answer · 2013-01-11 02:37:11Z

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

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