How do I build a python string from a raw (binary) ctype buffer?

Question

I'm playing with Python and ctypes and I can't figure out how to resolve this problem. I call to a C function which fills a raw binary data. My code looks like this:

class Client():
  def __init__(self):
    self.__BUFSIZE = 1024*1024
    self.__buf = ctypes.create_string_buffer(self.__BUFSIZE)
    self.client = ctypes.cdll.LoadLibrary(r"I:\bin\client.dll")


  def do_something(self):
    len_written = self.client.fill_raw_buffer(self.__buf, self.__BUFSIZE)
    my_string = repr(self.__buf.value)
    print my_string

The problem is that I'm receiving binary data (with 0x00) and it's truncated when I tried to build my_string. How can I build my_string if self._buf contains null bytes 0x00?

Any idea is welcome. Thanks

Sven Marnach · Accepted Answer · 2011-02-18 23:23:11Z

8

You can access a buffer returned by create_string_buffer() as a Python string by using its raw attribute:

a = ctypes.create_string_buffer(10)
a.raw 
# '\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'

To only access the first n bytes, use

a.raw[:n]

edited Feb 18, 2011 at 23:23

answered Feb 18, 2011 at 23:13

Sven Marnach

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1 Comment

Cartucho Over a year ago

Yes, but I have a big working buffer that is partially filled by function I'm calling for. I also know how many bytes were written (5 bytes for example). The question is how can I build a python string using the first 5 bytes considering some of those bytes can be null.

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