How do I build a python string from a ctype struct?

Question

I'm using ctypes and I've defined this struct in order to pass parameters

class my_struct(ctypes.Structure):
    _fields_ = [ ("buffer", ctypes.c_char * BUFSIZE),
                 ("size", ctypes.c_int )]

Then I call the C function using the following code, but I don't know how to create a string from the struct I've created.

class Client():

    def __init__(self):
        self.__proto = my_struct()
        self.client = ctypes.cdll.LoadLibrary(r"I:\bin\client.dll")

    def version(self):
        ret = self.client.execute(ctypes.byref(self.__proto))
        my_string = self.__proto.buffer[:self.__proto.size]

I want to create a python string using the first n bytes of the buffer (the buffer contains NULL characters but I have to handle this situation and create the string with /0x00 characters if necesary). The asignation

my_string = self.__proto.buffer[:self.__proto.size]

is not working bacause truncates the string if 0x00 appears. Any idea is welcome. Thanks in advance.

In Python 2.6.3, I've tested creating a ctypes array like your buffer, but I can't see that it gets truncated actually. Like this: ar = (ctypes.c_char*10)(); ar.value = "test"; assert ar[:6] == 'test\x00\x00'. Am I missing something? — aknuds1
– aknuds1, Commented Feb 26, 2011 at 23:52

llasram · Accepted Answer · 2011-03-18 13:19:45Z

Your problem is that ctypes tries to do some magic for you with char arrays, auto-converting them into NUL-terminated strings. You can get around this magic by using the ctypes.c_byte type instead of ctypes.c_char and retrieving the value as a string with ctypes.string_at. You can make accessing the member a little nicer with a helper property on the structure class, such as:

import ctypes
BUFSIZE = 1024

class my_struct(ctypes.Structure):
    _fields_ = [ ("_buffer", ctypes.c_byte * BUFSIZE),
                 ("size", ctypes.c_int )]

    def buffer():
        def fget(self):
            return ctypes.string_at(self._buffer, self.size)
        def fset(self, value):
            size = len(value)
            if size > BUFSIZE:
                raise ValueError("value %s too large for buffer",
                                 repr(value))
            self.size = size
            ctypes.memmove(self._buffer, value, size)
        return property(fget, fset)
    buffer = buffer()

proto = my_struct()
proto.buffer = "here\0are\0some\0NULs"
print proto.buffer.replace("\0", " ")

aknuds1 · Accepted Answer · 2011-02-27 00:41:28Z

0

I think you need to send a pointer to a C string in your my_struct, and not a C string directly, because C strings are null-terminated. Try to do it like this:

import ctypes

BUFSIZE = 10

class my_struct(ctypes.Structure):
    _fields_ = [ ("buffer", ctypes.POINTER(ctypes.c_char)),
                 ("size", ctypes.c_int )]

cstr = (ctypes.c_char * BUFSIZE)()
proto = my_struct(cstr)

answered Feb 27, 2011 at 0:41

aknuds1

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1 Comment

llasram Over a year ago

I see what you're trying to do, but that's not correct if the structure expected by the library the OP is interfacing with has the buffer directly embedded in the struct.

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