Python: How to pack different types of data into a string buffer using struct.pack_into

Question

I'm trying to pack some unsigned int data into a string buffer created using ctypes.create_string_buffer.

Here is the following code segment, and a running example showing the error on codepad:

import struct
import ctypes
import binascii

buf = ctypes.create_string_buffer(16)
struct.pack_into("=I=I=I", buf, 0, 1, 2, 3)
print binascii.hexlify(buf)

This yields the following error:

...
struct.error: bad char in struct format

The documentation doesn't allude to whether you can pack data of different types if the underlying buffer is of a specific C type. In this case, trying to pack unsigned int data into a string buffer with an underlying c_char type. Anyone know of a solution to do this, or is there a specific way to create a buffer that can pack any type of data?

Answer 1

Sorry for resurrecting old topic, but I get the point of "snap" - being hit by probably similar background habit.

"the first character of the format string can be used to indicate the byte order, size and alignment of the packed data" I agree. However:

• Python docs deliberately (?) omit even a single example of use of order formatters (All examples assume a native byte order, size, and alignment with a big-endian machine.)
• One may assume (as I and probably snap did), that "III" consists of three format strings and we may format every one of them at will. Hence =I=I=I. I shot myself in the foot after getting used to Ruby's array.pack, where one may freely change ordering along the expression (Ruby's equivalent is I_I_I_ in this case, as order selector comes after type).

Hence I guess it might be good to add a few lines to struct.pack/unpack docs, giving examples of order & padding use (meh, padding hit me even harder... I could live with native order, but padding ruined my protocol).