Python ctypesgen/ctypes: How to write struct fields to file in single byte alignment

Question

Using ctypesgen, I generated a struct (let\'s call it mystruct) with fields defined like so:

[(\'somelong\', ctypes.c_long),
 (\'somebyte\', ctypes.c_ubyte)
         


        

Answer 1

Define _pack_=1 before defining _fields_.

Example:

from ctypes import *
from io import BytesIO
from binascii import hexlify

def dump(o):
    s=BytesIO()
    s.write(o)
    s.seek(0)
    return hexlify(s.read())

class Test(Structure):
    _fields_ = [
        ('long',c_long),
        ('byte',c_ubyte),
        ('long2',c_long),
        ('str',c_char*5)]

class Test2(Structure):
    _pack_ = 1
    _fields_ = [
        ('long',c_long),
        ('byte',c_ubyte),
        ('long2',c_long),
        ('str',c_char*5)]

print dump(Test(1,2,3,'12345'))
print dump(Test2(1,2,3,'12345'))

Output:

0100000002000000030000003132333435000000
0100000002030000003132333435

Alternatively, use the struct module. Note it is important to define the endianness < which outputs the equivalent of _pack_=1. Without it, it will use default packing.

import struct
print hexlify(struct.pack('<LBL5s',1,2,3,'12345'))

Output:

0100000002030000003132333435