gistfile1.py

import ctypes
import os
import mmap


class Record(ctypes.Structure):
    _fields_ = [('index', ctypes.c_uint64),
                ('key_length', ctypes.c_uint16),
                ('key', ctypes.c_char * 256)]


RECORD_COUNT = 256

file_size = ctypes.sizeof(Record) * RECORD_COUNT

with open('somefile', 'w+b') as fp:
    os.ftruncate(fp.fileno(), file_size)
    mem = mmap.mmap(fp.fileno(), file_size)
    array = (Record * RECORD_COUNT).from_buffer(mem)
    for i, item in enumerate(array):
        item.index = i
        item.key = 'item ' + str(i)
        item.key_length = len(item.key)
    mem.flush()
    mem.close()

with open('somefile', 'r+b') as fp:
    mem = mmap.mmap(fp.fileno(), file_size)
    array = (Record * RECORD_COUNT).from_buffer(mem)
    for i, item in enumerate(array):
        assert(i == item.index)
        assert('item ' + str(i) == item.key)
        assert(len('item ' + str(i)) == item.key_length)