libaio 0.5

python 2 Notes

In python 2.7, a memoryview of a bytearray, despite being writable, is rejected by ctypes:

>>> from ctypes import c_char
>>> a = bytearray(b'foo')
>>> c_char.from_buffer(a)
c_char('f')
>>> b = memoryview(a)
>>> b.readonly
False
>>> c_char.from_buffer(b)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: expected a writeable buffer object

This means that it is not possible to only read or write a few bytes at the beginning of a large buffer without having to copy memory.

The same code works fine with python 3.x .

This is considered a python 2.7 ctypes or memoryview bug, and not a python-libaio bug.

Also, memoryview refuses to use an mmap object:

>>> import mmap
>>> a = mmap.mmap(-1, 16*1024)
>>> b = memoryview(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot make memory view because object does not have the buffer interface
>>>

…but ctypes is happy with it:

>>> import ctypes
>>> c = (ctypes.c_char * len(a)).from_buffer(a)
>>>

…and memoryview accepts being constructed over ctype objects:

>>> d = memoryview(c)
>>>

…and it really works !

>>> a[0]
'\x00'
>>> c[0]
'\x00'
>>> d[0]
'\x00'
>>> d[0] = '\x01'
>>> c[0]
'\x01'
>>> a[0]
'\x01'
>>> a[0] = '\x02'
>>> c[0]
'\x02'
>>> d[0]
'\x02'

This is considered a python 2.7 memoryview or mmap bug.