New submission from Eric Wieser <[email protected]>:
Discovered [here](https://github.com/numpy/numpy/issues/10528)
Consider the following structure, and a memoryview created from it:
class foo(ctypes.Structure):
_fields_ = [('one', ctypes.c_uint8),
('two', ctypes.c_uint32)]
f = foo()
mf = memoryview(f)
We'd expect this to insert padding, and it does:
>>> mf.itemsize
8
But that padding doesn't show up in the format string:
>>> mf.format
'T{<B:one:<I:two:}'
That format string describes the _packed_ version of the struct, with the `two`
field starting at offset 1, based on the `struct` documentation on how `<`
should be interpreted:
> No padding is added when using non-native size and alignment, e.g. with ‘<’,
> ‘>’
But ctypes doesn't even get it right for packed structs:
class foop(ctypes.Structure):
_fields_ = [('one', ctypes.c_uint8),
('two', ctypes.c_uint32)]
_pack_ = 1
f = foo()
mf = memoryview(f)
The size is what we'd expect:
>>> mf.itemsize
5
But the format is garbage:
>>> mf.format
'B' # sizeof(byte) == 5!?
----------
components: ctypes
messages: 311705
nosy: Eric.Wieser
priority: normal
severity: normal
status: open
title: ctypes: memoryview gives incorrect PEP3118 format strings for both
packed and unpacked structs
versions: Python 2.7, Python 3.4, Python 3.5, Python 3.6, Python 3.7, Python 3.8
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<https://bugs.python.org/issue32780>
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