Re: who can tell me the difference in ctypes.sizeof between Ironpython and python in using ctypes to create structure?

On Tue, Mar 10, 2015 at 9:49 PM, 狂禅 <[email protected]> wrote:
> recently i meet a problem in using Ironpython.
>     import ctypes as ct
>
>     class Data(ct.LittleEndianStructure):
>         _pack_      = 2
>         _fields_    = [
>
>             ('a',ct.c_ubyte),
>             ('b',ct.c_ubyte),
>             ('c',ct.c_ubyte),
>             ('d',ct.c_ushort),
>             ('e',ct.c_uint),
>             ('f',ct.c_ubyte),
>         ]
>     if __name__ == "__main__":
>         a = Data()
>         print ct.sizeof(a)
> as is show above.when i used this script running in python ,it prints 12.
> but when running in Ironpython ,it is 14.
> in fact ,i tried it in c like that:
>
>     #include<stdio.h>
>     void main()
>     {
>     #pragma pack(2)
>  struct{
>   unsigned char a;
>   unsigned char b;
>   unsigned char c;
>   unsigned short d;
>   unsigned int e;
>   unsigned char f;
>  }A;
>     #pragma pack()
>  printf("%d\n",sizeof(A));
>     }
> it is 12.
>
> can you tell me why it is and how i can crrect it in Ironpython???

Print the field reprs to see how it's defining the structure in terms
of offsets and sizes. Here's what I get in CPython on Windows:

    from ctypes import *

    class Data(LittleEndianStructure):
        _pack_ = 2
        _fields_ = (('a', c_ubyte),
                    ('b', c_ubyte),
                    ('c', c_ubyte),
                    ('d', c_ushort),
                    ('e', c_uint),
                    ('f', c_ubyte))

    >>> for field, dtype in Data._fields_:
    ...     print field, getattr(Data, field)
    ...
    a <Field type=c_ubyte, ofs=0, size=1>
    b <Field type=c_ubyte, ofs=1, size=1>
    c <Field type=c_ubyte, ofs=2, size=1>
    d <Field type=c_ushort, ofs=4, size=2>
    e <Field type=c_ulong, ofs=6, size=4>
    f <Field type=c_ubyte, ofs=10, size=1>

    >>> alignment(Data)
    2
    >>> sizeof(Data)
    12

The natural alignment is 4 bytes, which requires padding the structure
out to 16 bytes:

    class Data(LittleEndianStructure):
        _fields_ = (('a', c_ubyte),
                    ('b', c_ubyte),
                    ('c', c_ubyte),
                    ('d', c_ushort),
                    ('e', c_uint),
                    ('f', c_ubyte))

    >>> for field, dtype in Data._fields_:
    ...     print field, getattr(Data, field)
    ...
    a <Field type=c_ubyte, ofs=0, size=1>
    b <Field type=c_ubyte, ofs=1, size=1>
    c <Field type=c_ubyte, ofs=2, size=1>
    d <Field type=c_ushort, ofs=4, size=2>
    e <Field type=c_ulong, ofs=8, size=4>
    f <Field type=c_ubyte, ofs=12, size=1>

    >>> alignment(Data)
    4
    >>> sizeof(Data)
    16

HTH

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