Re: why sturct size different between C and Python ctypes?

On 22.02.2016 09:52, Yang Liu wrote:
> Hi,all
>
> I am using ctypes today and a sturct size is differet in C and Python
> ctypes code
> here is my code
>
> C:
> #include<linux/netlink.h>
> #include<linux/inet_diag.h>
>
> int main(int argc, char *argv[])
> {
>      struct {
>          struct nlmsghdr      nlh;
>          struct inet_diag_req req;
>       } nlreq;
>      printf("nlreq: %d\n", sizeof(nlreq));
>      return 0;
> }
>
> out:
> nlreq: 76
>
>
> Python:
> class nlmsghdr(ctypes.Structure):
>      _fields_ = [
>          ("len", ctypes.c_uint32),
>          ("type", ctypes.c_uint16),
>          ("flags", ctypes.c_uint16),
>          ("seq", ctypes.c_uint32),
>          ("pid", ctypes.c_uint32)]
>
> class Inet_diag_sockid(ctypes.Structure):
>      _fields_ = [
>          ("idiag_sport", ctypes.c_uint16.__ctype_be__),
>          ("idiag_dport", ctypes.c_uint16.__ctype_be__),
>          ("idiag_src", ctypes.c_uint32.__ctype_be__ * 4),
>          ("idiag_dst", ctypes.c_uint32.__ctype_be__ * 4),
>          ("idiag_if", ctypes.c_uint32),
>          ("idiag_cookie", ctypes.c_uint32 * 2),
>      ]
>
> class inet_diag_req(ctypes.Structure):
>      _fields_ = [
>          ("idiag_family", ctypes.c_uint8),
>          ("idiag_src_len", ctypes.c_uint8),
>          ("idiag_dst_len", ctypes.c_uint8),
>          ("idiag_ext", ctypes.c_uint8),
>          ("id", ctypes.POINTER(Inet_diag_sockid)),
>          ("idiag_states", ctypes.c_uint32),
>          ("idiag_dbs", ctypes.c_uint32)
>      ]
>
> class nlmsg(ctypes.Structure):
>      _fields_ = [
>          ("nlh", nlmsghdr),
>          ("req", Inet_diag_req)
>      ]
>
> print ctypes.sizeof(nlmsg)
>
> out:
> 24
>
>
> why the same struct size is 76 inC and 24 in ctypes ?

Usually this is due to structure packing. Try with _pack_ values in struct.

HTH
Niki

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