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

The reference I found to inet_diag_req online shows Inet_diag_sockid is 
a structure contained by value NOT by reference, .. I took away the 
POINTER and got a total of 76 bytes ( packing =1)

nlmsghdr     = 16     pack=1
inet_diag_req= 60     pack=1
===================
nlmsg        = 76     pack=1



Check to see if I am correct.

class inet_diag_req(ctypes.Structure):
     _pack_ = ww
     _fields_ = [
         ("idiag_family", ctypes.c_uint8),        # 1  = 1
         ("idiag_src_len", ctypes.c_uint8),       # 1  = 2
         ("idiag_dst_len", ctypes.c_uint8),       # 1  = 3
         ("idiag_ext", ctypes.c_uint8),           # 1  = 4
#        ("id", ctypes.POINTER(Inet_diag_sockid)),# 4  = 8
         ("id", Inet_diag_sockid), #<<<< by value, not reference
         ("idiag_states", ctypes.c_uint32),       # 4  = 12
         ("idiag_dbs", ctypes.c_uint32)           # 4  = 16
     ]                                         # total = 16


Steve



On 22/02/16 18:52, 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 ?
>
> Thanks
>
>
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