why sturct size different between C and Python ctypes?

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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