Question

我试图从结构if_data64中读取字段ifi_ibytes / ifi_obytes，如this header file中所述。我有ctypes工作，我可以进行sysctl调用，我可以从中获取看似可用的信息。但是，我从非零整数中得到了似乎是完全垃圾的东西。例如，我有一个接收0字节的接口，传输大约850万字节。但是，我的python代码表示已收到0并且传输了3,590,592,659,456个字节。

我正在从this known to work program复制方法，我从Python获取的字节数与上面显示的MenuMeter数字相近并不相似。我使用的是稍微不同的sysctl（NET_RT_IFLIST2），它提供了我在代码中考虑的64位整数。

这是我的代码：

from ctypes import *
from ctypes.util import find_library
from ctypes import sizeof as c_sizeof
import errno

libc = CDLL(find_library('c'), use_errno=True)

# hardcoded macosx constants!
CTL_NET = 4
PF_ROUTE = 17
NET_RT_IFLIST2 = 6

RTM_IFINFO2 = 0x12
IFF_LOOPBACK = 0x8
AF_LINK = 18

class if_data64(Structure):
   _pack_ = 4
   _fields_ = [('ifi_type', c_ubyte),
               ('ifi_typelen', c_ubyte),
               ('ifi_physical', c_ubyte),
               ('ifi_addrlen', c_ubyte),
               ('ifi_hdrlen', c_ubyte),
               ('ifi_recvquota', c_ubyte),
               ('ifi_xmitquota', c_ubyte),
               ('ifi_unused1', c_ubyte),
               ('ifi_mtu', c_uint32),
               ('ifi_metric', c_uint32),
               ('ifi_baudrate', c_uint32),
               ('ifi_ipackets', c_uint64),
               ('ifi_ierrors', c_uint64),
               ('ifi_opackets', c_uint64),
               ('ifi_oerrors', c_uint64),
               ('ifi_collisions', c_uint64),
               ('ifi_ibytes', c_uint64),
               ('ifi_obytes', c_uint64),
               ('ifi_imcasts', c_uint64),
               ('ifi_omcasts', c_uint64),
               ('ifi_iqdrops', c_uint64),
               ('ifi_noproto', c_uint64),
               ('ifi_recvtiming', c_uint32),
               ('ifi_xmittiming', c_uint32)]

class if_msghdr2(Structure):
   _fields_ = [('ifm_msglen', c_ushort),
               ('ifm_version', c_ubyte),
               ('ifm_type', c_ubyte),
               ('ifm_addrs', c_int),
               ('ifm_flags', c_int),
               ('ifm_index', c_ushort),
               ('ifm_snd_len', c_int),
               ('ifm_snd_maxlen', c_int),
               ('ifm_snd_drops', c_int),
               ('ifm_timer', c_int),
               ('ifm_data', if_data64)]

class sockaddr_dl(Structure):
   _fields_ = [('sdl_len', c_ubyte),
               ('sdl_family', c_ubyte),
               ('sdl_index', c_ushort),
               ('sdl_type', c_ubyte),
               ('sdl_nlen', c_ubyte),
               ('sdl_alen', c_ubyte),
               ('sdl_slen', c_ubyte),
               ('sdl_data', c_char * 12)] # for now

MIB_TYPE = c_int * 6
mib = MIB_TYPE(CTL_NET, PF_ROUTE, 0, 0, NET_RT_IFLIST2, 0)

sysctl_buf_len = c_uint(0)

rval = libc.sysctl(mib, 6, None, byref(sysctl_buf_len), None, 0)
if rval != 0:
   raise Exception(errno.errorcode[get_errno()])

sysctl_buf = create_string_buffer(sysctl_buf_len.value)
rval = libc.sysctl(mib, 6, sysctl_buf, byref(sysctl_buf_len), None, 0)
if rval != 0:
   raise Exception(errno.errorcode[get_errno()])

# walk the structure.  you need to know the length from ifm_msglen
idx = addressof(sysctl_buf)
end = idx + sysctl_buf_len.value
while idx < end:
   batch_off = idx - addressof(sysctl_buf)
   ifmsg = cast(c_void_p(idx), POINTER(if_msghdr2))
   if ifmsg.contents.ifm_type != RTM_IFINFO2:
      idx += ifmsg.contents.ifm_msglen
      continue
   if ifmsg.contents.ifm_flags & IFF_LOOPBACK:
      idx += ifmsg.contents.ifm_msglen
      continue
   # 12 bytes to compensate for 32 bit alignment
   sdl = cast(c_void_p(idx + c_sizeof(if_msghdr2) + 12), POINTER(sockaddr_dl))
   if sdl.contents.sdl_family != AF_LINK:
      idx += ifmsg.contents.ifm_msglen
      continue

   print sdl.contents.sdl_data[0:sdl.contents.sdl_nlen]
   print ifmsg.contents.ifm_data.ifi_ibytes, ifmsg.contents.ifm_data.ifi_obytes
   idx += ifmsg.contents.ifm_msglen

Answer 1

在if_data64中，ifi_baudrate字段应为c_uint64而非c_uint32。修复此问题应将ifi_bytes和ifo_bytes对齐到正确的偏移量。它还错过了最后一个字段ifi_lastchange。这是一个8字节timeval32结构。总之，这解释了为什么sdl需要额外的12字节偏移量。

class timeval32(Structure):
    _fields_ = [('tv_sec', c_int32), 
                ('tv_usec', c_int32)]

class if_data64(Structure):
    _pack_ = 4
    _fields_ = [('ifi_type', c_ubyte),
                ('ifi_typelen', c_ubyte),
                ('ifi_physical', c_ubyte),
                ('ifi_addrlen', c_ubyte),
                ('ifi_hdrlen', c_ubyte),
                ('ifi_recvquota', c_ubyte),
                ('ifi_xmitquota', c_ubyte),
                ('ifi_unused1', c_ubyte),
                ('ifi_mtu', c_uint32),
                ('ifi_metric', c_uint32),
                ('ifi_baudrate', c_uint64),    # was c_uint32
                ('ifi_ipackets', c_uint64),
                ('ifi_ierrors', c_uint64),
                ('ifi_opackets', c_uint64),
                ('ifi_oerrors', c_uint64),
                ('ifi_collisions', c_uint64),
                ('ifi_ibytes', c_uint64),
                ('ifi_obytes', c_uint64),
                ('ifi_imcasts', c_uint64),
                ('ifi_omcasts', c_uint64),
                ('ifi_iqdrops', c_uint64),
                ('ifi_noproto', c_uint64),
                ('ifi_recvtiming', c_uint32),
                ('ifi_xmittiming', c_uint32),
                ('ifi_lastchange', timeval32)] # was missing

我没有使用带有指针的cast，而是使用from_buffer方法（Python 2.6+）和偏移来编写循环。

buf_offset = 0
while buf_offset < sysctl_buf_len.value:
   msg_offset = buf_offset
   ifmsg = if_msghdr2.from_buffer(sysctl_buf, msg_offset)
   buf_offset += ifmsg.ifm_msglen
   if ifmsg.ifm_type != RTM_IFINFO2 or ifmsg.ifm_flags & IFF_LOOPBACK:
      continue
   sdl_offset = msg_offset + c_sizeof(if_msghdr2)
   sdl = sockaddr_dl.from_buffer(sysctl_buf, sdl_offset)
   if sdl.sdl_family != AF_LINK:
      continue
   print sdl.sdl_data[:sdl.sdl_nlen]
   print ifmsg.ifm_data.ifi_ibytes, ifmsg.ifm_data.ifi_obytes

来自ctypes的垃圾uint64值

1 个答案: