decoding ctypes structures

Question

I'm trying to get my head around ctypes in python, and I'm able to get some simple things working, but when it comes to unpacking c structs I find myself running into some difficulties. I decided that to learn this I should play around a bit, and while I know the socket standard library implements gethostbyname_ex(), I thought I'd make an attempt at implementing it using ctypes and libc.gethostbyname().

I'm able to execute libc.gethostbyname() fairly easily:

#!/usr/bin/env python
from ctypes import *

cdll.LoadLibrary('libc.so.6')
libc = CDLL('libc.so.6')
he = libc.gethostbyname("www.google.com")

But that gives me a hostent data-structure. I thought the best way to unpack this would be to grab the c struct and create a class which inherits from ctypes.Structure, and so I came up with this (I found the hostent struct definition in netdb.h):

class hostent(Structure):
    '''
    struct hostent
    {
      char *h_name;                 /* Official name of host.  */
      char **h_aliases;             /* Alias list.  */
      int h_addrtype;               /* Host address type.  */
      int h_length;                 /* Length of address.  */
      char **h_addr_list;           /* List of addresses from name server. */
    }
    '''
    _fields_ = [("h_name", c_char_p), ("h_aliases", POINTER(c_char_p)),
                ("h_addrtype", c_int), ("h_length", c_int),
                ("h_addr_list", POINTER(c_char_p))]

Where I'm unclear is whether I've setup the h_aliases and the h_addr_list fields correctly, because whenever I attempt to access them as arrays, even at the 0th index after a lookup of something that I know has at least one alias and at least one address, I get a NULL pointer access ValueError exception:

>>> he = hostent(libc.gethostbyname("www.google.com"))
>>> pprint.pprint(he.h_addr_list)
<__main__.LP_c_char_p object at 0xb75dae84>
>>> print he.h_addr_list[0]
Traceback (most recent call last):
  File "/tmp/py2659JxK", line 24, in <module>
    print he.h_addr_list[0]
ValueError: NULL pointer access

Any suggestions would be welcome.

Answer 1

You need to define what the return type of gethostbyname is:

>>> libc.gethostbyname.restype = POINTER(hostent)
>>> he = libc.gethostbyname("www.google.com")[0]
>>> he.h_aliases[0]
'www.google.com'

Also, h_addr_list should not be declared as POINTER(c_char_p), because c_char_p is used for null-terminated strings. In this case POINTER(POINTER(c_ubyte)) would be better, and then the first address would be he.h_addr_list[0][:4] if it's an IPv4 address.