从 Python 直接调用 getaddrinfo: ai_addr 是空指针

Question

我正在尝试通过 ctypes / libc 在 Mac OS 上从 Python 调用 getaddrinfo 以查找域的 IP 地址.

调用似乎成功：没有返回错误代码，并且ai_addrlen设置为28，据我所知这是适合IPv6地址的长度。但是，ai_addr 似乎是一个空指针，我不确定如何开始调试它。

如何使用 libc.getaddrinfo 查找域的 IP 地址？

from ctypes import (
    byref,
    c_char, c_char_p, c_int, c_size_t, c_void_p,
    CDLL,
    POINTER,
    pointer,
    Structure,
)

libc = CDLL(None)

class c_addrinfo(Structure):
    pass

c_addrinfo._fields_ = [
    ('ai_flags', c_int),
    ('ai_family', c_int),
    ('ai_socktype', c_int),
    ('ai_protocol', c_int),
    ('ai_addrlen', c_size_t),
    ('ai_addr', c_void_p),
    ('ai_canonname', c_char_p),
    ('ai_next', POINTER(c_addrinfo)),
]

c_addrinfo_p = POINTER(c_addrinfo)
result = c_addrinfo_p()
error = libc.getaddrinfo(
    c_char_p(b'www.google.com'),
    None,
    None,
    byref(result),
)

print(error)                          # 0
print(result.contents.ai_canonname)   # b'\x1c\x1e
print(result.contents.ai_addrlen)     # 28
print(bool(result.contents.ai_addr))  # False === null pointer

libc.freeaddrinfo(result)

Answer 1

根据 linux man page for getaddrinfo，存储 getaddrinfo 结果的 addrinfo 结构定义为

struct addrinfo {
    int              ai_flags;
    int              ai_family;
    int              ai_socktype;
    int              ai_protocol;
    socklen_t        ai_addrlen;
    struct sockaddr *ai_addr;
    char            *ai_canonname;
    struct addrinfo *ai_next;
};

并且根据相似的 FreeBSD man page for getaddrinfo (or one of Apple's man pages for getaddrinfo），它的 addrinfo 看起来是一样的，假设所有类型都匹配。

struct addrinfo {
     int ai_flags;             /* input flags */
     int ai_family;            /* address family for socket */
     int ai_socktype;          /* socket type */
     int ai_protocol;          /* protocol for socket */
     socklen_t ai_addrlen;     /* length of socket-address */
     struct sockaddr *ai_addr; /* socket-address for socket */
     char *ai_canonname;       /* canonical name for service location */
     struct addrinfo *ai_next; /* pointer to next in list */
};

然而查看相似的 FreeBSD source (or one of the open source Apple projects），我们看到一个微妙的不同定义：

struct addrinfo {
    int ai_flags;             /* AI_PASSIVE, AI_CANONNAME, AI_NUMERICHOST */
    int ai_family;            /* AF_xxx */
    int ai_socktype;          /* SOCK_xxx */
    int ai_protocol;          /* 0 or IPPROTO_xxx for IPv4 and IPv6 */
    socklen_t ai_addrlen;     /* length of ai_addr */
    char *ai_canonname;       /* canonical name for hostname */
    struct sockaddr *ai_addr; /* binary address */
    struct addrinfo *ai_next; /* next structure in linked list */
};

很容易错过，但 ai_canonname 和 ai_addr 与它们的记录方式相反 。这意味着 Python ctypes 定义，对于 Mac(/similar) 应该是

class c_addrinfo(Structure):
    pass

c_addrinfo._fields_ = [
    ('ai_flags', c_int),
    ('ai_family', c_int),
    ('ai_socktype', c_int),
    ('ai_protocol', c_int),
    ('ai_addrlen', c_size_t),
    ('ai_canonname', c_char_p),
    ('ai_addr', c_void_p),
    ('ai_next', POINTER(c_addrinfo)),
]

或适用于 Mac 和 Linux 的平台（在其他平台上无评论）

import platform

c_addrinfo._fields_ = [
    ('ai_flags', c_int),
    ('ai_family', c_int),
    ('ai_socktype', c_int),
    ('ai_protocol', c_int),
    ('ai_addrlen', c_size_t),
] + ([
    ('ai_canonname', c_char_p),
    ('ai_addr', c_void_p),
] if platform.system() == 'Darwin' else [
    ('ai_addr', c_void_p),
    ('ai_canonname', c_char_p),
]) + [
    ('ai_next', POINTER(c_addrinfo)),
]

对于这些版本，在 Mac 上，指针 ai_addr 不再为空。您还可以看到 early/experimental version that parses the addresses themselves that works in both Mac and Linux.

编辑：看起来像 documentation issue has already been reported to FreeBSD

从 Python 直接调用 getaddrinfo: ai_addr 是空指针

Calling getaddrinfo directly from Python: ai_addr is null pointer

python

ctypes

libc

python-3.x

getaddrinfo