直接从 Python 调用 getaddrinfo：ai_addr 为空指针

Question

我正在尝试在 Mac OS 上通过 ctypes / libc 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，ai_addrlen 设置为 28，据我所知，这是 IPv6 地址的适当长度。但是，ai_addr 似乎是一个空指针，我不知道如何开始调试它。

如何使用libc.getaddrinfo 找到域的 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 addrinfo 结构，其结果形式存储在getaddrinfo 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;
};

根据FreeBSD man page for getaddrinfo（或类似的Apple's man pages for getaddrinfo之一），假设所有类型都匹配，它的addrinfo看起来是一样的。

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FreeBSD source（或类似的one of the open source Apple projects），我们看到了一个微妙不同的定义：

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 Mac(/similar) 的 Python ctypes 定义应该是

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)),
]

或者一个可以在 Mac 和 Linux 上运行的（并且在其他平台上没有评论）

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。你也可以看到early/experimental version that parses the addresses themselves that works in both Mac and Linux。

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