c - recvfrom() 将 src_addr 保存为什么字节顺序？

Question

size_t recvfrom(int sockfd, void *buf, size_t len, int flags, struct sockaddr *src_addr, socklen_t *addrlen)

src_addr 参数以什么字节顺序写入？网络还是主机？我无法在 recvfrom 手册页或通过 google 和 SO 进行搜索时找到它。

Answer 1

假设套接字是 IPv4 或 IPv6 套接字，src_addr 中存储的主机和端口将按网络字节顺序排列。

这记录在 man page for IPv4 (man 7 ip) 中如下:

Address format

An IP socket address is defined as a combination of an IP interface address and a 16-bit port number. The basic IP protocol does not supply port numbers, they are implemented by higher level protocols
like udp(7) and tcp(7). On raw sockets sin_port is set to the IP protocol.

   struct sockaddr_in {
       sa_family_t    sin_family; /* address family: AF_INET */
       in_port_t      sin_port;   /* port in network byte order */
       struct in_addr sin_addr;   /* internet address */
   };

   /* Internet address. */
   struct in_addr {
       uint32_t       s_addr;     /* address in network byte order */
   };

sin_family is always set to AF_INET. This is required; in Linux 2.2 most networking functions return EINVAL when this setting is missing. sin_port contains the port in network byte order. The port numbers below 1024 are called privileged ports (or sometimes: reserved ports). Only a privileged process (on Linux: a process that has the CAP_NET_BIND_SERVICE capability in the user namespace governing its network namespace) may bind(2) to these sockets. Note that the raw IPv4 protocol as such has no concept of a port, they are implemented only by higher protocols like tcp(7) and udp(7).

sin_addr is the IP host address. The s_addr member of struct in_addr contains the host interface address in network byte order. in_addr should be assigned one of the INADDR_* values (e.g., INADDR_LOOPBACK) using htonl(3) or set using the inet_aton(3), inet_addr(3), inet_makeaddr(3) library functions or directly with the name resolver (see gethostbyname(3)).

ipv6 手册页有类似的措辞。

所以在读取端口号的时候，使用ntohs来提取。读取地址时，使用inet_ntop将其转换为文本形式。