recvfrom()保存src_addr的字节顺序是什么？

Question

recvfrom()保存src_addr的字节顺序是什么？

clinuxrecvfrom

3

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

src_addr参数采用哪种字节顺序进行编写？是网络字节顺序还是主机字节顺序？我无法在recvfrom手册页或通过Google和SO搜索找到相关信息。

- StoneThrow

1

可能是struct sockaddr_in成员字节顺序对于bind()的重复问题。 - bishop

1个回答

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- dbush · Accepted Answer

假设套接字是IPv4或IPv6套接字，则存储在src_addr中的主机和端口将以网络字节顺序呈现。

这在IPv4手册（man 7 ip）中有记录：

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手册的措辞类似。

因此，在读取端口号时，使用ntohs来提取它。在读取地址时，使用inet_ntop将其转换为文本形式。