#include <netinet/in.h>
struct in_addr {
    in_addr_t s_addr;                     /* 32-bit IPv4 address */
};                                        /* network byte ordered */

struct sockaddr_in {
    unit8_t        sin_len;               /* length of structure (16) */
    sa_family_t    sin_family;            /* AF_INET */
    in_port_t      sin_port;              /* 16-bit TCP or UDP port number */
                                          /* network byte ordered */
    struct in_addr sin_addr;              /* 32-bit IPv4 address */
                                          /* network byte ordered */
    char           sin_zero[8];           /* unused */
};

/* ============================================================================================================================ */

#include <arpa/inet.h>

int inet_aton(const char *strptr, struct in_addr *addrptr);
返回：若字符串有效则为1，否则为0

in_addr_t inet_addr(const char *strptr);
返回：若字符串有效则为32位二进制网络字节序的IPv4地址，否则为INADDR_NONE

char *inet_ntoa(struct in_addr inaddr);
返回：指向一个点分十进制数串的指针

/* =========================================================================================================================== */

#include <sys/socket.h>

ssize_t recvfrom(int sockfd, void *buff, size_t nbytes, int flags, struct sockaddr *from, socklen_t *addrlen);

ssize_t sendto(int sockfd, const void *buff, size_t nbytes, int flags, const struct sockaddr *to, socklen_t addrlen);

返回：若成功则为读或写的字节数，若出错则为-1