网络编程学习笔记(udp_client函数)

创建一个未连接的UDP套接口的客户函数，其代码如下：

#include <netdb.h>#include <sys/socket.h>#include <netinet/in.h>#include <arpa/inet.h>#include <unistd.h>#include <stdlib.h>#include <string.h>#include <stdio.h>#include <errno.h>#define MAXLINE 128int udp_client(const char *host, const char *serv, void **saptr, socklen_t *lenp){int sockfd;int n;struct addrinfo hints, *res, *ressave;bzero(&hints, sizeof(hints));hints.ai_family = AF_UNSPEC;hints.ai_socktype = SOCK_DGRAM;if ((n = getaddrinfo(host, serv, &hints, &res)) != 0) {printf("udp_client error for %s, %s:%s\n", host, serv, gai_strerror(n));return -1;}ressave = res;do {sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);if (sockfd >= 0) break;} while ((res = res->ai_next) != NULL);if (res == NULL) {printf("udp_client error for %s, %s\n", host, serv);return -1;}*saptr = malloc(res->ai_addrlen);memcpy(*saptr, res->ai_addr, sizeof(res->ai_addrlen));*lenp = res->ai_addrlen;freeaddrinfo(ressave);return sockfd;}char *sock_ntop(const struct sockaddr* sa, socklen_t len){char portstr[7];static char str[MAXLINE + 1];switch (sa->sa_family) {case AF_INET:{struct sockaddr_in *sin = (struct sockaddr_in*)sa;if (inet_ntop(AF_INET, &sin->sin_addr, str, sizeof(str)) == NULL) return NULL;if (ntohs(sin->sin_port) != 0) {snprintf(portstr, sizeof(portstr), ":%d", ntohs(sin->sin_port));strcat(str, portstr);}return str;}}return NULL;}int main(int argc, char **argv){int sockfd, n;char recvline[MAXLINE + 1];socklen_t salen;struct sockaddr *sa;if (argc != 3) {printf("usage:exename <hostname/ipaddress> <sevice/port>\n");return -1;}sockfd = udp_client(argv[1], argv[2], (void **)&sa, &salen);printf("sending to %s\n", sock_ntop(sa, salen));sendto(sockfd, " ", 1, 0, sa, salen);n = recvfrom(sockfd, recvline, MAXLINE, 0, NULL, NULL);recvline[n] = 0;fputs(recvline, stdout);return 0;}