unix代码示例：一个socket客户端程序
代码：模仿telnet访问newsmth bbs，打印第一次收到的应答后即退出。

#include <stdio.h>#include <unistd.h>#include <netdb.h> #include <errno.h> #include <sys/socket.h> #include <string.h> #define MAXADDRLEN  256  #define BUFLEN     500   void print_resp(int sockfd) {    printf("ready to print response\n");     char    buf[BUFLEN];     int n;     int i;     for(i = 0; i < 5; i ++){n = recv(sockfd, buf, BUFLEN, 0);        write(STDOUT_FILENO, buf, n);     }}  int main(int argc, char *argv[]) {     struct addrinfo *ailist, *aip;  //ai是addrinfo的缩写    struct addrinfo hint;  //从服务端拿到的addrinfo可能很多，需要使用hint来过滤。hint对象本身也定义为addrinfo类型    int             sockfd, err;      hint.ai_flags = 0;     hint.ai_family = 0;     hint.ai_socktype = SOCK_STREAM;     hint.ai_protocol = 0;     hint.ai_addrlen = 0;     hint.ai_canonname = NULL;     hint.ai_addr = NULL;     hint.ai_next = NULL;     if ((err = getaddrinfo("newsmth.org", "23", &hint, &ailist)) != 0) { //生成addrinfo        printf("getaddrinfo error: %s\n", gai_strerror(err)); exit(-1);    }    printf("getaddrinfo success\n");    for (aip = ailist; aip != NULL; aip = aip->ai_next) {         if ((sockfd = socket(aip->ai_family, SOCK_STREAM, 0)) < 0) //生成socket对象            err = errno; printf("make socket success\n");        if (connect(sockfd, aip->ai_addr, aip ->ai_addrlen) < 0) { //连接            err = errno;         } else {             printf("socket connect success\n");    print_resp(sockfd);             exit(0);         }     }     fprintf(stderr, "can't connect to the target: %s\n",        strerror(err));     exit(1); }