针对TIME服务的UDP客户软件的实现（附源码含注释）

Posted 2022-01-06 秒五

俗话虽说宇宙机人要善于复制，但博主还是希望各位能理解其中的原理，最好亲手实现一遍，毕竟三维空间中自己经历过的是记忆中最深刻的，且远比二维空间中的文字阅读要深刻。
鉴于博主的阅历有限，文章有失误或者疑惑的地方还望多多指出，开了提醒的我会第一时间交流回复，和各位同伴一起进步呀(ง •_•)ง

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实验原理：UDP客户端交互模式

步骤一：引入头文件

#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <errno.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <stdarg.h>
#include <time.h>

步骤二：Time服务客户端所需函数声明以及宏定义

#define BUFSIZE 64
#define UNIXEPOCH 2208988800UL
#define MSG "what time is it?\\n"
#define LINELEN 128

#ifndef INADDR_NONE
#define INADDR_NONE 0xffffffff
#endif

/*UDP链接*/
int connectUDP(const char* host, const char* service);
/*连接函数*/
int connectsock(const char* host, const char* service, const char* transport);
/*报错并退出*/
int errexit(const char* format, ...);

步骤三：主函数的实现，定义本机IP和服务名，连接套接字后向服务器发送数据（请求Time服务），接着接收数据（Time），并将接收到的数据从网络字节顺序转换成主机字节顺序，再转换成UNIX时间并打印输出

int main(int argc, char *argv[]) 
	char *host = "localhost";
	char *service = "time";
	time_t now;
	int s, n;
    switch (argc)
    
    case 1:
        host = "localhost";
        break;
    case 3:
        service = argv[2];
    case 2:
        host = argv[1];
        break;
    default:
        fprintf(stderr, "usage: UDPtime [host [prot]]\\n");
        exit(1);
    
    s = connectUDP(host, service);  //套接字
    (void)write(s, MSG, strlen(MSG));  //发送数据（请求）
    n = read(s, (char*)&now, sizeof(now));  //接收数据（时间）
    if (n < 0)
    
        errexit("read failed: %s\\n", strerror(errno));  //若无返回数据，报错
    
    now = ntohl((unsigned long)now);  //网络字节顺序转主机字节顺序
    now -= UNIXEPOCH;  //转换成UNIX时间
    printf("%s", ctime(&now));  //打印接收到的数据（时间）
    exit(0);

步骤四：connectUDP函数的实现，调用connectsock函数选择UDP连接

int connectUDP(const char* host, const char* service)

    return connectsock(host, service, "udp");

步骤五：connectsock函数的实现，实现所有分配和连接TCP/UDP套接字的操作（连接的实现）

int connectsock(const char* host, const char* service, const char* transport)

    struct hostent* phe;  //主机信息
    struct servent* pse;  //服务信息
    struct protoent* ppe;  //协议信息
    struct sockaddr_in sin;  //网络端点地址
    int s, type;
    memset(&sin, 0, sizeof(sin));
    sin.sin_family = AF_INET;

    /*匹配服务名和端口号*/
    if (pse = getservbyname(service, transport))
    
        //sin.sin_port = pse->s_port;
        sin.sin_port = htons(5188);
    
    else if ((sin.sin_port = htons((unsigned short)atoi(service))) == 0)
    
        errexit("can't get \\"%s\\" service entry\\n", service);
    

    /*匹配主机名至IP地址（允许点分十进制）*/
    if (phe = gethostbyname(host))
    
        memcpy(&sin.sin_addr, phe->h_addr, phe->h_length);
    
    else if ((sin.sin_addr.s_addr = inet_addr(host)) == INADDR_NONE)
    
        errexit("can't get \\"%s\\" host entry\\n", host);
    

    /*匹配传输协议名至协议号*/
    if ((ppe = getprotobyname(transport)) == 0)
    
        errexit("can't get \\"%s\\" protocol entry\\n", host);
    

    /*通过协议选择套接字类型*/
    if (strcmp(transport, "udp") == 0)
    
        type = SOCK_DGRAM;
    
    else
    
        type = SOCK_STREAM;
    

    /*分配套接字*/
    s = socket(PF_INET, type, ppe->p_proto);
    if (s < 0)
    
        errexit("can't create socket: %s\\n", strerror(errno));
    

    /*连接套接字*/
    if (connect(s, (struct sockaddr*)&sin, sizeof(sin)) < 0)
    
        errexit("can't connect to %s.%s: %s\\n", host, service, strerror(errno));
    
    return s;

步骤六：报错函数的实现

int errexit(const char* format, ...)

    va_list args;
    va_start(args, format);
    vfprintf(stderr, format, args);
    va_end(args);
	exit(1);

实验结果：成功访问Time服务~

完整源码

#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <errno.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <stdarg.h>
#include <time.h>

#define BUFSIZE 64
#define UNIXEPOCH 2208988800UL
#define MSG "what time is it?\\n"
#define LINELEN 128

#ifndef INADDR_NONE
#define INADDR_NONE 0xffffffff
#endif

/*UDP链接*/
int connectUDP(const char* host, const char* service);
/*连接函数*/
int connectsock(const char* host, const char* service, const char* transport);
/*报错并退出*/
int errexit(const char* format, ...);

int main(int argc, char *argv[]) 
	char *host = "localhost";
	char *service = "time";
	time_t now;
	int s, n;
    switch (argc)
    
    case 1:
        host = "localhost";
        break;
    case 3:
        service = argv[2];
    case 2:
        host = argv[1];
        break;
    default:
        fprintf(stderr, "usage: UDPtime [host [prot]]\\n");
        exit(1);
    
    s = connectUDP(host, service);  //套接字
    (void)write(s, MSG, strlen(MSG));  //发送数据（请求）
    n = read(s, (char*)&now, sizeof(now));  //接收数据（时间）
    if (n < 0)
    
        errexit("read failed: %s\\n", strerror(errno));  //若无返回数据，报错
    
    now = ntohl((unsigned long)now);  //网络字节顺序转主机字节顺序
    now -= UNIXEPOCH;  //转换成UNIX时间
    printf("%s", ctime(&now));  //打印接收到的数据（时间）
    exit(0);


/*第二过程 选择TCP或UDP连接*/
int connectUDP(const char* host, const char* service)

    return connectsock(host, service, "udp");



/*第三过程 连接的实现（分配和连接TCP/UDP套接字）*/
int connectsock(const char* host, const char* service, const char* transport)

    struct hostent* phe;  //主机信息
    struct servent* pse;  //服务信息
    struct protoent* ppe;  //协议信息
    struct sockaddr_in sin;  //网络端点地址
    int s, type;
    memset(&sin, 0, sizeof(sin));
    sin.sin_family = AF_INET;

    /*匹配服务名和端口号*/
    if (pse = getservbyname(service, transport))
    
        sin.sin_port = pse->s_port;
    
    else if ((sin.sin_port = htons((unsigned short)atoi(service))) == 0)
    
        errexit("can't get \\"%s\\" service entry\\n", service);
    

    /*匹配主机名至IP地址（允许点分十进制）*/
    if (phe = gethostbyname(host))
    
        memcpy(&sin.sin_addr, phe->h_addr, phe->h_length);
    
    else if ((sin.sin_addr.s_addr = inet_addr(host)) == INADDR_NONE)
    
        errexit("can't get \\"%s\\" host entry\\n", host);
    

    /*匹配传输协议名至协议号*/
    if ((ppe = getprotobyname(transport)) == 0)
    
        errexit("can't get \\"%s\\" protocol entry\\n", host);
    

    /*通过协议选择套接字类型*/
    if (strcmp(transport, "udp") == 0)
    
        type = SOCK_DGRAM;
    
    else
    
        type = SOCK_STREAM;
    

    /*分配套接字*/
    s = socket(PF_INET, type, ppe->p_proto);
    if (s < 0)
    
        errexit("can't create socket: %s\\n", strerror(errno));
    

    /*连接套接字*/
    if (connect(s, (struct sockaddr*)&sin, sizeof(sin)) < 0)
    
        errexit("can't connect to %s.%s: %s\\n", host, service, strerror(errno));
    
    return s;


/*报错*/
int errexit(const char* format, ...)

    va_list args;
    va_start(args, format);
    vfprintf(stderr, format, args);
    va_end(args);
    exit(1);