Linux程序设计之套接字：UDP

    编写与客户端之间维持连接的应用程序，我们用面向连接的TCP套接字来完成这一工作。但在有些情况下，在程序中花费时间来建立和维持一个套接字连接是不必要的。比如 daytime 服务，我们首先创建一个套接字，然后建立连接，读取一个响应，最后关闭连接。在这一过程中，我们使用了很多操作步骤，仅仅为了获取一个日期。

     daytime 服务还可以用数据报通过UDP来访问。为了访问它，发送一个数据报给该服务，然后在响应中获取一个包含日期和时间的数据报。这一过程非常简单。当客户需要发送一个短小的查询请求给服务器，并且期望接受到一个短小的响应时，我们一般使用由UDP提供的服务。如果服务器处理客户请求的时间足够短，服务器就可以通过一次处理一个客户请求的方式来提供服务，从而允许操作系统将客户进入的请求放入队列。这简化服务器程序的编写。

    为了使用UDP提供的服务，你需要像往常一样使用套接字和 close 系统调用，但你需要用两个数据报专用的系统调用 sendto 和 recvfrom 来代替使用TCP服务时的 read 和 write 调用。下面是一个简单的示范程序，如有需要可以扩展其功能：

 

/* server.cc */
 
#include <cstdio>                                                                 
#include <cstdlib> 
                                                                                  
#include "sys/socket.h"                                                           
#include "netinet/in.h"                                                           
#include "unistd.h"                                                               
                                                                                  
#define SRV_PORT 6500 

using namespace std;                                                              
                                                                                  
char buffer[512];                                                                 
                                                                                  
int main(int argc, char *argv[])                                                  
{                                                                                 
    /* Create a UDP socket */                                                     
    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);                                  
                                                                                  
    /* Construct the address for use with sendto/recvfrom */                      
    struct sockaddr_in address;                                                   
    address.sin_family = AF_INET;                                                 
    address.sin_addr.s_addr = htonl(INADDR_ANY);                                  
    address.sin_port = htons(SRV_PORT);                                           
    int len = sizeof(address);                                                    
                                                                                  
    /* Bind address to socket */                                                  
    bind(sockfd, (struct sockaddr *)&address, sizeof(address));                   
                                                                                  
    /* serving */                                                                 
    while(1) {                                                                    
        /* receive */                                                             
        recvfrom(sockfd, buffer, sizeof(buffer), 0,                               
                 (struct sockaddr *)&address, (socklen_t *)&len);                 
                                                                                  
        /* handle */                                                              
        sprintf(buffer, "i am server");                                           
                                                                                  
        /* response */                                                            
        sendto(sockfd, buffer, sizeof(buffer), 0,                                 
               (struct sockaddr *)&address, sizeof(address));                     
                                                                                  
    }                                                                             
                                                                                  
    /* close */                                                                   
    close(sockfd);                                                                
                  
    return 0;                                                                     
} 

/* client.cc */
 
#include <cstdio>                                                                 
#include <cstdlib>                                                                
                                                                                  
#include "arpa/inet.h"                                                            
#include "sys/socket.h"                                                           
#include "netinet/in.h"                                                           
#include "unistd.h"                                                               
                        
#define SRV_ADDR "127.0.0.1"                                                 
#define SRV_PORT 6500 
 
#include "client.h"                                                               
                                                                                  
using namespace std;                                                              
                                                                                  
char buffer[512];                                                                 
                                                                                  
int main(int argc, char *argv[])                                                  
{                                                                                 
    /* Create a UDP socket */                                                     
    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);                                  
                                                                                  
    /* Construct the address for use with sendto/recvfrom */                      
    struct sockaddr_in address;                                                   
    address.sin_family = AF_INET;                                                 
    address.sin_addr.s_addr = inet_addr(SRV_ADDR);                                
    address.sin_port = htons(SRV_PORT);                                           
    int len = sizeof(address);                                                    
                                                                                  
    /* sendto */                                                                  
    sendto(sockfd, buffer, sizeof(buffer), 0,                                     
           (struct sockaddr *)&address, sizeof(address));                         
                                                                                  
    /* receive */                                                                 
    recvfrom(sockfd, buffer, sizeof(buffer), 0,                                   
             (struct sockaddr *)&address, (socklen_t *)&len);                     
                                                                                  
    /* debug */                                                                   
    printf("RECV: %s", buffer);                                                   
                                                                                  
    /* close */                                                                   
    close(sockfd);                                                                
 
    return 0;                                                                     
}