UDP编程注意事项

客户端程序
#include    
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#include

int main(int argc,char *argv[]){
 char buf[65536];
 memset(buf,0,65536);
 int sockfd;
 int fd_server;
 int maxsockfd;
 int nread = 0;
 int yes = 1;
 fd_set fset;
 struct sockaddr_in server;//两种常见地址
 struct sockaddr addr;
 int len = sizeof(struct sockaddr);
/*
 server.sin_port = htons(12345);
 server.sin_family = AF_INET;
 server.sin_addr.s_addr = htonl("inet_addr("10.1.1.1")");
*/
 struct timeval timeout;
 
 sockfd = socket(AF_INET,SOCK_DGRAM, 0);
 setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes));//i常用设置
 server.sin_port = htons(12345);
 server.sin_family = AF_INET;
 server.sin_addr.s_addr = htonl(inet_addr("10.1.1.1"));//inet_addr的使用，htonl htons的使用
 if(bind(sockfd, (struct sockaddr*)&server, sizeof(struct sockaddr))<0)
 {
  printf("error:%s\n",strerror(errno));
 }
 
 FD_ZERO(&fset);
 FD_SET(sockfd,&fset);
 timeout.tv_sec = 100;
 timeout.tv_usec = 0;
 maxsockfd = sockfd + 1;
 while(select(maxsockfd,&fset,NULL,NULL,&timeout)< 0){
  printf("errno = %d,error:%s\n",errno,strerror(errno));
  exit(1);
 }
 if(FD_ISSET(sockfd,&fset)){
  nread = recvfrom(sockfd,buf,65507,0,&addr,&len);//recvfrom 参数 6个，第四个flag与第六个len尤其第六个要尤其主意
  printf("nead = %d\n",nread); 
 }
 fd_server = open("/mnt/server.log",O_RDWR|O_CREAT|O_APPEND);//mode O_RDWD O_CREAT O_APPEND需要注意所在头文件fcntl sys/stat
 if(fd_server < 0){
  printf("errno = %d,error:%s\n",errno,strerror(errno));
  exit(1);
 }
 if((nread = write(fd_server,buf,strlen((char*)buf)))<0){
  printf("errno = %d,error:%s\n",errno,strerror(errno));
  exit(1);
 }
 printf("nread = %d\n",nread);
 close(fd_server);
 close(sockfd);
}
客户端程序：

#include    
#include    
#include    
#include    
#include
#include
#include    
#include    
#include    
#include    
#include    
#include

int main(int argc,char *argv[]){
 char buf[65536];
 int sockfd;
 int fd_client;
 int maxsockfd;
 int nread = 0;
 int yes = 1;
 fd_set fset;
 struct sockaddr_in client;
// struct sockaddr addr;

 struct sockaddr_in server;

 server.sin_port = htons(12345);
 server.sin_family = AF_INET;
 server.sin_addr.s_addr = htonl(inet_addr("10.1.1.1"));

 struct timeval timeout;

 
 fd_client = open("/mnt/client.log",O_RDWR|O_CREAT|O_APPEND);
 if(fd_client < 0){
  printf("errno = %d,error:%s\n",errno,strerror(errno));
  exit(1);
 }
 if((nread = read(fd_client,buf,65535))<0){
  printf("errno = %d,error:%s\n",errno,strerror(errno));
  exit(1);
 }
 printf("nread = %d\n",nread);

 
 sockfd = socket(AF_INET,SOCK_DGRAM, 0);
 setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes));
 client.sin_port = htons(12345);
 client.sin_family = AF_INET;
 client.sin_addr.s_addr = htonl(INADDR_ANY);
 bind(sockfd, (struct sockaddr*)&client, sizeof(&client));
 
 FD_ZERO(&fset);
 FD_SET(sockfd,&fset);
 timeout.tv_sec = 2;
 timeout.tv_usec = 0;
 maxsockfd = sockfd + 1;
 while(select(maxsockfd,NULL,&fset,NULL,&timeout)< 0){
  printf("errno = %d,error:%s\n",errno,strerror(errno));
  exit(1);
 }
 if(FD_ISSET(sockfd,&fset)){
  nread = sendto(sockfd,buf,65507,0,(struct sockaddr*)&server,sizeof(struct sockaddr));//sendto 与recvfrom在参数上的最大不同在于第六个参数，不是以地址的形式出现
  printf("nead = %d\n",nread); 
 }

 close(fd_client);
 close(sockfd);
}
本程序设计主要是为了测试sendto最多可发送字节长度：UDP报文的最大长是65535，UDP头长8个字节，IP头长20个字节
故sendto最长可发送长度为65507个字节