udp sendto函数的疑惑

时间：2010-04-12 来源：zhanghonghu84

测试一个udp服务器时，写了如下程序。 #include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/select.h>
#include <netdb.h> int main(int argc,char *argv[])
{
int srv_sock;
socklen_t clt_len;
struct sockaddr_in srv_addr;
struct sockaddr_in clt_addr;
struct timeval wait_time;
int port;
int ret;
fd_set read_fds;
int num;
char recv_buf[1024]; if(argc!=2){
  printf("Usage: %s port_name\n",argv[0]);
  return 1;
}

port=atoi(argv[1]); srv_sock=socket(AF_INET,SOCK_DGRAM,0);
if(srv_sock<0){
  perror("cannot create socket");
  return 1;
} memset(&srv_addr,0,sizeof(srv_addr));
srv_addr.sin_family=AF_INET;
srv_addr.sin_addr.s_addr=htonl(INADDR_ANY);
srv_addr.sin_port=htons(port); ret=bind(srv_sock,(struct sockaddr*)&srv_addr,sizeof(srv_addr));
if(ret<0){
  perror("cannot bind the socket");
  return 1;
}    // clt_len = sizeof(struct sockaddr_in);

while(1){
  wait_time.tv_sec=0;
  wait_time.tv_usec=0;
  FD_ZERO(&read_fds);
  FD_SET(srv_sock,&read_fds);

  num=select(srv_sock+1,&read_fds,NULL,NULL,&wait_time);
  if(num<0){
   perror("select fail");
   continue;
  }

  if(FD_ISSET(srv_sock,&read_fds)){
   int n=recvfrom(srv_sock,recv_buf,sizeof(recv_buf),0,(struct sockaddr*)&clt_addr,&clt_len);
   printf("clt_len is %d\n",clt_len);
   if(n<0){
    perror("cannot receive client message");
    close(srv_sock);
    return 1;
   }
   recv_buf[n] = 0;
   printf("server receive: %s\n",recv_buf);
   memset(recv_buf,0,sizeof(recv_buf));

   n = sendto(srv_sock,"hello\n",6,0,(struct sockaddr*)&clt_addr,clt_len);
   if(n < 0)
   {
       perror("can't sent client message");
   }


  }
}
} 测试代码时，发现服务器第一次收到数据发送总是提示 can't sent client message: Address family not supported by protocol 后面的就会一直成功。不明白什么原因。比较了几个udp例子之后，发现把程序里面红色部分不注释掉，就可以成功了。始终找不到原因，到后续再看看源代码看怎么解决？希望知道的兄弟能说下。

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