sock编程，发送一个文件后，接受完毕，要等S端和C端都关闭后才能读取文件

我发送一个图片，接收完后，在客户端和服务器端没有关闭之前，打开此图片显示无法读取。当关闭客户端和服务器端后，就可以正常打开了。为什么？
客户端发送代码：FILE   *pRead;      
    pRead=fopen("f:\\1.jpg","rb+");   
    while(!feof(pRead)) 
//while(fread(BUFFER,1,1024,pRead))
    {   
       fread(BUFFER,1,1024,pRead);   
       clientStream.send_n(BUFFER,sizeof(BUFFER));
   memset(BUFFER,0,sizeof(BUFFER));
      }

fclose(pRead);
memset(BUFFER,0,sizeof(BUFFER));
    strcpy(BUFFER,"END");cout<<BUFFER;
clientStream.send_n(BUFFER,sizeof(BUFFER));
服务器端接收代码： FILE *pWrite;
 char BUFFER[1024];
 memset(BUFFER,0,sizeof(BUFFER));
  pWrite=fopen("d:\\1.jpg","wb+");
 while(severStream.recv_n(BUFFER,sizeof(BUFFER))!=-1)
 {
 
 if(strcmp(BUFFER,"END")!=0)
              {
                fwrite(BUFFER,1024,1,pWrite);
memset(BUFFER,0,sizeof(BUFFER));
              }