Problem with LWIP_ UDP can receice can't send!

My project is to transmit data to pc as quick as possible . Now I can transfer data to PC with lwip tcp/ip ,the max rate is about 920KB/s, So I think if I use the udp ,the speed will be much high. But now ,the problem is I make a udp server I can get the data from the PC, but I can’t transmit my data to the PC!

 # include <stdio.h># include <stdlib.h># include <string.h># include <ctype.h># include <sys/param.h># include <sys/fcntl.h># include "lwip/netif.h"# include "lwip/sockets.h"# include "arch/sys_arch.h"# include "sys/alt_alarm.h"# include "alt_types.h"# include "http.h"# include "user.h"
 # include "system.h"# include <string.h># include "io.h"# include "altera_avalon_pio_regs.h"# include "alt_types.h"# include "sys/alt_irq.h"# include "priv/alt_busy_sleep.h"# include "sys/alt_dev.h"
 
volatile alt_u32 done=0;  // SRH
alt_u32 data1=0;   // SRH
alt_u32 data_a,data_i;
alt_u32 data={0};//gsl
 
 
void http_task()
{
  int     i, fd_listen, max_socket;
  int     socket_send,len_send;                    //srh
  int     socket_recv,len_recv;                    //srh
  struct  sockaddr_in addr,sDestAddr;
  struct  timeval select_timeout;
  struct sockaddr_in  send_rem;                //srh
  struct sockaddr_in  recv_rem;                //srh
  fd_set  readfds, writefds;
  static  http_conn     conn;
  unsigned char    *dddata;               //srh
  unsigned char err;                      //srh
  unsigned char buff="12345";       //srh
 
  len_send = sizeof(send_rem);                     //srh
  len_recv = sizeof(recv_rem);                     //srh
 
  //if ((fd_listen = socket(AF_INET, SOCK_STREAM, 0)) < 0)
  if ((fd_listen = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
  {
    die_with_error(" Listening socket creation failed");
  }
 
 
  addr.sin_family = AF_INET;
  addr.sin_len=sizeof(addr);
  addr.sin_port = htons(12345);   // 80  HTTP_PORT
  addr.sin_addr.s_addr = INADDR_ANY;
 
 
  sDestAddr.sin_family=AF_INET;
  sDestAddr.sin_len=sizeof(sDestAddr);
  sDestAddr.sin_port=htons(23456);
  sDestAddr.sin_addr.s_addr=htonl(INADDR_BROADCAST);
 
  if ((bind(fd_listen,(struct sockaddr *)&addr,sizeof(addr))) < 0)
  {
    die_with_error(" Bind failed");
  }
  /*
   * Sockets primer, continued...
   * The listen socket is a socket which is waiting for incoming connections.
   * This call to listen will block (i.e. not return) until someone tries to 
   * connect to this port.
   */ 
  //if ((listen(fd_listen,1)) < 0)
  //{
  //  die_with_error(" Listen failed");
  //}
  /* 
   * At this point we have successfully created a socket which is listening
   * on HTTP_PORT for connection requests from any remote address.
   */
 // for(i=0; i<HTTP_NUM_CONNECTIONS; i++)
  //{
  //  http_reset_connection(&conn, i);
  //}
 
  while(1)
  {
    /*****************************************SRH**********************************************/ 
 /*   printf("receive test\n");
      socket_recv=accept(fd_listen,(struct sockaddr*)&recv_rem,&len_recv);
      while(1)
      {
       printf("test0");
       if(0!=done)
       {
         printf("I am working!");
       recv(socket_recv,data1,strlen(data1),0);
       send(socket_send,data1,strlen(data1),0);
       msleep(500);
       done--;
       }
      }
      close(socket_send);*/
    /*****************************************SRH**********************************************/ 
 
 
    /**************************srh****************************/
        printf("\nreceive test\n");
        //send(socket_send,(unsigned char *)&data,60,0);
       // socket_send= accept(fd_listen,(struct sockaddr*)&send_rem,&len_send);
        //sprintf(dddata,"welcome to cugb");
        char buffer;
        int nbytes;
        while(1)
        {
           nbytes=lwip_recv(fd_listen, buffer, sizeof(buffer),8);
           if(nbytes>0)
          { printf("test1->%s\n",buffer);
           lwip_sendto(fd_listen,"UDP_DATA", sizeof("UDP_DATA")+1, 0, (struct sockaddr *)&sDestAddr, sizeof(sDestAddr));
           msleep(500);
          }
 
           //lwip_sendto(fd_listen, "udp_data", sizeof("udp_data"), 0, (struct sockaddr *)&sDestAddr, sizeof(sDestAddr));
          // if(0!=done)
           //{
            //send(socket_send,buff,strlen(buff),0);
         //   data=data_i;
          //  send(socket_send,(unsigned char *)&data,4000,0);//60
          //  data_i++;
          //  for(i=0;i<500;i++);
            printf("waiting.....");
            msleep(50);
 
          //  done--;
          // }      
        }
 
        close(fd_listen);
 
 
 
  }
}