error LNK2001: unresolved external symbol _main等.该如何解决

error LNK2001: unresolved external symbol _main等.
#include "stdafx.h"
#include <WinSock.h>
#include <time.h>
#include <windows.h>


#pragma comment (lib,"Ws2_32")  
   
struct NTP_Packet  
{  
  int Control_Word;  
  int root_delay;  
  int root_dispersion;  
  int reference_identifier;  
  __int64 reference_timestamp;  
  __int64 originate_timestamp;  
  __int64 receive_timestamp;  
  int transmit_timestamp_seconds;  
  int transmit_timestamp_fractions;  
};

/************************************************************************/  
/* 函数说明:自动与时间服务器同步更新  
/* 参数说明:无  
/* 返 回 值:成功返回TRUE，失败返回FALSE  
/************************************************************************/  
BOOL UpdateSysTime()  
{  
  WORD wVersionRequested;  
  WSADATA wsaData;  
   
  // 初始化版本  
  wVersionRequested = MAKEWORD( 1, 1 );  
  if (0!=WSAStartup(wVersionRequested, &wsaData))  
  {  
  WSACleanup();  
  return FALSE;  
  }  
  if (LOBYTE(wsaData.wVersion)!=1 || HIBYTE(wsaData.wVersion)!=1)  
  {  
  WSACleanup( );  
  return FALSE;  
  }  
   
  // 这个IP是*时间同步服务器地址，可自行修改  
  SOCKET soc=socket(AF_INET,SOCK_DGRAM,IPPROTO_UDP);  
  struct sockaddr_in addrSrv;  
  addrSrv.sin_addr.S_un.S_addr=inet_addr("192.43.244.18");  
  addrSrv.sin_family=AF_INET;  
  addrSrv.sin_port=htons(123);  
   
  NTP_Packet NTP_Send,NTP_Recv;  
  NTP_Send.Control_Word = htonl(0x0B000000);  
  NTP_Send.root_delay = 0;  
  NTP_Send.root_dispersion = 0;  
  NTP_Send.reference_identifier = 0;  
  NTP_Send.reference_timestamp = 0;  
  NTP_Send.originate_timestamp = 0;  
  NTP_Send.receive_timestamp = 0;  
  NTP_Send.transmit_timestamp_seconds = 0;  
  NTP_Send.transmit_timestamp_fractions = 0;  
   
  if(SOCKET_ERROR==sendto(soc,(const char*)&NTP_Send,sizeof(NTP_Send),  
  0,(struct sockaddr*)&addrSrv,sizeof(addrSrv)))  
  {  
  closesocket(soc);  
  return FALSE;  
  }  
  int sockaddr_Size =sizeof(addrSrv);  
  if(SOCKET_ERROR==recvfrom(soc,(char*)&NTP_Recv,sizeof(NTP_Recv),  
  0,(struct sockaddr*)&addrSrv,&sockaddr_Size))  
  {  
  closesocket(soc);  
  return FALSE;  
  }  
  closesocket(soc);  
  WSACleanup();  
   
  SYSTEMTIME newtime;  
  float Splitseconds;  
  struct tm *lpLocalTime;  
  time_t ntp_time;  
   
  // 获取时间服务器的时间  
  ntp_time = ntohl(NTP_Recv.transmit_timestamp_seconds)-2208988800;  
  lpLocalTime = localtime(&ntp_time);  
  if(lpLocalTime == NULL)  
  {  
  return FALSE;  
  }  
   
  // 获取新的时间  
  newtime.wYear =lpLocalTime->tm_year+1900;  
  newtime.wMonth =lpLocalTime->tm_mon+1;