problem with GetIpForwardTable Implementation in Delphi 7

I have put together this code for viewing Routing Table in My delphi App.

  PMIB_IPFORWARDROW = ^TMibIpForwardRow;
TMibIpForwardRow=packed record
dwForwardDest:DWORD;
dwForwardMask:DWORD;
dwForwardPolicy:DWORD;
dwForwardNextHop:DWORD;
dwForwardIfIndex:DWORD;
dwForwardType:DWORD;
dwForwardProto:DWORD;
dwForwardAge:DWORD;
dwForwardNextHopAS:DWORD;
dwForwardMetric1:DWORD;
dwForwardMetric2:DWORD;
dwForwardMetric3:DWORD;
dwForwardMetric4:DWORD;
dwForwardMetric5:DWORD;
end;

PMIB_IPFORWARDTABLE=^TMibIpForwardTable ;
TMibIpForwardTable=packed record
dwNumEntries:DWORD;
table:array [0..ANY_SIZE-1] of TMibIpForwardRow;
end;

The Declaration of the Function is :

 getIpForwardTable: function (pIpForwardTable:PMIB_IPFORWARDTABLE; pdwSize:PULONG; bOrder:BOOL):DWORD; stdcall;

And the Function for Implementation is:

  procedure GetRouteTable;
 var
 Error:DWORD;
 pRouteTable:PMIB_IPFORWARDTABLE ;
   dwSize:ULONG;
  //dwSize:ULONG;
  TableSize:Integer;
 begin
  try
    dwSize:=0;
    GetMem(pRouteTable,SizeOf(TMibIpForwardTable));
   Error:=GetIpForwardTable(pRouteTable,@dwSize,FALSE);// Error at this line
   if Error=NO_ERROR t开发者_JAVA百科hen
   begin
     ShowMessage(IntToStr(Error));

    end
    else
        ShowMessage(IntToStr(Error));
   except
     on E:Exception do
     ShowMessage(E.ClassName+':'+E.Message);
     end;
   end;

I am getting Error code=122. Please help in correcting this Error.

Thanks in Advance

edit

I have tried this combo too but it didnt work

  Error:=GetIpForwardTable(nil,@dwSize,FALSE);
  if Error=ERROR_INSUFFICIENT_BUFFER then
    ShowMessage(IntToStr(Error))
  else
  begin
    GetMem(pRouteTable,dwSize);
  Error:=getIpForwardTable(pRouteTable,@dwSize,False);
   if Error=NO_ERROR then
  begin
    ShowMessage(IntToStr(Error));
  end
  else
    ShowMessage(IntToStr(Error));

Error code 122 is ERROR_INSUFFICIENT_BUFFER. The documentation for GetIpForwardTable states that this error code is returned when the following occurs.

The buffer pointed to by the pIpForwardTable parameter is not large enough. The required size is returned in the DWORD variable pointed to by the pdwSize parameter.

The solution is to read the value of dwSize that is returned, and call again with a buffer of that size.

The other problem with your code is that you do not pass in a valid buffer. Notice that the variable pRouteTable is never assigned to. In your edit, you do call GetMem to allocate it, but in the wrong place. You need something like this:

Error := GetIpForwardTable(nil, @dwSize, False);
if Error<>ERROR_INSUFFICIENT_BUFFER then
  RaiseLastOSError(Error);
GetMem(pRouteTable,dwSize);
Error := GetIpForwardTable(pRouteTable, @dwSize, False);
if Error<>ERROR_SUCCESS then
  RaiseLastOSError(Error);