Delphi + Synapse: How to check if I am still connected

Question

I am using TTCPBlockSocket for a TCP/IP Application . The problem is that I can't determine when the connection is lost.

GetLastError returns 0
Read returns 0

But I encounter these values eaven if the application is connected so I can get it how can I be notified when the TCP/IP connection is lost.

Thanks

Answer 1

I would say, you can use the CanRead method, passing 0 to its Timeout parameter. This function internally performs select function for the socket and returns False when an error occurs. It should be enough to use just this method since the select function returns SOCKET_ERROR if you lose the connection, and it causes the CanRead method return False as a result. So I think, it's enough to check if the socket connection is alive this way:

function IsSocketAlive(ASocket: TTCPBlockSocket): Boolean;
begin
  Result := ASocket.CanRead(0);
end;

Answer 2

As opC0de says, to check if the client is still connected you need to both check TCPBlockSocket.CanRead(0) and TCPBlockSocket.WaitingData = 0

function IsSocketAlive(ASocket: TTCPBlockSocket): Boolean;
begin
  Result := (ASocket.Socket = INVALID_SOCKET) or ((ASocket.WaitingData = 0) and ASocket.CanRead(0));
end;

I'm using this way in my projects so I can confirm that it works.

Answer 3

I was using kattunga's answer, but encountered a lot of false positive triggers during my stress test. My assumption is that the packet will sometimes arrive at a server between WaitingData and CanRead conditions, leading to server thinking that a socket is dead. I changed the order of conditions and problem was gone.

function TTCPSynWorkThread.IsSocketDead(ASocket: TTCPBlockSocket): boolean;
begin 
  Result := (ASocket.Socket = INVALID_SOCKET) or (ASocket.CanRead(0) and (ASocket.WaitingData = 0));
end;