Reading SIO_KEEPALIVE_VALS fields on a Windows socket (for keepalive idle and interval times)

Question

Given a Windows socket, I want to determine which values it is using for the TCP keepalive idle time and the TCP keepalive interval time (roughly equivalent to the TCP_KEEPIDLE and TCP_KEEPINTVL settings on Berkeley sockets).

I see that you can set these values using a WSAIoctl call (see http://msdn.microsoft.com/en-us/library/windows/desktop/dd877220%28v=vs.85%29.aspx ). However, there does not appear to be any API for reading their current values. I tried calling WSAIoctl with a populated output parameter but NULL input parameter, like this:

DWORD bytes_returned;
struct tcp_keepalive keepalive_opts;
int rv = WSAIoctl(socket, SIO_KEEPALIVE_VALS, NULL, 0, &keepalive_opts, sizeof(keepalive_opts), &bytes_returned, NULL, NULL);

But this returns me a WSAEFAULT ("The system detected an invalid pointer address in attempting to use a pointer argument in a call.").

I could call WSAIoctl with both an input and an output parameter, but I don't want to set the values, I just want to read them. And as far as I can tell, providing any non-NULL input parameter would cause the parameters to be set to whatever values happen to be in that memory space (defined by the struct tcp_keepalive; again see http://msdn.microsoft.com/en-us/library/windows/desktop/dd877220%28v=vs.85%29.aspx ).

The above also highlights another problem with not knowing what the current values are: I can't set just one of the keepalive idle time or the keepalive interval time - I must blow away both (unknown) values at the same time since they're both members of the struct I'm required to provide.

I know that I could assume things about what values are set based on Windows documentation, but I'd rather not assume. I see that http://technet.microsoft.com/en-us/library/bb726981.aspx#EDAA defines KeepAliveInterval and KeepAliveTime default values. However, the Parameters folder in my Windows 7 registry does not contain either of those keys, so I really have to rely on the documentation being 100% correct here (to know the default values a socket will receive), which is much worse than programmatically retrieving them (even retrieving them from the registry might be ok, but the above experience shows I can't).

Is there any way to get the current TCP keepalive idle time and the TCP keepalive interval time values for a Windows socket?

Answer 1

Unlike TCP_KEEPIDLE and TCP_KEEPINTVL, which can be used with getsockopt(), there is no way to read the current SIO_KEEPALIVE_VALS values for a socket, only to set them.