Re: Using custom Winsock socket functions with Cygwin

[Bruce Cran <bruce at cran dot org dot uk>]

On Tue, 1 Jun 2010 20:06:08 +0100
Bruce Cran <bruce@cran.org.uk> wrote:

> I have a Winsock provider DLL which communicates with a kernel SCTP
> stack. I'd like to enable applications built with both Visual Studio
> and Cygwin to run because there's lots of code that's been ported from
> Linux. However, the problem I've come across is that because I have a
> few custom socket functions such as sctp_sendmsg and sctp_recvmsg that
> don't go through the Winsock layer but still get a SOCKET
> descriptor, I get passed Cygwin's internal file descriptor - i.e. I
> can see my WSPAccept function returns a value of 161 up the stack but
> it gets translated to 4 by Cygwin's socket code, and the 161 value
> will be stored in the io_handle member of the fhandler_socket class.
> I thought I could maybe use GetFileType and _get_osfhandle to deal
> with running under Cygwin but it turns out that GetFileType tells me
> I have a valid handle, and converting it using _get_osfhandle anyway
> gives me an invalid handle back.
> 
> Is there some way to detect that the handle's wrong and fetch the
> proper one without my code needing detailed knowledge of the
> fhandler_socket class?
> 

I think I can see what the problem is now: since I call _get_osfhandle
from within the Winsock provider, it seems to be bypassing the Cygwin
layer; likewise, if I call getsockopt cygwin1.dll doesn't show up in
the call stack either, and WSAENOTSOCK gets returned.

The workaround I've come up with is to redefine my custom sctp_recvmsg
function as being inline so it gets called within the context of the
application; that means I can call getsockopt using a custom 'optname'
which can return the SOCKET handle, having gone through cygwin1.dll.
This seems to work, but is such a hack that I'm hoping there's some
better solution.

-- 
Bruce Cran

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