[lldb-dev] select() on Windows

[Matthew Gardiner]

Zachary Turner wrote:
> So I've run into a potentially difficult issue on Windows.
>
> I have the test suite actually running.  A few tests even passed 
> surprisingly.  But what doesn't work is one-shot script commands. 
>  e.g. the following:
>
> (lldb) script print "foo"
>
> Running this deadlocks.  The reason is related to the 
> ConnectionFileDescriptor class.  Basically, on Windows the select() 
> API will only accept sockets, nothing else.  It seems select() is only 
> used from the BytesAvailable() function, but that it attempts to 
> simultaneously check if data is available on a) the read file 
> descriptor, which could be an arbitrary object (pipe, socket, file, 
> etc), and b) optionally, a pipe.
>
> Is this correct?  There isn't a great equivalent for this on Windows.
>
> It's possible to make it work, but at a minimum it's going to involve 
> re-implementing Pipe in terms of HANDLEs on Windows, because it's the 
> only way you can do non-blocking I/O on a Pipe.
>
> For starters, we could just stub out an entirely separate 
> implementation of the function on Windows that makes use of this new 
> HANDLE-based Pipe class, but I imagine that's going to get old pretty 
> fast if we need to use select() in other places.  I did search the 
> code and only saw select() used in one other place, but perhaps at 
> some point we'll need to abstract this out into an actual 
> platform-specific version of select.  FWIW Windows does provide a 
> rough equivalent, it just uses an entirely different API, and is 
> completely HANDLE-based, so this approach will probably require some 
> more work to make other types of objects (files, sockets, processes, 
> etc) HANDLE-based instead of fd-based on Windows.
>
> Thoughts?  Suggestions?
>
Yes. As you allude to above: you'd need to reimplement in terms of 
WaitForMultipleObjects and friends using HANDLEs. My previous experience 
of doing a lot of cross-platform was that you always ended up with a 
chunk of *nix code using select/poll and Windows code with 
WaitForMultipleObjects. But it's not a straight swap (i.e. select for 
WFMO) - since with windows there are a few disparate APIs which map the 
object of choice (i.e the socket etc.) to a waitable HANDLE.

Matt



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