Unix Domain Socket Limitation?

Norton Allen allen@huarp.harvard.edu
Mon Nov 30 17:19:16 GMT 2020

On 11/26/2020 12:13 PM, Ken Brown wrote:
> [Adding the Cygwin list back to the Cc.]
> On 11/26/2020 11:27 AM, Norton Allen wrote:
>> On 11/25/2020 5:27 PM, Ken Brown via Cygwin wrote:
>>> On 11/25/2020 4:47 PM, Norton Allen wrote:
>>>> In my recent tests, it appears as though it is not possible to 
>>>> successfully connect via two Unix Domain sockets from one client 
>>>> application to one server application.
>>>> Specifically, if I create a server which listens on a Unix Domain 
>>>> socket and a client, which attempts to connect() twice, both seem 
>>>> to lock up. This is not the behavior under Linux.
>>>> I will be happy to work up a minimal example if it is helpful in 
>>>> tracking this down. I wanted to start by asking whether this is a 
>>>> known limitation and/or if there is something about the Cygwin 
>>>> implementation that makes this sort of thing very difficult.
>>> A minimal example would be extremely helpful.
>>> Corinna can answer questions about limitations in the current 
>>> implementation. But there is a new implementation under development. 
>>> It's in the topic/af_unix branch of the Cygwin git repository if 
>>> you're interested in looking at it.
>>> Corinna began working on this a couple years ago, and I've recently 
>>> been trying to finish it.  I've made quite a bit of progress, but 
>>> there's still more to do and undoubtedly many bugs. So any test 
>>> cases you have would be very useful. 
>> Thanks Ken,
>> As it happens, attempting to produce a minimal example suggests my 
>> problem may be somewhere else. I think I've worked in most of the 
>> features of my application one by one but have not yet revealed a 
>> failure.
> OK.  But if you ever do have occasion to write small test programs 
> involving AF_UNIX sockets, please send them on.  The new AF_UNIX code 
> needs as much testing as it can get.
I have finally put together a start of a minimal example, although it 
seems to require a certain level of complexity before tripping on the 
bug. At the moment, I do not believe the issue is related to having 
multiple sockets between the client and server. I am thinking it is some 
sort of race condition related to non-blocking sockets, since I have 
only observed it when both the client and server are using non-blocking 

I have yet to plunge into cygwin.dll, but I think I have reached that point.

Here is the code: https://github.com/nthallen/cygwin_unix

Since I have only exercised this on my machine, I would be very 
interested to know if it is reproducible on anyone else's.

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