Questions about select for sockets

[Ken Brown]

On 4/6/2021 12:28 PM, Corinna Vinschen wrote:
> On Apr  6 11:44, Ken Brown wrote:
>> On 4/6/2021 10:33 AM, Corinna Vinschen wrote:
>>> On Apr  6 16:20, Corinna Vinschen wrote:
>>>> On Apr  3 14:16, Ken Brown via Cygwin-developers wrote:
>>>>> 4. Why do we set
>>>>>
>>>>>     except_ready = saw_shutdown_write () || saw_shutdown_read () ?
>>>>>
>>>>> I can't find any documentation that says that a shutdown should be
>>>>> considered an exceptional condition.  On the other hand, POSIX does say,
>>>>> "Other circumstances under which a socket may be considered to have an
>>>>> exceptional condition pending are protocol-specific and
>>>>> implementation-defined."  So maybe there's some Cygwin-specific reason for
>>>>> doing this?
>>>>
>>>> Nope, this is old (and wrong) cruft.  Neither Steven's book nor the
>>>> Linux man pages, nor testing on Linux imply that ready for exception
>>>> is used to indicate anything other than out-of-band data.  This should
>>>> be fixed.
>>>
>>> We may also have to change the saw_shutdown_read/saw_shutdown_write
>>> handling.  I checked this on Linux and what happens is:
>>>
>>>     After shutdown (fd, SHUT_RD), the socket is ready for reading and writing
>>
>> This seems surprising to me.  Is it really the shutdown that caused it to be
>> ready for writing in your test, or was it ready for writing anyway (e.g.,
>> because the relevant buffer was empty)?
> 
> I guess so, too.  How to make sure the socket isn't ready for writing
> without going to great lengths?

I guess you could have a subprocess write to the socket in a loop, so that its 
buffer will quickly fill up and a further write will block.

Ken