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Re: Problem with select() on console
Christopher Faylor wrote:
> On Thu, Jul 15, 2010 at 01:44:19AM +0100, Cliff Hones wrote:
>> When select() is used to test for input availability on the standard
>> cygwin console in normal (cooked) mode, it indicates input is available
>> as soon as any key is pressed. However, a call to read(0,...)
>> will (correctly) block until a terminating RETURN is entered.
>> select() should only indicate input is available when a call
>> to read would *not* block - ie when a read call will immediately
>> return at least one character or an error such as EOF.
> Since, AFAIK, Windows has no way to do this, I don't see how it could be
> done easily. You could, I guess, pull characters into a buffer until a
> newline was found but that would be pretty error-prone and any use of
> select() would potentially invalidate console i/o for subprocesses.
> So, I don't see this changing anytime soon.
Hmm. Having looked further, it's not just select() which is affected.
If you set the NONBLOCK flag on the console, and perform a read(),
you get error EWOULDBLOCK/EAGAIN until a key is hit. When a key
is hit, the read blocks until RETURN is entered.
I must look at the console source - but it seems from behaviour and what
you are saying that Windows provides a way for the user to determine that
console input has started (since read() and select() behaviour changes
when a key is pressed) but no way to determine in advance that a call
to input a complete line will block because an incomplete line is present.
I'm surprised this hasn't come up more frequently. In my case, I have an
app which needs to respond to user input line-based commands, but also
does other processing. I don't want the app to block every time the user
starts to enter a command. Of course I could use threads, but that's
an unnecessary change (at least unnecessary on Linux etc). And I could
insist users use mintty, but that's rather presumptuous.
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