[BRLTTY] how to use readKey on Windows

[James Teh]

On 27/11/2008 4:30 AM, Thijs Koerselman wrote:
> My plan was to spawn a
> new thread from my main app. This thread then calls readKey() in an
> infinite loop (as long as some flag is valid). The keys returned by
> brlapi are buffered, and my main app is reading this buffer at fixed
> intervals.
This won't work. If you call readKey in another thread like this (at 
least on Windows), any other calls to BRLAPI (e.g. to write text to the 
display) will block. I *think* this is because you can't have two 
operations occurring simultaneously on the same bidirectional pipe. This 
does make some sense.

The solution I used in NVDA was to set up a timer in my main thread to 
poll for keys. This also avoids the need to spawn another thread.

Another gotcha you should be aware of is that if you write to the 
display while there are keys waiting to be read, the write operation 
will block (and thus your application will hang). This is because 
Windows local named pipes (which BRLAPI uses) don't queue input. This 
issue is a bug in the Windows implementation of BRLAPI, but for the 
aforementioned reason, it is very difficult to solve; the brlapi server 
would need to do its own queuing. To get around this, you need to read 
all keys before each write. There is still a possible race condition 
here, but it usually won't occur.

-- 
James Teh
Email/MSN Messenger/Jabber: jamie at jantrid.net
Web site: http://www.jantrid.net/