On 07/17/2014 10:03 AM, Grant Edwards wrote:
On 2014-07-17, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
tcflow(TCOxxx) flow control is independent of IXON flow control.
The union of both flow states determines if the tty can output;
IXON = true IXON = false
START STOP
tcflow(TCOON) on off on
tcflow(TCOOFF) off off off
Thanks, that's pretty much what I had decided based on tests and
browsing the source code.
Just to confirm:
tcflow(TCION/TCIOFF): overrides the "input" side of xon/xoff flow
control and forces the sending of XON/XOFF.
tcflow(TCOON/TCOOFF): does not have anything to do with the "output"
side of xon/xoff flow control, but controls
something completely orthogonal.
That rather counter-intuitive (not that counter-intuitive is exactly a
novel thing when it comes to Unix serial ports).
That rasies this question: what does an application use to control the
"output" side of xon/xoff flow control? There is a Windows API for
doing that, and I get asked how to do it in Linux. I always tell them
they can't.
I didn't explain this properly.
Both tcflow(TCOxxx) and receiving START/STOP when IXON == true control
the output flow.
For example,
ttyS0 = open("/dev/ttyS0", O_RDWR);
/* Disable ttyS0 output */
tcflow(ttyS0, TCOOFF);
/* writes to ttyS0 will now be buffered but not sent */
/* remote terminal sends START which is received, however sending is
* still disabled by tcflow()
*/
/* Enable ttyS0 output */
tcflow(ttyS0, TCOON);
/* ttyS0 output enabled, previously buffered writes are now sent */
/* remote terminal sends STOP which is received and ttyS0 output
* is now disabled (writes to ttyS0 will be buffered but not sent)
*/
tcflow(ttyS0, TCOON); <--- has no effect because flow was not previously
disabled by tcflow()
/* remote terminal sends START which is received and ttyS0 output
* is now enabled, previously buffered writes are now sent
*/
I did gloss over one special case: tcflow(TCOON) will re-enable output
_even if the remote terminal last sent STOP_ but only if output is also
disabled by tcflow(TCOOFF).
To me, the separate state tracking of tcflow() and START/STOP makes sense.
Regards,
Peter Hurley
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