Setting termios VMIN > 0 and VTIME > 0 on non blocking file

[Brian Inglis]

On 2020-03-11 14:48, Åke Rehnman via Cygwin wrote:
> opening a file (serial port) with O_NONBLOCK and subsequently setting termios
> VMIN and VTIME > 0 makes read() never ever return any data (returns EAGAIN
> indefinitely).
> Don't ask my why one would want to do something like this but apparently the
> "screen" program think this is a good way of doing things...
> Nevertheless not returning any data ever seems wrong.
> Any insight on how it is supposed to work is welcome.

VMIN > 0 || VTIME > 0 implies blocking; O_NONBLOCK implies SIGIO delivery; see:

	https://www.tldp.org/HOWTO/pdf/Serial-Programming-HOWTO.pdf
	https://www.cmrr.umn.edu/~strupp/serial.html
	https://en.wikibooks.org/wiki/Serial_Programming/termios

also read the man pages carefully and *assume* nothing; functions should work
*exactly* as documented: there be dragons!

You have to set up for, or adapt to, the expected input speed and quantity, the
interrupt response overhead (hardware, OS, and C signal), and what else you want
to be able to do at the same time.

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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