Re: [BUG] drivers/tty: read() on a noncanonical blocking tty randomly fails when VMIN > received >= buf

From: Julio Guerra
Date: Wed May 04 2016 - 19:27:37 EST

When a tty (here a slave pty) is set in noncanonical input and blocking read modes, a read() randomly blocks when:
"VMIN > kernel received >= user buffer size > 0".

The standard says that read() should block until VMIN bytes are received [1][2]. Whether this is an implementation defined case not really specified by POSIX or not, it should not behave randomly (otherwise it really should be documented in termios manpage).

This is not a bug.

From the termios(3) man page:

* MIN > 0; TIME == 0: read(2) blocks until the lesser of MIN bytes or the number of bytes requested are availâ
able, and returns the lesser of these two values.

This does not appear in my man...

Anyway, how do you explain the random behavior then?

Julio Guerra