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From: Peter Hurley <peter@hurleysoftware.com>
Newsgroups: linux.kernel
Subject: Re: [BUG] drivers/tty: read() on a noncanonical blocking tty randomly  fails when VMIN > received >= buf
Date: Thu, 05 May 2016 03:00:02 +0200
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On 05/04/2016 04:27 PM, Julio Guerra wrote:
>>> 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 avail‐
>>          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?

A long standing bug in this read mode allows the asynchronous input
processing thread to race with the read() thread and become confused
about how much data remains.

I fixed this in 4.6; when I run your test on 4.6, it consistently
returns the full user buffer.

Regards,
Peter Hurley