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

From: One Thousand Gnomes
Date: Thu May 05 2016 - 06:08:58 EST

On Wed, 4 May 2016 16:07:44 -0700
Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:

> Hi Julio,
> On 05/04/2016 04:00 PM, Julio Guerra wrote:
> > Hi,
> >
> > 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.

The standard says

Case B: MIN>0, TIME=0

In case B, since the value of TIME is zero, the timer plays no
role and only MIN is significant. A pending read shall not be
satisfied until MIN bytes are received (that is, the pending read
shall block until MIN bytes are received), or a signal is
received. A program that uses case B to read record-based
terminal I/O may block indefinitely in the read operation.

That is if you do

read(fd, buf, 3)

and MIN is 5, the read should not return until there are 5 bytes in the
queue. The following code is guaranteed to work reliably by the standard
with TIME 0 MIN 5 (ignoring signals for the moment)

read(fd, buf, 3);
fcntl(fd, F_SETFL, FNDELAY);
assert(read(fd, buf, 2) == 2);

Historically this behaviour was useful for things like block transfer
protocols, especially with offloaded serial processing.

So actually I think we do have a bug, the behaviuour is not standards
compliant, and the man page documents the erroneous behaviour.