Messages in this thread | | | Date | Thu, 5 May 2016 11:08:31 +0100 | From | One Thousand Gnomes <> | Subject | Re: [BUG] drivers/tty: read() on a noncanonical blocking tty randomly fails when VMIN > received >= buf |
| |
On Wed, 4 May 2016 16:07:44 -0700 Peter Hurley <peter@hurleysoftware.com> 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 avail‐ > 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.
Alan
| |