Re: VMIN/VTIME and read expectations

[Joel Sherrill <joel dot sherrill at oarcorp dot com>]

On 12/17/2009 04:30 PM, Eric Norum wrote:
> When you're using VMIN=0, VTIME=255 you have ICANON=0 so 'end-of-file' doesn't really exist.
>    

I was kind of guessing that.  Unfortunately  this is in the RTEMS login 
code.
I can make it work OK for serial ports but when attached to a socket, I
have no idea how to detect when the socket is dropped.

I switched to calling read() and as documented, it always returns 0
when there is no data.  When the connection is dropped, it returns 0
VERY quickly.  Do you think this part is a bug in the read on a socket?

If this turns into an RTEMS specific issue, we need to switch lists.
But I would really like to know what is correct. :)

--joel
> On Dec 17, 2009, at 2:16 PM, Joel Sherrill wrote:
>
>    
> > Hi,
> >
> > I have been looking into a program on RTEMS that
> > uses VMIN=0, VTIME=255. The program calls
> > fgetc() which calls read().
> >
> > fp->_flags == 0x2806 before the fgetc() call.
> > read() times out and returns 0.
> > fgetc() returns -1 and fp->_flags == 0x2826
> >
> >
> > The 0x0020 being set is _SEOF.
> >
> > My questions are:
> >
> > + Is this correct behaviour?
> >
> > + How does the application programmer distinguish
> > between a real EOF and an unsatisfied read?  Even the
> > Fedora read(2) man page says:
> >
> >        On success, the number of bytes read is returned (zero indicates end of
> >        file), and the file position is advanced by this number.
> >
> > So even if I dropped down to read(2), I still don't know
> > how to distinguish EOF from a read timeout.
> >
> > Thanks.
> >
> > --
> > Joel Sherrill, Ph.D.             Director of Research&   Development
> > joel.sherrill@OARcorp.com        On-Line Applications Research
> > Ask me about RTEMS: a free RTOS  Huntsville AL 35805
> >    Support Available             (256) 722-9985
> >
> >
> >      
>    


--
Joel Sherrill, Ph.D.             Director of Research&  Development
joel.sherrill@OARcorp.com        On-Line Applications Research
Ask me about RTEMS: a free RTOS  Huntsville AL 35805
   Support Available             (256) 722-9985