VMIN/VTIME and read expectations

[Joel Sherrill]

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