termios & non-blocking I/O

[Alex Semenyaka]

On Wed, Apr 09, 2003 at 04:52:51AM -0700, Terry Lambert wrote:
>> On Tue, Apr 08, 2003 at 11:49:44PM +0400, Andrey A. Chernov wrote:
> >> IEEE Std 1003.1-200x does not specify whether the setting of O_NONBLOCK
> >> takes precedence over MIN or TIME settings. Therefore, if O_NONBLOCK is
> >> set, read( ) may return immediately, regardless of the setting of MIN or
> >> TIME. Also, if no data is available, read( ) may either return 0, or
> >> return -1 with errno set to [EAGAIN].
>> Right you are, but the question is a little bit different, namely - what is
>> _better_ to do: to return 0 or to return -1/EAGAIN.
> As was pointed out in Andrey Chernov's posting, the place to fix
> this is libc_r, not the kernel, since the kernel is doing something
> permissable, and the side effect you are seeing is a result of your
> use of libc_r.

Yes it is permissable, it is just in the disagreement with the behaviour
when MIN>0 and TIME=0. Don't you think the behaviour in the (TIME=0, MIN>0)
and (TIME>0, MIN=0) should be consistent somehow?

Then, changing the kernel behavour is simple, straightforward and also
PERFECTLY permissable while tracing the situation with the descriptor mode
in libc_r is not so simple. To be fair such tracing might be a difficult
task. 

>> Now the first choice
>> is implemented. And the point IS that in multi-thread environment with
>> utheads such implementation leads to BAD results.
> The main problem you will face, if you try to do this in the
> kernel instead of libc_r, is that each time you call the read,
> if it behaved the second way, you would stall all threads in
> the process for VTIME, and you're going to have a hard time

No, sorry, you have missed my point. Sure I do NOT propose to block
the input when TIME>0. I propose to return -1/EAGAIN (as in the
MIN>0 case) instead of 0 (as it is done now). That is what I called
"the second way". And in this case both libc_r and program can
easely distinct between EOF and nothing-to-read situation and handle
them properly. Also all other threads will not be stucked. 

Once again: -1/EAGAIN is absolutely legal as well generally and more
correct here, that's what I am saying.

							SY, Alex