Re: EAGAIN with read

["Saurabh Sehgal" <saurabh.r.s@gmail.com>]

Hi,

using select solved the problem ... I was not using wait/waitpid ...
it works fine now :) Thanks for all the help. Really appreciate it.

Best Regards,

Saurabh

On Dec 8, 2007 2:12 PM, Steve Graegert <graegerts@gmail.com> wrote:
> On Dec 7, 2007 12:27 AM, Saurabh Sehgal <saurabh.r.s@gmail.com> wrote:
> > Hi,
> >
> > Thanks for the reply .. right now, i was reading and checking for the
> > errno EAGAIN, but my loop would prematurely exit. I later found out
> > that the errnor returned by read was ECHILD ... looking at the read(3)
> > manpages ... this errno is not mentioned
> >
> > But since the process that is supposed to write to the pipe is a
> > forked processs .. does that change anything ?
>
> Sorry for the delay...
>
> Looks like you have other problems with your code.  I suppose you're
> calling wait(2)/waitpid(2)?  Could you please post the code in
> question? Otherwise I fail to see what could be wrong.
>
>         \Steve
>
> PS: Please, do not top post as it makes it diffcult to follow the
> conversion.  Thanks.
>
>
> > On Dec 6, 2007 4:29 PM, Steve Graegert <graegerts@gmail.com> wrote:
> > >
> > > On Dec 6, 2007 9:14 PM, Saurabh Sehgal <saurabh.r.s@gmail.com> wrote:
> > > > Hi,
> > > >
> > > > I had a basic question about read . I have a file descriptor marked
> > > > with non blocking I/O , and I want to read data from the file
> > > > descriptor. This file descriptor is the read end of a UNIX pipe.
> > > >
> > > >  The process that the pipe reads from is a very slow process. Hence I
> > > > need to poll  and keep on trying to read from the fd until the process
> > > > has actually written something to the pipe. I execute read while the
> > > > errno condition EAGAIN is true. Will this ever result in an infinite
> > > > loop ? (lets say the remote process dies and doesnt write anything to
> > > > the pipe, will I go into an infinite loop since I am polling while
> > > > EAGAIN is true ?).
> > >
> > > Hi,
> > >
> > > I'd suggest taking a look at select(2) which allows for the
> > > specification of a timeout.  Use pselect(2) if you are waiting for a
> > > signal as well as data from a file descriptor or otherwise the
> > > select(2) call may block indefinitely due to a nasty race condition
> > > that may occur.
> > >
> > > To your question: There are actually a couple of reasons why your
> > > process would or would not run indefinitely in that loop.  There are
> > > also a couple of other errors read(3) can return that might cause your
> > > condition to return false, thus breaking the loop (if I understood
> > > correctly, code example is welcome). What about EINTR which is
> > > returned when the call was interrupted by a signal before any data was
> > > read?  (Please note that for a FIFO or pipe it will never return EINTR
> > > if any data has been read.)
> > >
> > >         \Steve
> > >
> > > --
> > >
> > > Steve Grägert
> > > DigitalEther.de
> > >
> >
>
-
To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html