Re: [Boost-users] [asio] Problem Assignin a File Descriptor to posix:basic_stream_descriptor

Subject: Re: [Boost-users] [asio] Problem Assignin a File Descriptor to posix:basic_stream_descriptor
From: Nader Salehi (Nader.Salehi_at_[hidden])
Date: 2010-03-18 16:54:31

Correction!  There's actually a better solution than what I said in my
previous reply; You could explicitly disable 'epoll' in asio and
switch to 'select' by defining BOOST_ASIO_DISABLE_EPOLL.

Cheers,
Nader

On 3/18/2010 10:22 Nader Salehi writes:
> I see.  At this time the path of least resistance would be to use
> libevent which allows me --albiet awkwardly via setenv()-- to choose
> the polling method.
> 
> Thanks for the reply.
> 
> Nader
> 
> On 3/17/2010 21:19 Nick Jones writes:
> > On Wed, 2010-03-17 at 14:42 -0700, Nader Salehi wrote:
> > 
> > > I traced down the epoll_reactor::register_descriptor() where the
> > > function fails to add the descriptor to the poll list.
> > > 
> > > Any idea what I am doing wrong?  I would appreciate any comment.
> > > 
> > 
> > Filesystem fds do not support epoll, as they lack the poll function
> > internally in the linux kernel.  otoh, select does supports filesystem
> > fds.
> > 
> > If you do an strace of your sample program, you will see that the call
> > to epoll_ctl fails with the error number EPERM.  Yes, the value chosen
> > is quite unfortunate, I always thought it would have been better to
> > return ENOTSUP or EOPNOTSUPP... anyhow, that means that epoll is
> > rejecting the fd you supplied, in this case because it was a filesystem
> > fd and in Linux, this type of fd lacks a poll function pointer in its
> > file_operations structure.  I remember looking through the epoll sources
> > in an older version of the kernel, and finding that this was the only
> > reason that EPERM is returned as an error.
> > 
> > You may have to find another method of managing IO from and to the file,
> > perhaps you can use another standard or asio provided stream type?
> > 
> > Thanks
> > 
> >   Nick
> > 
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