This is the mail archive of the
mailing list for the eCos project.
Re: Update on the availability of socketpair()?
Andrew Lunn <email@example.com> writes:
> On Wed, Jan 21, 2004 at 03:28:20PM -0600, Philip.Fleege@gd-ais.com wrote:
> > Hi All-
> > I read in an earlier posting;
> > http://sources.redhat.com/ml/ecos-discuss/2003-07/msg00485.html that
> > the function: socketpair() was intentionally left out of the 2.0
> > release. But that it could (and therefore might) be added. Is
> > there any short term (or long term) plan to add this call?
> Not that i know of. Nobody has contributed patchs as far as i know.
> > Just as is described in the reference above, I'm porting some unix
> > code that uses a "tickle" pipe to wakeup a thread blocking on
> > select(). And yes, I could use a 'localhost' socket. However, I am
> > advocating that socketpair() be added since it makes the application
> > code so simple and elegant.
> If you want it, then i suggest you implement it (or contract somebody
> to do it for you). It should not be too difficult.
> > Finally, if there is an effort to add this to eCos, what is the
> > chances that it could be kept in without having to have all the
> > network code; or it be a "configurable" item. I guess what I'm
> > really asking is what are the chances that a pipe() system call be
> > added. Our embedded application will actually be using serial
> > lines...but we use the network during development (to fake out
> > devices) so our final software will not have a network stack (thus
> > no socketpair() when we remove network support).
> The current socket interface has a plugin architecture. There is a
> generic socket layer which different stacks register there
> implementations of socket functions to. You should implement your code
> as a little stack which registers itself and only implements AF_LOCAL
> socketpair(). You will need to extend the socket interface a little
> since it currently does not have socketpair(), but that should not be
> too hard. With this architecture it should be easy to make it run
> with and without the AF_INET TCP/IP stack.
Everything Andrew says is true. My own thoughts on the issue:
- I was perhaps being a little over-optimistic when I suggested before
that it might be easy to add socketpair() back to the TCP/IP
stack. The entire AF_INET family code has not been ported, the only
way to do it in the current stack would be to generate a pair of
back-to-back TCP sockets, but they would end up doing the full TCP
protocol through the loopback interface. Not nice, but it would
- In BSD pipe() is really just a wrapper for socketpair(), so there is
no pipe implementation we could import. Writing a pipe
implementation from scratch, and getting it to work correctly in all
situations is not an afternoon's job. It also raises all sorts of
issues about memory allocation and dynamic buffer management which
we would prefer to avoid.
- The main requirement here is to kick a select() out of its wait. So
one option is to implement a simple "semaphore" filesystem. This
allows named "files" to be created and opened. Writes to the file
increment a counter, reads decrement it, selects wait until the
counter is greater than zero. No data is transferred or stored, so
there is no memory allocation issue. Something like this could
probably be tinkered together fairly quickly.
Nick Garnett eCos Kernel Architect
http://www.ecoscentric.com The eCos and RedBoot experts
Before posting, please read the FAQ: http://sources.redhat.com/fom/ecos
and search the list archive: http://sources.redhat.com/ml/ecos-discuss