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Date: Mon, 30 Mar 2015 14:36:43 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: Handles returned by mq_open are not valid file descriptors as supposed to be under native linux distributions
Message-ID: <20150330123643.GC10785@calimero.vinschen.de>
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References: <556891A1F0F6154CB3DD7811A49431534D3B8027 AT FOWEXMC003 DOT de DOT astrium DOT corp>
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On Mar 27 09:30, KEREP, Mladen wrote:
> We're using POSIX message queues to pass messages between processes.
> For this we've build a library layer to be able to use message queues
> on different platforms. Basically linux (debian, Ubuntu, archlinux,
> rasbian) is the development platform, but also vxworks platforms are
> supported.
>=20
> Several message queues are opened through mq_open calls and the
> returned handles are organized in file descriptor sets (fd_set). The
> set is then passed over to a select(2) call, which blocks processing
> and returns as soon as any message in any of the queues arrives.
>=20
> This works under linux, since the handles returned are valid file
> descriptors and the macros FD_ZERO / FD_SET are able to handle that
> handles.
>=20
> However, under Cygwin,

For a start, since what you're doing sounds pretty commercial to me,
may I point out https://cygwin.com/licensing.html?  May I assume your
code is open source?

> mq_open does not directly return file handles,
> but pointers to (unknown) data structures in memory (ref.
> http://sourceware.org/ml/cygwin/2013-07/msg00179.html), which cannot
> be used with FD_ZERO, FD_SET, so not with select(2). I know from the
> man pages (mq_open): "Polling message queue descriptors On Linux, a
> message queue descriptor is actually a file descriptor, and can be
> monitored using select(2), poll(2), or epoll(7).  This is not
> portable."
>=20
> mq_open() and select() conform to POSIX.1-2001, at least under linux,
> but also under Cygwin ?  How can this be modified, so that it works
> under Cygwin as well ?

"This is not portable" is the important hint in the Linux man page.  On
Cygwin we're using basically the implementation from W. Richard Stevens,
as published for his book "UNIX Network Programming, Volume 2,
Interprocess communication".  It doesn't support the Linux extensions
but, yes, it's POSIX compliant, see POSIX.1-2008:

 "A message queue descriptor *may* be implemented using a file descriptor,
  in which case applications can open up to at least {OPEN_MAX} file and
  message queues." [emphasis mine]

Note that mq_open doesn't return an int, but a special message queue type
mqd_t for a reason.  Again, if you use message queues with select, you're
creating non-portable code.

Having said that, it *would* be possible to change the implementation to
make mqd_t a file descriptor and to support the Linux extension, but I'm
not planning to do so for the time being.

But, as always, patches are welcome.


Corinna

--=20
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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