From: Corinna Vinschen <corinna-cygwin@cygwin.com>
To: cygwin@cygwin.com
Subject: Re: Persistence of file implemented objects
Date: Mon, 02 Jul 2012 08:03:00 -0000 [thread overview]
Message-ID: <20120702080303.GA17618@calimero.vinschen.de> (raw)
In-Reply-To: <4FF0942B.1030201@gmail.com>
On Jul 1 19:17, Richard H Lee wrote:
> >Fifos persist on reboot on Linux or Cygwin. They live on the
> >filesystem. I don't see how POSIX IPC shared memory and semaphores
> >could persist.
>
> Sorry, I meant unix/bsd sockets.
AF_UNIX/AF_LOCAL sockets are implemented on filesystem level as well,
not only on Cygwin. If they are not explicitely unlinked, they will be
in the way when trying to create a new socket of the same name.
> Regarding the POSIX IPC's, they are stored in /dev . In regular
> *nix, /dev do not represent "physical" files on the filesystem,
> hence they do not persist over boot.
>
> In cygwin, they actually do represent physical files. So if they are
> not freed correctly by the program, the persist over to the next
> boot.
Right. The named POSIX IPC objects are created as normal files under
/dev/shm and /dev/mqueue. These objects are supposed to follow POSIX
file name naming rules, and they are supposed to be persistent until you
call mq_unlink, sem_unlink, or shm_unlink respectively. They are also
meant to persist if a process crashes(*).
So in contrast to XSI IPC implemented by running cygserver they also
persist when all Cygwin processes have stopped and even over reboot,
because they don't live on a virtual filesystem like on Linux, but on
the real filesystem.
From the POSIX point of view that's ok. If you have a problems with
that, you might consider to create some autostart script which deletes
all files below /dev/shm and /dev/mqueue.
Corinna
(*) See http://pubs.opengroup.org/onlinepubs/007908799/xsh/mq_open.html
http://pubs.opengroup.org/onlinepubs/007908799/xsh/sem_open.html
http://pubs.opengroup.org/onlinepubs/007908799/xsh/shm_open.html
--
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Project Co-Leader cygwin AT cygwin DOT com
Red Hat
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prev parent reply other threads:[~2012-07-02 8:03 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-06-30 22:29 Richard H Lee
2012-07-01 0:35 ` Christopher Faylor
2012-07-01 18:17 ` Richard H Lee
2012-07-01 23:35 ` Christopher Faylor
2012-07-02 8:03 ` Corinna Vinschen [this message]
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