Persistence of file implemented objects
Mon Jul 2 08:03:00 GMT 2012
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
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.
(*) See http://pubs.opengroup.org/onlinepubs/007908799/xsh/mq_open.html
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