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Re: cygwin stable and cvs snapshot - fork() bug
On Nov 5 07:43, Lev Bishop wrote:
> On 11/5/07, Corinna Vinschen wrote:
> > Ouch, ouch, ouch. shmctl(IPC_RMID) closed the handle to the shared
> > memory, but neglected to remove the actual mappings as well as the
> > bookkeeping structure. The result is that after a fork the child thinks
> > there are still mappings which have to be duplicated into its own
> > memory. But the handle has already been closed in the parent, so the
> > MapViewOfFile call fails with "invalid handle".
> > Unfortunately not many applications use shmctl(IPC_RMID) before creating
> > a child process since usually the shared memory is meant to be... well,
> > shared. That's why this didn't crop up more often, obviously.
> Are you sure that you're interpreting IPC_RMID correctly? My
> understanding is that you can still share the memory until you
> actually remove the mapping. (Sort of like how you can unlink() a temp
> file immediately after you open it, and continue to use it). I assumed
> this was the reason for the create-map-remove pattern used by mpd.
> From the linux man page:
> IPC_RMID Mark the segment to be destroyed. The segment will only
> actually be destroyed after the last process detaches it
> (i.e., when the shm_nattch member of the associated strucâ
> ture shmid_ds is zero). The caller must be the owner or
> creator, or be privileged. If a segment has been marked
> for destruction, then the (non-standard) SHM_DEST flag of
> the shm_perm.mode field in the associated data structure
> retrieved by IPC_STAT will be set.
Well, when I created the patch, I had read the SUSv3 man page
It says something different:
Remove the shared memory identifier specified by shmid from the
system and destroy the shared memory segment and shmid_ds data
structure associated with it. IPC_RMID can only be executed by a
process that has an effective user ID equal to either that of a
process with appropriate privileges or to the value of shm_perm.cuid
or shm_perm.uid in the shmid_ds data structure associated with
OTOH, the implementation within cygserver is more along the lines of
the Linux manpage. Oh well. I guess I'll have a look into implementing
it more Linux-like.
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Project Co-Leader cygwin AT cygwin DOT com
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