X-Recipient: archive-cygwin@delorie.com X-Spam-Check-By: sourceware.org Message-ID: Date: Mon, 5 Nov 2007 07:43:43 -0500 From: "Lev Bishop" To: cygwin@cygwin.com Subject: Re: cygwin stable and cvs snapshot - fork() bug In-Reply-To: <20071105102147.GI31224@calimero.vinschen.de> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Disposition: inline References: <4721DFCC.8070100@cygwin.com> <20071029083512.GA4224@calimero.vinschen.de> <4725D656.5090303@cygwin.com> <20071101095835.GG31224@calimero.vinschen.de> <20071105102147.GI31224@calimero.vinschen.de> Mailing-List: contact cygwin-help@cygwin.com; run by ezmlm Precedence: bulk List-Id: List-Unsubscribe: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: cygwin-owner@cygwin.com Mail-Followup-To: cygwin@cygwin.com Delivered-To: mailing list cygwin@cygwin.com Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by delorie.com id lA5Ci6sQ015045 On 11/5/07, Corinna Vinschen wrote: > On Nov 1 10:58, Corinna Vinschen wrote: > > On Oct 31 14:26, Lev Bishop wrote: > > > $ cat lev.c && gcc -o lev lev.c -Wall -Wextra && CYGWIN=server ./lev > > > #include > > > #include > > > #include > > > > > > int main(void) > > > { > > > int shmid; > > > if ((shmid = shmget(IPC_PRIVATE, 100,IPC_CREAT | 0600 )) < 0 || > > > !shmat(shmid, NULL, 0) || > > > shmctl(shmid, IPC_RMID, NULL) < 0) > > > puts("problems with shm!"); > > > fork(); > > > } > > > lev.c: In function `main': > > > lev.c:13: warning: control reaches end of non-void function > > > 3 [main] lev 1924 c:\Documents and > > > Settings\Lev\Desktop\mpd-0.13.0\lev.exe: *** fatal error - > > > MapViewOfFileEx (0x3E0000), Win32 error 6. Terminating. > > > 124 [main] lev 5076 fork: child 1924 - died waiting for dll > > > loading, errno 11 > > > > Thanks for the testcase. I'm surprised that nobody experienced this > > problem before. Sorta holiday here, so I'll look into it next week. > > 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 strucā 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. Lev -- Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple Problem reports: http://cygwin.com/problems.html Documentation: http://cygwin.com/docs.html FAQ: http://cygwin.com/faq/