92001b9497
This removes the need for manually changing this flag for Google Chrome users. It also improves compatibility with Linux applications running under Linuxulator compatibility layer, and possibly also helps in porting software from Linux. Generally speaking, the flag allows applications to create the shared memory segment, attach it, remove it, and then continue to use it and to reattach it later. This means that the kernel will automatically "clean up" after the application exits. It could be argued that it's against POSIX. However, SUSv3 says this about IPC_RMID: "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." From my reading, we break it in any case by deferring removal of the segment until it's detached; we won't break it any more by also deferring removal of the identifier. This is the behaviour exhibited by Linux since... probably always, and also by OpenBSD since the following commit: revision 1.54 date: 2011/10/27 07:56:28; author: robert; state: Exp; lines: +3 -8; Allow segments to be used even after they were marked for deletion with the IPC_RMID flag. This is permitted as an extension beyond the standards and this is similar to what other operating systems like linux do. MFC after: 1 month Relnotes: yes Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D3603
142 lines
4.3 KiB
Groff
142 lines
4.3 KiB
Groff
.\"
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.\" Copyright (c) 1995 David Hovemeyer <daveho@infocom.com>
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.\"
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd October 10, 2015
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.Dt SHMCTL 2
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.Os
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.Sh NAME
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.Nm shmctl
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.Nd shared memory control
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/ipc.h
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.In sys/shm.h
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.Ft int
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.Fn shmctl "int shmid" "int cmd" "struct shmid_ds *buf"
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.Sh DESCRIPTION
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Performs the action specified by
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.Fa cmd
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on the shared memory segment identified by
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.Fa shmid :
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.Bl -tag -width SHM_UNLOCKX
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.It Dv IPC_STAT
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Fetch the segment's
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.Fa "struct shmid_ds" ,
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storing it in the memory pointed to by
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.Fa buf .
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.\"
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.\" XXX need to make sure that this is correct for FreeBSD
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.\"
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.It Dv IPC_SET
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Changes the
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.Fa shm_perm.uid ,
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.Fa shm_perm.gid ,
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and
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.Fa shm_perm.mode
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members of the segment's
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.Fa "struct shmid_ds"
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to match those of the struct pointed to by
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.Fa buf .
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The calling process's effective uid must
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match either
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.Fa shm_perm.uid
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or
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.Fa shm_perm.cuid ,
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or it must have superuser privileges.
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.It Dv IPC_RMID
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Removes the segment from the system.
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The removal will not take
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effect until all processes having attached the segment have exited.
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For the operation
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to succeed, the calling process's effective uid must match
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.Fa shm_perm.uid
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or
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.Fa shm_perm.cuid ,
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or the process must have superuser privileges.
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If the
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.Va kern.ipc.shm_allow_removed
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.Xr sysctl 3
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variable is set to 0, once the IPC_RMID operation has taken place,
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no further processes will be allowed to attach the segment.
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.\" .It Dv SHM_LOCK
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.\" Locks the segment in memory. The calling process must have
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.\" superuser privileges. Not implemented in FreeBSD.
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.\" .It Dv SHM_UNLOCK
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.\" Unlocks the segment from memory. The calling process must
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.\" have superuser privileges. Not implemented in FreeBSD.
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.El
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.Pp
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The
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.Vt shmid_ds
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structure is defined as follows:
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.\"
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.\" I fiddled with the spaces a bit to make it fit well when viewed
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.\" with nroff, but otherwise it is straight from sys/shm.h
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.\"
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.Bd -literal
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struct shmid_ds {
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struct ipc_perm shm_perm; /* operation permission structure */
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size_t shm_segsz; /* size of segment in bytes */
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pid_t shm_lpid; /* process ID of last shared memory op */
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pid_t shm_cpid; /* process ID of creator */
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int shm_nattch; /* number of current attaches */
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time_t shm_atime; /* time of last shmat() */
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time_t shm_dtime; /* time of last shmdt() */
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time_t shm_ctime; /* time of last change by shmctl() */
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};
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.Ed
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.Sh RETURN VALUES
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.Rv -std shmctl
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.Sh ERRORS
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The
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.Fn shmctl
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system call
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will fail if:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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Invalid operation, or
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no shared memory segment was found corresponding to
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.Fa shmid .
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.\"
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.\" XXX I think the following is right: ipcperm() only returns EPERM
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.\" when an attempt is made to modify (IPC_M) by a non-creator
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.\" non-owner
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.It Bq Er EPERM
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The calling process's effective uid does not match the uid of
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the shared memory segment's owner or creator.
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.It Bq Er EACCES
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Permission denied due to mismatch between operation and mode of
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shared memory segment.
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.El
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.Sh "SEE ALSO"
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.Xr shmat 2 ,
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.Xr shmdt 2 ,
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.Xr shmget 2 ,
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.Xr ftok 3
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