.\" $OpenBSD: shmctl.2,v 1.14 2011/10/27 07:56:28 robert Exp $ .\" $NetBSD: shmctl.2,v 1.3 1997/03/27 08:20:39 mikel Exp $ .\" .\" Copyright (c) 1995 Frank van der Linden .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed for the NetBSD Project .\" by Frank van der Linden .\" 4. The name of the author may not be used to endorse or promote products .\" derived from this software without specific prior written permission .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. .\"/ .Dd $Mdocdate: October 27 2011 $ .Dt SHMCTL 2 .Os .Sh NAME .Nm shmctl .Nd shared memory control operations .Sh SYNOPSIS .Fd #include .Fd #include .Fd #include .Ft int .Fn shmctl "int shmid" "int cmd" "struct shmid_ds *buf" .Sh DESCRIPTION The .Fn shmctl system call performs some control operations on the shared memory area specified by .Fa shmid . .Pp Each shared memory segment has a data structure associated with it, parts of which may be altered by .Fn shmctl and parts of which determine the actions of .Fn shmctl . .Pp This structure is defined as follows in .Aq Pa sys/shm.h : .Bd -literal struct shmid_ds { struct ipc_perm shm_perm; /* operation permissions */ int shm_segsz; /* size of segment in bytes */ pid_t shm_lpid; /* pid of last shm op */ pid_t shm_cpid; /* pid of creator */ short shm_nattch; /* # of current attaches */ time_t shm_atime; /* last shmat() time*/ time_t shm_dtime; /* last shmdt() time */ time_t shm_ctime; /* last change by shmctl() */ void *shm_internal; /* sysv stupidity */ }; .Ed .Pp The .Bf -literal ipc_perm .Ef structure used inside the .Bf -literal shmid_ds .Ef structure is defined in .Aq Pa sys/ipc.h and looks like this: .Bd -literal struct ipc_perm { uid_t cuid; /* creator user id */ gid_t cgid; /* creator group id */ uid_t uid; /* user id */ gid_t gid; /* group id */ mode_t mode; /* r/w permission (see chmod(2)) */ u_short seq; /* sequence # (to generate unique msg/sem/shm id) */ key_t key; /* user specified msg/sem/shm key */ }; .Ed .Pp The operation to be performed by .Fn shmctl is specified in .Fa cmd and is one of: .Bl -tag -width IPC_RMIDX .It Dv IPC_STAT Gather information about the shared memory segment and place it in the structure pointed to by .Fa buf . .It Dv IPC_SET Set the value of the .Va shm_perm.uid , .Va shm_perm.gid and .Va shm_perm.mode fields in the structure associated with .Fa shmid . The values are taken from the corresponding fields in the structure pointed to by .Fa buf . This operation can only be executed by the superuser, or a process that has an effective user ID equal to either .Va shm_perm.cuid or .Va shm_perm.uid in the data structure associated with the shared memory segment. .Pp .It Dv IPC_RMID Mark the shared memory segment specified by .Fa shmid for removal when it is no longer in use by any process. When it is removed, all data associated with it will be destroyed too. Only the superuser or a process with an effective UID equal to the .Va shm_perm.cuid or .Va shm_perm.uid values in the data structure associated with the queue can do this. .El .Pp The read and write permissions on a shared memory identifier are determined by the .Va shm_perm.mode field in the same way as is done with files (see .Xr chmod 2 ) , but the effective UID can match either the .Va shm_perm.cuid field or the .Va shm_perm.uid field, and the effective GID can match either .Va shm_perm.cgid or .Va shm_perm.gid . .Sh RETURN VALUES Upon successful completion, a value of 0 is returned. Otherwise, \-1 is returned and the global variable .Va errno is set to indicate the error. .Sh ERRORS .Fn shmctl will fail if: .Bl -tag -width Er .It Bq Er EPERM .Fa cmd is equal to .Dv IPC_SET or .Dv IPC_RMID and the caller is not the superuser, nor does the effective UID match either the .Va shm_perm.uid or .Va shm_perm.cuid fields of the data structure associated with the shared memory segment. .Pp An attempt is made to increase the value of .Va shm_qbytes through .Dv IPC_SET but the caller is not the superuser. .It Bq Er EACCES The command is .Dv IPC_STAT and the caller has no read permission for this shared memory segment. .It Bq Er EINVAL .Fa shmid is not a valid shared memory segment identifier. .Pp .Va cmd is not a valid command. .It Bq Er EFAULT .Fa buf specifies an invalid address. .El .Sh SEE ALSO .Xr shmat 2 , .Xr shmget 2 .Sh STANDARDS Segments which are marked for removal (but not yet removed since they are still in use) can be attached to by new callers using .Xr shmat 2 . This is permitted as an extension beyond the standards.