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.\"
.\" Copyright (c) 1995 David Hovemeyer <daveho@infocom.com>
.\"
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.\" $FreeBSD: release/9.1.0/lib/libc/sys/shmctl.2 216508 2010-12-17 21:10:08Z brucec $
.\"
.Dd July 17, 1995
.Dt SHMCTL 2
.Os
.Sh NAME
.Nm shmctl
.Nd shared memory control
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/ipc.h
.In sys/shm.h
.Ft int
.Fn shmctl "int shmid" "int cmd" "struct shmid_ds *buf"
.Sh DESCRIPTION
Performs the action specified by
.Fa cmd
on the shared memory segment identified by
.Fa shmid :
.Bl -tag -width SHM_UNLOCKX
.It Dv IPC_STAT
Fetch the segment's
.Fa "struct shmid_ds" ,
storing it in the memory pointed to by
.Fa buf .
.\"
.\" XXX need to make sure that this is correct for FreeBSD
.\"
.It Dv IPC_SET
Changes the
.Fa shm_perm.uid ,
.Fa shm_perm.gid ,
and
.Fa shm_perm.mode
members of the segment's
.Fa "struct shmid_ds"
to match those of the struct pointed to by
.Fa buf .
The calling process's effective uid must
match either
.Fa shm_perm.uid
or
.Fa shm_perm.cuid ,
or it must have superuser privileges.
.It Dv IPC_RMID
Removes the segment from the system.
The removal will not take
effect until all processes having attached the segment have exited;
however, once the IPC_RMID operation has taken place, no further
processes will be allowed to attach the segment.
For the operation
to succeed, the calling process's effective uid must match
.Fa shm_perm.uid
or
.Fa shm_perm.cuid ,
or the process must have superuser privileges.
.\" .It Dv SHM_LOCK
.\" Locks the segment in memory.  The calling process must have
.\" superuser privileges. Not implemented in FreeBSD.
.\" .It Dv SHM_UNLOCK
.\" Unlocks the segment from memory.  The calling process must
.\" have superuser privileges.  Not implemented in FreeBSD.
.El
.Pp
The
.Vt shmid_ds
structure is defined as follows:
.\"
.\" I fiddled with the spaces a bit to make it fit well when viewed
.\" with nroff, but otherwise it is straight from sys/shm.h
.\"
.Bd -literal
struct shmid_ds {
    struct ipc_perm shm_perm;   /* operation permission structure */
    size_t          shm_segsz;  /* size of segment in bytes */
    pid_t           shm_lpid;   /* process ID of last shared memory op */
    pid_t           shm_cpid;   /* process ID of creator */
    int             shm_nattch; /* number of current attaches */
    time_t          shm_atime;  /* time of last shmat() */
    time_t          shm_dtime;  /* time of last shmdt() */
    time_t          shm_ctime;  /* time of last change by shmctl() */
};
.Ed
.Sh RETURN VALUES
.Rv -std shmctl
.Sh ERRORS
The
.Fn shmctl
system call
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
Invalid operation, or
no shared memory segment was found corresponding to
.Fa shmid .
.\"
.\" XXX I think the following is right: ipcperm() only returns EPERM
.\"	when an attempt is made to modify (IPC_M) by a non-creator
.\"	non-owner
.It Bq Er EPERM
The calling process's effective uid does not match the uid of
the shared memory segment's owner or creator.
.It Bq Er EACCES
Permission denied due to mismatch between operation and mode of
shared memory segment.
.El
.Sh "SEE ALSO"
.Xr shmat 2 ,
.Xr shmdt 2 ,
.Xr shmget 2 ,
.Xr ftok 3