.\"	$NetBSD: shmctl.2,v 1.15 2003/04/16 13:34:56 wiz Exp $
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.Dd June 17, 2002
.Dt SHMCTL 2
.Os
.Sh NAME
.Nm shmctl
.Nd shared memory control operations
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/shm.h
.Ft int
.Fn shmctl "int shmid" "int cmd" "struct shmid_ds *buf"
.Sh DESCRIPTION
The
.Fn shmctl
system call performs control operations on the shared memory segment
specified by
.Fa shmid .
.Pp
Each shared memory segment has a
.Sy shmid_ds
structure associated with it which contains the following members:
.Bd -literal
    struct ipc_perm shm_perm;   /* operation permissions */
    size_t          shm_segsz;  /* size of segment in bytes */
    pid_t           shm_lpid;   /* pid of last shm op */
    pid_t           shm_cpid;   /* pid of creator */
    shmatt_t        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() */
.Ed
.Pp
The
.Sy ipc_perm
structure used inside the
.Sy shmid_ds
structure is defined in
.Aq Pa sys/ipc.h
and contains the following members:
.Bd -literal
    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; /* permission (lower 9 bits) */
.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 super-user, 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.
.It Dv IPC_RMID
Remove the shared memory segment specified by
.Fa shmid
and destroy the data associated with it.
Only the super-user 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 super-user, 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 was made to increase the value of
.Va shm_qbytes
through
.Dv IPC_SET
but the caller is not the super-user.
.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 ipcrm 1 ,
.Xr ipcs 1 ,
.Xr shmat 2 ,
.Xr shmget 2
.Sh STANDARDS
The
.Nm
system call conforms to
.St -xsh5 .
.Sh HISTORY
Shared memory segments appeared in the first release of
.At V .