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.Dd November 25, 2006
.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
.In 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 segment can do this.
.It Dv SHM_LOCK
Lock the shared memory segment specified by
.Fa shmid
in memory.
This operation can only be executed by the super-user.
.It Dv SHM_UNLOCK
Unlock the shared memory segment specified by
.Fa shmid .
This operation can only be executed by the super-user.
.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 EACCES
The command is
.Dv IPC_STAT
and the caller has no read permission for this shared memory segment.
.It Bq Er EFAULT
.Fa buf
specifies an invalid address.
.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 ENOMEM
The
.Fa cmd
is equal to
.Dv SHM_LOCK
and there is not enough physical memory.
.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.
.Pp
The
.Fa cmd
is equal to
.Dv SHM_LOCK
or
.Dv SHM_UNLOCK
and the caller is not the super-user.
.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 .