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.\"   $OpenBSD: shmctl.2,v 1.2 1996/10/08 01:20:15 michaels Exp $
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.Dd August 17, 1995
.Dt SHMCTL 2
.Os
.Sh NAME
.Nm shmctl
.Nd shared memory control operations
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/ipc.h>
.Fd #include <sys/msg.h>
.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 .
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 .
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
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 {
    ushort cuid; /* creator user id */
    ushort cgid; /* creator group id */
    ushort uid;	 /* user id */
    ushort gid;	 /* group id */
    ushort mode; /* r/w permission (see chmod(2)) */
    ushort seq;	 /* sequence # (to generate unique msg/sem/shm id) */
    key_t key;	 /* user specified msg/sem/shm key */
};
.Ed
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
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 IPC_SET or 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.
An attempt is made to increase the value of
.Va shm_qbytes
through IPC_SET
but the caller is not the super-user.
.It Bq Er EACCES
The command is 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.
.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 shmdt 2 ,
.Xr shmget 2