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.\" Copyright (c) 1995 Frank van der Linden
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.Dd $Mdocdate: November 15 2014 $
.Dt SEMCTL 2
.Os
.Sh NAME
.Nm semctl
.Nd semaphore control operations
.Sh SYNOPSIS
.In sys/sem.h
.Ft int
.Fn semctl "int semid" "int semnum" "int cmd" "union semun arg"
.Sh DESCRIPTION
The
.Fn semctl
system call provides a number of control operations on the semaphore specified
by
.Fa semnum
and
.Fa semid .
The operation to be performed is specified in
.Fa cmd
(see below).
.Fa arg
is a union of the following fields:
.Bd -literal
	int     val;			/* value for SETVAL */
	struct  semid_ds *buf;		/* buffer for IPC_{STAT,SET} */
	u_short *array;			/* array for GETALL & SETALL */
.Ed
.Pp
The
.Bf -literal
semid_ds
.Ef
structure used in the
.Dv IPC_SET
and
.Dv IPC_STAT
commands is defined as follows in
.In sys/sem.h :
.Bd -literal
struct semid_ds {
	struct ipc_perm sem_perm;	/* operation permissions */
	struct  sem *sem_base;		/* semaphore set */
	u_short sem_nsems;		/* number of sems in set */
	time_t  sem_otime;		/* last operation time */
	time_t  sem_ctime;		/* last change time */
};
.Ed
.Pp
The
.Bf -literal
ipc_perm
.Ef
structure used inside the
.Bf -literal
semid_ds
.Ef
structure is defined in
.In 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
.Fn semctl
provides the following operations:
.Bl -tag -width IPC_RMIDX
.It Dv GETVAL
Return the value of the semaphore.
.It Dv SETVAL
Set the value of the semaphore to
.Va arg.val .
.It Dv GETPID
Return the pid of the last process that did an operation on this semaphore.
.It Dv GETNCNT
Return the number of processes waiting to acquire the semaphore.
.It Dv GETZCNT
Return the number of processes waiting for the value of the semaphore to
reach 0.
.It Dv GETALL
Return the values for all the semaphores associated with
.Fa semid .
.It Dv SETALL
Set the values for all the semaphores that are associated with the semaphore
identifier
.Fa semid
to the corresponding values in
.Va arg.array .
.It Dv IPC_STAT
Gather statistics about a semaphore and place the information in the
.Bf -literal
semid_ds
.Ef
structure pointed to by
.Fa arg.buf
(see above).
.It Dv IPC_SET
Set the value of the
.Va sem_perm.uid ,
.Va sem_perm.gid
and
.Va sem_perm.mode
fields in the structure associated with the semaphore.
The values are taken from the corresponding fields in the structure
pointed to by
.Fa arg.buf .
This operation can only be executed by the superuser, or a process that
has an effective user ID equal to either
.Va sem_perm.cuid
or
.Va sem_perm.uid
in the data structure associated with the message queue.
.It Dv IPC_RMID
Remove the semaphores associated with
.Fa semid
from the system and destroy the data structures associated with it.
Only the superuser or a process with an effective UID equal to the
.Va sem_perm.cuid
or
.Va sem_perm.uid
values in the data structure associated with the semaphore can do this.
.El
.Pp
The permission to read or change a message queue (see
.Xr semop 2 )
is determined by the
.Va sem_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 sem_perm.cuid
field or the
.Va sem_perm.uid
field, and the
effective GID can match either
.Va sem_perm.cgid
or
.Va sem_perm.gid .
.Sh RETURN VALUES
For the
.Dv GETVAL ,
.Dv GETPID ,
.Dv GETNCNT ,
and
.Dv GETZCNT
operations,
.Fn semctl
returns one of the values described above if successful.
All other operations will make
.Fn semctl
return 0 if no errors occur.
Otherwise \-1 is returned and
.Va errno
set to reflect the error.
.Sh ERRORS
.Fn semctl
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 sem_perm.uid
or
.Va sem_perm.cuid
fields of the data structure associated with the message queue.
.It Bq Er EACCES
The caller has no operation permission for this semaphore.
.It Bq Er EINVAL
.Fa semid
is not a valid message semaphore identifier.
.Pp
.Va cmd
is not a valid command.
.It Bq Er EFAULT
.Fa arg.buf
or
.Fa arg.array
specify an invalid address.
.It Bq Er ERANGE
.Fa cmd
is equal to
.Dv SETVAL
or
.Dv SETALL
and
.Va arg.val
or the values in
.Va arg.array
are greater than the system-imposed limit.
.El
.Sh SEE ALSO
.Xr semget 2 ,
.Xr semop 2