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.Dd August 25, 1999
.Dt SEMCTL 2
.Os
.Sh NAME
.Nm semctl
.Nd semaphore control operations
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.Fd #include \*[Lt]sys/sem.h\*[Gt]
.Ft int
.Fn semctl "int semid" "int semnum" "int cmd" "..."
.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).
The fourth argument is optional and depends upon the operation requested.
If required, it 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 \*[Am] SETALL */
.Ed
.Pp
The
.Sy semid_ds
structure used in the
.Dv IPC_SET
and
.Dv IPC_STAT
commands is defined in
.Aq Pa sys/sem.h
and contains the following members:
.Bd -literal
    struct ipc_perm sem_perm; /* operation permissions */
    unsigned 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
.Sy ipc_perm
structure used inside the
.Sy semid_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
.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
.Fa arg.val ,
where
.Fa arg
is the fourth argument to
.Fn semctl .
.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 of all the semaphores associated with
.Fa semid .
.It Dv SETALL
Set the values of all the semaphores that are associated with the semaphore
identifier
.Fa semid
to the corresponding values in
.Fa arg.array ,
where
.Fa arg
is the fourth argument to
.Fn semctl .
.It Dv IPC_STAT
Gather information about a semaphore and place the information in the
structure pointed to by
.Fa arg.buf ,
where
.Fa arg
is the fourth argument to
.Fn semctl .
.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 ,
there
.Fa arg
is the fourth argument to
.Fn semctl .
This operation can only be executed by the super-user, 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 super-user 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 super-user, 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
specifies an invalid address.
.El
.Sh SEE ALSO
.Xr semget 2 ,
.Xr semop 2
.Sh STANDARDS
The
.Nm
system call conforms to
.St -xsh5 .
.Sh HISTORY
Semaphores appeared in the first release of
.At V .