.\"	$OpenBSD: semget.2,v 1.19 2014/11/15 22:19:53 guenther Exp $
.\"	$NetBSD: semget.2,v 1.2 1997/03/27 08:20:41 mikel Exp $
.\"
.\" Copyright (c) 1995 Frank van der Linden
.\" All rights reserved.
.\"
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.\" modification, are permitted provided that the following conditions
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.\"    must display the following acknowledgement:
.\"      This product includes software developed for the NetBSD Project
.\"      by Frank van der Linden
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.\"/
.Dd $Mdocdate: November 15 2014 $
.Dt SEMGET 2
.Os
.Sh NAME
.Nm semget
.Nd get semaphore set
.Sh SYNOPSIS
.In sys/sem.h
.Ft int
.Fn semget "key_t key" "int nsems" "int semflg"
.Sh DESCRIPTION
The
.Fn semget
system call returns the semaphore identifier associated with
.Fa key .
.Pp
A new set containing
.Fa nsems
semaphores is created if either
.Fa key
is equal to
.Dv IPC_PRIVATE ,
or
.Fa key
does not have a semaphore set associated with it and the
.Dv IPC_CREAT
bit is set in
.Fa semflg .
.Pp
The access modes of the created semaphores is specified in
.Fa semflg
as a bitwise OR of zero or more of the following values:
.Bd -literal -offset indent
SEM_A        alter permission for owner
SEM_R        read permission for owner

SEM_A >> 3   alter permission for group
SEM_R >> 3   read permission for group

SEM_A >> 6   alter permission for other
SEM_R >> 6   read permission for other
.Ed
.Pp
If a new set of semaphores is created, the data structure associated with it
(the
.Va semid_ds
structure, see
.Xr semctl 2 )
is initialized as follows:
.Bl -bullet
.It
.Va sem_perm.cuid
and
.Va sem_perm.uid
are set to the effective UID of the calling process.
.It
.Va sem_perm.gid
and
.Va sem_perm.cgid
are set to the effective GID of the calling process.
.It
.Va sem_perm.mode
is set to the lower 9 bits of
.Fa semflg .
.It
.Va sem_nsems
is set to the value of
.Fa nsems .
.It
.Va sem_ctime
is set to the current time.
.It
.Va sem_otime
is set to 0.
.El
.Sh RETURN VALUES
.Fn semget
returns a non-negative semaphore identifier if successful.
Otherwise, \-1 is returned and
.Va errno
is set to reflect the error.
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EACCES
The caller has no permission to access a semaphore set already associated with
.Fa key .
.It Bq Er EEXIST
Both
.Dv IPC_CREAT
and
.Dv IPC_EXCL
are set in
.Fa semflg ,
and a semaphore set is already associated with
.Fa key .
.It Bq Er EINVAL
.Va nsems
is less than or equal to 0 or greater than the system limit for the
number in a semaphore set.
.Pp
A semaphore set associated with
.Fa key
exists, but has fewer semaphores than the number specified in
.Fa nsems .
.It Bq Er ENOSPC
A new set of semaphores could not be created because the system limit
for the number of semaphores or the number of semaphore sets has been
reached.
.It Bq Er ENOENT
.Dv IPC_CREAT
was not set in
.Fa semflg
and no semaphore set associated with
.Fa key
was found.
.El
.Sh SEE ALSO
.Xr semctl 2 ,
.Xr semop 2 ,
.Xr ftok 3