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 $NetBSD: semget.2,v 1.18 2006/10/08 08:22:34 rillig Exp $

 Copyright (c) 1995 Frank van der Linden
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.Dd May 13, 2004
.Dt SEMGET 2
.Os
.Sh NAME
.Nm semget
.Nd get set of semaphores
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
n 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 .

p
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 .
If both the
.Dv IPC_CREAT
bit and the
.Dv IPC_EXCL
bit are set in
.Fa semflg ,
and
.Fa key
has a semaphore set associated with it already,
the operation will fail.

p
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:
l -bullet t .Va sem_perm.cuid
and
.Va sem_perm.uid
are set to the effective uid of the calling process.
t .Va sem_perm.gid
and
.Va sem_perm.cgid
are set to the effective gid of the calling process.
t .Va sem_perm.mode
is set to the lower 9 bits of
.Fa semflg .
t .Va sem_nsems
is set to the value of
.Fa nsems .
t .Va sem_ctime
is set to the current time.
t .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
l -tag -width Er t Bq Er EACCES The caller has no permission to access a semaphore set already associated with
.Fa key .
t 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 .
t Bq Er EINVAL .Va nsems
is less than 0 or greater than the system limit for the number in a semaphore
set.

p
A semaphore set associated with
.Fa key
exists, but has fewer semaphores than the number specified in
.Fa nsems .
t 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.
t Bq Er ENOENT .Dv IPC_CREAT
is not set in
.Fa semflg
and no semaphore set associated with
.Fa key
was found.
.El
.Sh SEE ALSO
.Xr ipcs 1 ,
.Xr semctl 2 ,
.Xr semop 2 ,
.Xr ftok 3
.Sh STANDARDS
The
.Nm
system call conforms to
.St -xsh5 .
.Sh HISTORY
Semaphores appeared in the first release of
.At V .