.\"
.\" Copyright (c) 1995 David Hovemeyer <daveho@infocom.com>
.\"
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.\" $FreeBSD: head/lib/libc/sys/semget.2 295384 2016-02-07 21:25:08Z jilles $
.\"
.Dd February 7, 2016
.Dt SEMGET 2
.Os
.Sh NAME
.Nm semget
.Nd obtain a semaphore id
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/ipc.h
.In sys/sem.h
.Ft int
.Fn semget "key_t key" "int nsems" "int flag"
.Sh DESCRIPTION
Based on the values of
.Fa key
and
.Fa flag ,
.Fn semget
returns the identifier of a newly created or previously existing
set of semaphores.
.\"
.\" This is copied verbatim from the shmget manpage.  Perhaps
.\" it should go in a common manpage, such as .Xr ipc 2
.\"
The key
is analogous to a filename: it provides a handle that names an
IPC object.
There are three ways to specify a key:
.Bl -bullet
.It
IPC_PRIVATE may be specified, in which case a new IPC object
will be created.
.It
An integer constant may be specified.
If no IPC object corresponding
to
.Fa key
is specified and the IPC_CREAT bit is set in
.Fa flag ,
a new one will be created.
.It
The
.Xr ftok 3
function
may be used to generate a key from a pathname.
.El
.\"
.\" Likewise for this section, except SHM_* becomes SEM_*.
.\"
.Pp
The mode of a newly created IPC object is determined by
.Em OR Ns 'ing
the following constants into the
.Fa flag
argument:
.Bl -tag -width XSEM_WXX6XXX
.It Dv SEM_R
Read access for user.
.It Dv SEM_A
Alter access for user.
.It Dv ( SEM_R>>3 )
Read access for group.
.It Dv ( SEM_A>>3 )
Alter access for group.
.It Dv ( SEM_R>>6 )
Read access for other.
.It Dv ( SEM_A>>6 )
Alter access for other.
.El
.Pp
If a new set of semaphores is being created,
.Fa nsems
is used to indicate the number of semaphores the set should contain.
Otherwise,
.Fa nsems
may be specified as 0.
.Sh RETURN VALUES
The
.Fn semget
system call
returns the id of a semaphore set if successful; otherwise, -1
is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn semget
system call
will fail if:
.Bl -tag -width Er
.\" ipcperm could fail (we are opening to read and write, as it were)
.It Bq Er EACCES
Access permission failure.
.\"
.\" sysv_sem.c is quite explicit about these, so I'm pretty sure
.\" this is accurate
.\"
.It Bq Er EEXIST
IPC_CREAT and IPC_EXCL were specified, and a semaphore set
corresponding to
.Fa key
already exists.
.It Bq Er EINVAL
The number of semaphores requested exceeds the system imposed maximum
per set.
.It Bq Er EINVAL
A semaphore set corresponding to
.Fa key
already exists and contains fewer semaphores than
.Fa nsems .
.It Bq Er EINVAL
A semaphore set corresponding to
.Fa key
does not exist and
.Fa nsems
is 0 or negative.
.It Bq Er ENOSPC
Insufficiently many semaphores are available.
.It Bq Er ENOSPC
The kernel could not allocate a
.Fa "struct semid_ds" .
.It Bq Er ENOENT
No semaphore set was found corresponding to
.Fa key ,
and IPC_CREAT was not specified.
.El
.Sh SEE ALSO
.Xr semctl 2 ,
.Xr semop 2 ,
.Xr ftok 3