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.\"   $OpenBSD: shmget.2,v 1.2 1996/10/08 01:20:16 michaels Exp $
.\"	$NetBSD: shmget.2,v 1.1 1995/10/16 23:49:32 jtc Exp $	
.\"
.\" Copyright (c) 1995 Frank van der Linden
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.\"      by Frank van der Linden
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.Dd August 17, 1995
.Dt SHMGET 2
.Os
.Sh NAME
.Nm shmget
.Nd get shared memory area identifier
.Sh SYNOPSIS
.Fd #include <sys/shm.h>
.Ft int
.Fo shmget
.Fa "key_t key"
.Fa "size_t size"
.Fa "int shmflg"
.Fc
.Sh DESCRIPTION
.Fn shmget
returns the shared memory identifier associated with the key
.Fa key .
.Pp
A shared memory segment is created if either
.Fa key
is equal to IPC_PRIVATE, or
.Fa key
does not have a shared memory segment identifier associated with it, and the IPC_CREAT
bit is set in
.Fa shmflg.
.Pp
If a new shared memory segment is created, the data structure associated with it (the
.Va shmid_ds
structure, see
.Xr shmctl 2 )
is initialized as follows:
.Bl -bullet
.It 
.Va shm_perm.cuid
and
.Va shm_perm.uid
are set to the effective uid of the calling process.
.It
.Va shm_perm.gid
and
.Va shm_perm.cgid
are set to the effective gid of the calling process.
.It
.Va shm_perm.mode
is set to the lower 9 bits of
.Fa shmflg .
.It
.Va shm_lpid ,
.Va shm_nattch ,
.Va shm_atime ,
and 
.Va shm_dtime
are set to 0
.It
.Va shm_ctime
is set to the current time.
.It
.Va shm_segsz
is set to the value of
.Fa size .
.It
The
.Xr ftok 3
function may be used to generate a key from a pathname.
.El
.Sh RETURN VALUES
Upon successful completion a positive shared memory segment identifier is returned.
Otherwise, -1 is returned and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn shmget
system call will fail if:
.Bl -tag -width Er
.\" ===========
.It Bq Er EACCES
A shared memory segment is already associated with
.Fa key
and the caller has no permission to access it.
.\" ===========
.It Bq Er EEXIST
Both IPC_CREAT and IPC_EXCL are set in
.Fa shmflg ,
and a shared memory segment is already associated with
.Fa key .
.\" ===========
.It Bq Er EINVAL
No shared memory segment is to be created,
and a shared memory segment exists for
.Fa key ,
but the size of the segment associated with it
is less than
.Fa size ,
which is non-zero.
.\" ===========
.It Bq Er ENOENT
IPC_CREAT was not set in
.Fa shmflg
and no shared memory segment associated with
.Fa key
was found.
.\" ===========
.It Bq Er ENOMEM
There is not enough memory left to created a shared memory segment of the
requested size.
.\" ===========
.It Bq Er ENOSPC
A new shared memory identifier could not be created because the system limit
for the number of shared memory identifiers has been reached.
.El
.Sh LEGACY SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/ipc.h>
.Fd #include <sys/shm.h>
.Pp
.Ft int
.br
.Fo shmget
.Fa "key_t key"
.Fa "int size"
.Fa "int shmflg"
.Fc ;
.Pp
All of these include files are necessary.
The type of
.Fa size
has changed.
.Sh SEE ALSO
.Xr ftok 3 ,
.Xr shmat 2 ,
.Xr shmctl 2 ,
.Xr shmdt 2 ,
.Xr compat 5