.\"
.\" Copyright (c) 1995 David Hovemeyer <daveho@infocom.com>
.\"
.\" All rights reserved.
.\"
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.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" $FreeBSD: releng/12.2/lib/libc/sys/shmget.2 330409 2018-03-04 20:06:02Z brooks $
.\"
.Dd March 4, 2018
.Dt SHMGET 2
.Os
.Sh NAME
.Nm shmget
.Nd obtain a shared memory identifier
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/shm.h
.Ft int
.Fn shmget "key_t key" "size_t size" "int flag"
.Sh DESCRIPTION
Based on the values of
.Fa key
and
.Fa flag ,
.Fn shmget
returns the identifier of a newly created or previously existing shared
memory segment.
.\"
.\" The following bit about keys and modes also applies to semaphores
.\" and message queues.
.\"
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
may be used to generate a key from a pathname.
.El
.Pp
The mode of a newly created IPC object is determined by
which are set by ORing these constants into the
.Fa flag
argument:
.Bl -tag -width 0000
.It Dv 0400
Read access for owner.
.It Dv 0200
Write access for owner.
.It Dv 0040
Read access for group.
.It Dv 0020
Write access for group.
.It Dv 0004
Read access for other.
.It Dv 0002
Write access for other.
.El
.\"
.\" XXX - we should also mention how uid, euid, and gid affect ownership
.\"	  and use
.\"
.\" end section about keys and modes
.\"
.Pp
When creating a new shared memory segment,
.Fa size
indicates the desired size of the new segment in bytes.
The size
of the segment may be rounded up to a multiple convenient to the
kernel (i.e., the page size).
.Sh RETURN VALUES
Upon successful completion,
.Fn shmget
returns the positive integer identifier of a shared memory segment.
Otherwise, -1 is returned and
.Va errno
set to indicate the error.
.Sh ERRORS
The
.Fn shmget
system call
will fail if:
.Bl -tag -width Er
.\"
.\" XXX What about ipcperm failing?
.\"
.It Bq Er EINVAL
Size specified is greater than the size of the previously existing segment.
Size specified is less than the system imposed minimum, or greater than
the system imposed maximum.
.It Bq Er ENOENT
No shared memory segment was found matching
.Fa key ,
and IPC_CREAT was not specified.
.It Bq Er ENOSPC
The kernel was unable to allocate enough memory to
satisfy the request.
.It Bq Er EEXIST
IPC_CREAT and IPC_EXCL were specified, and a shared memory segment
corresponding to
.Fa key
already exists.
.El
.Sh "SEE ALSO"
.Xr shmat 2 ,
.Xr shmctl 2 ,
.Xr shmdt 2 ,
.Xr ftok 3