.\"
.\" Copyright (c) 1995 David Hovemeyer <daveho@infocom.com>
.\"
.\" All rights reserved.
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.\" $FreeBSD: src/lib/libc/sys/shmat.2,v 1.8.2.6 2001/12/14 18:34:01 ru Exp $
.\"
.Dd January 4, 2014
.Dt SHMAT 2
.Os
.Sh NAME
.Nm shmat ,
.Nm shmdt
.Nd attach or detach shared memory
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In machine/param.h
.In sys/types.h
.In sys/ipc.h
.In sys/shm.h
.Ft void *
.Fn shmat "int shmid" "const void *addr" "int flag"
.Ft int
.Fn shmdt "const void *addr"
.Sh DESCRIPTION
.Fn Shmat
attaches the shared memory segment identified by
.Fa shmid
to the calling process's address space.  The address where the segment
is attached is determined as follows:
.\"
.\" These are cribbed almost exactly from Stevens, _Advanced Programming in
.\" the UNIX Environment_.
.\"
.Bl -bullet
.It
If
.Fa addr
is 0, the segment is attached at an address selected by the
kernel.
.It
If
.Fa addr
is nonzero and SHM_RND is not specified in
.Fa flag ,
the segment is attached the specified address.
.It
If
.Fa addr
is specified and SHM_RND is specified,
.Fa addr
is rounded down to the nearest multiple of SHMLBA.
.El
.Pp
.Fn Shmdt
detaches the shared memory segment at the address specified by
.Fa addr
from the calling process's address space.
.Sh RETURN VALUES
Upon success,
.Fn shmat
returns the address where the segment is attached; otherwise, -1
is returned and
.Va errno
is set to indicate the error.
.Pp
.Rv -std shmdt
.Sh ENVIRONMENT
The XSI Interprocess Communication family of functions is also available
as an implementation in userspace.
To use it, the
.Xr sysvipcd 8
daemon has to be running.
.Pp
If the
.Ev USR_SYSVIPC
variable is set in a process' environment, the process and its children
will use the userspace implementation.
.Sh ERRORS
.Fn Shmat
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
No shared memory segment was found corresponding to
.Fa shmid .
.It Bq Er EINVAL
.Fa addr
was not an acceptable address.
.El
.Pp
.Fn Shmdt
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa addr
does not point to a shared memory segment.
.El
.Sh "SEE ALSO"
.Xr shmctl 2 ,
.Xr shmget 2
.Sh AUTHORS
.An -nosplit
The
.Dx
specific userspace implementation (see
.Sx ENVIRONMENT )
was written by
.An Larisa Grigore .