.\"
.\" 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:
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\" $FreeBSD$
.\"
.Dd January 14, 2019
.Dt SHMAT 2
.Os
.Sh NAME
.Nm shmat ,
.Nm shmdt
.Nd attach or detach shared memory
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.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
The
.Fn shmat
system call
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
.Va SHM_RND
is not specified in
.Fa flag ,
the segment is attached the specified address.
.It
If
.Fa addr
is specified and
.Va SHM_RND
is specified,
.Fa addr
is rounded down to the nearest multiple of SHMLBA.
.El
.Pp
If the
.Va SHM_REMAP
flag is specified and the passed
.Fa addr
is not
.Dv NULL ,
any existing mappings in the virtual addresses range are
cleared before the segment is attached.
If the flag is not specified,
.Fa addr
is not
.Dv NULL ,
and the virtual address range contains
some pre-existing mappings, the
.Fn shmat
call fails.
.Pp
The
.Fn shmdt
system call
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 ERRORS
The
.Fn shmat
system call
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
The
.Fa addr
argument
was not an acceptable address.
.It Bq Er ENOMEM
The specified
.Fa addr
cannot be used for mapping, for instance due to the amount of available
space being smaller than the segment size,
or because pre-existing mappings are in the range and no
.Va SHM_REMAP
flag was provided.
.It Bq Er EMFILE
Failed to attach the shared memory segment because the per-process
.Va kern.ipc.shmseg
.Xr sysctl 3
limit was reached.
.El
.Pp
The
.Fn shmdt
system call
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa addr
argument
does not point to a shared memory segment.
.El
.Sh "SEE ALSO"
.Xr shmctl 2 ,
.Xr shmget 2
