plan9fox/sys/man/3/segment

.TH SEGMENT 3 
.SH NAME
segment \- long lived memory segments
.SH SYNOPSIS
.nf
.B bind -c '#g' /mnt/segment

.BI #g/ seg1
.BI #g/ seg1 /ctl
.BI #g/ seg1 /data
.BI #g/ seg2
.BI #g/ seg2 /ctl
.BI #g/ seg2 /data
 ...
.fi
.SH DESCRIPTION
.PP
The
.I segment
device provides a 2-level file system representing
long-lived sharable segments that processes may
.IR segattach (2).
The name of the directory is the
.I class
argument to
.IR segattach .
.PP
New segments are created under the top level
using
.B create
(see
.IR open (2)).
The
.B DMDIR
bit must be set in the permissions.
.IR Remove (2)'ing
the directory makes the segment no longer
available for
.IR segattach .
However, the segment will continue to exist until all
processes using it either exit or
.I segdetach
it.
.PP
Within each segment directory are two files,
.B data
and
.BR ctl .
Reading and writing
.B data
affects the contents of the segment.
Reading and writing 
.B ctl
retrieves and sets the segment's properties.
.PP
There is only one control message, which sets the segment's
virtual address and length in bytes:
.EX
	va \fIaddress length type\fP
.EE
.I Address
is automatically rounded down to a page boundary and
.I length
is rounded up to end the segment at a page boundary.
The segment will reside at the same virtual address in
all processes sharing it.
Optionally,
.I type
can be specified as
.B fixed
or
.BR sticky .
Fixed segments are uncached and physically continuous
with a fixed physical base address suitable for hardware
DMA access.
Sticky segments are like normal shared segments but
preallocated at creation time and never swapped out.
Only the hostower is allowed to create
.B fixed
or
.B sticky
segments.
.PP
When the segment is attached using
.IR segattach,
the address and length arguments are ignored in the call;
they are defined only by the
.B va
control message.
Once the address and length are set, they cannot be reset.
.PP
Reading the control file
returns a message of the same format with the segment's actual
start address and length. For
.B fixed
segments, the type and physical base address are appended.
.PP
Opening
.B data
or reading
.B ctl
before setting the virtual address yields the error
``segment not yet allocated''.
.PP
The permissions check when
.IR segattach ing
is equivalent to the one performed when opening
.B data
with mode ORDWR.
.SH EXAMPLE
.PP
Create a one megabyte segment at address 0x10000000:
.EX
	% bind -c '#g' /mnt/segment
	% mkdir /mnt/segment/example
	% echo 'va 0x10000000 0x100000' > /mnt/segment/example/ctl
.EE
.PP
Put the string ``hi mom'' at the start of the segment:
.EX
	% echo -n hi mom > /mnt/segment/example/data
.EE
.PP
Attach the segment to a process:
.EX
{
	ulong va;

	va = segattach(0, "example", 0, 0);
}
.EE
.SH "SEE ALSO
.IR segattach (2)
.SH SOURCE
.B /sys/src/9/port/devsegment.c