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.TH SEGMENT 3
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.SH NAME
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segment \- long lived memory segments
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.SH SYNOPSIS
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.nf
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.B bind '#g' /mnt/segment
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.BI #g/ seg1
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.BI #g/ seg1 /ctl
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.BI #g/ seg1 /data
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.BI #g/ seg2
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.BI #g/ seg2 /ctl
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.BI #g/ seg2 /data
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 ...
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.fi
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.SH DESCRIPTION
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.PP
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The
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.I segment
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device provides a 2-level file system representing
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long-lived sharable segments that processes may
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.IR segattach (2).
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The name of the directory is the
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.I class
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argument to
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.IR segattach .
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.PP
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New segments are created under the top level
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using
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.B create
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(see
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.IR open (2)).
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The
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.B DMDIR
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bit must be set in the permissions.
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.IR Remove (2)'ing
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the directory makes the segment no longer
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available for
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.IR segattach .
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However, the segment will continue to exist until all
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processes using it either exit or
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.I segdetach
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it.
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.PP
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Within each segment directory are two files,
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.B data
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and
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.BR ctl .
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Reading and writing
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.B data
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affects the contents of the segment.
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Reading and writing
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.B ctl
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retrieves and sets the segment's properties.
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.PP
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There is only one control message, which sets the segment's
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virtual address and length in bytes:
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.EX
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	va \fIaddress length\fP
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.EE
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.I Address
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is automatically rounded down to a page boundary and
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.I length
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is rounded up to end the segment at a page boundary.
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The segment will reside at the same virtual address in
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all processes sharing it.
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When the segment
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is attached using
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.IR segattach,
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the address and length arguments are ignored in the call;
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they are defined only by the
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.B va
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control message.
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Once the address and length are set, they cannot be reset.
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.PP
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Reading the control file
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returns a message of the same format with the segment's actual
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start address and length.
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.PP
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Opening
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.B data
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or reading
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.B ctl
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before setting the virtual address yields the error
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``segment not yet allocated''.
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.PP
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The permissions check when
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.IR segattach ing
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is equivalent to the one performed when opening
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.B data
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with mode ORDWR.
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.SH EXAMPLE
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.PP
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Create a one megabyte segment at address 0x10000000:
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.EX
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	% bind '#g' /mnt/segment
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	% mkdir /mnt/segment/example
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	% echo 'va 0x10000000 0x100000' > /mnt/segment/example/ctl
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.EE
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.PP
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Put the string ``hi mom'' at the start of the segment:
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.EX
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	% echo -n hi mom > /mnt/segment/example/data
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.EE
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.PP
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Attach the segment to a process:
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.EX
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{
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	ulong va;
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	va = segattach(0, "example", 0, 0);
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}
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.EE
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.SH "SEE ALSO
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.IR segattach (2)
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.SH SOURCE
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.B /sys/src/9/port/devsegment.c