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From: rbk@sequent.UUCP (Bob Beck)
Newsgroups: comp.unix.wizards
Subject: mmap() implementation
Message-ID: <2711@sequent.UUCP>
Date: Wed, 24-Jun-87 12:14:56 EDT
Article-I.D.: sequent.2711
Posted: Wed Jun 24 12:14:56 1987
Date-Received: Fri, 26-Jun-87 07:21:57 EDT
Organization: Sequent Computer Systems, Beaverton, OR
Lines: 22
Keywords: mapped files, shared memory

Sequent's version of UNIX (Dynix) implements a version of mmap(), with some
of the "blanks" form the original 4.2bsd system description filled in.  This
supports paged shared memory (nice for parallel programming ;-), and can
support SystemV shared-memory via library code (libc).

If more than one process maps overlapping portions of a file for PROT_RDWR
and MAP_SHARED you get shared memory.  MAP_PRIVATE ==> local mods to the
data.  No address-space alignment restrictions other than page boundaries.
Mapped files are "coherent" with normal reads and writes, so "cp mapped-file
somewhere" will get the latest data.  Mapped address space is dumped into
core files, providing a "snapshot" of the data when/if the program dies.  In
the upcomming release of Dynix (v3.0) "shared text" is finally gone and is
done as a sub-case of mapped files.  Also mmap() can map over any previously
existing address space (or it can populate new address space) -- the last
thing mapped is what's there.  munmap() makes address space go away (ie,
creates a "hole").  mmap() also supports device-drivers providing physical or
paged maps for various "custom" reasons.

If anyone wants more details, let me know.

					Bob Beck
					Sequent Computer Systems