memory management -- mmap()
Brandon Allbery
allbery at ncoast.UUCP
Sat Mar 8 10:16:46 AEST 1986
Expires:
Quoted from <4479 at think.ARPA> ["shared memory: a poll"], by rose at think.ARPA (John Rose)...
+---------------
| The "4.2BSD System Manual", in the section on memory management
| (/usr/doc/sysman/1.2.t), sketches out a future extension to Unix*
| called "mmap()". This feature allows an "object" represented by
| a file-descriptor to be mapped into the per-process address space;
| there is provision for modifications made to the "object" to be
| shared by more than one process.
|
| I haven't seen much discussion of this kind of memory management
| on the net, but I have heard of vendors implementing it. So
| there must be people out there who have used it in interesting ways.
+---------------
Xenix 3.0 (I don't know about 5.0) does something that sounds like this.
There is a special type of file which is a shared-memory file. A process
opens the file with a special system call (the file must exist or be created),
then any process which opens the file gets it attached to its memory space.
The problems: (1) Xenix runs very slowly when a shared memory is open for
writing by more than one process (so say the docs); (2) they say NEVER leave
the shared memory attached while making a system call, as it can crash
Xenix(!). There are system calls (aside from create/open and close of the
shared-memory file) which attach and detach the shared memory from the
process's address space.
Anyone with more Xenix and/or 4.2/4.3 experience know anything to prove or
disprove this?
--Brandon
(P.S. The shared-memory file is an S_IFNAM; there is a special flag in the
inode which distinguishes between FIFOs and shared-memory files. That may
give some idea what Xenix is up to.)
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