Re: [9fans] shared memory

[Geoff Collyer <geoff@collyer.net>]

I wouldn't have called shmat `user friendly' and I don't think it's
any easier to use than the Plan 9 machinery.  shmat, shmget, etc.
don't really do much for you; they just do what segattach does, but in
a more complicated way.  You still have to arrange to coordinate
access by the processes sharing the memory.  I think

	char *ss = (char *)segattach(0, "shared", nil, length);

is all you need to allocate a shared segment of size `length', though
you still have to actually share it, probably by forking.

If the processes in question are related, you don't even need
segattach, just be careful to keep per-process data on the stack (in
automatic variables) and when you rfork(), be sure to include the
RFMEM flag.  After the rfork, parent and child will share their data
segment.

If you're looking for a way for unrelated processes to share memory, I
think your best bet is to use the undocumented S option of ramfs:

	ramfs -S shared1

to create a sharable /tmp, then have all the programs that want to
share the memory do:

	mkdir /tmp/shared1
	mount -c /srv/shared1 /tmp/shared1

The 'l' permission bit on files in /tmp/shared1 can implement locks.

How are you trying to share memory (or what are you trying to
accomplish)?  All of the above assumes that you want something like
System V shared memory.