Shared memory and IPC
Luke Kenneth Casson Leighton
lkcl at samba-tng.org
Thu Sep 6 13:21:26 GMT 2001
the only tricky bit is, and this is relevant i should
imagine to doing shared, complex data structures in samba,
an also a possible solution to some of the issues
in APR, how do you guarantee that multiple processes -
no, _all_ processes - get access to the same fixed address
memory segment / segments?
i wasn't aware that mmap could do fixed memory segments,
otherwise i would have suggested (on apr dev) an sms_shmem
that used shared memory.
[fyi: sms - Smart Memory system. it's a wrapper around
memory management that presents the same API to mlock()ed,
mmap()ed, kernel-page-mapped, etc. memory, that has
support for either malloc/realloc/free style or pool-style
such as Trivial Alloc and APR pools management].
a few months back or so, doing an sms_shmem was considered
[with a view to then presenting that via an apr_pool_t
however, the trouble with shared memory is [was!] that
the offsets in the different processes for the shared
memory segment are different.
so, placing a linked list or any data structures with
pointers in it is a waste of time.
now, i know that in samba there are shmem-start-relative
linked lists to data structures *without* pointers in
them., so i know this can be done.
and doing a linked list in MAP_FIXED segments would
be an awful lot easier, in fact it would be trivial.
the only thing, then, that is of concern, is how to link
several segments together: it's not a good idea to
mmap a massive segment of memory, have to do several
what happens if you run out and have to get some more
MMAP_FIXED segments? how do you guarantee that all
processes will all successfully mmap the same shared
memory segment at the same address? how do you
communicate the _need_ to get more MMAPED_FIXED
segments to all processes? :)
[please, someone tell me if i am explaining this clearly enough]
or, should the first implementation avoid this by
specifying the required segment size at apr_initialise()
time or as a parameter to apr_sms_shmem_create()?
On Thu, Sep 06, 2001 at 12:37:27PM -0700, Jeremy Allison wrote:
> Luke Kenneth Casson Leighton wrote:
> > is it possible to get memory allocated in one process to be shared
> > with another process *at the same address* on unix, os/2, nt, vms etc?
> mmap has the MAP_FIXED flag that forces the mmap to fail
> if it can't be done at the given address. You could use
> a different IPC method to pass that address to other
> This is UNIX/POSIX only though (as far as I know).
> dcom-dev mailing list
> dcom-dev at lists.dcerpc.net
More information about the samba-technical