Re: [Boost-users] [shmem] Help with shmem and fixed segments

From: Roman Neuhauser (neuhauser_at_[hidden])
Date: 2006-07-15 06:54:33

# bblasi_at_[hidden] / 2006-07-15 09:16:34 +0100:
> Ion,
> 
> thank you for your reply.
> 
> As you suggested mmap returns a different address than the one specified,
> behaving as a named_shared_object under the hood instead of a
> fixed_named_shared_object.
> 
> I have used the MAP_FIXED flag for mmap with disastrous results (the man
> pages already discourage you to use this flag). The computer hanged, very
> strange behaviours, etc... It just forces a segment the size you want at the
> address you specify overwriting anything in it's way. Lots of fun as you can
> foresee...

    What operating system was that? mmap fails if it cannot satisfy the
    request.
 
> So as I see it:
> 
> 	- You cannot store raw pointers in a shared memory even if you use
> fixed_named_shared_object. They all have to be offset_ptr always as it may
> not map where you want.
> 	- Documentation under 'Using STL containers and mapping the memory at a
> fixed address' should reflect this.
> 
> A possible workaround, as you suggested, would be to somehow write down the
> address where the memory was mapped to so that other processes know about
> it. IMHO this is a named_shared_object in the end, so why use
> fixed_named_shared_object?
> 
> I think that due to the inner behavior of mmap as long as you are aware that
> a fixed_named_shared_object is in fact an
> OCCASIONALY_fixed_named_shared_object everything sould be alright. Notice
> that the same binary run twice on the same box may map to a different
> address each time which may or may not be the one specified! And even if it
> returns false during creation of the segment you have already screwed your
> box overwritting protected memory areas...
> 
> Do you agree or have I got the wrong end of the stick?
> 
> (I use Linux Fedora Core 5)
> 
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