Re: [Boost-users] [interprocess] another newbie question: increasing size of segment

From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2007-01-24 12:41:39

Wolfgang Hottgenroth wrote:
> Hi,
> 
> 
> first of all: many thanks to Ion, it was very helpful.
> 
> But here is my next problem: Can I and how can I increase the size of a
> shared memory segment?

You can't do that with current Interprocess functions. Maybe I should 
add that as a new to-do. There are two difficulties:

-> Should we maintain the base address?
-> What if extending fails? Should I preserve the old shared memory?
-> Should the expansion be atomic? (And this maybe can't be achieved, 
because another process can be doing something in the segment and we 
have no locks to stop it)

I will think about it when I have a bit of time. But I'm afraid it will 
take some time. If you know the objects your shared memory contains, you 
can create a new managed_shared_memory and create a new map taking the 
old map as an argument. Something like:

managed_shared_memory new_shm(...);

new_shm.construct<MyMap>("name")(old_map);

This would reproduce the old map in the new map. On the other hand, you 
will need to notify other processes about this. Shared memory is fixed 
size and I'm afraid we can't do much to improve this... I've tried to 
experiment a bit with multi-segment shared memory (mapping more shared 
memory when needed) but the problem is that somehow other processes must 
be notified atomically. And I have no solution for this.

So I'm afraid I have no solution for a dynamically growable shared 
memory ;-(

Regards,

Ion