Shared memory and IPC

[Branko Čibej]

Luke Kenneth Casson Leighton wrote:

>On Mon, Sep 10, 2001 at 08:44:24PM +0200, Branko E`ibej wrote:
>
>>You're passing around structured defined in IDL, right? 
>>
>
>yep.
>
>>Therefore you 
>>know exactly where the pointers are. 
>>
>
>... urrr...   yes.
>
>>I assume pointers are the only bits 
>>that will cause problems if the segment isn't mapped to the same 
>>addresses in both apps.
>>
>>If all of the above is correct, and MMAP_FIXED fails (as it sometimes 
>>will) or isn't available (as it often isn't), you can simply fixup the 
>>pointers by adding an offset -- the difference between the addresses of 
>>the mapped area in the two apps. That would still be faster than a 
>>memcpy (except if you're passing arrays of poitners).
>>
>
>[arrays of pointers are surprisingly common, or rather, complex
>data structures in enumeration arrays of users/groups/SIDs/printers
>etc. with several pointers to strings, SIDs etc. in the Windows
>NT Domains IDL structures i've seen - are surprisingly common.
>
>that's not to say that someone doing their own DCOM app would
>have a lot of arrays of pointers]
>
>
>okay, i get it - combined with dave cb's and jim d's posts.
>
>what you're _not_ recommending is to be marshalling and unmarshalling
>the data into a flat stream, but instead:
>
Right,

>to walk the data structures using the IDL definition, changing
>all pointers *in-place*.
>
and right.

>okay.
>
>whilst in theory this is an extremely good idea, in practice i
>think it will be about eight to ten man-weeks' full-time work
>to action, which discourages me but might not discourage other
>people.
>
Oh, I didn't mean to imply that it's a small task. In this case it would 
probably make sense to make it work first and worry about performance later.


-- 
Brane Čibej <brane at xbc.nu> http://www.xbc.nu/brane/