[gmpi] Re: ABI's / Nutshell

Thanks for the careful reply, Tim -- yes, it helps clarify very much. I'll try to rewrite the nutshell to incorporate the new info in the next day or two. FWIW, in deference to the more C-oriented among us I was trying to avoid "object"-oriented terminology w/r/t the packaging/factory items themselves, reserving that for the plug objects.

Here's what's currently throwing me: 
To instantiate a plugin, the host calls a method of the IGMPI_Factory
"object", passing a GUID.  The resulting "object" is an IGMPI_Unknown.
The IGMPI_Unknown "object" can be queried for compliance with the
GMPI_Plugin interface.

In practical terms, this means that you can't assume that a DLL complies
with any specific GMPI interface unless you ask it explicitly.

Since we are not using COM per se, what is the motivation for the "Unknown" step? Is it that the factory going to be making something other than plug instances? Is it for managing plug API version changes? Or is this just a ptr typing thing?


Does any of this happen automatically?  I don't know, nor do I know if it
is cross-platform.

It would be good to get a clearer picture soon, while we can still make changes easily if need be.

        -- Chris G.


Tim said:
On Thu, Feb 24, 2005 at 01:55:36PM -0800, Chris Grigg wrote: 
 Every package file has exactly one factory function, whose offset
 into the package file is itself found at a known offset.  To


As stands today:

Every package file has a single entry point which is a well-known symbol.
That entry point is a function which returns a pointer to a IGMPI_Factory
"object".  IGMPI "objects" are really an ANSI C struct with a pointer to a
table of ANSI C functions.

This makes these "objects" usable from any language that can wrap C.

 instantiate a plug product, the host loads the package file into
 memory and executes its factory function, passing in the GUID of the
 desired plug product; factory allocates and initializes any static


To instantiate a plugin, the host calls a method of the IGMPI_Factory
"object", passing a GUID.  The resulting "object" is an IGMPI_Unknown.
The IGMPI_Unknown "object" can be queried for compliance with the
GMPI_Plugin interface.

In practical terms, this means that you can't assume that a DLL complies
with any specific GMPI interface unless you ask it explicitly.

When the host is done with an "object", it call the release method of that
"object" (a well-known offset into the function table of that "object").
Once released, the caller may not use the "object" anymore.

 b) How exactly does the factory, or a plug, call back into the host
 for services like memory, files, etc.?  Doesn't the factory need a
 bundle of host callback function pointers too?


I assume that, eventually, the host will pass an "object" to the plugin.
The host "object" will have functions to do stuff.

 c) Is the plug prohibited from calling anything outside itself (other
 than the host-provided services callbacks)?  If not, since we aren't
 using a dynamic linker to load plugs, how exactly are those symbols
 (system lib calls etc.) resolved by the time the plug needs to call
 them?


If a plugin needs libraries beyond the standard C++ runtime or C runtime,
it should link them itself.  It's safest to statically link them into the
plugin.

Example:  if my plugin requires libfoo v3.14, I should statically link
libfoo v3.14 into my plugin, so I don't get version mismatch.
Alternatively, I can dynamically load another shared lib.

Does any of this happen automatically?  I don't know, nor do I know if it
is cross-platform.

Does that clarify?

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