[squeak-dev] FFI pinning examples?

Eliot Miranda eliot.miranda at gmail.com
Thu Aug 24 22:06:30 UTC 2017

Hi Phil,

On Thu, Aug 24, 2017 at 1:36 PM, Phil B <pbpublist at gmail.com> wrote:

> I remember reading a while back that Spur added memory pinning and was
> curious if any Squeak packages has made use of it yet? (Looking for an
> example or two to get me started)

The protocol i in Object, with the selectors pin, unpin and isPinned.  You
can send any of these to any non-immediate object.  It is your
responsibility to avoid calling unpin until external code no longer holds
references to any pinned object.  It is your responsibility to arrange that
the objects are still accessible within the system so that they are not
GCed while external code holds onto them.  Pinning is orthogonal to GC.  It
prevents the GC from moving objects, but does not prevent them from being
GCed if no references to them exist in the Smalltalk system.  Note that
also pinning may cause an object to move from newSpace to oldSpace, as
objects are only pinball in oldSpace.  Pinning a new object causes it to
become a copy of the object in oldSpace, so do not pass an object to
external code and then pin it; it may move.

 So the idea is
- create some objects you want to share with external code and pin them
- pass them to that external code through the FFI
- allow that external code to run (e.g spawn a thread in the external code,
or make subsequent call though the FFI exercise the code)
- shut down the external code
- unpin your objects, or merely allow them to be GCed.

Alternatively external code can pin the objects it is given, providing it
updates their location afterwards.  See pinObject:.

Note that you should use the interpreterProxy accessors
firstIndexableField: and firstFixedField: to find out the start of the body
of an object (pinned or otherwise).  byteSizeOf:, instanceSizeOf:,
slotSizeOf: and stSizeOf: can all be used to find out various sizes.  See
platforms/Cross/vm/sqVirtualMachine.h, which also provides pinObject: and

Clearly some work is needed to add documentation in the right places.  LMK
where you looked, where you would expect to find such information, etc.
best, Eliot
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