At Sat, 30 Sep 2017 09:45:02 -0700, Eric Dobson wrote: > I'm trying to write some racket code which interfaces with a foreign > library and provides a safe interface. Some of the functions I'm calling > allocate memory and need to have this explicitly freed by the caller. The > 'allocator' binding from ffi/unsafe/alloc seems to solve this problem, but > I'm running into issues using it from places other than the main one. The > issue is that if a place gets shut down then the finalizer that was > registered don't seem to run, and I can observe the leaked memory at the > process level. > > I took a look at ffi/unsafe/custodian and it seems that > register-custodian-shutdown would allow this to be implemented. Is there a > reason that such allocator doesn't do this by default?
The simple answer is that I didn't think about places originally, but there are some challenges beyond just wiring in a custodian hook. If we change `ffi/unsafe/alloc` so that a custodian shutdown runs all deallocators, that would work in many cases. It would create a problem, however, if there are objects to deallocate where the deallocation function references other objects that themselves must be deallocated. That is, an allocated value A might depend on an allocated value B up until the point that A has been deallocated. That dependency can be expressed to `ffi/unsafe/alloc` (via the garbage collector) by including a reference to B in the deallocation closure for A. Setting up dependencies that way is not common, but I'm pretty sure I've used that pattern once or twice. Meanwhile, there's no similar way to order callbacks that are registered with a custodian. I'm tempted to say that `ffi/unsafe/alloc` should only be used for objects where the deallocation order doesn't matter, so that deallocators can be triggered by a custodian shutdown. But an additional constraint is needed: values to be deallocated cannot be used by any other custodian-shutdown callbacks. That additional constraint seems too limiting. Maybe we need to add some sort of ordering constraints to custodians, but I'm not immediately sure of the right way to do that. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
