Ability to unload an ffi-lib?

[Chris P]

Is it possible to unload an ffi-lib? I'm doing development on Windows where loading a DLL locks it and prevents the DLL from being recompiled.

There was another post on this, but it was about 7 years old, so I thought I would see if anything has changed.

Thanks,

Chris

[Matthew Flatt]

There's still no mechanism to unload a DLL.

For a long time, it wasn't practical to try to unload a DLL, because
the finalization mechanisms were too asynchronous --- making it
difficult to detect the point where the DLL would no longer be
referenced and could be unloaded safely. But support for
custodian-managed finalization has improved to the point where this
problem is worth revisiting, and I'll take another look soon.

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[Chris Pettitt]

That sounds great! I would love to take advantage of it when it becomes available.

Thanks,

Chris

On Fri, Sep 6, 2019 at 7:09 AM Matthew Flatt <mfl...@cs.utah.edu> wrote:

There's still no mechanism to unload a DLL.

For a long time, it wasn't practical to try to unload a DLL, because
the finalization mechanisms were too asynchronous --- making it
difficult to detect the point where the DLL would no longer be
referenced and could be unloaded safely. But support for
custodian-managed finalization has improved to the point where this
problem is worth revisiting, and I'll take another look soon.

At Thu, 5 Sep 2019 19:59:19 -0700 (PDT), Chris P wrote:

[Matthew Flatt]

The next snapshot build (available in 18 hours or so at
pre.racket-lang.org) includes a `#:custodian` optional argument to
`ffi-lib`. Probably you want `#:custodian (current-custodian)`, which
will unload the foreign library for a program run in DrRacket in
reaction to "Kill" or "Run".

Beware that `#:custodian (current-custodian)` will not interact nicely
with GC-based finalization via `register-finalizer` or
`ffi/unsafe/alloc`. If you have values to finalize,
`register-finalizer-and-custodian-shutdown` could be the way to go, but
beware that `register-finalizer-and-custodian-shutdown` provides no
ordering guarantee (unlike the `ffi/unsafe/alloc` interface). In other
words, there's still no simple answer to all problems, just a new
answer for some simple problems.

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