[racket-dev] Problem with threads in Racket C API?

From: Justin Phillips (jjustinphillipss at gmail.com)
Date: Wed Oct 13 15:41:31 EDT 2010

Bingo thank you so much.

On Wed, Oct 13, 2010 at 5:32 AM, Matthew Flatt <mflatt at cs.utah.edu> wrote:

> At Tue, 12 Oct 2010 21:17:45 -0700, Justin Phillips wrote:
> > I have a function that gets called by OS X's CoreMIDI framework. This
> > function adds some data to a queue. I have registered the queue and the
> > function with scheme_add_evt and scheme_add_evt_through_sema and even
> > explicitly tried to block with scheme_block_until on separate occasions.
> In
> > all instances, when the function gets called from CoreMIDI to add to my
> > queue DrRacket dies because of EXC_BAD_ACCESS -- KERN_PROTECTION_FAILURE
> at
> > 0x00 and the thread that crashed is the highest numbered thread (which is
> > the DrRacket main thread no?) and it is in my function code. Am I setting
> > something up wrong or could there be an issue with CoreMIDI? Any help
> would
> > be greatly appreciated.
> The lowest-numbered thread is normally the DrRacket main thread, so it
> sounds like a thread mismatch is the likely the problem.
> With a thread mismatch in 3m-cooperating code, you would get a crash
> almost immediately, because the thread-local table identifying the GC
> instance won't be there. If your callback is not using any Racket
> functions, then annotate the function with `XFORM_SKIP_PROC', like
> this:
>  void my_callback_in_wrong_thread(int arg)
>  {
>    ...
>  }
> The `XFORM_SKIP_PROC' annotation tells the 3m-cooperation transformer
> to leave the function alone.
> If your callback is using Racket functions, then you have to change it
> so that's not the case. Although it sounds like you're not using
> `ffi/unsafe', you might be able to use it to help bridge threads. If
> you pass a callback to a foreign library where the callback will be
> invoked in a thread other than the main one, the `#:async-apply' option
> of `_fun' avoids the thread mismatch by sending the Racket callback
> back to the main thread for invocation.
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