[racket] Continuation in embedding Racket

From: Matthew Flatt (mflatt at cs.utah.edu)
Date: Fri Dec 5 07:27:07 EST 2014

At Thu, 04 Dec 2014 00:23:37 +0300, Nesterov Kirill wrote:
> >  (call-with-continuation-prompt (lambda () (generate-digit)))
> With this I'm getting segfault.

Sorry for the delay! Looking back at your original explanation:

> > At Wed, 03 Dec 2014 23:45:28 +0300, Nesterov Kirill wrote:
> >>  I'm trying to embed racket 3m version as interpreter into GUI
> >> application. For  custom interpreters this application have two
> >> required APIs - first one is init  and the second one is eval
> >> string. So in init I'm calling  scheme_register_tls_space and
> >> scheme_main_setup after. And in eval I'm using  custom version of
> >> Racket's read-eval-print-loop without recursion for  evaluation of
> >> thrings.

I think you're saying that Racket_cli_execute_line() is not called
within the function passed to scheme_main_setup(), but is instead
called after scheme_main_setup() returns.

If so, that would explain the crash when trying to use continuations.
The Racket run-time system expects to be within the call to
scheme_main_setup() always, so that it can manipulate the C stack to
implement continuations, threads, etc.

Having to put all calls to Racket inside scheme_main_setup() can be
awkward for an embedding application. When the embedding application
can't be organized so that all calls are within scheme_main_setup(), a
possible approach is to put Racket into its own OS-level thread. The
scheme_main_setup() function would be called in that thread, and the
function passed to scheme_main_setup() would wait for evaluation
requests from other threads. Then, Racket_cli_execute_line() would send
a request to that thread and wait for the response.

Of course, putting Racket into its own OS thread can complicate
interactions between Racket and the embedding application, especially
if Racket calls back to the embedding environment. Unfortunately, we
don't have a better answer at the moment.

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