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Unbound variable across threads in C++ application

From: Lluís Vilanova
Subject: Unbound variable across threads in C++ application
Date: Mon, 03 Oct 2016 20:53:01 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

Hi! I'm not sure if this is the proper place to ask about this, but I saw no
other mailing list.

I have a C++ program that I want to extend with guile. It have two threads, the
first one defines a variable, while the second one prints its
value. Nonetheless, the second thread barfs saying that the variable is not

I've ensured the ordering is correct, and reading through the reference manual
seems to point to this being correct (pseudo-pseudo-code):

  int main() {
      scm_c_eval("(define variable 1)");
      scm_c_eval("(format #t \">> 1 >> ~s\" variable)");

      std::thread t1([]() {
          scm_c_eval("(format #t \">> 2 >> ~s\" variable)");

I've actually wrapped all my calls to guile to print helpful exception
backtraces. Here's what I get when accessing variable on the second thread (the
variable is named "qi-args"):

  Guile backtrace:
  In unknown file:
     ?: 6 [call-with-input-string "(format #t \">> 2 >> ~s\\n\" qi-args)\n" ...]
  In ice-9/boot-9.scm:
  2401: 5 [save-module-excursion #<procedure 55a3af3d5db0 at 
ice-9/eval-string.scm:65:9 ()>]
  In ice-9/eval-string.scm:
    44: 4 [read-and-eval #<input: string 55a3af464ea0> #:lang ...]
    37: 3 [lp (format #t ">> 2 >> ~s\n" qi-args)]
  In ice-9/eval.scm:
   387: 2 [eval # ()]
   393: 1 [eval #<memoized qi-args> ()]
  In unknown file:
     ?: 0 [memoize-variable-access! #<memoized qi-args> #<directory # 
  qdbi-guile: unbound-variable: (#f "Unbound variable: ~S" (qi-args) #f)

Any ideas what might be going wrong?


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