[SOLVED] WITH-OUTPUT-TO-STRING with multithreading in Common Lisp

WITH-OUTPUT-TO-STRING with multithreading in Common Lisp

I want to do something that means the following:

(with-output-to-string (*standard-output*)
  (bt:join-thread
   (bt:make-thread
    (lambda ()
      (format *standard-output* "Hello World")))))
;=> "" (actual output)
;=> "Hello World" (expected output)

In my understanding, this does not work because the *standard-output* that gets dynamically rebound by with-output-to-string outside the thread does not take effect inside the thread. What are the possible and recommedable ways?

In essence, I want to capture the output that was written to *standard-output* by another thread.

Solution

One can rebind the special variable to be thread local:

(with-output-to-string (*standard-output*)
  (bt:join-thread
    (bt:make-thread
      (lambda ()
        (format *standard-output* "Hello World"))
      :initial-bindings `((*standard-output* . ,*standard-output*)))))

*initial-bindings* is an alist with (variable . value) elements.