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#lang racket
(provide displayln-safe)
;; Several threads may want to print to stdout, so lets make things civil
; constant always available
(define stdout (make-semaphore 1))
; prints to stdout with an optional output port
; requires a specified semaphore for the optional output port
(define displayln-safe
(lambda (a-string [a-semaphore stdout] [a-output-port (current-output-port)])
(cond [(not (and (eq? a-semaphore stdout) (eq? a-output-port (current-output-port))))
(semaphore-wait a-semaphore)
(semaphore-wait stdout)
(displayln a-string a-output-port)
(flush-output a-output-port)
(displayln a-string)
(semaphore-post stdout)
(semaphore-post a-semaphore)]
[else
(semaphore-wait stdout)
(displayln a-string)
(semaphore-post stdout)])))
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