gistfile1.txt

shiro@scherzo:~/src$ gosh ./tt.scm a b c gosh ./t.scm
shiro@scherzo:~/src$ cat a
out
out
out
out
out
out
out
out
out
out
shiro@scherzo:~/src$ cat b
err
err
err
err
err
err
err
err
err
err
shiro@scherzo:~/src$ cat c
oerurt

out
err
ouet
rr
oute
rr
oeut
rr
oeut
rr
oerurt

oute
rr
oeut
rr
oute
rr
shiro@scherzo:~/src$ 

tt.scm

(use gauche.process)
(use gauche.threads)
(use util.match)

(define (tee in out1 out2)
  (generator-for-each (^c (write-char c out1)
                          (write-char c out2))
                      (cut read-char in)))

(define (main args)
  (match-let1 (file-a file-b file-c . command) (cdr args)
    (let ([a (open-output-file file-a)]
          [b (open-output-file file-b)]
          [c (open-output-file file-c)]
          [p (run-process command :output :pipe :error :pipe :wait #f)])
      (unwind-protect
          (let ([cout (process-output p)]
                [cerr (process-error p)])
            (set! (port-buffering cout) :none)
            (set! (port-buffering cerr) :none)
            (let ([t0 (thread-start! (make-thread (^[] (tee cout a c))))]
                  [t1 (thread-start! (make-thread (^[] (tee cerr b c))))])
              (thread-join! t0)
              (thread-join! t1)))
        (close-output-port a)
        (close-output-port b)
        (close-output-port c)
        (process-kill p)))))