[Openmcl-devel] timely output from an external-process-output-stream

Paul Krueger plkrueger at comcast.net
Wed Mar 23 10:01:44 PDT 2011

Try something like "fflush(stdout);" after your printf. I suspect that it's just being buffered on the output side.

On Mar 23, 2011, at 11:17 AM, Sol Swords wrote:

> Hi,
> I'm running an external program via run-program, and that program semi-regularly prints output that I'd like to see on the screen, but also save to operate on later.  So I'm using :output :stream, reading from the stream using read-line, and both printing and saving the read strings:
> (let* ((extprog (ccl::run-program "./foox" nil
>                                  :wait nil
>                                  :input nil
>                                  :output :stream))
>       (out (ccl::external-process-output-stream prog))
>       line     ;; lines read from program output
>       buf      ;; saved program output
>       (keep-going t))
>  (loop while keep-going do
>        (write-line "Waiting for line") (force-output)
>        (setq line (read-line out nil)) ;; no eof-error-p
>        (if line
>            (progn (push line buf)
>                   (write-line line)
>                   (force-output))
>          (setq keep-going nil))))
> But depending on the program, this actually sometimes ends up just sitting for a while after writing one "Waiting for line", and then dumping all the output when the program exits.  So, for example, if foox is a little c program like this:
> #include <stdio.h>
> int main () {
>  int i;
>  for (i=0; i<5; i++) {
>    printf("line %i\n", i);
>    sleep(1);
>  }
>  return 0;
> }
> Then I get, as expected, the following output:
> Waiting for line
> line 0
> Waiting for line
> line 1
> Waiting for line
> line 2
> Waiting for line
> line 3
> Waiting for line
> line 4
> Waiting for line
> But this output actually occurs all at once after the 5 seconds, instead of getting one new line every second.  On the other hand, if I run "yes" instead, I get immediate output, and if I run foox with :output t, I also get immediate output (but then don't have a mechanism to save the strings.)
> Is there some way to ensure (perhaps by changing the behavior of the external program, but preferably instead changing something in the Lisp) that I get the program's output more frequently?
> Thanks,
> - Sol
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