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

Sol Swords sswords at centtech.com
Wed Mar 23 09:17:54 PDT 2011


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)
           (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);
   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?

  - Sol

More information about the Openmcl-devel mailing list