stpd and stdout

[Martin Hunt]

On Tue, 2006-05-09 at 15:21 -0700, Hien Nguyen wrote:
> Hi Martin,
> 
> While working on the the systemtap GUI we run into the problem of 
> getting the stdout to the gui app in a timely fashion. We does get the 
> output as a blob of text once in a while (looks like data were only 
> flushed out when the buffer is full). To recreate the problem you can 
> try this
> 
> stap -e 'probe syscall.open{ log(filename) }' | tee
> 
> Anyway, I added one line of code to flushout the data as soon as the 
> stpd gets it. It seems to solve the above problem. To avoid unnecessary 
> performance issues for others, perhaps we should have that as an option. 
> What do you think?

I think that in this case (stdout from procfs) it should always be
line-buffered. No option is necessary.  In fact I tried it and couldn't
measure any significant performance advantage for full buffering.  I've
gone ahead and committed the following patch.  Let me know how it works.

Index: librelay.c
===================================================================
RCS file: /cvs/systemtap/src/runtime/stpd/librelay.c,v
retrieving revision 1.37
diff -u -r1.37 librelay.c
--- librelay.c  8 Apr 2006 21:59:36 -0000       1.37
+++ librelay.c  18 May 2006 17:51:31 -0000
@@ -722,6 +722,7 @@
        int type;
        FILE *ofp = stdout;

+       setvbuf(ofp, (char *)NULL, _IOLBF, 0);
        pthread_mutex_init(&processing_mutex, NULL);

        signal(SIGINT, sigproc);