Re: [Minios-devel] [UNIKRAFT PATCH] lib/ukboot: fflush() after banner is printed

[Simon Kuenzer <simon.kuenzer@xxxxxxxxx>]

On 29.01.20 10:46, Gaulthier Gain wrote:
> Hi Simon,
>
> I tested with musl. Now, we can see the "Unikraft banner" however application 
> output is not displayed.
>
> I think that we need to fix this issue in the musl implementation.
I tested that doing fflush to stdout and stderr in the application 
works, too. The issue with putting an fflush after calling main() is 
that we can't know what it does with the file descriptors. Probably, the 
fix is to do an fflush() in helloworld app or to get musl somehow flush 
the buffers on Unikernel exit. It anyways surprises me that the fflush 
does not happen automatically on '\n' with this musl port. Is msul 
behaving the same when on Linux?
However, independent of all this, I still think this patch alone makes 
sense since it is ensuring the order when the banner is printed during 
boot. Since uk_pr_* variants suppose to bypass the normal libc printing 
system, we may have different behavior between printf and uk_pr_* that 
could cause interleaved or out-of-order output.
> Gaulthier
>
> > On 27 Jan 2020, at 10:16, Simon Kuenzer <simon.kuenzer@xxxxxxxxx> wrote:
> >
> > For the case that a libc is adding buffering `stdout`, we are flushing
> > `stdout` after printing the "Welcome to Unirkaft" banner. This is to
> > make sure that the output stays in order and does not get mixed with
> > the kernel and debug output.
> >
> > Signed-off-by: Simon Kuenzer <simon.kuenzer@xxxxxxxxx>
> > ---
> > lib/ukboot/boot.c | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > diff --git a/lib/ukboot/boot.c b/lib/ukboot/boot.c
> > index 3f5046ca..9bd15ca8 100644
> > --- a/lib/ukboot/boot.c
> > +++ b/lib/ukboot/boot.c
> > @@ -100,6 +100,7 @@ static void main_thread_func(void *arg)
> >         printf("\\_,_/_//_/_/_/\\_\\/_/  \\_,_/_/ \\__/\n");
> >         printf("%35s\n",
> >                STRINGIFY(UK_CODENAME) " " STRINGIFY(UK_FULLVERSION));
> > +       fflush(stdout);
> > #endif
> >         /*
> >          * Application
> > --
> > 2.20.1
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