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

[Gaulthier Gain <Gaulthier.Gain@xxxxxxxxx>]

> On 29 Jan 2020, at 11:34, Simon Kuenzer <simon.kuenzer@xxxxxxxxx> wrote:
> 
> 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?

I’ll investigate 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.

In that case, effectively it makes sense.

> 
>> 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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