Hello Stefan,

oh wow. What a beginner's mistake. Thank you. Of course I cast the message register wrong (missing a & there). It seems to be working now:

time = *(l4_kernel_clock_t*)&l4_utcb_mr()->mr[0];

Thanks a lot.

Best regards,

Georg

On 23 February 2016 at 09:53, Stefan Kalkowski <stefan.kalkowski@...1...> wrote:

Hello Georg,

On 02/22/2016 06:38 PM, Georg Guba wrote:
> I'm trying to implement Platform_thread::execution_time() for Fiasco.OC.
> However, when using the code below, I'm getting a null pointer error when
> trying to read the UTCB's message register:
>
> Possible null pointer READ at 0 IP 704c0d7c
>
> unsigned long long Platform_thread::execution_time() const
> {
> unsigned long long time = 0;
>
> if (_utcb)
> {
> l4_thread_stats_time(_thread.local.dst());
> time = *(l4_kernel_clock_t*)l4_utcb_mr()->mr[0];
> }
>
> return time;
> }
>
> The doc for l4_thread_stats_time(...) says
>
> /**
> * \brief Get consumed timed of thread in Âµs.
> * \ingroup l4_thread_api
> * \param thread Thread to get the consumed time from.
> *
> * The consumed time is returned as l4_kernel_clock_t at UTCB message
> * register 0.
> */
> L4_INLINE l4_msgtag_t
> l4_thread_stats_time(l4_cap_idx_t thread) L4_NOTHROW;
>
> So my usage should be about right.
> What am I missing?

although, this is a bit off-topic and I did not used the mentioned
kernel feature till now (you might refer to the l4-hackers mailing-list
when having further questions regarding Fiasco.OC and L4Re), I'll try to
help you.

It seems you expected a pointer in the UTCB to the time structure, but
it is surely a value (the kernel wouldn't know where to deposit the
value aside of the UTCB, it doesn't do memory allocations on behalf of
user tasks).

When searching for the right usage of "l4_thread_stats_time", I came
across this post on the l4-hackers mailinglist:
http://os.inf.tu-dresden.de/pipermail/l4-hackers/2012/005419.html

I hope it helps.

Regards
Stefan

>
>
>
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--
Stefan Kalkowski
Genode Labs

http://www.genode-labs.com/ Â· http://genode.org/

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