[lttng-dev] Printing bit fields with LTTng

[Chidhu R]

Great. Thanks for sharing your inputs. I too learned something new.

On Wed, Jun 10, 2015 at 3:20 PM, Simon Marchi <simon.marchi at polymtl.ca>
wrote:

> On 10 June 2015 at 17:28, Simon Marchi <simon.marchi at polymtl.ca> wrote:
> > On 10 June 2015 at 17:16, Chidhu R <chid1989 at gmail.com> wrote:
> >> Hello,
> >>
> >> I am getting compilation issues while trying to print bit field values
> with
> >> LTTng.
> >>
> >> Error:
> >>
> >> hello.c: In function ‘main’:
> >> hello.c:36:1: error: ‘typeof’ applied to a bit-field
> >> hello.c:36:1: error: ‘typeof’ applied to a bit-field
> >> hello.c:36:1: error: ‘typeof’ applied to a bit-field
> >> hello.c:36:1: error: ‘sizeof’ applied to a bit-field
> >>
> >> Snippet:
> >>
> >> struct abc{
> >>         int a:16;
> >>         int b:8;
> >> };
> >>
> >> Line 36: tracepoint(hello_world, my_third_tracepoint, aa.a, "welcome");
> >> Line 37: printf("val = %x\n",aa.a);
> >>
> >> tracepoint line results in compilation error.
> >> printf succeeds.
> >>
> >> Tp definition looks like this.
> >>
> >> TRACEPOINT_EVENT(
> >>     hello_world,
> >>     my_third_tracepoint,
> >>     TP_ARGS(
> >>         uint32_t, arg,
> >>         const char * , my_string_arg
> >>     ),
> >>     TP_FIELDS(
> >>         ctf_integer_hex(uint32_t, my_arg, arg)
> >>         ctf_string(field, my_string_arg)
> >>     )
> >> )
> >>
> >> How to print bit field values?
> >>
> >> Thanks
> >> Chid
> >
> > I have no idea what the right fix would be, but an easy workaround
> > could be to add 0 to your value:
> >
> >     tracepoint(hello_world, my_third_tracepoint, aa.a + 0, "welcome");
> >
> > I just tried it and it works.
> >
> > Simon
>
> Alright, after a bit more investigation (thanks to Philippe Proulx),
> we found that this is caused by the SystemTap integration. If the
> SystemTap integration is enabled, each LTTng tracepoint also defines a
> SystemTap tracepoint.
>
> SystemTap uses sizeof() on the tracepoint's input arguments to find
> their sizes and typeof() to find their signedness. It encodes this
> info into a special section that then allows tools like gdb to re-use
> the tracepoints and get the argument values. For more information,
> look at the definitions of _SDT_ARGSIZE and _SDT_ARGSIGNED in
> /usr/include/sys/sdt.h.
>
> Pragmatically, I think that the "+ 0" trick is good enough, since it
> pleases SystemTap and is otherwise harmless. You could get the same
> result by casting your value to an integer type. However, you would
> need to carefully choose the size and signedness of the type you cast
> to, or it might alter your value. With + 0, it should work with any
> type.
>
> Thanks for raising this, I learned something new today.
>
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