[lttng-dev] decoding tracef msg string via babeltrace API
Milian Wolff
milian.wolff at kdab.com
Tue Apr 2 15:54:09 EDT 2019
On Dienstag, 2. April 2019 21:40:02 CEST Milian Wolff wrote:
> On Dienstag, 2. April 2019 21:03:41 CEST Milian Wolff wrote:
> > Hey all
> >
> > I added a tracef tracepoint and want to decode it's `msg` field in the
> > BT_EVENT_FIELDS scope. Babeltrace on the command line seems to handle this
> > nicely, but whatever I try, it doesn't work with the API.
> >
> > I'm using babeltrace from the stable-1.5 branch.
> >
> > First, the output from babeltrace on the command line:
> >
> > ```
> > event {
> >
> > name = "lttng_ust_tracef:event";
> > id = 9;
> > stream_id = 0;
> > loglevel = 14;
> > fields := struct {
> >
> > integer { size = 32; align = 8; signed = 0; encoding =
> > none;
> >
> > base = 10; } __msg_length;
> >
> > integer { size = 8; align = 8; signed = 1; encoding =
> > UTF8;
> >
> > base = 10; } _msg[ __msg_length ];
> >
> > };
> >
> > };
> > ...
> > timestamp = 15:56:15.404973865, delta = +0.000023594, trace = 1000/64-bit,
> > trace:hostname = agathebauer, trace:domain = ust, loglevel = TRACE_DEBUG
> > (14), name = lttng_ust_tracef:event, stream.packet.context = {
> > timestamp_begin = 4382372875047, timestamp_end = 4462741641629,
> > content_size = 47177888, packet_size = 47185920, packet_seq_num = 0,
> > events_discarded = 0, cpu_id = 3 }, stream.event.header = { id = (
> > "compact" : container = 9 ), v = { compact = { timestamp = 70385656 } } },
> > event.fields = { _msg_length = 33, msg = "run_event_entry: ImageCache,
> > 1000" }
> > ...
> > ```
> >
> > Now, in my code I find the event and then I've tried:
> >
> > ```
> >
> > auto definition = bt_ctf_get_field(event, scope, "msg");
> > auto declaration = bt_ctf_get_decl_from_def(definition);
> > auto type = bt_ctf_field_type(declaration);
> > auto encoding = bt_ctf_get_encoding(declaration);
> > auto string = bt_ctf_get_string(definition);
> > auto char_array = bt_ctf_get_char_array(definition);
> > const bt_definition* const* list = nullptr;
> > unsigned num_list = 0;
> > auto field_list_ret = bt_ctf_get_field_list(ctf_event,
> >
> > definition, &list, &num_list);
> >
> > fprintf(stderr, "tracef dbg: %p %p | %d %d | %s | %s | %p %u
> >
> > %d\n", definition, declaration, type, encoding, string, char_array, list,
> > num_list, field_list_ret);
> > ```
> >
> > the output is:
> >
> > ```
> > tracef dbg: 0x55e4c5b084f0 0x55e4c5b3f220 | 9 1 | (null) | (null) | (nil)
> > 0
> > -1
> >
> > ```
> >
> > So, it's a sequence (type 9), but I can't get the sequence... Actually,
> > when I tried this the firs ttime without patching babeltrace, then it
> > crashed in events.c:256 [1]. The def_sequence is non-null, but
> > def_sequence->elems is null and that's not checked...
> >
> > [1]: https://github.com/efficios/babeltrace/blob/stable-1.5/formats/ctf/
> > events.c#L256
> >
> > I'm quite stumped - what am I doing wrong? How does babeltrace handle the
> > string-decoding of tracef's msg arg?
>
> I've found out that babeltrace seems to not consume the public API. Instead,
> in `sequence.c` it's directly accessing the sequence's string member. If I
> patch `bt_ctf_get_string` like this:
>
> ```
> diff --git a/formats/ctf/events.c b/formats/ctf/events.c
> index bd195b93..f0ea49c8 100644
> --- a/formats/ctf/events.c
> +++ b/formats/ctf/events.c
> @@ -599,12 +599,26 @@ end:
> char *bt_ctf_get_string(const struct bt_definition *field)
> {
> char *ret = NULL;
> + const struct bt_declaration *decl = field ?
> bt_ctf_get_decl_from_def(field) : NULL;
> + enum ctf_type_id type = decl ? bt_ctf_field_type(decl) :
> CTF_TYPE_UNKNOWN;
>
> - if (field && bt_ctf_field_type(bt_ctf_get_decl_from_def(field)) ==
> CTF_TYPE_STRING)
> + if (type == CTF_TYPE_SEQUENCE) {
> + enum ctf_string_encoding encoding = bt_ctf_get_encoding(decl);
> + struct definition_sequence *sequence_definition;
> + if (encoding == CTF_STRING_UTF8 || encoding ==
> CTF_STRING_ASCII) {
> + sequence_definition = container_of(field, struct
> definition_sequence, p);
> + ret = sequence_definition->string->str;
> + goto end;
> + }
> + } else if (type == CTF_TYPE_STRING) {
> ret = bt_get_string(field);
> - else
> - bt_ctf_field_set_error(-EINVAL);
> + goto end;
> + }
>
> +error:
> + bt_ctf_field_set_error(-EINVAL);
> +
> +end:
> return ret;
> }
> ```
>
> it seems to be doing what I was hoping for. Would something like that be
> accepted upstream, or do we need a third string-returning function
> `bt_ctf_get_string_sequence` or similar?
>
> Generally, I already dislike it very much that there is a distinction
> between `bt_ctf_get_string` and `bt_ctf_get_char_array` - both should - imo
> - be handled through `bt_ctf_get_string`.
And now I found out that this is known and a patch is even available:
https://github.com/efficios/babeltrace/pull/98/commits/
3ab89a428ce2a8a4f76218f79c9eb6c26ae5aeee
It's been nearly 1.5 years and babeltrace 2.0 is still not ready. Can you
please merge this into babeltrace 1.5 and release a new version of that stable
branch?
Thanks
--
Milian Wolff | milian.wolff at kdab.com | Senior Software Engineer
KDAB (Deutschland) GmbH, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt, C++ and OpenGL Experts
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