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`.
Cheers
--
Milian Wolff | milian.wo...@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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