[lttng-dev] Reading CTF trace using Babeltrace API

[Salman Rafiq]

Hi Jérémie,

Thanks for your prompt reply.


Just to add a little to your answer:

In order to get the event "id" field, we need to call another function as well. 
Since the "id" field is declared as an enumerated data type (uint16_t for large header and uint5_t for compact). 
For example, to read the event id, it works only by calling bt_ctf_get_enum_int(id_field), otherwise it returns 0.

Slight modification to the code sample did the job:

while ((ctf_event = bt_ctf_iter_read_event(iter))) {

        const struct bt_definition *stream_event_header_scope =
                bt_ctf_get_top_level_scope(
                        ctf_event, BT_STREAM_EVENT_HEADER);
        const struct bt_definition *id_field =
                bt_ctf_get_field(
                        ctf_event, stream_event_header_scope, "id");

        printf("id = %" PRIu64 "\n", bt_ctf_get_uint64(bt_ctf_get_enum_int(id_field)));  	// change here!!!
		


But I am getting strange behavior only for the "sched_switch" event. 
Each time I read it, it returns 31 in case the header is compact (31 events max) and 65535 when the header is large instead of the correct value.

For all other "id's" of the event, it returns correct value :-s

I am unable to figure out the reason. Please let me know, if you have any information in this regard. 
Thanks a lot!

Regards,
Salman

--
Salman Rafiq
Geschäftsfeld Industrial Communication
Fraunhofer-Institut  für Eingebettete Systeme und Kommunikationstechnik ESK

Hansastraße 32 | 80686 München
Telefon, Fax:  +49 89 547088-356 | +49 89 547088-221
E-Mail:   salman.rafiq at esk.fraunhofer.de
  
Internet: 
http://www.esk.fraunhofer.de
http://www.twitter.com/FraunhoferESK


-----Original Message-----
From: jeremie.galarneau at gmail.com [mailto:jeremie.galarneau at gmail.com] On Behalf Of Jérémie Galarneau
Sent: Mittwoch, 2. Oktober 2013 20:25
To: Salman Rafiq
Cc: jdesfossez at efficios.com; lttng-dev at lists.lttng.org; Mathieu Desnoyers
Subject: Re: Reading CTF trace using Babeltrace API

On Wed, Oct 2, 2013 at 12:55 PM, Salman Rafiq <salman.rafiq at esk.fraunhofer.de> wrote:
> Dear LTTngers,
>
> I would like to ask few questions related to the API's for reading CTF traces.
>
> 1.  Is there any way to read CTF metadata independently, like we have certain API's to read the events.
>

Not from the public API, unfortunately.

> 2. There are certain scopes defined in ctf/events.h like:
>         BT_TRACE_PACKET_HEADER          = 0,
>         BT_STREAM_PACKET_CONTEXT        = 1,
>         BT_STREAM_EVENT_HEADER          = 2,
>         BT_STREAM_EVENT_CONTEXT         = 3,
>         BT_EVENT_CONTEXT                = 4,
>         BT_EVENT_FIELDS                 = 5,
>
> I could not figure out the way to ready the fields 'id' and 'stream_id', for a particular event while iterating through the trace.
> Though I can ready the event name using an API and fields using BT_EVENT_FIELDS as a scope.
> For e.g. consider the
>
> event {
>         name = sched_migrate_task;
>         id = 1;
>         stream_id = 0;
>         fields := struct {
>                 integer { size = 8; align = 8; signed = 1; encoding = UTF8; base = 10; } _comm[16];
>                 integer { size = 32; align = 8; signed = 1; encoding = none; base = 10; } _tid;
>                 integer { size = 32; align = 8; signed = 1; encoding = none; base = 10; } _prio;
>                 integer { size = 32; align = 8; signed = 1; encoding = none; base = 10; } _orig_cpu;
>                 integer { size = 32; align = 8; signed = 1; encoding = none; base = 10; } _dest_cpu;
>         };
> };
>

The "id" field is part of the BT_STREAM_EVENT_HEADER scope, while the "stream_id" field is part of the BT_TRACE_PACKET_HEADER scope.

The code below is adapted from converter/babeltrace.c's
convert_trace() loop to print both values.

while ((ctf_event = bt_ctf_iter_read_event(iter))) {

        const struct bt_definition *stream_event_header_scope =
                bt_ctf_get_top_level_scope(
                        ctf_event, BT_STREAM_EVENT_HEADER);
        const struct bt_definition *id_field =
                bt_ctf_get_field(
                        ctf_event, stream_event_header_scope, "id");

        printf("id = %" PRIu64 "\n", bt_ctf_get_uint64(id_field));

        const struct bt_definition *packet_header_scope =
                bt_ctf_get_top_level_scope(
                        ctf_event, BT_TRACE_PACKET_HEADER);
        const struct bt_definition *stream_id_field =
                bt_ctf_get_field(
                        ctf_event, packet_header_scope, "stream_id");

        printf("stream_id = %" PRIu64 "\n", bt_ctf_get_uint64(
                       stream_id_field)); [...] }

Regards,
Jérémie

>
> Any help or suggestion related to this issue will be really appreciated.
> Thanks!
>
> Regards,
> Salman
>
> --
> Salman Rafiq
> Geschäftsfeld Industrial Communication Fraunhofer-Institut  für 
> Eingebettete Systeme und Kommunikationstechnik ESK
>
> Hansastraße 32 | 80686 München
> Telefon, Fax:  +49 89 547088-356 | +49 89 547088-221
> E-Mail:   salman.rafiq at esk.fraunhofer.de
>
> Internet:
> http://www.esk.fraunhofer.de
> http://www.twitter.com/FraunhoferESK
>



--
Jérémie Galarneau
EfficiOS Inc.
http://www.efficios.com