[lttng-dev] Reading CTF trace using Babeltrace API

Salman Rafiq salman.rafiq at esk.fraunhofer.de
Wed Oct 23 10:50:03 EDT 2013


Hello again :)

While reading Kernel trace events (system calls), I came across an issue related to reading the field 'args' inside 'sys_unknown' system call event.

Below is the information I get using public babeltrace API's for the event field type.
Ctf_type_id = 8  (CTF_TYPE_ARRAY)


But when I try to read the value for the field using the following API
char * bt_ctf_get_char_array(const struct bt_definition *field);

I receive null value. Also it gives me warning that I am instead extracting a String.
[warning] Extracting String

Also, I am unable to read the value using the following API.
char *bt_ctf_get_string(const struct bt_definition *field); 


below is the sample event recorded:
[00002934900435406801] (+0.000009630) sys_unknown: { cpu_id = 3 }, { id = 277, args = [ [0] = 22, [1] = 0, [2] = 4096, [3] = 2, [4] = 4096, [5] = 5 ] }

AND the event definition:
event {
	name = sys_unknown;
	id = 2;
	stream_id = 0;
	fields := struct {
		integer { size = 32; align = 8; signed = 0; encoding = none; base = 10; } _id
		integer { size = 64; align = 8; signed = 0; encoding = none; base = 10; } _args[6];
	};
};


Any suggestion related to that will be very helpful.
Thanks!

Best 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, 9. Oktober 2013 01:50
To: Mathieu Desnoyers
Cc: Salman Rafiq; Julien Desfossez; lttng-dev at lists.lttng.org
Subject: Re: Reading CTF trace using Babeltrace API

On Tue, Oct 8, 2013 at 7:14 PM, Mathieu Desnoyers <mathieu.desnoyers at efficios.com> wrote:
> ----- Original Message -----
>> From: "Jérémie Galarneau" <jeremie.galarneau at efficios.com>
>> To: "Salman Rafiq" <salman.rafiq at esk.fraunhofer.de>
>> Cc: jdesfossez at efficios.com, lttng-dev at lists.lttng.org, "Mathieu 
>> Desnoyers" <mathieu.desnoyers at efficios.com>
>> Sent: Tuesday, October 8, 2013 2:52:38 PM
>> Subject: Re: Reading CTF trace using Babeltrace API
> [...]
>>
>> The problem is that the "id" fields in both struct 
>> event_header_compact and event_header_large acts as a variant's tag 
>> enumeration.
>>
>> For instance, if we take a look at the event_header_compact 
>> structure,
>>
>> struct event_header_compact {
>>         enum : uint5_t { compact = 0 ... 30, extended = 31 } id;
>>         variant <id> {
>>                 struct {
>>                         uint27_clock_monotonic_t timestamp;
>>                 } compact;
>>                 struct {
>>                         uint32_t id;
>>                         uint64_clock_monotonic_t timestamp;
>>                 } extended;
>>         } v;
>> } align(8);
>>
>> The "enum id" field will indicate the event's id if it has a value of 
>> [0, 30]. It also indicates that the "compact" structure is selected 
>> in the variant that follows. If id == 31, the "extended" structure is 
>> selected, which means that you should retrieve the "id" field's value 
>> from that structure.
>>
>> Since code is worth a thousand words, here's a more complete code 
>> sample to read the id fields.
>>
>> while ((ctf_event = bt_ctf_iter_read_event(iter))) {
>>         uint64_t event_id = 0;
>>         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_definition =
>>                 bt_ctf_get_field(ctf_event, stream_event_header_scope,
>>                                  "id");
>>         const struct bt_definition *v_definition =
>>                 bt_ctf_get_field(ctf_event, stream_event_header_scope,
>>                                  "v");
>>
>>         /* "id may be an enum, get the underlying integer if it's the case */
>>         if (id_definition &&
>> bt_ctf_field_type(bt_ctf_get_decl_from_def(id_definition)) ==
>> CTF_TYPE_ENUM) {
>>                 id_definition = bt_ctf_get_enum_int(id_definition);
>>         }
>>
>>         /* Check if there is a "v" field and if it contains an "id" field */
>>         if (v_definition &&
>> bt_ctf_field_type(bt_ctf_get_decl_from_def(v_definition)) ==
>> CTF_TYPE_STRUCT) {
>>                 struct bt_definition const * const *list = NULL;
>>                 unsigned int count = 0;
>>                 unsigned int i;
>>
>>                 ret = bt_ctf_get_field_list(ctf_event, v_definition, 
>> &list, &count);
>>                 if (ret) {
>>                         goto end;
>>                 }
>>
>>                 for (i = 0; i < count; i++) {
>>                         const char *name = 
>> bt_ctf_field_name(list[i]);
>>
>>                         if (strcmp("id", name) == 0) {
>>                                 id_definition = list[i];
>>                                 break;
>>                         }
>>                 }
>
> Instead of doing this bt_ctf_get_field_list() and iteration, can't we simply use bt_ctf_get_field() to try getting the struct bt_definition ?

Right. I was under the impression the bt_ctf_get_field() would only work on scopes, but it's not the case.

Thanks!
Jérémie

>
> Thanks,
>
> Mathieu
>
>>         }
>>
>>         if (id_definition &&
>> bt_ctf_field_type(bt_ctf_get_decl_from_def(id_definition)) ==
>> CTF_TYPE_INTEGER) {
>>                 event_id = bt_ctf_get_uint64(id_definition);
>>         }
>>
>>         printf("id = %" PRIu64 "\n", event_id); }
>>
>> Regards,
>> Jérémie
>>
>> > 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
>>
>>
>>
>> --
>> Jérémie Galarneau
>> EfficiOS Inc.
>> http://www.efficios.com
>>
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com



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



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