[lttng-dev] how to generate CTF metadata

[Philippe Proulx]

On Sun, Mar 15, 2015 at 6:14 PM, Dominik Taborsky <bremby at seznam.cz> wrote:
>
>
> On 03/15/2015 10:31 PM, Philippe Proulx wrote:
>>
>> On Sun, Mar 15, 2015 at 5:25 PM, Dominik Taborsky <bremby at seznam.cz>
>> wrote:
>>>
>>> Hello,
>>>
>>> I am working on a project that involves generating CTF metadata, but I am
>>> unsure about how to proceed. I am not sure about how to get information
>>> about types,
>>
>>
>> What "types" are you talking about exactly? What are you intending to
>> write to binary streams?
>
>
> Oh, sorry for not specifying exactly: I mean any defined, non-standard
> types. For example, when you do:
>
> typedef uint32_t key_t;
> typedef struct {
>         size_t len;
>         char *data;
> } wire_data_t;
>
> You use these types by their typedef'd name. Suppose then, you want to trace
> a function like:
>
> int do_work(key_t k, wire_data_t data);
>
> Then how do you know what those types are without looking at the typedefs?
> The runtime tracer does not have access to those header files. And even if
> it did, there may be tons of layers of typedefs. You've got to do this at
> runtime, which obviously leaves me with DWARF. Otherwise I'd have to specify
> the type info myself before or after the trace.
>
> So my main question is: how does the LTTng linux kernel module understand
> all the types so it may describe them in the CTF metadata?

LTTng-modules knows each field of event payload structures. They are described,
with a name and a basic C type, in instrumentation headers (see [1]). So
LTTng-modules does not have this unknown typedef constraint. The actual TSDL
metadata is produced in lttng-events.c; see [2] for an example of how a basic
C type is "converted" to a TSDL integer: __alignof__(x) is used for alignment,
(sizeof(x) * CHAR_BIT) for the size in bits, and this clever macro:

    #define lttng_is_signed_type(type) (((type)(-1)) < 0)

to know if the type is signed or not.

[1] http://git.lttng.org/?p=lttng-modules.git;a=blob;f=instrumentation/events/lttng-module/sched.h;h=1bea326135a428e0db10c42388a9683d58edd745;hb=HEAD#l231
[2] http://git.lttng.org/?p=lttng-modules.git;a=blob;f=lttng-events.h;h=118ea3b0db52d82e6072e17aa901c1850e22e43c;hb=HEAD#l74

Phil
>
>
> Dominik
>
>
>>
>> Phil
>>
>>> i.e. their size, signedness, alignment and possibly other. I
>>> know this information can be gathered from DWARF. Is that the primary way
>>> to
>>> do this? Is there any other, reasonable way? Can you point me at some
>>> code
>>> that does this? How does the LTTng linux kernel module work?
>>>
>>> Thanks.
>>>
>>> Dominik
>>>
>>> _______________________________________________
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>>> lttng-dev at lists.lttng.org
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