[lttng-dev] [Patch LTTng-ust 6/7] Add the macros to generate the data structures for CTF named enumerations

[Stefan Seefeld]

On 02/03/2014 10:28 AM, Mathieu Desnoyers wrote:

> Hi Stefan,
> 
> Let's try to make sure I understand your intent here. Let's say we have
> an event with a "weight" field (uint32_t). You would like to have
> a multiplicative factor (e.g. *1000 for "kg") as a second field, that would
> be entirely known from the metadata (pretty much a C literal in the metadata,
> so it does not have to be repeated for each event).

Right. The analogy with a C literal isn't entirely sufficient, as there
is a range of situations where in fact "constant" doesn't map to
compile-time constant in C/C++. Thus...

> There are various ways to do this I imagine: e.g.:
> 
> * Using the typing system


[...]

...the typing system can't quite capture values that are immutable
within a running session, but not quite constant in the sense that they
can be captured in a type system.

So perhaps another way to look at it is like a static class member, for
which storage isn't part of each instance of the class, but rather
somewhere that's associated with all instances of the class...

> But I don't see anything there that requires adding any top-level construct
> to the CTF metadata.

...and that "somewhere" would naturally be the metadata section of the
traces.

> Hence, I am tempted to keep the name "global type declaration"
> for the part of metadata located in the "global" section (opposed to within a
> trace, env, stream, and event scope). Anyway, all we can declare there are types,
> so it makes sense to use this name. If we ever add something to the CTF spec in the
> future that is global but is not a type, then we'll have to extend the interfaces
> and implementations (which should be expected if we extend the CTF spec).

Right. I wasn't arguing so much against the use of "global type
declaration" for the case of named enums, but merely wanted to point out
that the mechanism could be extended to cover non-type-related metadata.

Thanks,
		Stefan


-- 
Stefan Seefeld
CodeSourcery / Mentor Graphics
http://www.mentor.com/embedded-software/