[lttng-dev] Limit on the number of arguments in TP_ARGS

[Kienan Stewart]

Hi Shahar,

On 8/7/24 10:03 AM, Shahar Hochma via lttng-dev wrote:
> Hi,
> I've been using LTTNG traces and I was trying to create events with more 
> than 10 arguments. But, I'm having an issue where this seems to fail in 
> compilation with a cryptic error when I define more than 10 arguments in 
> TP_ARGS. >
> I assume the issue is that there are some macros that are only defined 
> up to 10 arguments. I didn't see any documentation of this limit or any 
> mention of this anywhere, and I wanted to know if there is maybe a way 
> to get around this.

That's correct the macros are only defined for up to 10 arguments. This 
was initially due to a desire to maintain compatibility with SDT, which 
also had a 10 argument limit. That limit is now slightly higher (12) in 
recent versions of SDT.

The limit is documented here[1]. There's also previous mailing list 
threads[2], and here is the relevant portion[3] of the source code.

> 
> Of course I can combine arguments to struts or define 2 events instead 
> of one. But I wanted to know if there is a native way to resolve this.
> 

The canonical way to do this is to use a structure. Using two events 
would also work, but I think it's slightly more desirable to use fewer 
events (especially if you want to keep the data together).

> Thanks.

thanks,
kienan

[1]: https://lttng.org/man/3/lttng-ust/v2.13/#doc-tracepoint-event
[2]: https://lists.lttng.org/pipermail/lttng-dev/2023-November/030673.html
[3]: 
https://github.com/lttng/lttng-ust/blob/5d10459b6b5182fcbc98240d9dace026c87a5037/include/lttng/tracepoint.h#L76

> 
> 
> 
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