Problems with arrays of floats (lttng 2.13)

[Herman Bruyninckx]

On Fri, 9 May 2025, Kienan Stewart wrote:

> Hi Herman,
>
> On 5/9/25 6:03 AM, Herman Bruyninckx via lttng-dev wrote:
>> Dear all,
>> 
>> I want to trace an array of floats, but I always get bogus output at the
>> (Python) Babeltrace2 site...
>
> could you give an example of the tracepoint you have setup in your code that 
> doesn't work, with the corresponding output?

Thank you for your swift reaction. Much appreaciated.

First of all, my apologies: I should have provided the required information the first time around already.

---
This is the information about the installed versions of the relevant software:

$ uname -a
Linux SET-L-ME-P22061 6.11.0-24-generic #24-Ubuntu SMP PREEMPT_DYNAMIC Fri Mar 14 18:13:56 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux

$ lttng -V
lttng (LTTng Trace Control) 2.13.13 - Nordicité

$ babeltrace2 -V
Babeltrace 2.0.6 "Amqui"
---


---
My application code generates a trace as follows:

Tracepoint "-tp.h":
LTTNG_UST_TRACEPOINT_EVENT(
     comm_activity,
     range_scan,

     /* Input arguments */
     LTTNG_UST_TP_ARGS(
         int *, distances,
     ),

     /* Output event fields */
     LTTNG_UST_TP_FIELDS(
         lttng_ust_field_array(int, distances,distances,5)
)


int testmeas[5] = {1, 2, 3, 4, 5};
// double testmeas[5] = {1., 2., 3., 4., 5.};
// (one array for test with integers, one with doubles):

lttng_ust_tracepoint(comm_activity,range_scan,5,testmeas, "range_scan");

---

> Could you explain what you mean by (Python) babeltrace2 site?
This is the Python code I use to display the trace generated above:
(inspired by <https://babeltrace.org/docs/v2.0/python/bt2/examples.html>)

---
file tt.py:

import bt2
import sys

# Get the trace path from the first command-line argument.
path = sys.argv[1]

# Create a trace collection message iterator with this path.
msg_it = bt2.TraceCollectionMessageIterator(path)

# Iterate the trace messages.
for msg in msg_it:
     # `bt2._EventMessageConst` is the Python type of an event message.
     if type(msg) is bt2._EventMessageConst:
         # An event message holds a trace event.
         event = msg.event
         # Only check `range_scan` events.
         if event.name != 'comm_activity:range_scan':
             continue
         print(event.payload_field)
---

and these are the resulting traces (for int and double, respectively, and with the C and Python versions of Babeltrace):

$ babeltrace2  ~/git-repositories/communication_5c/build/test_session/
[12:19:11.381152129] (+?.?????????) SET-L-ME-P22061 comm_activity:range_scan: { cpu_id = 6 }, { distances = [ [0] = 1, [1] = 2, [2] = 3, [3] = 4, [4] = 5 ] }

$ python3 tt.py  ~/git-repositories/communication_5c/build/test_session/
{'distances': [1, 2, 3, 4, 5]}

----

bt2$ babeltrace2  ~/git-repositories/communication_5c/build/test_session/
[10:04:51.068836215] (+?.?????????) SET-L-ME-P22061 comm_activity:range_scan: { cpu_id = 6 }, { distances = [ [0] = 0, [1] = 1072693248, [2] = 0, [3] = 1073741824, [4] = 0 ] }

$ python3 tt.py  ~/git-repositories/communication_5c/build/test_session/
{'distances': [0, 1072693248, 0, 1073741824, 0]}

> Are you instrumenting python code, or are you instrumenting a C/C++ 
> application with lttng-ust?

The latter.

>> I have no problems with arrays of integers, though.
>> 
>> Is the float array feature not yet implemented in lttng 2.13?
>> Will it be in 2.14? (Also the "BLOB" feature is useful for me.)
>> 
>
> I don't see any changes in lttng-ust for 2.14 that allow for tracepoints that 
> have float arrays or blobs. lttng-modules 2.14 does add support for CTF 2. 
> The support for CTF 2 in lttng-tools 2.14 will remain at an experimental 
> level.
>
> From the release notes:
>
>      CTF 2 output isn't enabled by default and is intended solely for
>      preview and feedback purposes: having an experimental support will
>      allow users to test and evaluate CTF 2 until we make it the
>      default output format.
>
>
>> Thanks for your help.
>> 
>> Herman
>
> thanks,
> kienan

Herman

> [1]: 
> https://git.lttng.org/?p=lttng-tools.git;a=blob_plain;f=ChangeLog;hb=11f963148df9597de6d891d96d748bae5ced7149
>
>