Re: Problems with arrays of floats (lttng 2.13)

[Herman Bruyninckx via lttng-dev]

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