lttng_ust_tracelog(3) (v2.13)


lttng_ust_tracelog, lttng_ust_vtracelog — LTTng-UST printf(3)-like interface with a log level


#include <lttng/tracelog.h>
#define lttng_ust_tracelog(level, fmt, …)
#define lttng_ust_vtracelog(level, fmt, ap)

Link with:

  • -llttng-ust

  • If you define _LGPL_SOURCE before including <lttng/tracelog.h> (directly or indirectly): -llttng-ust-common


The LTTng-UST lttng_ust_tracelog() and lttng_ust_vtracelog() API allows you to trace your application with the help of simple printf(3)-like and vprintf(3)-like macros, with an additional parameter for the desired log level.

The fmt argument is passed directly as the fmt parameter of vasprintf(3), as well as:

For lttng_ust_tracelog()

The optional parameters following fmt.

For lttng_ust_vtracelog()

The ap parameter as the ap parameter of vasprintf(3) (va_list type).

The purpose of lttng_ust_tracelog() and lttng_ust_vtracelog() is to ease the migration from logging to tracing.

The available values for the level parameter are:


System is unusable.


Action must be taken immediately.


Critical conditions.


Error conditions.


Warning conditions.


Normal, but significant, condition.


Informational message.


Debug information with system-level scope (set of programs).


Debug information with program-level scope (set of processes).


Debug information with process-level scope (set of modules).


Debug information with module (executable/library) scope (set of units).


Debug information with compilation unit scope (set of functions).


Debug information with function-level scope.


Debug information with line-level scope (default log level).


Debug-level message.

To use lttng_ust_tracelog() or lttng_ust_vtracelog(), include <lttng/tracelog.h> where you need it, and link your application with liblttng-ust and liblttng-ust-common. See the EXAMPLE section below for a complete usage example.

Once your application is instrumented with lttng_ust_tracelog() and/or lttng_ust_vtracelog() calls and ready to run, use lttng-enable-event(1) to enable the lttng_ust_tracelog:* event. You can isolate specific log levels with the --loglevel and --loglevel-only options of this command.

The lttng_ust_tracelog() and lttng_ust_vtracelog() events contain the following fields:

Field name Description


Line in source file where lttng_ust_tracelog() was called.


Source file from which lttng_ust_tracelog() was called.


Function name from which lttng_ust_tracelog() was called.


Formatted string output.

If you do not need to attach a specific log level to a lttng_ust_tracelog()/lttng_ust_vtracelog() call, use lttng_ust_tracef(3) instead.

See also the LIMITATIONS section below for important limitations to consider when using lttng_ust_tracelog() or lttng_ust_vtracelog().


Here’s a usage example of lttng_ust_tracelog():

#include <stdlib.h>
#include <lttng/tracelog.h>

int main(int argc, char *argv[])
    int i;

    if (argc < 2) {
                           "Not enough arguments: %d", argc);
        return EXIT_FAILURE;

                       "Starting app with %d arguments", argc);

    for (i = 0; i < argc; i++) {
                           "Argument %d: %s", i, argv[i]);

                       "Exiting app");
    return EXIT_SUCCESS;

This C source file, saved as app.c, can be compiled into a program like this:

cc -o app app.c -llttng-ust -llttng-ust-common

You can create an LTTng tracing session, enable all the lttng_ust_tracelog() events, and start the created tracing session like this:

lttng create my-session
lttng enable-event --userspace 'lttng_ust_tracelog:*'
lttng start

Or you can enable lttng_ust_tracelog() events matching a log level at least as severe as a given log level:

lttng enable-event --userspace 'lttng_ust_tracelog:*' \

Next, start the program to be traced:

./app a few arguments passed to this application

Finally, stop the tracing session, and inspect the recorded events:

lttng stop
lttng view


The lttng_ust_tracelog() and lttng_ust_vtracelog() utility macros were developed to make user space tracing super simple, albeit with notable disadvantages compared to custom, full-fledged tracepoint providers:

  • All generated events have the same provider/event names.

  • There’s no static type checking.

  • The only event field with user data you actually get, named msg, is a string potentially containing the values you passed to the macro using your own format. This also means that you cannot use filtering using a custom expression at run time because there are no isolated fields.

  • Since lttng_ust_tracelog() and lttng_ust_vtracelog() use C standard library’s vasprintf(3) function in the background to format the strings at run time, their expected performance is lower than using custom tracepoint providers with typed fields, which do not require a conversion to a string.

  • Generally, a string containing the textual representation of the user data fields is not as compact as binary fields in the resulting trace.

Thus, lttng_ust_tracelog()/lttng_ust_vtracelog() are useful for quick prototyping and debugging, but should not be considered for any permanent/serious application instrumentation.

lttng_ust_vtracelog() does not have a STAP_PROBEV() call, because STAP_PROBEV() does not support va_list. If you need it, you should emit this call yourself.

See lttng-ust(3) to learn more about custom tracepoint providers.


If you encounter any issue or usability problem, please report it on the LTTng bug tracker.



This macro is part of the LTTng-UST project.

This macro is distributed under the GNU Lesser General Public License, version 2.1. See the COPYING file for more details.


Thanks to Ericsson for funding this work, providing real-life use cases, and testing.

Special thanks to Michel Dagenais and the DORSAL laboratory at École Polytechnique de Montréal for the LTTng journey.


LTTng-UST was originally written by Mathieu Desnoyers, with additional contributions from various other people. It is currently maintained by Mathieu Desnoyers.