tracef — LTTng-UST printf(3)-like interface


#include <lttng/tracef.h>
#define tracef(fmt, ...)

Link with -llttng-ust.


The LTTng-UST tracef() API allows you to trace your application with the help of a simple printf(3)-like macro. The fmt argument is passed directly to the fmt parameter of vasprintf(3), as well as the optional parameters following fmt.

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

Once your application is instrumented with tracef() calls and ready to run, use lttng-enable-event(1) to enable the lttng_ust_tracef:* event.

The tracef() events contain a single field, named msg, which is the formatted string output.

If you need to attach a specific log level to a tracef() call, use tracelog(3) instead.

See also the Limitations section below for important limitations to consider when using tracef().


Here’s a usage example of tracef():

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

int main(void)
    int i;

    for (i = 0; i < 25; i++) {
        tracef("my message: %s, this integer: %d", "a message", i);

    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

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

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

Next, start the program to be traced:

$ ./app

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

$ lttng stop
$ lttng view


The tracef() utility macro was developed to make user space tracing super simple, albeit with notable disadvantages compared to custom, full-fledged tracepoint providers:

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

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 <>.

See Also

tracelog(3), lttng-ust(3), lttng(1), printf(3)

Referenced By

lttng-ust(3), tracelog(3).

10/17/2019 LTTng 2.10.6 LTTng Manual