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LTTNG_UST_TRACEF(3) LTTng Manual LTTNG_UST_TRACEF(3) NAME lttng_ust_tracef, lttng_ust_vtracef - LTTng-UST printf(3)-like interface SYNOPSIS #include <lttng/tracef.h> #define lttng_ust_tracef(fmt, ...) #define lttng_ust_vtracef(fmt, ap) Link with: * -llttng-ust * If you define _LGPL_SOURCE before including <lttng/tracef.h> (directly or indirectly): -llttng-ust-common DESCRIPTION The LTTng-UST lttng_ust_tracef() and lttng_ust_vtracef() API allows you to trace your application with the help of simple printf(3)-like and vprintf(3)-like macros. The fmt argument is passed directly as the fmt parameter of vasprintf(3), as well as: For lttng_ust_tracef() The optional parameters following fmt. For lttng_ust_vtracef() The ap parameter as the ap parameter of vasprintf(3) (va_list type). To use lttng_ust_tracef() or lttng_ust_vtracef(), include <lttng/tracef.h> where you need it, and link your application with liblttng-ust and liblt- tng-ust-common. See the EXAMPLE section below for a complete usage example. Once your application is instrumented with lttng_ust_tracef() and/or lt- tng_ust_vtracef() calls and ready to run, use lttng-enable-event(1) to en- able the lttng_ust_tracef:* event. The lttng_ust_tracef() and lttng_ust_vtracef() events contain a single field, named msg, which is the formatted string output. If you need to attach a specific log level to a lttng_ust_tracef()/lt- tng_ust_vtracef() call, use lttng_ust_tracelog(3) and lt- tng_ust_vtracelog(3) instead. See also the LIMITATIONS section below for important limitations to con- sider when using lttng_ust_tracef() or lttng_ust_vtracef(). EXAMPLE Here's a usage example of lttng_ust_tracef(): #include <stdlib.h> #include <lttng/tracef.h> int main(void) { int i; for (i = 0; i < 25; i++) { lttng_ust_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 -llttng-ust-common You can create an LTTng tracing session, enable the lttng_ust_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 LIMITATIONS The lttng_ust_tracef() and lttng_ust_vtracef() utility macros were devel- oped to make user space tracing super simple, albeit with notable disadvan- tages 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_tracef() and lttng_ust_vtracef() use C standard li- brary's vasprintf(3) function in the background to format the strings at run time, their expected performance is lower than using custom tra- cepoint 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_tracef()/lttng_ust_vtracef() are useful for quick prototyp- ing and debugging, but should not be considered for any permanent/serious application instrumentation. lttng_ust_vtracef() 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. BUGS If you encounter any issue or usability problem, please report it on the LTTng bug tracker <https://bugs.lttng.org/projects/lttng-ust>. RESOURCES * LTTng project website <http://lttng.org> * LTTng documentation <http://lttng.org/docs> * Git repositories <http://git.lttng.org> * GitHub organization <http://github.com/lttng> * Continuous integration <http://ci.lttng.org/> * Mailing list <http://lists.lttng.org> for support and development: lt- tng-dev@lists.lttng.org * IRC channel <irc://irc.oftc.net/lttng>: #lttng on irc.oftc.net COPYRIGHTS This macro is part of the LTTng-UST project. This macro is distributed under the GNU Lesser General Public License, ver- sion 2.1 <http://www.gnu.org/licenses/old-licenses/lgpl-2.1.en.html>. See the COPYING <https://github.com/lttng/lttng-ust/blob/v2.13/COPYING> file for more details. THANKS Thanks to Ericsson for funding this work, providing real-life use cases, and testing. Special thanks to Michel Dagenais and the DORSAL laboratory <http://www.dorsal.polymtl.ca/> at Ecole Polytechnique de Montreal for the LTTng journey. AUTHORS LTTng-UST was originally written by Mathieu Desnoyers, with additional con- tributions from various other people. It is currently maintained by Mathieu Desnoyers <mailto:mathieu.desnoyers@efficios.com>. SEE ALSO lttng_ust_tracelog(3), lttng_ust_vtracelog(3), lttng-ust(3), lttng(1), printf(3) LTTng 2.13.9 08/02/2026 LTTNG_UST_TRACEF(3)
NAME | SYNOPSIS | DESCRIPTION | EXAMPLE | LIMITATIONS | BUGS | RESOURCES | COPYRIGHTS | THANKS | AUTHORS | SEE ALSO
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