NAME

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

SYNOPSIS

       #include <lttng/tracef.h>

       #define tracef(fmt, ...)

       Link with -llttng-ust.

DESCRIPTION

       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().

EXAMPLE

       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

LIMITATIONS

       The tracef() utility macro was 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 tracef() uses C standard library’s vasprintf(3) function in the background to
           format the strings at run time, its 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, 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.

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: lttng-
           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, version 2.1
       <http://www.gnu.org/licenses/old-licenses/lgpl-2.1.en.html>. See the COPYING
       <https://github.com/lttng/lttng-ust/blob/master/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 École Polytechnique de Montréal for the LTTng journey.

AUTHORS

       LTTng-UST was originally written by Mathieu Desnoyers, with additional contributions from
       various other people. It is currently maintained by Mathieu Desnoyers
       <mailto:mathieu.desnoyers@efficios.com>.

SEE ALSO

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