tracelog(3) — Linux manual page


TRACELOG(3)                   LTTng Manual                   TRACELOG(3)

NAME         top

       tracelog - LTTng-UST printf(3)-like interface with a log level

SYNOPSIS         top

       #include <lttng/tracelog.h>

       #define tracelog(level, fmt, ...)

       Link with -llttng-ust.

DESCRIPTION         top

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

       The purpose of tracelog() 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

           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

           Debug information with function-level scope.

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

           Debug-level message.

       To use tracelog(), include <lttng/tracelog.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 tracelog() 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 tracelog() events contain the following fields:

       │Field name Description              │
       │           │                          │
       │line       │ Line in source file      │
       │           │ where tracelog() was     │
       │           │ called.                  │
       │           │                          │
       │file       │ Source file from which   │
       │           │ tracelog() was called.   │
       │           │                          │
       │func       │ Function name from which │
       │           │ tracelog() was called.   │
       │           │                          │
       │msg        │ Formatted string output. │

       If you do not need to attach a specific log level to a tracelog()
       call, use tracef(3) instead.

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

EXAMPLE         top

       Here’s a usage example of tracelog():

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

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

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

               tracelog(TRACE_INFO, "Starting app with %d arguments", argc);

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

               tracelog(TRACE_INFO, "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

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

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

       Or you can enable 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

           $ lttng stop
           $ lttng view

LIMITATIONS         top

       The tracelog() 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 tracelog() 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, tracelog() 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         top

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

RESOURCES         top

       •   LTTng project website <>

       •   LTTng documentation <>

       •   Git repositories <>

       •   GitHub organization <>

       •   Continuous integration <>

       •   Mailing list <> for support and

       •   IRC channel <irc://>: #lttng on

COPYRIGHTS         top

       This macro is part of the LTTng-UST project.

       This macro is distributed under the GNU Lesser General Public
       License, version 2.1 <
       licenses/lgpl-2.1.en.html>. See the COPYING
       <> file
       for more details.

THANKS         top

       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.

AUTHORS         top

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

SEE ALSO         top

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

COLOPHON         top

       This page is part of the LTTng-UST (    LTTng Userspace Tracer)
       project.  Information about the project can be found at 
       ⟨⟩.  It is not known how to report bugs for this
       man page; if you know, please send a mail to
       This page was obtained from the tarball lttng-ust-2.11.0.tar.bz2
       fetched from ⟨⟩ on 2019-11-19.
       If you discover any rendering problems in this HTML version of
       the page, or you believe there is a better or more up-to-date
       source for the page, or you have corrections or improvements to
       the information in this COLOPHON (which is not part of the
       original manual page), send a mail to

LTTng 2.10.6                   10/17/2019                    TRACELOG(3)

Pages that refer to this page: lttng-ust(3)tracef(3)