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TRACELOG(3) LTTng Manual TRACELOG(3)
tracelog - LTTng-UST printf(3)-like interface with a log level
#include <lttng/tracelog.h>
#define tracelog(level, fmt, ...)
Link with -llttng-ust.
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:
TRACE_EMERG
System is unusable.
TRACE_ALERT
Action must be taken immediately.
TRACE_CRIT
Critical conditions.
TRACE_ERR
Error conditions.
TRACE_WARNING
Warning conditions.
TRACE_NOTICE
Normal, but significant, condition.
TRACE_INFO
Informational message.
TRACE_DEBUG_SYSTEM
Debug information with system-level scope (set of programs).
TRACE_DEBUG_PROGRAM
Debug information with program-level scope (set of processes).
TRACE_DEBUG_PROCESS
Debug information with process-level scope (set of modules).
TRACE_DEBUG_MODULE
Debug information with module (executable/library) scope (set of
units).
TRACE_DEBUG_UNIT
Debug information with compilation unit scope (set of functions).
TRACE_DEBUG_FUNCTION
Debug information with function-level scope.
TRACE_DEBUG_LINE
Debug information with line-level scope (default log level).
TRACE_DEBUG
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().
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 this:
$ 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:*' \
--loglevel=TRACE_INFO
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 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.
If you encounter any issue or usability problem, please report it on
the LTTng bug tracker <https://bugs.lttng.org/projects/lttng-ust>.
· 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
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 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.
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>.
tracef(3), lttng-ust(3), lttng(1), printf(3)
This page is part of the LTTng-UST ( LTTng Userspace Tracer)
project. Information about the project can be found at
⟨http://lttng.org/⟩. It is not known how to report bugs for this man
page; if you know, please send a mail to man-pages@man7.org. This
page was obtained from the project's upstream Git repository
⟨git://git.lttng.org/lttng-ust.git⟩ on 2018-02-02. (At that time,
the date of the most recent commit that was found in the repository
was 2017-12-19.) 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 man-pages@man7.org
LTTng 2.10.0-rc1 08/02/2017 TRACELOG(3)
Pages that refer to this page: lttng-ust(3), tracef(3)
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