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/v2.10.6/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.