NAME

lttng_ust_tracelog, lttng_ust_vtracelog - LTTng-UST printf(3)-like interface with a log level

SYNOPSIS

#include <lttng/tracelog.h>

#define lttng_ust_tracelog(level, fmt, ...)
#define lttng_ust_vtracelog(level, fmt, ap)

Link with:

DESCRIPTION

The LTTng-UST lttng_ust_tracelog() and lttng_ust_vtracelog() API allows you to trace your application with the help of simple printf(3)-like and vprintf(3)-like macros, with an additional parameter for the desired log level.

The fmt argument is passed directly as the fmt parameter of vasprintf(3), as well as:

For lttng_ust_tracelog()

For lttng_ust_vtracelog()

The purpose of lttng_ust_tracelog() and lttng_ust_vtracelog() is to ease the migration from logging to tracing.

The available values for the level parameter are:

LTTNG_UST_TRACEPOINT_LOGLEVEL_EMERG

LTTNG_UST_TRACEPOINT_LOGLEVEL_ALERT

LTTNG_UST_TRACEPOINT_LOGLEVEL_CRIT

LTTNG_UST_TRACEPOINT_LOGLEVEL_ERR

LTTNG_UST_TRACEPOINT_LOGLEVEL_WARNING

LTTNG_UST_TRACEPOINT_LOGLEVEL_NOTICE

LTTNG_UST_TRACEPOINT_LOGLEVEL_INFO

LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_SYSTEM

LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_PROGRAM

LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_PROCESS

LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_MODULE

LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_UNIT

LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_FUNCTION

LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_LINE

LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG

To use lttng_ust_tracelog() or lttng_ust_vtracelog(), include <lttng/tracelog.h> where you need it, and link your application with liblttng-ust and liblttng-ust-common. See the EXAMPLE section below for a complete usage example.

Once your application is instrumented with lttng_ust_tracelog() and/or lttng_ust_vtracelog() 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 lttng_ust_tracelog() and lttng_ust_vtracelog() events contain the following fields:

If you do not need to attach a specific log level to a lttng_ust_tracelog()/lttng_ust_vtracelog() call, use lttng_ust_tracef(3) instead.

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

EXAMPLE

Here’s a usage example of lttng_ust_tracelog():

#include <stdlib.h>
#include <lttng/tracelog.h>
int main(int argc, char *argv[])
{
    int i;
    if (argc < 2) {
        lttng_ust_tracelog(LTTNG_UST_TRACEPOINT_LOGLEVEL_CRIT,
                           "Not enough arguments: %d", argc);
        return EXIT_FAILURE;
    }
    lttng_ust_tracelog(LTTNG_UST_TRACEPOINT_LOGLEVEL_INFO,
                       "Starting app with %d arguments", argc);
    for (i = 0; i < argc; i++) {
        lttng_ust_tracelog(LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG,
                           "Argument %d: %s", i, argv[i]);
    }
    lttng_ust_tracelog(LTTNG_UST_TRACEPOINT_LOGLEVEL_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 -llttng-ust-common

You can create an LTTng tracing session, enable all the lttng_ust_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 lttng_ust_tracelog() events matching a log level at least as severe as a given log level:

$ lttng enable-event --userspace 'lttng_ust_tracelog:*' \
                     --loglevel=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

LIMITATIONS

The lttng_ust_tracelog() and lttng_ust_vtracelog() utility macros were developed to make user space tracing super simple, albeit with notable disadvantages compared to custom, full-fledged tracepoint providers:

Thus, lttng_ust_tracelog()/lttng_ust_vtracelog() are useful for quick prototyping and debugging, but should not be considered for any permanent/serious application instrumentation.

lttng_ust_vtracelog() 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

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 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

lttng_ust_tracef(3), lttng_ust_vtracef(3), lttng-ust(3), lttng(1), printf(3)