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 in source file where |
| Source file from which |
| Function name from which |
| 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.
Mailing list for support and
development: lttng-dev@lists.lttng.org
IRC channel: #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. See the
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 at École Polytechnique de Montréal for the LTTng journey.