tracelog(3)
===========
:object-type: macro


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


SYNOPSIS
--------
[verse]
*#include <lttng/tracelog.h>*

[verse]
#define *tracelog*('level', 'fmt', ...)

Link with `-llttng-ust`.


DESCRIPTION
-----------
The LTTng-UST `tracelog()` API allows you to trace your application with
the help of a simple man:printf(3)-like macro, with an additional
parameter for the desired log level. The 'fmt' argument is passed
directly to the 'fmt' parameter of man: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:

include::log-levels.txt[]

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

Once your application is instrumented with `tracelog()` calls and
ready to run, use man:lttng-enable-event(1) to enable the
`lttng_ust_tracelog:*` event. You can isolate specific log levels with
the nloption:--loglevel and nloption:--loglevel-only options of this
command.

The `tracelog()` events contain the following fields:

[options="header"]
|===============================================================
| 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 man:tracef(3) instead.

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


[[example]]
EXAMPLE
-------
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:

[role="term"]
---------------------------
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:

[role="term"]
---------------------------------------------------
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:

[role="term"]
-------------------------------------------------------
lttng enable-event --userspace 'lttng_ust_tracelog:*' \
                   --loglevel=TRACE_INFO
-------------------------------------------------------

Next, start the program to be traced:

[role="term"]
------------------------------------------------
./app a few arguments passed to this application
------------------------------------------------

Finally, stop the tracing session, and inspect the recorded events:

[role="term"]
----------
lttng stop
lttng view
----------


[[limitations]]
LIMITATIONS
-----------
:macro-name: tracelog

include::tracef-tracelog-limitations.txt[]


include::common-footer.txt[]

include::common-copyrights.txt[]

include::common-authors.txt[]


SEE ALSO
--------
man:tracef(3),
man:lttng-ust(3),
man:lttng(1),
man:printf(3)