Introduce vtracelog

[lttng-ust.git] / doc / man / tracelog.3.txt

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', ...)
#define *vtracelog*('level', 'fmt', 'va_list' ap)

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()` or `vtracelog()`, 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()` or `vtracelog()`.


[[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)