// SPDX-FileCopyrightText: 2016 Philippe Proulx <pproulx@efficios.com>
// SPDX-License-Identifier: CC-BY-4.0
//
lttng-ust(3)
============
:object-type: library


NAME
----
lttng-ust - LTTng user space tracing


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

[verse]
#define *LTTNG_UST_TP_ARGS*('args'...)
#define *LTTNG_UST_TP_ENUM_VALUES*('values'...)
#define *LTTNG_UST_TP_FIELDS*('fields'...)
#define *LTTNG_UST_TRACEPOINT_ENUM*('prov_name', 'enum_name', 'mappings')
#define *LTTNG_UST_TRACEPOINT_EVENT*('prov_name', 't_name', 'args', 'fields')
#define *LTTNG_UST_TRACEPOINT_EVENT_CLASS*('cls_prov_name', 'cls_name',
                                         'args', 'fields')
#define *LTTNG_UST_TRACEPOINT_EVENT_INSTANCE*('cls_prov_name', 'cls_name',
                                            'inst_prov_name', 't_name', 'args')
#define *LTTNG_UST_TRACEPOINT_LOGLEVEL*('prov_name', 't_name', 'level')
#define *lttng_ust_do_tracepoint*('prov_name', 't_name', ...)
#define *lttng_ust_field_array*('int_type', 'field_name', 'expr', 'count')
#define *lttng_ust_field_array_nowrite*('int_type', 'field_name', 'expr', 'count')
#define *lttng_ust_field_array_hex*('int_type', 'field_name', 'expr', 'count')
#define *lttng_ust_field_array_nowrite_hex*('int_type', 'field_name', 'expr',
                                          'count')
#define *lttng_ust_field_array_network*('int_type', 'field_name', 'expr', 'count')
#define *lttng_ust_field_array_network_nowrite*('int_type', 'field_name',
                                              'expr', 'count')
#define *lttng_ust_field_array_network_hex*('int_type', 'field_name', 'expr',
                                          'count')
#define *lttng_ust_field_array_network_nowrite_hex*('int_type', 'field_name',
                                                  'expr', 'count')
#define *lttng_ust_field_array_text*(char, 'field_name', 'expr', 'count')
#define *lttng_ust_field_array_text_nowrite*(char, 'field_name', 'expr',
                                           'count')
#define *lttng_ust_field_enum*('prov_name', 'enum_name', 'int_type', 'field_name',
                             'expr')
#define *lttng_ust_field_enum_nowrite*('prov_name', 'enum_name', 'int_type',
                                     'field_name', 'expr')
#define *lttng_ust_field_enum_value*('label', 'value')
#define *lttng_ust_field_enum_range*('label', 'start', 'end')
#define *lttng_ust_field_float*('float_type', 'field_name', 'expr')
#define *lttng_ust_field_float_nowrite*('float_type', 'field_name', 'expr')
#define *lttng_ust_field_integer*('int_type', 'field_name', 'expr')
#define *lttng_ust_field_integer_hex*('int_type', 'field_name', 'expr')
#define *lttng_ust_field_integer_network*('int_type', 'field_name', 'expr')
#define *lttng_ust_field_integer_network_hex*('int_type', 'field_name', 'expr')
#define *lttng_ust_field_integer_nowrite*('int_type', 'field_name', 'expr')
#define *lttng_ust_field_sequence*('int_type', 'field_name', 'expr',
                                 'len_type', 'len_expr')
#define *lttng_ust_field_sequence_nowrite*('int_type', 'field_name', 'expr',
                                         'len_type', 'len_expr')
#define *lttng_ust_field_sequence_hex*('int_type', 'field_name', 'expr',
                                     'len_type', 'len_expr')
#define *lttng_ust_field_sequence_nowrite_hex*('int_type', 'field_name', 'expr',
                                             'len_type', 'len_expr')
#define *lttng_ust_field_sequence_network*('int_type', 'field_name', 'expr',
                                         'len_type', 'len_expr')
#define *lttng_ust_field_sequence_network_nowrite*('int_type', 'field_name',
                                                 'expr', 'len_type',
                                                 'len_expr')
#define *lttng_ust_field_sequence_network_hex*('int_type', 'field_name', 'expr',
                                             'len_type', 'len_expr')
#define *lttng_ust_field_sequence_network_nowrite_hex*('int_type',
                                                     'field_name',
                                                     'expr', 'len_type',
                                                     'len_expr')
#define *lttng_ust_field_sequence_text*(char, 'field_name', 'expr', 'len_type',
                                      'len_expr')
#define *lttng_ust_field_sequence_text_nowrite*(char, 'field_name', 'expr',
                                              'len_type', 'len_expr')
#define *lttng_ust_field_string*('field_name', 'expr')
#define *lttng_ust_field_string_nowrite*('field_name', 'expr')
#define *lttng_ust_tracepoint*('prov_name', 't_name', ...)
#define *lttng_ust_tracepoint_enabled*('prov_name', 't_name')

Link with, following this manual page:

* `-llttng-ust -ldl`
* If you define `_LGPL_SOURCE` before including
  `<lttng/tracepoint.h>` (directly or indirectly): `-llttng-ust-common`


DESCRIPTION
-----------
The http://lttng.org/[_Linux Trace Toolkit: next generation_] is an open
source software package used for correlated tracing of the Linux kernel,
user applications, and user libraries.

LTTng-UST is the user space tracing component of the LTTng project. It
is a port to user space of the low-overhead tracing capabilities of the
LTTng Linux kernel tracer. The `liblttng-ust` library is used to trace
user applications and libraries.

NOTE: This man page is about the `liblttng-ust` library. The LTTng-UST
project also provides Java and Python packages to trace applications
written in those languages. How to instrument and trace Java and Python
applications is documented in
http://lttng.org/docs/[the online LTTng documentation].

There are three ways to use `liblttng-ust`:

  * Using the man:lttng_ust_tracef(3) API, which is similar to
    man:printf(3).
  * Using the man:lttng_ust_tracelog(3) API, which is
    man:lttng_ust_tracef(3) with a log level parameter.
  * Defining your own tracepoints. See the
    <<creating-tp,Creating a tracepoint provider>> section below.


Compatibility with previous APIs
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Since LTTng-UST{nbsp}2.13, the `LTTNG_UST_COMPAT_API_VERSION` definition
controls which LTTng-UST APIs are available (compiled):

Undefined::
    All APIs are available.

'N' (0 or positive integer)::
    API version{nbsp}__N__, and all the following existing APIs, are
    available. Previous APIs are not available (not compiled).

The following table shows the mapping from LTTng-UST versions (up to
LTTng-UST{nbsp}{manversion}) to available API versions:

[options="header"]
|====
|LTTng-UST version |Available API versions
|2.0 to 2.12 |0
|2.13 |0 and 1
|====

This manual page **only** documents version{nbsp}1 of the API.

If you wish to have access to version{nbsp}0 of the API (for example,
the `tracepoint()`, `ctf_integer()`, and `TRACEPOINT_EVENT()` macros),
then either don't define `LTTNG_UST_COMPAT_API_VERSION`, or define it to
`0` before including any LTTng-UST header.


[[creating-tp]]
Creating a tracepoint provider
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Creating a tracepoint provider is the first step of using
`liblttng-ust`. The next steps are:

  * <<tracepoint,Instrumenting your application with
    `lttng_ust_tracepoint()` calls>>
  * Building your application with LTTng-UST support, either
    <<build-static,statically>> or <<build-dynamic,dynamically>>.

A *tracepoint provider* is a compiled object containing the event probes
corresponding to your custom tracepoint definitions. A tracepoint
provider contains the code to get the size of an event and to serialize
it, amongst other things.

To create a tracepoint provider, start with the following
_tracepoint provider header_ template:

------------------------------------------------------------------------
#undef LTTNG_UST_TRACEPOINT_PROVIDER
#define LTTNG_UST_TRACEPOINT_PROVIDER my_provider

#undef LTTNG_UST_TRACEPOINT_INCLUDE
#define LTTNG_UST_TRACEPOINT_INCLUDE "./tp.h"

#if !defined(_TP_H) || \
    defined(LTTNG_UST_TRACEPOINT_HEADER_MULTI_READ)
#define _TP_H

#include <lttng/tracepoint.h>

/*
 * LTTNG_UST_TRACEPOINT_EVENT(), LTTNG_UST_TRACEPOINT_EVENT_CLASS(),
 * LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(),
 * LTTNG_UST_TRACEPOINT_LOGLEVEL(), and `LTTNG_UST_TRACEPOINT_ENUM()`
 * are used here.
 */

#endif /* _TP_H */

#include <lttng/tracepoint-event.h>
------------------------------------------------------------------------

In this template, the tracepoint provider is named `my_provider`
(`LTTNG_UST_TRACEPOINT_PROVIDER` definition). The file needs to bear the
name of the `LTTNG_UST_TRACEPOINT_INCLUDE` definition (`tp.h` in this case).
Between `#include <lttng/tracepoint.h>` and `#endif` go
the invocations of the <<tracepoint-event,`LTTNG_UST_TRACEPOINT_EVENT()`>>,
<<tracepoint-event-class,`LTTNG_UST_TRACEPOINT_EVENT_CLASS()`>>,
<<tracepoint-event-class,`LTTNG_UST_TRACEPOINT_EVENT_INSTANCE()`>>,
<<tracepoint-loglevel,`LTTNG_UST_TRACEPOINT_LOGLEVEL()`>>, and
<<tracepoint-enum,`LTTNG_UST_TRACEPOINT_ENUM()`>> macros.

NOTE: You can avoid writing the prologue and epilogue boilerplate in the
template file above by using the man:lttng-gen-tp(1) tool shipped with
LTTng-UST.

The tracepoint provider header file needs to be included in a source
file which looks like this:

------------------------------------------------------------------------
#define LTTNG_UST_TRACEPOINT_CREATE_PROBES

#include "tp.h"
------------------------------------------------------------------------

Together, those two files (let's call them `tp.h` and `tp.c`) form the
tracepoint provider sources, ready to be compiled.

You can create multiple tracepoint providers to be used in a single
application, but each one must have its own header file.

The <<tracepoint-event,`LTTNG_UST_TRACEPOINT_EVENT()` usage>> section below
shows how to use the `LTTNG_UST_TRACEPOINT_EVENT()` macro to define the actual
tracepoints in the tracepoint provider header file.

See the <<example,EXAMPLE>> section below for a complete example.


[[tracepoint-event]]
`LTTNG_UST_TRACEPOINT_EVENT()` usage
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The `LTTNG_UST_TRACEPOINT_EVENT()` macro is used in a template provider
header file (see the <<creating-tp,Creating a tracepoint provider>>
section above) to define LTTng-UST tracepoints.

The `LTTNG_UST_TRACEPOINT_EVENT()` usage template is as follows:

------------------------------------------------------------------------
LTTNG_UST_TRACEPOINT_EVENT(
    /* Tracepoint provider name */
    my_provider,

    /* Tracepoint/event name */
    my_tracepoint,

    /* List of tracepoint arguments (input) */
    LTTNG_UST_TP_ARGS(
        ...
    ),

    /* List of fields of eventual event (output) */
    LTTNG_UST_TP_FIELDS(
        ...
    )
)
------------------------------------------------------------------------

The `LTTNG_UST_TP_ARGS()` macro contains the input arguments of the tracepoint.
Those arguments can be used in the argument expressions of the output
fields defined in `LTTNG_UST_TP_FIELDS()`.

The format of the `LTTNG_UST_TP_ARGS()` parameters is: C type, then argument name;
repeat as needed, up to ten times. For example:

------------------------------------------------------------------------
LTTNG_UST_TP_ARGS(
    int, my_int,
    const char *, my_string,
    FILE *, my_file,
    double, my_float,
    struct my_data *, my_data
)
------------------------------------------------------------------------

The `LTTNG_UST_TP_FIELDS()` macro contains the output fields of the tracepoint,
that is, the actual data that can be recorded in the payload of an event
emitted by this tracepoint.

The `LTTNG_UST_TP_FIELDS()` macro contains a list of
`lttng_ust_field_*()` macros :not: separated by commas.
The available macros are documented in the
<<ctf-macros,Available `lttng_ust_field_*()` field type macros>>
section below.


[[ctf-macros]]
Available field macros
~~~~~~~~~~~~~~~~~~~~~~
This section documents the available `lttng_ust_field_*()` macros that
can be inserted in the `LTTNG_UST_TP_FIELDS()` macro of the
<<tracepoint-event,`LTTNG_UST_TRACEPOINT_EVENT()` macro>>.

Standard integer, displayed in base 10:

[verse]
*lttng_ust_field_integer*('int_type', 'field_name', 'expr')
*lttng_ust_field_integer_nowrite*('int_type', 'field_name', 'expr')

Standard integer, displayed in base 16:

[verse]
*lttng_ust_field_integer_hex*('int_type', 'field_name', 'expr')

Integer in network byte order (big endian), displayed in base 10:

[verse]
*lttng_ust_field_integer_network*('int_type', 'field_name', 'expr')

Integer in network byte order, displayed in base 16:

[verse]
*lttng_ust_field_integer_network_hex*('int_type', 'field_name', 'expr')

Floating point number:

[verse]
*lttng_ust_field_float*('float_type', 'field_name', 'expr')
*lttng_ust_field_float_nowrite*('float_type', 'field_name', 'expr')

Null-terminated string:

[verse]
*lttng_ust_field_string*('field_name', 'expr')
*lttng_ust_field_string_nowrite*('field_name', 'expr')

Statically-sized array of integers (`_hex` versions displayed in
hexadecimal, `_network` versions in network byte order):

[verse]
*lttng_ust_field_array*('int_type', 'field_name', 'expr', 'count')
*lttng_ust_field_array_nowrite*('int_type', 'field_name', 'expr', 'count')
*lttng_ust_field_array_hex*('int_type', 'field_name', 'expr', 'count')
*lttng_ust_field_array_nowrite_hex*('int_type', 'field_name', 'expr', 'count')
*lttng_ust_field_array_network*('int_type', 'field_name', 'expr', 'count')
*lttng_ust_field_array_network_nowrite*('int_type', 'field_name', 'expr',
                                      'count')
*lttng_ust_field_array_network_hex*('int_type', 'field_name', 'expr', 'count')
*lttng_ust_field_array_network_nowrite_hex*('int_type', 'field_name',
                                          'expr', 'count')

Statically-sized array, printed as text; no need to be null-terminated:

[verse]
*lttng_ust_field_array_text*(char, 'field_name', 'expr', 'count')
*lttng_ust_field_array_text_nowrite*(char, 'field_name', 'expr', 'count')

Dynamically-sized array of integers (`_hex` versions displayed in
hexadecimal, `_network` versions in network byte order):

[verse]
*lttng_ust_field_sequence*('int_type', 'field_name', 'expr', 'len_type',
                         'len_expr')
*lttng_ust_field_sequence_nowrite*('int_type', 'field_name', 'expr',
                                 'len_type', 'len_expr')
*lttng_ust_field_sequence_hex*('int_type', 'field_name', 'expr', 'len_type',
                             'len_expr')
*lttng_ust_field_sequence_nowrite_hex*('int_type', 'field_name', 'expr',
                                     'len_type', 'len_expr')
*lttng_ust_field_sequence_network*('int_type', 'field_name', 'expr',
                                 'len_type', 'len_expr')
*lttng_ust_field_sequence_network_nowrite*('int_type', 'field_name', 'expr',
                                         'len_type', 'len_expr')
*lttng_ust_field_sequence_network_hex*('int_type', 'field_name', 'expr',
                                     'len_type', 'len_expr')
*lttng_ust_field_sequence_network_nowrite_hex*('int_type', 'field_name',
                                             'expr', 'len_type',
                                             'len_expr')

Dynamically-sized array, displayed as text; no need to be null-terminated:

[verse]
*lttng_ust_field_sequence_text*(char, 'field_name', 'expr', 'len_type',
                              'len_expr')
*lttng_ust_field_sequence_text_nowrite*(char, 'field_name', 'expr',
                                      'len_type', 'len_expr')

Enumeration. The enumeration field must be defined before using this
macro with the `LTTNG_UST_TRACEPOINT_ENUM()` macro. See the
<<tracepoint-enum,`LTTNG_UST_TRACEPOINT_ENUM()` usage>> section for more
information.

[verse]
*lttng_ust_field_enum*('prov_name', 'enum_name', 'int_type', 'field_name',
                     'expr')
*lttng_ust_field_enum_nowrite*('prov_name', 'enum_name', 'int_type',
                             'field_name', 'expr')

The parameters are:

'count'::
    Number of elements in array/sequence. This must be known at
    compile time.

'enum_name'::
    Name of an enumeration field previously defined with the
    `LTTNG_UST_TRACEPOINT_ENUM()` macro. See the
    <<tracepoint-enum,`LTTNG_UST_TRACEPOINT_ENUM()` usage>> section for more
    information.

'expr'::
    C expression resulting in the field value. This expression can
    use one or more arguments passed to the tracepoint. The arguments
    of a given tracepoint are defined in the `LTTNG_UST_TP_ARGS()` macro (see
    the <<creating-tp,Creating a tracepoint provider>> section above).

'field_name'::
    Event field name (C identifier syntax, :not: a literal string).

'float_type'::
    Float C type (`float` or `double`). The size of this type determines
    the size of the floating point number field.

'int_type'::
    Integer C type. The size of this type determines the size of the
    integer/enumeration field.

'len_expr'::
    C expression resulting in the sequence length. This expression
    can use one or more arguments passed to the tracepoint.

'len_type'::
    Unsigned integer C type of sequence length.

'prov_name'::
    Tracepoint provider name. This must be the same as the tracepoint
    provider name used in a previous field definition.

The `_nowrite` versions omit themselves from the recorded trace, but are
otherwise identical. Their primary purpose is to make some of the
event context available to the event filters without having to commit
the data to sub-buffers. See man:lttng-enable-event(1) to learn more
about dynamic event filtering.

See the <<example,EXAMPLE>> section below for a complete example.


[[tracepoint-enum]]
`LTTNG_UST_TRACEPOINT_ENUM()` usage
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
An enumeration field is a list of mappings between an integers, or a
range of integers, and strings (sometimes called _labels_ or
_enumerators_). Enumeration fields can be used to have a more compact
trace when the possible values for a field are limited.

An enumeration field is defined with the `LTTNG_UST_TRACEPOINT_ENUM()`
macro:

------------------------------------------------------------------------
LTTNG_UST_TRACEPOINT_ENUM(
    /* Tracepoint provider name */
    my_provider,

    /* Enumeration name (unique in the whole tracepoint provider) */
    my_enum,

    /* Enumeration mappings */
    LTTNG_UST_TP_ENUM_VALUES(
        ...
    )
)
------------------------------------------------------------------------

`LTTNG_UST_TP_ENUM_VALUES()` contains a list of enumeration mappings,
:not: separated by commas. Two macros can be used in the
`LTTNG_UST_TP_ENUM_VALUES()`: `lttng_ust_field_enum_value()` and
`lttng_ust_field_enum_range()`.

`lttng_ust_field_enum_value()` is a single value mapping:

[verse]
*lttng_ust_field_enum_value*('label', 'value')

This macro maps the given 'label' string to the value 'value'.

`lttng_ust_field_enum_range()` is a range mapping:

[verse]
*lttng_ust_field_enum_range*('label', 'start', 'end')

This macro maps the given 'label' string to the range of integers from
'start' to 'end', inclusively. Range mappings may overlap, but the
behaviour is implementation-defined: each trace reader handles
overlapping ranges as it wishes.

See the <<example,EXAMPLE>> section below for a complete example.


[[tracepoint-event-class]]
`LTTNG_UST_TRACEPOINT_EVENT_CLASS()` usage
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A *tracepoint class* is a class of tracepoints sharing the
same field types and names. A tracepoint instance is one instance of
such a declared tracepoint class, with its own event name.

LTTng-UST creates one event serialization function per tracepoint class.
Using `LTTNG_UST_TRACEPOINT_EVENT()` creates one tracepoint class per
tracepoint definition, whereas using
`LTTNG_UST_TRACEPOINT_EVENT_CLASS()` and
`LTTNG_UST_TRACEPOINT_EVENT_INSTANCE()` creates one tracepoint class,
and one or more tracepoint instances of this class. In other words, many
tracepoints can reuse the same serialization code. Reusing the same
code, when possible, can reduce cache pollution, thus improve
performance.

The `LTTNG_UST_TRACEPOINT_EVENT_CLASS()` macro accepts the same
parameters as the `LTTNG_UST_TRACEPOINT_EVENT()` macro, except that
instead of an event name, its second parameter is the _tracepoint class
name_:

------------------------------------------------------------------------
#define LTTNG_UST_TRACEPOINT_PROVIDER my_provider

/* ... */

LTTNG_UST_TRACEPOINT_EVENT_CLASS(
    /* Tracepoint class provider name */
    my_provider,

    /* Tracepoint class name */
    my_tracepoint_class,

    /* List of tracepoint arguments (input) */
    LTTNG_UST_TP_ARGS(
        ...
    ),

    /* List of fields of eventual event (output) */
    LTTNG_UST_TP_FIELDS(
        ...
    )
)
------------------------------------------------------------------------

Once the tracepoint class is defined, you can create as many tracepoint
instances as needed:

-------------------------------------------------------------------------
#define LTTNG_UST_TRACEPOINT_PROVIDER natality

/* ... */

LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(
    /* Name of the tracepoint class provider */
    my_provider,

    /* Tracepoint class name */
    my_tracepoint_class,

    /* Name of the local (instance) tracepoint provider */
    natality,

    /* Tracepoint/event name */
    my_tracepoint,

    /* List of tracepoint arguments (input) */
    LTTNG_UST_TP_ARGS(
        ...
    )
)
------------------------------------------------------------------------

As you can see, the `LTTNG_UST_TRACEPOINT_EVENT_INSTANCE()` does not
contain the `LTTNG_UST_TP_FIELDS()` macro, because they are defined at
the `LTTNG_UST_TRACEPOINT_EVENT_CLASS()` level.

Note that the `LTTNG_UST_TRACEPOINT_EVENT_INSTANCE()` macro requires two
provider names:

* The name of the tracepoint class provider (`my_provider` in the
  example above).
+
This is the same as the first argument of the
`LTTNG_UST_TRACEPOINT_EVENT_CLASS()` expansion to refer to.

* The name of the local, or instance, provider (`natality` in the
  example above).
+
This is the provider name which becomes the prefix part of the name of
the events which such a tracepoint creates.

The two provider names may be different if the tracepoint class and the
tracepoint instance macros are in two different translation units.

See the <<example,EXAMPLE>> section below for a complete example.


[[tracepoint-loglevel]]
`LTTNG_UST_TRACEPOINT_LOGLEVEL()` usage
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Optionally, a *log level* can be assigned to a defined tracepoint.
Assigning different levels of severity to tracepoints can be useful:
when controlling tracing sessions, you can choose to only enable
events falling into a specific log level range using the
nloption:--loglevel and nloption:--loglevel-only options of the
man:lttng-enable-event(1) command.

Log levels are assigned to tracepoints that are already defined using
the `LTTNG_UST_TRACEPOINT_LOGLEVEL()` macro. The latter must be used
after having used `LTTNG_UST_TRACEPOINT_EVENT()` or
`LTTNG_UST_TRACEPOINT_EVENT_INSTANCE()` for a given tracepoint. The
`LTTNG_UST_TRACEPOINT_LOGLEVEL()` macro is used as follows:

------------------------------------------------------------------------
LTTNG_UST_TRACEPOINT_LOGLEVEL(
    /* Tracepoint provider name */
    my_provider,

    /* Tracepoint/event name */
    my_tracepoint,

    /* Log level */
    LTTNG_UST_TRACEPOINT_LOGLEVEL_INFO
)
------------------------------------------------------------------------

The available log level definitions are:

include::log-levels.txt[]

See the <<example,EXAMPLE>> section below for a complete example.


[[tracepoint]]
Instrumenting your application
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Once the tracepoint provider is created (see the
<<creating-tp,Creating a tracepoint provider>> section above), you can
instrument your application with the defined tracepoints thanks to the
`lttng_ust_tracepoint()` macro:

[verse]
#define *lttng_ust_tracepoint*('prov_name', 't_name', ...)

With:

'prov_name'::
    Tracepoint provider name.

't_name'::
    Tracepoint/event name.

`...`::
    Tracepoint arguments, if any.

Make sure to include the tracepoint provider header file anywhere you
use `lttng_ust_tracepoint()` for this provider.

NOTE: Even though LTTng-UST supports `lttng_ust_tracepoint()` call site
duplicates having the same provider and tracepoint names, it is
recommended to use a provider/tracepoint name pair only once within the
application source code to help map events back to their call sites when
analyzing the trace.

Sometimes, arguments to the tracepoint are expensive to compute (take
call stack, for example). To avoid the computation when the tracepoint
is disabled, you can use the `lttng_ust_tracepoint_enabled()` and
`lttng_ust_do_tracepoint()` macros:

[verse]
#define *lttng_ust_tracepoint_enabled*('prov_name', 't_name')
#define *lttng_ust_do_tracepoint*('prov_name', 't_name', ...)

`lttng_ust_tracepoint_enabled()` returns a non-zero value if the tracepoint
named 't_name' from the provider named 'prov_name' is enabled at
run time.

`lttng_ust_do_tracepoint()` is like `lttng_ust_tracepoint()`, except that it doesn't check
if the tracepoint is enabled. Using `lttng_ust_tracepoint()` with
`lttng_ust_tracepoint_enabled()` is dangerous since `lttng_ust_tracepoint()` also contains
the `lttng_ust_tracepoint_enabled()` check, thus a race condition is possible
in this situation:

------------------------------------------------------------------------
if (lttng_ust_tracepoint_enabled(my_provider, my_tracepoint)) {
    stuff = prepare_stuff();
}

lttng_ust_tracepoint(my_provider, my_tracepoint, stuff);
------------------------------------------------------------------------

If the tracepoint is enabled after the condition, then `stuff` is not
prepared: the emitted event will either contain wrong data, or the
whole application could crash (segmentation fault, for example).

NOTE: Neither `lttng_ust_tracepoint_enabled()` nor
`lttng_ust_do_tracepoint()` have a `STAP_PROBEV()` call, so if you need
it, you should emit this call yourself.

Tracing C/$$C++$$ constructors and destructors
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If one of the following is true:

* You compile your $$C++$$ application with GCC{nbsp}≤{nbsp}4.8.

* You compile your C/$$C++$$ application with a $$C++$$ compiler and
  define `LTTNG_UST_ALLOCATE_COMPOUND_LITERAL_ON_HEAP`.

Then LTTng won't trace:

* C constructors and destructors in the application itself or in
  statically linked archives.

* Some $$C++$$ constructors and destructors in the application
  itself or in statically linked archives.
+
In this case, which exact $$C++$$ constructors and destructors won't be
traced depends on the initialization order within each translation unit
and across the entire program when all translation units are linked
together.

[[build-static]]
Statically linking the tracepoint provider
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
With the static linking method, compiled tracepoint providers are copied
into the target application.

Define `LTTNG_UST_TRACEPOINT_DEFINE` definition below the
`LTTNG_UST_TRACEPOINT_CREATE_PROBES` definition in the tracepoint
provider source:

------------------------------------------------------------------------
#define LTTNG_UST_TRACEPOINT_CREATE_PROBES
#define LTTNG_UST_TRACEPOINT_DEFINE

#include "tp.h"
------------------------------------------------------------------------

Create the tracepoint provider object file:

[role="term"]
----
$ cc -c -I. tp.c
----

NOTE: Although an application instrumented with LTTng-UST tracepoints
can be compiled with a $$C++$$ compiler, tracepoint probes
should be compiled with a C compiler.

At this point, you _can_ archive this tracepoint provider object file,
possibly with other object files of your application or with other
tracepoint provider object files, as a static library:

[role="term"]
----
$ ar rc tp.a tp.o
----

Using a static library does have the advantage of centralising the
tracepoint providers objects so they can be shared between multiple
applications. This way, when the tracepoint provider is modified, the
source code changes don't have to be patched into the source code tree
of each application. The applications need to be relinked after each
change, but need not to be otherwise recompiled (unless the API of the
tracepoint provider changes).

Then, link your application with this object file (or with the static
library containing it) and with `liblttng-ust` and `libdl` (`libc` on a
BSD system):

[role="term"]
----
$ cc -o app tp.o app.o -llttng-ust -ldl
----


[[build-dynamic]]
Dynamically loading the tracepoint provider
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The second approach to package the tracepoint provider is to use the
dynamic loader: the library and its member functions are explicitly
sought, loaded at run time.

In this scenario, the tracepoint provider is compiled as a shared
object.

The process to create the tracepoint provider shared object is pretty
much the same as the <<build-static,static linking method>>, except
that:

  * Since the tracepoint provider is not part of the application,
    `LTTNG_UST_TRACEPOINT_DEFINE` must be defined, for each tracepoint
    provider, in exactly one source file of the _application_
  * `LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE` must be defined next to
    `LTTNG_UST_TRACEPOINT_DEFINE`

Regarding `LTTNG_UST_TRACEPOINT_DEFINE` and
`LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE`, the recommended practice
is to use a separate C source file in your application to define them,
then include the tracepoint provider header files afterwards. For
example, as `tp-define.c`:

------------------------------------------------------------------------
#define LTTNG_UST_TRACEPOINT_DEFINE
#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE

#include "tp.h"
------------------------------------------------------------------------

The tracepoint provider object file used to create the shared library is
built like it is using the static linking method, but with the
nloption:-fpic option:

[role="term"]
----
$ cc -c -fpic -I. tp.c
----

It is then linked as a shared library like this:

[role="term"]
----
$ cc -shared -Wl,--no-as-needed -o tp.so tp.o -llttng-ust
----

This tracepoint provider shared object isn't linked with the user
application: it must be loaded manually. This is why the application is
built with no mention of this tracepoint provider, but still needs
libdl:

[role="term"]
----
$ cc -o app app.o tp-define.o -ldl
----

There are two ways to dynamically load the tracepoint provider shared
object:

  * Load it manually from the application using man:dlopen(3)
  * Make the dynamic loader load it with the `LD_PRELOAD`
    environment variable (see man:ld.so(8))

If the application does not dynamically load the tracepoint provider
shared object using one of the methods above, tracing is disabled for
this application, and the events are not listed in the output of
man:lttng-list(1).

Note that it is not safe to use man:dlclose(3) on a tracepoint provider
shared object that is being actively used for tracing, due to a lack of
reference counting from LTTng-UST to the shared object.

For example, statically linking a tracepoint provider to a shared object
which is to be dynamically loaded by an application (a plugin, for
example) is not safe: the shared object, which contains the tracepoint
provider, could be dynamically closed (man:dlclose(3)) at any time by
the application.

To instrument a shared object, either:

  * Statically link the tracepoint provider to the application, or
  * Build the tracepoint provider as a shared object (following the
    procedure shown in this section), and preload it when tracing is
    needed using the `LD_PRELOAD` environment variable.


Using LTTng-UST with daemons
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Some extra care is needed when using `liblttng-ust` with daemon
applications that call man:fork(2), man:clone(2), or BSD's man:rfork(2)
without a following man:exec(3) family system call. The library
`liblttng-ust-fork.so` needs to be preloaded before starting the
application with the `LD_PRELOAD` environment variable (see
man:ld.so(8)).

To use `liblttng-ust` with a daemon application which closes file
descriptors that were not opened by it, preload the `liblttng-ust-fd.so`
library before you start the application. Typical use cases include
daemons closing all file descriptors after man:fork(2), and buggy
applications doing ``double-closes''.


Context information
~~~~~~~~~~~~~~~~~~~
Context information can be prepended by the LTTng-UST tracer before
each event, or before specific events.

Context fields can be added to specific channels using
man:lttng-add-context(1).

The following context fields are supported by LTTng-UST:

General context fields::
+
`cpu_id`:::
    CPU ID.
+
NOTE: With the default per-CPU buffer allocation policy (see
man:lttng-enable-channel(1)), this context field is always enabled, and
it cannot be added with man:lttng-add-context(1): its main purpose is to
be used for dynamic event filtering. See man:lttng-enable-event(1) for
more information about event filtering.

`ip`:::
    Instruction pointer: enables recording the exact address from which
    an event was emitted. This context field can be used to
    reverse-lookup the source location that caused the event
    to be emitted.

`pthread_id`:::
    POSIX thread identifier.
+
Can be used on architectures where `pthread_t` maps nicely to an
`unsigned long` type.

Process context fields::
+
`procname`:::
    Thread name, as set by man:exec(3) or man:prctl(2). It is
    recommended that programs set their thread name with man:prctl(2)
    before hitting the first tracepoint for that thread.

`vpid`:::
    Virtual process ID: process ID as seen from the point of view of the
    current process ID namespace (see man:pid_namespaces(7)).

`vtid`:::
    Virtual thread ID: thread ID as seen from the point of view of the
    current process ID namespace (see man:pid_namespaces(7)).

perf context fields::
+
`perf:thread:COUNTER`:::
    perf counter named 'COUNTER'. Use `lttng add-context --list` to
    list the available perf counters.
+
Only available on IA-32 and x86-64 architectures.

`perf:thread:raw:rN:NAME`:::
    perf counter with raw ID 'N' and custom name 'NAME'. See
    man:lttng-add-context(1) for more details.

Namespace context fields (see man:namespaces(7))::
+
`cgroup_ns`:::
    Inode number of the current control group namespace (see
    man:cgroup_namespaces(7)) in the proc file system.

`ipc_ns`:::
    Inode number of the current IPC namespace (see
    man:ipc_namespaces(7)) in the proc file system.

`mnt_ns`:::
    Inode number of the current mount point namespace (see
    man:mount_namespaces(7)) in the proc file system.

`net_ns`:::
    Inode number of the current network namespace (see
    man:network_namespaces(7)) in the proc file system.

`pid_ns`:::
    Inode number of the current process ID namespace (see
    man:pid_namespaces(7)) in the proc file system.

`time_ns`:::
    Inode number of the current clock namespace (see
    man:time_namespaces(7)) in the proc file system.

`user_ns`:::
    Inode number of the current user namespace (see
    man:user_namespaces(7)) in the proc file system.

`uts_ns`:::
    Inode number of the current UTS namespace (see
    man:uts_namespaces(7)) in the proc file system.

Credential context fields (see man:credentials(7))::
+
`vuid`:::
    Virtual real user ID: real user ID as seen from the point of view of
    the current user namespace (see man:user_namespaces(7)).

`vgid`:::
    Virtual real group ID: real group ID as seen from the point of view
    of the current user namespace (see man:user_namespaces(7)).

`veuid`:::
    Virtual effective user ID: effective user ID as seen from the point
    of view of the current user namespace (see man:user_namespaces(7)).

`vegid`:::
    Virtual effective group ID: effective group ID as seen from the
    point of view of the current user namespace (see
    man:user_namespaces(7)).

`vsuid`:::
    Virtual saved set-user ID: saved set-user ID as seen from the point
    of view of the current user namespace (see man:user_namespaces(7)).

`vsgid`:::
    Virtual saved set-group ID: saved set-group ID as seen from the
    point of view of the current user namespace (see
    man:user_namespaces(7)).


[[state-dump]]
LTTng-UST state dump
~~~~~~~~~~~~~~~~~~~~
If an application that uses `liblttng-ust` becomes part of a tracing
session, information about its currently loaded shared objects, their
build IDs, and their debug link information are emitted as events
by the tracer.

The following LTTng-UST state dump events exist and must be enabled
to record application state dumps. Note that, during the state dump
phase, LTTng-UST can also emit _shared library load/unload_ events
(see <<ust-lib,Shared library load/unload tracking>> below).

`lttng_ust_statedump:start`::
    Emitted when the state dump begins.
+
This event has no fields.

`lttng_ust_statedump:end`::
    Emitted when the state dump ends. Once this event is emitted, it
    is guaranteed that, for a given process, the state dump is
    complete.
+
This event has no fields.

`lttng_ust_statedump:bin_info`::
    Emitted when information about a currently loaded executable or
    shared object is found.
+
Fields:
+
[options="header"]
|===
|Field name |Description

|`baddr`
|Base address of loaded executable.

|`memsz`
|Size of loaded executable in memory.

|`path`
|Path to loaded executable file.

|`is_pic`
|Whether or not the executable is position-independent code.

|`has_build_id`
|Whether or not the executable has a build ID. If this field is 1, you
can expect that an `lttng_ust_statedump:build_id` event record follows
this one (not necessarily immediately after).

|`has_debug_link`
|Whether or not the executable has debug link information. If this field
is 1, you can expect that an `lttng_ust_statedump:debug_link` event
record follows this one (not necessarily immediately after).
|===

`lttng_ust_statedump:build_id`::
    Emitted when a build ID is found in a currently loaded shared
    library. See
    https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html[Debugging Information in Separate Files]
    for more information about build IDs.
+
Fields:
+
[options="header"]
|===
|Field name |Description

|`baddr`
|Base address of loaded library.

|`build_id`
|Build ID.
|===

`lttng_ust_statedump:debug_link`::
    Emitted when debug link information is found in a currently loaded
    shared library. See
    https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html[Debugging Information in Separate Files]
    for more information about debug links.
+
Fields:
+
[options="header"]
|===
|Field name |Description

|`baddr`
|Base address of loaded library.

|`crc`
|CRC of debug link file.

|`filename`
|Debug link file name.
|===

`lttng_ust_statedump:procname`::
    The process procname at process start.
+
Fields:
+
[options="header"]
|===
|Field name |Description

|`procname`
|The process name.

|===


[[ust-lib]]
Shared library load/unload tracking
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The <<state-dump,LTTng-UST state dump>> and the LTTng-UST helper library
to instrument the dynamic linker (see man:liblttng-ust-dl(3)) can emit
**shared library load/unload tracking** events.

The following shared library load/unload tracking events exist and must
be enabled to track the loading and unloading of shared libraries:

`lttng_ust_lib:load`::
    Emitted when a shared library (shared object) is loaded.
+
Fields:
+
[options="header"]
|===
|Field name |Description

|`baddr`
|Base address of loaded library.

|`memsz`
|Size of loaded library in memory.

|`path`
|Path to loaded library file.

|`has_build_id`
|Whether or not the library has a build ID. If this field is 1, you
can expect that an `lttng_ust_lib:build_id` event record follows
this one (not necessarily immediately after).

|`has_debug_link`
|Whether or not the library has debug link information. If this field
is 1, you can expect that an `lttng_ust_lib:debug_link` event
record follows this one (not necessarily immediately after).
|===

`lttng_ust_lib:unload`::
    Emitted when a shared library (shared object) is unloaded.
+
Fields:
+
[options="header"]
|===
|Field name |Description

|`baddr`
|Base address of unloaded library.
|===

`lttng_ust_lib:build_id`::
    Emitted when a build ID is found in a loaded shared library (shared
    object). See
    https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html[Debugging Information in Separate Files]
    for more information about build IDs.
+
Fields:
+
[options="header"]
|===
|Field name |Description

|`baddr`
|Base address of loaded library.

|`build_id`
|Build ID.
|===

`lttng_ust_lib:debug_link`::
    Emitted when debug link information is found in a loaded
    shared library (shared object). See
    https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html[Debugging Information in Separate Files]
    for more information about debug links.
+
Fields:
+
[options="header"]
|===
|Field name |Description

|`baddr`
|Base address of loaded library.

|`crc`
|CRC of debug link file.

|`filename`
|Debug link file name.
|===


Detect if LTTng-UST is loaded
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To detect if `liblttng-ust` is loaded from an application:

. Define the `lttng_ust_loaded` weak symbol globally:
+
------------------------------------------------------------------------
int lttng_ust_loaded __attribute__((weak));
------------------------------------------------------------------------
+
This weak symbol is set by the constructor of `liblttng-ust`.

. Test `lttng_ust_loaded` where needed:
+
------------------------------------------------------------------------
/* ... */

if (lttng_ust_loaded) {
    /* LTTng-UST is loaded */
} else {
    /* LTTng-UST is NOT loaded */
}

/* ... */
------------------------------------------------------------------------


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

NOTE: A few examples are available in the
https://github.com/lttng/lttng-ust/tree/stable-{lttng_version}/doc/examples[`doc/examples`]
directory of LTTng-UST's source tree.

This example shows all the features documented in the previous
sections. The <<build-static,static linking>> method is chosen here
to link the application with the tracepoint provider.

You can compile the source files and link them together statically
like this:

[role="term"]
----
$ cc -c -I. tp.c
$ cc -c app.c
$ cc -o app tp.o app.o -llttng-ust -ldl
----

Using the man:lttng(1) tool, create an LTTng tracing session, enable
all the events of this tracepoint provider, and start tracing:

[role="term"]
----
$ lttng create my-session
$ lttng enable-event --userspace 'my_provider:*'
$ lttng start
----

You may also enable specific events:

[role="term"]
----
$ lttng enable-event --userspace my_provider:big_event
$ lttng enable-event --userspace my_provider:event_instance2
----

Run the application:

[role="term"]
----
$ ./app some arguments
----

Stop the current tracing session and inspect the recorded events:

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


Tracepoint provider header file
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
`tp.h`:

------------------------------------------------------------------------
#undef LTTNG_UST_TRACEPOINT_PROVIDER
#define LTTNG_UST_TRACEPOINT_PROVIDER my_provider

#undef LTTNG_USTTRACEPOINT_INCLUDE
#define LTTNG_USTTRACEPOINT_INCLUDE "./tp.h"

#if !defined(_TP_H) || \
    defined(LTTNG_UST_TRACEPOINT_HEADER_MULTI_READ)
#define _TP_H

#include <lttng/tracepoint.h>
#include <stdio.h>

#include "app.h"

LTTNG_UST_TRACEPOINT_EVENT(
    my_provider,
    simple_event,
    LTTNG_UST_TP_ARGS(
        int, my_integer_arg,
        const char *, my_string_arg
    ),
    LTTNG_UST_TP_FIELDS(
        lttng_ust_field_string(argc, my_string_arg)
        lttng_ust_field_integer(int, argv, my_integer_arg)
    )
)

LTTNG_UST_TRACEPOINT_ENUM(
    my_provider,
    my_enum,
    LTTNG_UST_TP_ENUM_VALUES(
        lttng_ust_field_enum_value("ZERO", 0)
        lttng_ust_field_enum_value("ONE", 1)
        lttng_ust_field_enum_value("TWO", 2)
        lttng_ust_field_enum_range("A RANGE", 52, 125)
        lttng_ust_field_enum_value("ONE THOUSAND", 1000)
    )
)

LTTNG_UST_TRACEPOINT_EVENT(
    my_provider,
    big_event,
    LTTNG_UST_TP_ARGS(
        int, my_integer_arg,
        const char *, my_string_arg,
        FILE *, stream,
        double, flt_arg,
        int *, array_arg
    ),
    LTTNG_UST_TP_FIELDS(
        lttng_ust_field_integer(int, int_field1, my_integer_arg * 2)
        lttng_ust_field_integer_hex(long int, stream_pos,
                                    ftell(stream))
        lttng_ust_field_float(double, float_field, flt_arg)
        lttng_ust_field_string(string_field, my_string_arg)
        lttng_ust_field_array(int, array_field, array_arg, 7)
        lttng_ust_field_array_text(char, array_text_field,
                                   array_arg, 5)
        lttng_ust_field_sequence(int, seq_field, array_arg, unsigned int,
                                 my_integer_arg / 10)
        lttng_ust_field_sequence_text(char, seq_text_field,
                                      array_arg, unsigned int,
                                      my_integer_arg / 5)
        lttng_ust_field_enum(my_provider, my_enum, int,
                             enum_field, array_arg[1])
    )
)

LTTNG_UST_TRACEPOINT_LOGLEVEL(my_provider, big_event,
                              LTTNG_UST_TRACEPOINT_LOGLEVEL_WARNING)

LTTNG_UST_TRACEPOINT_EVENT_CLASS(
    my_provider,
    my_tracepoint_class,
    LTTNG_UST_TP_ARGS(
        int, my_integer_arg,
        struct app_struct *, app_struct_arg
    ),
    LTTNG_UST_TP_FIELDS(
        lttng_ust_field_integer(int, a, my_integer_arg)
        lttng_ust_field_integer(unsigned long, b, app_struct_arg->b)
        lttng_ust_field_string(c, app_struct_arg->c)
    )
)

LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(
    my_provider,
    my_tracepoint_class,
    my_provider,
    event_instance1,
    LTTNG_UST_TP_ARGS(
        int, my_integer_arg,
        struct app_struct *, app_struct_arg
    )
)

LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(
    my_provider,
    my_tracepoint_class,
    my_provider,
    event_instance2,
    LTTNG_UST_TP_ARGS(
        int, my_integer_arg,
        struct app_struct *, app_struct_arg
    )
)

LTTNG_UST_TRACEPOINT_LOGLEVEL(my_provider, event_instance2,
                              LTTNG_UST_TRACEPOINT_LOGLEVEL_INFO)

LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(
    my_provider,
    my_tracepoint_class,
    my_provider,
    event_instance3,
    LTTNG_UST_TP_ARGS(
        int, my_integer_arg,
        struct app_struct *, app_struct_arg
    )
)

#endif /* _TP_H */

#include <lttng/tracepoint-event.h>
------------------------------------------------------------------------


Tracepoint provider source file
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
`tp.c`:

------------------------------------------------------------------------
#define LTTNG_UST_TRACEPOINT_CREATE_PROBES
#define LTTNG_UST_TRACEPOINT_DEFINE

#include "tp.h"
------------------------------------------------------------------------


Application header file
~~~~~~~~~~~~~~~~~~~~~~~
`app.h`:

------------------------------------------------------------------------
#ifndef _APP_H
#define _APP_H

struct app_struct {
    unsigned long b;
    const char *c;
    double d;
};

#endif /* _APP_H */
------------------------------------------------------------------------


Application source file
~~~~~~~~~~~~~~~~~~~~~~~
`app.c`:

------------------------------------------------------------------------
#include <stdlib.h>
#include <stdio.h>

#include "tp.h"
#include "app.h"

static int array_of_ints[] = {
    100, -35, 1, 23, 14, -6, 28, 1001, -3000,
};

int main(int argc, char* argv[])
{
    FILE *stream;
    struct app_struct app_struct;

    lttng_ust_tracepoint(my_provider, simple_event, argc, argv[0]);
    stream = fopen("/tmp/app.txt", "w");

    if (!stream) {
        fprintf(stderr,
                "Error: Cannot open /tmp/app.txt for writing\n");
        return EXIT_FAILURE;
    }

    if (fprintf(stream, "0123456789") != 10) {
        fclose(stream);
        fprintf(stderr, "Error: Cannot write to /tmp/app.txt\n");
        return EXIT_FAILURE;
    }

    lttng_ust_tracepoint(my_provider, big_event, 35,
                         "hello tracepoint", stream, -3.14,
                         array_of_ints);
    fclose(stream);
    app_struct.b = argc;
    app_struct.c = "[the string]";
    lttng_ust_tracepoint(my_provider, event_instance1, 23,
                         &app_struct);
    app_struct.b = argc * 5;
    app_struct.c = "[other string]";
    lttng_ust_tracepoint(my_provider, event_instance2, 17,
                         &app_struct);
    app_struct.b = 23;
    app_struct.c = "nothing";
    lttng_ust_tracepoint(my_provider, event_instance3, -52,
                         &app_struct);
    return EXIT_SUCCESS;
}
------------------------------------------------------------------------


ENVIRONMENT VARIABLES
---------------------
`LTTNG_HOME`::
    Alternative user's home directory. This variable is useful when the
    user running the instrumented application has a non-writable home
    directory. This path is where Unix sockets for communication with
    the per-user session daemon are located.

`LTTNG_UST_ABORT_ON_CRITICAL`::
    If set, abort the instrumented application on a critical
    error message.

`LTTNG_UST_ALLOW_BLOCKING`::
    If set, allow the application to retry event tracing when there's
    no space left for the event record in the sub-buffer, therefore
    effectively blocking the application until space is made available
    or the configured timeout is reached.
+
To allow an application to block during tracing, you also need to
specify a blocking timeout when you create a channel with the
nloption:--blocking-timeout option of the man:lttng-enable-channel(1)
command.
+
This option can be useful in workloads generating very large trace data
throughput, where blocking the application is an acceptable trade-off to
prevent discarding event records.
+
WARNING: Setting this environment variable may significantly
affect application timings.

`LTTNG_UST_APP_PATH`::
    Path of the directory where to expect the LTTng session daemon to
    place the following files to receive instrumented application
    connection requests:
+
--
* The application registration Unix socket.

* The "wait" shared memory files to wake up instrumented applications
  that are waiting for a session daemon to start.

* The agent port file.
+
This file contains the TCP port the agents must connect to in order to
enable LTTng tracing.
--
+
This directory must exist when you start the application for LTTng
tracing to be enabled.
+
When `LTTNG_UST_APP_PATH` is set, liblttng-ust:
+
--
* Only considers this path to connect to a session daemon.
* Won't connect to root and/or user session daemons as usual.
--
+
See the corresponding `LTTNG_UST_CTL_PATH` environment variable of
man:lttng-sessiond(8).

`LTTNG_UST_CLOCK_PLUGIN`::
    Path to the shared object which acts as the clock override plugin.
    An example of such a plugin can be found in the LTTng-UST
    documentation under
    https://github.com/lttng/lttng-ust/tree/stable-{lttng_version}/doc/examples/clock-override[`examples/clock-override`].

`LTTNG_UST_DEBUG`::
    If set, enable the debug and error output of `liblttng-ust`.

`LTTNG_UST_GETCPU_PLUGIN`::
    Path to the shared object which acts as the `getcpu()` override
    plugin. An example of such a plugin can be found in the LTTng-UST
    documentation under
    https://github.com/lttng/lttng-ust/tree/stable-{lttng_version}/doc/examples/getcpu-override[`examples/getcpu-override`].

`LTTNG_UST_MAP_POPULATE_POLICY`::
    If set, override the policy used to populate shared memory pages
    within the application.
+
The possible values are:

`none` (default):::
  Do not pre-populate any page: take minor faults on first access
  while tracing.

`cpu_possible`:::
  Pre-populate pages for all possible CPUs in the system, as
  shown in `/sys/devices/system/cpu/possible`.

`LTTNG_UST_REGISTER_TIMEOUT`::
    Waiting time for the _registration done_ session daemon command
    before proceeding to execute the main program (milliseconds).
+
The value `0` means _do not wait_. The value `-1` means _wait forever_.
Setting this environment variable to `0` is recommended for applications
with time constraints on the process startup time.
+
Default: 3000.

`LTTNG_UST_WITHOUT_BADDR_STATEDUMP`::
    If set, prevents `liblttng-ust` from performing a base address state
    dump (see the <<state-dump,LTTng-UST state dump>> section above).

`LTTNG_UST_WITHOUT_PROCNAME_STATEDUMP`::
    If set, prevents `liblttng-ust` from performing a procname state
    dump (see the <<state-dump,LTTng-UST state dump>> section above).


include::common-footer.txt[]

include::common-copyrights.txt[]

include::common-authors.txt[]


SEE ALSO
--------
man:lttng_ust_tracef(3),
man:lttng_ust_tracelog(3),
man:lttng-gen-tp(1),
man:lttng-ust-dl(3),
man:lttng-ust-cyg-profile(3),
man:lttng(1),
man:lttng-enable-event(1),
man:lttng-list(1),
man:lttng-add-context(1),
man:babeltrace(1),
man:dlopen(3),
man:ld.so(8)
