Document LTTng 2.13

[lttng-docs.git] / 2.13 / lttng-docs-2.13.txt

The LTTng Documentation
=======================
Philippe Proulx <pproulx@efficios.com>
v2.13, 15 June 2021


include::../common/copyright.txt[]


include::../common/welcome.txt[]


include::../common/audience.txt[]


[[chapters]]
=== What's in this documentation?

The LTTng Documentation is divided into the following sections:

* ``**<<nuts-and-bolts,Nuts and bolts>>**'' explains the
  rudiments of software tracing and the rationale behind the
  LTTng project.
+
Skip this section if you’re familiar with software tracing and with the
LTTng project.

* ``**<<installing-lttng,Installation>>**'' describes the steps to
  install the LTTng packages on common Linux distributions and from
  their sources.
+
Skip this section if you already properly installed LTTng on your target
system.

* ``**<<getting-started,Quick start>>**'' is a concise guide to
  get started quickly with LTTng kernel and user space tracing.
+
We recommend this section if you're new to LTTng or to software tracing
in general.
+
Skip this section if you're not new to LTTng.

* ``**<<core-concepts,Core concepts>>**'' explains the concepts at
  the heart of LTTng.
+
It's a good idea to become familiar with the core concepts
before attempting to use the toolkit.

* ``**<<plumbing,Components of LTTng>>**'' describes the various
  components of the LTTng machinery, like the daemons, the libraries,
  and the command-line interface.

* ``**<<instrumenting,Instrumentation>>**'' shows different ways to
  instrument user applications and the Linux kernel for LTTng tracing.
+
Instrumenting source code is essential to provide a meaningful
source of events.
+
Skip this section if you don't have a programming background.

* ``**<<controlling-tracing,Tracing control>>**'' is divided into topics
  which demonstrate how to use the vast array of features that
  LTTng{nbsp}{revision} offers.

* ``**<<reference,Reference>>**'' contains API reference tables.

* ``**<<glossary,Glossary>>**'' is a specialized dictionary of terms
  related to LTTng or to the field of software tracing.


include::../common/convention.txt[]


include::../common/acknowledgements.txt[]


[[whats-new]]
== What's new in LTTng{nbsp}{revision}?

LTTng{nbsp}{revision} bears the name _Nordicité_, the product of a
collaboration between https://champlibre.co/[Champ Libre] and
https://champlibre.co/[Boréale]. This farmhouse IPA is brewed with
https://en.wikipedia.org/wiki/Kveik[Kveik] yeast and Québec-grown
barley, oats, and juniper branches. The result is a remarkable, fruity,
hazy golden IPA that offers a balanced touch of resinous and woodsy
bitterness.

New features and changes in LTTng{nbsp}{revision}:

General::
+
* The LTTng trigger API of <<liblttng-ctl-lttng,`liblttng-ctl`>> now
  offers the ``__event rule matches__'' condition (an <<event-rule,event
  rule>> matches an event) as well as the following new actions:
+
--
* <<basic-tracing-session-control,Start or stop>> a recording session.
* <<session-rotation,Archive the current trace chunk>> of a
  recording session (rotate).
* <<taking-a-snapshot,Take a snapshot>> of a recording session.
--
+
As a reminder, a <<trigger,trigger>> is a condition-actions pair. When
the condition of a trigger is satisfied, LTTng attempts to execute its
actions.
+
This feature is also available with the new man:lttng-add-trigger(1),
man:lttng-remove-trigger(1), and man:lttng-list-triggers(1)
<<lttng-cli,cmd:lttng>> commands.
+
Starting from LTTng{nbsp}{revision}, a trigger may have more than one
action.
+
See “<<add-event-rule-matches-trigger,Add an ``event rule matches''
trigger to a session daemon>>” to learn more.

* The LTTng <<lttng-ust,user space>> and <<lttng-modules,kernel>>
  tracers offer the new namespace context field `time_ns`, which is the
  inode number, in the proc file system, of the current clock namespace.
+
See man:lttng-add-context(1), man:lttng-ust(3), and
man:time_namespaces(7).

* The link:/man[manual pages] of LTTng-tools now have a terminology and
  style which match the LTTng Documentation, many fixes, more internal
  and manual page links, clearer lists and procedures, superior
  consistency, and usage examples.
+
The new man:lttng-event-rule(7) manual page explains the new, common
way to specify an event rule on the command line.
+
The new man:lttng-concepts(7) manual page explains the core concepts of
LTTng. Its contents is essentially the ``<<core-concepts,Core
concepts>>'' section of this documentation, but more adapted to the
manual page style.

User space tracing::
+
[IMPORTANT]
====
The major version part of the `liblttng-ust`
https://en.wikipedia.org/wiki/Soname[soname] is bumped, which means you
**must recompile** your instrumented applications/libraries and
<<tracepoint-provider,tracepoint provider packages>> to use
LTTng-UST{nbsp}{revision}.

This change became a necessity to clean up the library and for
`liblttng-ust` to stop exporting private symbols.

Also, LTTng{nbsp}{revision} prepends the `lttng_ust_` and `LTTNG_UST_`
prefix to all public macro/definition/function names to offer a
consistent API namespace. The LTTng{nbsp}2.12 API is still available;
see the ``Compatibility with previous APIs'' section of
man:lttng-ust(3).
====
+
Other notable changes:
+
* The `liblttng-ust` C{nbsp}API offers the new man:lttng_ust_vtracef(3)
  and man:lttng_ust_vtracelog(3) macros which are to
  man:lttng_ust_tracef(3) and man:lttng_ust_tracelog(3) what
  man:vprintf(3) is to man:printf(3).

* LTTng-UST now only depends on https://liburcu.org/[`liburcu`] at build
  time, not at run time.

Kernel tracing::
+
* The preferred display base of event record integer fields which
  contain memory addresses is now hexadecimal instead of decimal.

* The `pid` field is removed from `lttng_statedump_file_descriptor`
  event records and the `file_table_address` field is added.
+
This new field is the address of the `files_struct` structure which
contains the file descriptor.
+
See the
``https://github.com/lttng/lttng-modules/commit/e7a0ca7205fd4be7c829d171baa8823fe4784c90[statedump: introduce `file_table_address`]''
patch to learn more.

* The `flags` field of `syscall_entry_clone` event records is now a
  structure containing two enumerations (exit signal and options).
+
This change makes the flag values more readable and meaningful.
+
See the
``https://github.com/lttng/lttng-modules/commit/d775625e2ba4825b73b5897e7701ad6e2bdba115[syscalls: Make `clone()`'s `flags` field a 2 enum struct]''
patch to learn more.

* The memory footprint of the kernel tracer is improved: the latter only
  generates metadata for the specific system call recording event rules
  that you <<enabling-disabling-events,create>>.


[[nuts-and-bolts]]
== Nuts and bolts

What is LTTng? As its name suggests, the _Linux Trace Toolkit: next
generation_ is a modern toolkit for tracing Linux systems and
applications. So your first question might be:
**what is tracing?**


[[what-is-tracing]]
=== What is tracing?

As the history of software engineering progressed and led to what
we now take for granted--complex, numerous and
interdependent software applications running in parallel on
sophisticated operating systems like Linux--the authors of such
components, software developers, began feeling a natural
urge to have tools that would ensure the robustness and good performance
of their masterpieces.

One major achievement in this field is, inarguably, the
https://www.gnu.org/software/gdb/[GNU debugger (GDB)],
an essential tool for developers to find and fix bugs. But even the best
debugger won't help make your software run faster, and nowadays, faster
software means either more work done by the same hardware, or cheaper
hardware for the same work.

A _profiler_ is often the tool of choice to identify performance
bottlenecks. Profiling is suitable to identify _where_ performance is
lost in a given piece of software. The profiler outputs a profile, a
statistical summary of observed events, which you may use to discover
which functions took the most time to execute. However, a profiler won't
report _why_ some identified functions are the bottleneck. Bottlenecks
might only occur when specific conditions are met, conditions that are
sometimes impossible to capture by a statistical profiler, or impossible
to reproduce with an application altered by the overhead of an
event-based profiler. For a thorough investigation of software
performance issues, a history of execution is essential, with the
recorded values of variables and context fields you choose, and with as
little influence as possible on the instrumented application. This is
where tracing comes in handy.

_Tracing_ is a technique used to understand what goes on in a running
software system. The piece of software used for tracing is called a
_tracer_, which is conceptually similar to a tape recorder. When
recording, specific instrumentation points placed in the software source
code generate events that are saved on a giant tape: a _trace_ file. You
can record user application and operating system events at the same
time, opening the possibility of resolving a wide range of problems that
would otherwise be extremely challenging.

Tracing is often compared to _logging_. However, tracers and loggers are
two different tools, serving two different purposes. Tracers are
designed to record much lower-level events that occur much more
frequently than log messages, often in the range of thousands per
second, with very little execution overhead. Logging is more appropriate
for a very high-level analysis of less frequent events: user accesses,
exceptional conditions (errors and warnings, for example), database
transactions, instant messaging communications, and such. Simply put,
logging is one of the many use cases that can be satisfied with tracing.

The list of recorded events inside a trace file can be read manually
like a log file for the maximum level of detail, but it's generally
much more interesting to perform application-specific analyses to
produce reduced statistics and graphs that are useful to resolve a
given problem. Trace viewers and analyzers are specialized tools
designed to do this.

In the end, this is what LTTng is: a powerful, open source set of
tools to trace the Linux kernel and user applications at the same time.
LTTng is composed of several components actively maintained and
developed by its link:/community/#where[community].


[[lttng-alternatives]]
=== Alternatives to noch:{LTTng}

Excluding proprietary solutions, a few competing software tracers
exist for Linux:

https://github.com/dtrace4linux/linux[dtrace4linux]::
    A port of Sun Microsystems' DTrace to Linux.
+
The cmd:dtrace tool interprets user scripts and is responsible for
loading code into the Linux kernel for further execution and collecting
the outputted data.

https://en.wikipedia.org/wiki/Berkeley_Packet_Filter[eBPF]::
    A subsystem in the Linux kernel in which a virtual machine can
    execute programs passed from the user space to the kernel.
+
You can attach such programs to tracepoints and kprobes thanks to a
system call, and they can output data to the user space when executed
thanks to different mechanisms (pipe, VM register values, and eBPF maps,
to name a few).

https://www.kernel.org/doc/Documentation/trace/ftrace.txt[ftrace]::
    The de facto function tracer of the Linux kernel.
+
Its user interface is a set of special files in sysfs.

https://perf.wiki.kernel.org/[perf]::
    A performance analysis tool for Linux which supports hardware
    performance counters, tracepoints, as well as other counters and
    types of probes.
+
The controlling utility of perf is the cmd:perf command line/text UI
tool.

https://linux.die.net/man/1/strace[strace]::
    A command-line utility which records system calls made by a
    user process, as well as signal deliveries and changes of process
    state.
+
strace makes use of https://en.wikipedia.org/wiki/Ptrace[ptrace] to
fulfill its function.

https://www.sysdig.org/[sysdig]::
    Like SystemTap, uses scripts to analyze Linux kernel events.
+
You write scripts, or _chisels_ in the jargon of sysdig, in Lua and
sysdig executes them while it traces the system or afterwards. The
interface of sysdig is the cmd:sysdig command-line tool as well as the
text UI-based cmd:csysdig tool.

https://sourceware.org/systemtap/[SystemTap]::
    A Linux kernel and user space tracer which uses custom user scripts
    to produce plain text traces.
+
SystemTap converts the scripts to the C language, and then compiles them
as Linux kernel modules which are loaded to produce trace data. The
primary user interface of SystemTap is the cmd:stap command-line tool.

The main distinctive features of LTTng is that it produces correlated
kernel and user space traces, as well as doing so with the lowest
overhead amongst other solutions. It produces trace files in the
https://diamon.org/ctf[CTF] format, a file format optimized
for the production and analyses of multi-gigabyte data.

LTTng is the result of more than 10{nbsp}years of active open source
development by a community of passionate developers. LTTng is currently
available on major desktop and server Linux distributions.

The main interface for tracing control is a single command-line tool
named cmd:lttng. The latter can create several recording sessions, enable
and disable recording event rules on the fly, filter events efficiently
with custom user expressions, start and stop tracing, and much more.
LTTng can write the traces on the file system or send them over the
network, and keep them totally or partially. You can make LTTng execute
user-defined actions when LTTng emits an event. You can view the traces
once tracing becomes inactive or as LTTng records events.

<<installing-lttng,Install LTTng now>> and
<<getting-started,start tracing>>!


[[installing-lttng]]
== Installation

**LTTng** is a set of software <<plumbing,components>> which interact to
<<instrumenting,instrument>> the Linux kernel and user applications, and
to <<controlling-tracing,control tracing>> (start and stop
recording, create recording event rules, and the rest). Those
components are bundled into the following packages:

LTTng-tools::
    Libraries and command-line interface to control tracing.

LTTng-modules::
    Linux kernel modules to instrument and trace the kernel.

LTTng-UST::
    Libraries and Java/Python packages to instrument and trace user
    applications.

Most distributions mark the LTTng-modules and LTTng-UST packages as
optional when installing LTTng-tools (which is always required). In the
following sections, we always provide the steps to install all three,
but note that:

* You only need to install LTTng-modules if you intend to use
  the Linux kernel LTTng tracer.

* You only need to install LTTng-UST if you intend to use the user
  space LTTng tracer.

[NOTE]
====
As of 10{nbsp}June{nbsp}2021, LTTng{nbsp}{revision} is not yet available
in any major non-enterprise Linux distribution.

For https://www.redhat.com/[RHEL] and https://www.suse.com/[SLES]
packages, see https://packages.efficios.com/[EfficiOS Enterprise
Packages].

For other distributions, <<building-from-source,build LTTng from
source>>.
====


[[building-from-source]]
=== Build from source

To build and install LTTng{nbsp}{revision} from source:

. Using the package manager of your distribution, or from source,
  install the following dependencies of LTTng-tools and LTTng-UST:
+
--
* https://sourceforge.net/projects/libuuid/[libuuid]
* https://directory.fsf.org/wiki/Popt[popt]
* https://liburcu.org/[Userspace RCU]
* http://www.xmlsoft.org/[libxml2]
* **Optional**: https://github.com/numactl/numactl[numactl]
--

. Download, build, and install the latest LTTng-modules{nbsp}{revision}:
+
--
[role="term"]
----
$ cd $(mktemp -d) &&
  wget https://lttng.org/files/lttng-modules/lttng-modules-latest-2.13.tar.bz2 &&
  tar -xf lttng-modules-latest-2.13.tar.bz2 &&
  cd lttng-modules-2.13.* &&
  make &&
  sudo make modules_install &&
  sudo depmod -a
----
--

. Download, build, and install the latest LTTng-UST{nbsp}{revision}:
+
--
[role="term"]
----
$ cd $(mktemp -d) &&
  wget https://lttng.org/files/lttng-ust/lttng-ust-latest-2.13.tar.bz2 &&
  tar -xf lttng-ust-latest-2.13.tar.bz2 &&
  cd lttng-ust-2.13.* &&
  ./configure &&
  make &&
  sudo make install &&
  sudo ldconfig
----
--
+
Add `--disable-numa` to `./configure` if you don't have
https://github.com/numactl/numactl[numactl].
+
--
[IMPORTANT]
.Java and Python application tracing
====
If you need to instrument and have LTTng trace <<java-application,Java
applications>>, pass the `--enable-java-agent-jul`,
`--enable-java-agent-log4j`, or `--enable-java-agent-all` options to the
`configure` script, depending on which Java logging framework you use.

If you need to instrument and have LTTng trace
<<python-application,Python applications>>, pass the
`--enable-python-agent` option to the `configure` script. You can set
the env:PYTHON environment variable to the path to the Python interpreter
for which to install the LTTng-UST Python agent package.
====
--
+
--
[NOTE]
====
By default, LTTng-UST libraries are installed to
dir:{/usr/local/lib}, which is the de facto directory in which to
keep self-compiled and third-party libraries.

When <<building-tracepoint-providers-and-user-application,linking an
instrumented user application with `liblttng-ust`>>:

* Append `/usr/local/lib` to the env:LD_LIBRARY_PATH environment
  variable.

* Pass the `-L/usr/local/lib` and `-Wl,-rpath,/usr/local/lib` options to
  man:gcc(1), man:g++(1), or man:clang(1).
====
--

. Download, build, and install the latest LTTng-tools{nbsp}{revision}:
+
--
[role="term"]
----
$ cd $(mktemp -d) &&
  wget https://lttng.org/files/lttng-tools/lttng-tools-latest-2.13.tar.bz2 &&
  tar -xf lttng-tools-latest-2.13.tar.bz2 &&
  cd lttng-tools-2.13.* &&
  ./configure &&
  make &&
  sudo make install &&
  sudo ldconfig
----
--

TIP: The https://github.com/eepp/vlttng[vlttng tool] can do all the
previous steps automatically for a given version of LTTng and confine
the installed files to a specific directory. This can be useful to try
LTTng without installing it on your system.


[[getting-started]]
== Quick start

This is a short guide to get started quickly with LTTng kernel and user
space tracing.

Before you follow this guide, make sure to <<installing-lttng,install>>
LTTng.

This tutorial walks you through the steps to:

. <<tracing-the-linux-kernel,Record Linux kernel events>>.

. <<tracing-your-own-user-application,Record the events of a user
  application>> written in C.

. <<viewing-and-analyzing-your-traces,View and analyze the
  recorded events>>.


[[tracing-the-linux-kernel]]
=== Record Linux kernel events

NOTE: The following command lines start with the `#` prompt because you
need root privileges to control the Linux kernel LTTng tracer. You can
also control the kernel tracer as a regular user if your Unix user is a
member of the <<tracing-group,tracing group>>.

. Create a <<tracing-session,recording session>> to write LTTng traces
  to dir:{/tmp/my-kernel-trace}:
+
--
[role="term"]
----
# lttng create my-kernel-session --output=/tmp/my-kernel-trace
----
--

. List the available kernel tracepoints and system calls:
+
--
[role="term"]
----
# lttng list --kernel
# lttng list --kernel --syscall
----
--

. Create <<event,recording event rules>> which match events having
  the desired names, for example the `sched_switch` and
  `sched_process_fork` tracepoints, and the man:open(2) and man:close(2)
  system calls:
+
--
[role="term"]
----
# lttng enable-event --kernel sched_switch,sched_process_fork
# lttng enable-event --kernel --syscall open,close
----
--
+
Create a recording event rule which matches _all_ the Linux kernel
tracepoint events with the opt:lttng-enable-event(1):--all option
(recording with such a recording event rule generates a lot of data):
+
--
[role="term"]
----
# lttng enable-event --kernel --all
----
--

. <<basic-tracing-session-control,Start recording>>:
+
--
[role="term"]
----
# lttng start
----
--

. Do some operation on your system for a few seconds. For example,
  load a website, or list the files of a directory.

. <<creating-destroying-tracing-sessions,Destroy>> the current
  recording session:
+
--
[role="term"]
----
# lttng destroy
----
--
+
The man:lttng-destroy(1) command doesn't destroy the trace data; it
only destroys the state of the recording session.
+
The man:lttng-destroy(1) command also runs the man:lttng-stop(1) command
implicitly (see ``<<basic-tracing-session-control,Start and stop a
recording session>>''). You need to stop recording to make LTTng flush
the remaining trace data and make the trace readable.

. For the sake of this example, make the recorded trace accessible to
  the non-root users:
+
--
[role="term"]
----
# chown -R $(whoami) /tmp/my-kernel-trace
----
--

See ``<<viewing-and-analyzing-your-traces,View and analyze the
recorded events>>'' to view the recorded events.


[[tracing-your-own-user-application]]
=== Record user application events

This section walks you through a simple example to record the events of
a _Hello world_ program written in{nbsp}C.

To create the traceable user application:

. Create the tracepoint provider header file, which defines the
  tracepoints and the events they can generate:
+
--
[source,c]
.path:{hello-tp.h}
----
#undef LTTNG_UST_TRACEPOINT_PROVIDER
#define LTTNG_UST_TRACEPOINT_PROVIDER hello_world

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

#if !defined(_HELLO_TP_H) || defined(LTTNG_UST_TRACEPOINT_HEADER_MULTI_READ)
#define _HELLO_TP_H

#include <lttng/tracepoint.h>

LTTNG_UST_TRACEPOINT_EVENT(
    hello_world,
    my_first_tracepoint,
    LTTNG_UST_TP_ARGS(
        int, my_integer_arg,
        char *, my_string_arg
    ),
    LTTNG_UST_TP_FIELDS(
        lttng_ust_field_string(my_string_field, my_string_arg)
        lttng_ust_field_integer(int, my_integer_field, my_integer_arg)
    )
)

#endif /* _HELLO_TP_H */

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

. Create the tracepoint provider package source file:
+
--
[source,c]
.path:{hello-tp.c}
----
#define LTTNG_UST_TRACEPOINT_CREATE_PROBES
#define LTTNG_UST_TRACEPOINT_DEFINE

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

. Build the tracepoint provider package:
+
--
[role="term"]
----
$ gcc -c -I. hello-tp.c
----
--

. Create the _Hello World_ application source file:
+
--
[source,c]
.path:{hello.c}
----
#include <stdio.h>
#include "hello-tp.h"

int main(int argc, char *argv[])
{
    unsigned int i;

    puts("Hello, World!\nPress Enter to continue...");

    /*
     * The following getchar() call only exists for the purpose of this
     * demonstration, to pause the application in order for you to have
     * time to list its tracepoints. You don't need it otherwise.
     */
    getchar();

    /*
     * An lttng_ust_tracepoint() call.
     *
     * Arguments, as defined in `hello-tp.h`:
     *
     * 1. Tracepoint provider name (required)
     * 2. Tracepoint name (required)
     * 3. `my_integer_arg` (first user-defined argument)
     * 4. `my_string_arg` (second user-defined argument)
     *
     * Notice the tracepoint provider and tracepoint names are
     * C identifiers, NOT strings: they're in fact parts of variables
     * that the macros in `hello-tp.h` create.
     */
    lttng_ust_tracepoint(hello_world, my_first_tracepoint, 23,
                         "hi there!");

    for (i = 0; i < argc; i++) {
        lttng_ust_tracepoint(hello_world, my_first_tracepoint,
                             i, argv[i]);
    }

    puts("Quitting now!");
    lttng_ust_tracepoint(hello_world, my_first_tracepoint,
                         i * i, "i^2");
    return 0;
}
----
--

. Build the application:
+
--
[role="term"]
----
$ gcc -c hello.c
----
--

. Link the application with the tracepoint provider package,
  `liblttng-ust` and `libdl`:
+
--
[role="term"]
----
$ gcc -o hello hello.o hello-tp.o -llttng-ust -ldl
----
--

Here's the whole build process:

[role="img-100"]
.Build steps of the user space tracing tutorial.
image::ust-flow.png[]

To record the events of the user application:

. Run the application with a few arguments:
+
--
[role="term"]
----
$ ./hello world and beyond
----
--
+
You see:
+
--
----
Hello, World!
Press Enter to continue...
----
--

. Start an LTTng <<lttng-sessiond,session daemon>>:
+
--
[role="term"]
----
$ lttng-sessiond --daemonize
----
--
+
NOTE: A session daemon might already be running, for example as a
service that the service manager of your distribution started.

. List the available user space tracepoints:
+
--
[role="term"]
----
$ lttng list --userspace
----
--
+
You see the `hello_world:my_first_tracepoint` tracepoint listed
under the `./hello` process.

. Create a <<tracing-session,recording session>>:
+
--
[role="term"]
----
$ lttng create my-user-space-session
----
--

. Create a <<event,recording event rule>> which matches user space
  tracepoint events named `hello_world:my_first_tracepoint`:
+
--
[role="term"]
----
$ lttng enable-event --userspace hello_world:my_first_tracepoint
----
--

. <<basic-tracing-session-control,Start recording>>:
+
--
[role="term"]
----
$ lttng start
----
--

. Go back to the running `hello` application and press **Enter**.
+
The program executes all `lttng_ust_tracepoint()` instrumentation
points, emitting events as the event rule you created in step{nbsp}5
matches them, and
exits.

. <<creating-destroying-tracing-sessions,Destroy>> the current
  recording session:
+
--
[role="term"]
----
$ lttng destroy
----
--
+
The man:lttng-destroy(1) command doesn't destroy the trace data; it
only destroys the state of the recording session.
+
The man:lttng-destroy(1) command also runs the man:lttng-stop(1) command
implicitly (see ``<<basic-tracing-session-control,Start and stop a
recording session>>''). You need to stop recording to make LTTng flush
the remaining trace data and make the trace readable.

By default, LTTng saves the traces to the
+$LTTNG_HOME/lttng-traces/__NAME__-__DATE__-__TIME__+ directory, where
+__NAME__+ is the recording session name. The env:LTTNG_HOME environment
variable defaults to `$HOME` if not set.


[[viewing-and-analyzing-your-traces]]
=== View and analyze the recorded events

Once you have completed the <<tracing-the-linux-kernel,Record Linux
kernel events>> and <<tracing-your-own-user-application,Record user
application events>> tutorials, you can inspect the recorded events.

There are many tools you can use to read LTTng traces:

https://babeltrace.org/[Babeltrace{nbsp}2]::
    A rich, flexible trace manipulation toolkit which includes
    a versatile command-line interface
    (man:babeltrace2(1)),
    a https://babeltrace.org/docs/v2.0/libbabeltrace2/[C{nbsp}library],
    and https://babeltrace.org/docs/v2.0/python/bt2/[Python{nbsp}3 bindings]
    so that you can easily process or convert an LTTng trace with
    your own script.
+
The Babeltrace{nbsp}2 project ships with a plugin
(man:babeltrace2-plugin-ctf(7)) which supports the format of the traces
which LTTng produces, https://diamon.org/ctf/[CTF].

http://tracecompass.org/[Trace Compass]::
    A graphical user interface for viewing and analyzing any type of
    logs or traces, including those of LTTng.

https://github.com/lttng/lttng-analyses[LTTng analyses]::
    An experimental project which includes many high-level analyses of
    LTTng kernel traces, like scheduling statistics, interrupt
    frequency distribution, top CPU usage, and more.

NOTE: This section assumes that LTTng wrote the traces it recorded
during the previous tutorials to their default location, in the
dir:{$LTTNG_HOME/lttng-traces} directory. The env:LTTNG_HOME
environment variable defaults to `$HOME` if not set.


[[viewing-and-analyzing-your-traces-bt]]
==== Use the cmd:babeltrace2 command-line tool

The simplest way to list all the recorded events of an LTTng trace is to
pass its path to man:babeltrace2(1), without options:

[role="term"]
----
$ babeltrace2 ~/lttng-traces/my-user-space-session*
----

The cmd:babeltrace2 command finds all traces recursively within the
given path and prints all their events, sorting them chronologically.

Pipe the output of cmd:babeltrace2 into a tool like man:grep(1) for
further filtering:

[role="term"]
----
$ babeltrace2 /tmp/my-kernel-trace | grep _switch
----

Pipe the output of cmd:babeltrace2 into a tool like man:wc(1) to count
the recorded events:

[role="term"]
----
$ babeltrace2 /tmp/my-kernel-trace | grep _open | wc --lines
----


[[viewing-and-analyzing-your-traces-bt-python]]
==== Use the Babeltrace{nbsp}2 Python bindings

The <<viewing-and-analyzing-your-traces-bt,text output of
cmd:babeltrace2>> is useful to isolate event records by simple matching
using man:grep(1) and similar utilities. However, more elaborate
filters, such as keeping only event records with a field value falling
within a specific range, are not trivial to write using a shell.
Moreover, reductions and even the most basic computations involving
multiple event records are virtually impossible to implement.

Fortunately, Babeltrace{nbsp}2 ships with
https://babeltrace.org/docs/v2.0/python/bt2/[Python{nbsp}3 bindings]
which make it easy to read the event records of an LTTng trace
sequentially and compute the desired information.

The following script accepts an LTTng Linux kernel trace path as its
first argument and prints the short names of the top five running
processes on CPU{nbsp}0 during the whole trace:

[source,python]
.path:{top5proc.py}
----
import bt2
import sys
import collections


def top5proc():
    # Get the trace path from the first command-line argument
    it = bt2.TraceCollectionMessageIterator(sys.argv[1])

    # This counter dictionary will hold execution times:
    #
    #     Task command name -> Total execution time (ns)
    exec_times = collections.Counter()

    # This holds the last `sched_switch` timestamp
    last_ts = None

    for msg in it:
        # We only care about event messages
        if type(msg) is not bt2._EventMessageConst:
            continue

        # Event of the event message
        event = msg.event

        # Keep only `sched_switch` events
        if event.cls.name != 'sched_switch':
            continue

        # Keep only records of events which LTTng emitted from CPU 0
        if event.packet.context_field['cpu_id'] != 0:
            continue

        # Event timestamp (ns)
        cur_ts = msg.default_clock_snapshot.ns_from_origin

        if last_ts is None:
            # Start here
            last_ts = cur_ts

        # (Short) name of the previous task command
        prev_comm = str(event.payload_field['prev_comm'])

        # Initialize an entry in our dictionary if not done yet
        if prev_comm not in exec_times:
            exec_times[prev_comm] = 0

        # Compute previous command execution time
        diff = cur_ts - last_ts

        # Update execution time of this command
        exec_times[prev_comm] += diff

        # Update last timestamp
        last_ts = cur_ts

    # Print top 5
    for name, ns in exec_times.most_common(5):
        print('{:20}{} s'.format(name, ns / 1e9))


if __name__ == '__main__':
    top5proc()
----

Run this script:

[role="term"]
----
$ python3 top5proc.py /tmp/my-kernel-trace/kernel
----

Output example:

----
swapper/0           48.607245889 s
chromium            7.192738188 s
pavucontrol         0.709894415 s
Compositor          0.660867933 s
Xorg.bin            0.616753786 s
----

Note that `swapper/0` is the ``idle'' process of CPU{nbsp}0 on Linux;
since we weren't using the CPU that much when recording, its first
position in the list makes sense.


[[core-concepts]]
== [[understanding-lttng]]Core concepts

From a user's perspective, the LTTng system is built on a few concepts,
or objects, on which the <<lttng-cli,cmd:lttng command-line tool>>
operates by sending commands to the <<lttng-sessiond,session daemon>>
(through <<liblttng-ctl-lttng,`liblttng-ctl`>>).

Understanding how those objects relate to each other is key to master
the toolkit.

The core concepts of LTTng are:

* <<"event-rule","Instrumentation point, event rule, and event">>
* <<trigger,Trigger>>
* <<tracing-session,Recording session>>
* <<domain,Tracing domain>>
* <<channel,Channel and ring buffer>>
* <<event,Recording event rule and event record>>

NOTE: The man:lttng-concepts(7) manual page also documents the core
concepts of LTTng, with more links to other LTTng-tools manual pages.


[[event-rule]]
=== Instrumentation point, event rule, and event

An _instrumentation point_ is a point, within a piece of software,
which, when executed, creates an LTTng _event_.

LTTng offers various <<instrumentation-point-types,types of
instrumentation>>.

An _event rule_ is a set of conditions to match a set of events.

When LTTng creates an event{nbsp}__E__, an event rule{nbsp}__ER__ is
said to __match__{nbsp}__E__ when{nbsp}__E__ satisfies _all_ the
conditions of{nbsp}__ER__. This concept is similar to a
https://en.wikipedia.org/wiki/Regular_expression[regular expression]
which matches a set of strings.

When an event rule matches an event, LTTng _emits_ the event, therefore
attempting to execute one or more actions.

[IMPORTANT]
====
[[event-creation-emission-opti]]The event creation and emission
processes are documentation concepts to help understand the journey from
an instrumentation point to the execution of actions.

The actual creation of an event can be costly because LTTng needs to
evaluate the arguments of the instrumentation point.

In practice, LTTng implements various optimizations for the Linux kernel
and user space <<domain,tracing domains>> to avoid actually creating an
event when the tracer knows, thanks to properties which are independent
from the event payload and current context, that it would never emit
such an event. Those properties are:

* The <<instrumentation-point-types,instrumentation point type>>.

* The instrumentation point name.

* The instrumentation point log level.

* For a <<event,recording event rule>>:
** The status of the rule itself.
** The status of the <<channel,channel>>.
** The activity of the <<tracing-session,recording session>>.
** Whether or not the process for which LTTng would create the event is
   <<pid-tracking,allowed to record events>>.

In other words: if, for a given instrumentation point{nbsp}__IP__, the
LTTng tracer knows that it would never emit an event,
executing{nbsp}__IP__ represents a simple boolean variable check and,
for a Linux kernel recording event rule, a few process attribute checks.
====

As of LTTng{nbsp}{revision}, there are two places where you can find an
event rule:

<<event,Recording event rule>>::
    A specific type of event rule of which the action is to record the
    matched event as an event record.
+
See ``<<enabling-disabling-events,Create and enable a recording event
rule>>'' to learn more.

``Event rule matches'' <<trigger,trigger>> condition (since LTTng{nbsp}2.13)::
    When the event rule of the trigger condition matches an event, LTTng
    can execute user-defined actions such as sending an LTTng
    notification,
    <<basic-tracing-session-control,starting a recording session>>,
    and more.
+
See “<<add-event-rule-matches-trigger,Add an ``event rule matches''
trigger to a session daemon>>” to learn more.

For LTTng to emit an event{nbsp}__E__,{nbsp}__E__ must satisfy _all_ the
basic conditions of an event rule{nbsp}__ER__, that is:

* The instrumentation point from which LTTng
  creates{nbsp}__E__ has a specific
  <<instrumentation-point-types,type>>.

* A pattern matches the name of{nbsp}__E__ while another pattern
  doesn't.

* The log level of the instrumentation point from which LTTng
  creates{nbsp}__E__ is at least as severe as some value, or is exactly
  some value.

* The fields of the payload of{nbsp}__E__ and the current context fields
  satisfy a filter expression.

A <<event,recording event rule>> has additional, implicit conditions to
satisfy.


[[instrumentation-point-types]]
==== Instrumentation point types

As of LTTng{nbsp}{revision}, the available instrumentation point
types are, depending on the <<domain,tracing domain>>:

Linux kernel::
    LTTng tracepoint:::
        A statically defined point in the source code of the kernel
        image or of a kernel module using the
        <<lttng-modules,LTTng-modules>> macros.

    Linux kernel system call:::
        Entry, exit, or both of a Linux kernel system call.

    Linux https://www.kernel.org/doc/html/latest/trace/kprobes.html[kprobe]:::
        A single probe dynamically placed in the compiled kernel code.
+
When you create such an instrumentation point, you set its memory
address or symbol name.

    Linux user space probe:::
        A single probe dynamically placed at the entry of a compiled
        user space application/library function through the kernel.
+
When you create such an instrumentation point, you set:
+
--
With the ELF method::
    Its application/library path and its symbol name.

With the USDT method::
    Its application/library path, its provider name, and its probe name.
+
``USDT'' stands for _SystemTap User-level Statically Defined Tracing_,
a http://dtrace.org/blogs/about/[DTrace]-style marker.
--
+
As of LTTng{nbsp}{revision}, LTTng only supports USDT probes which
are _not_ reference-counted.

    Linux https://www.kernel.org/doc/html/latest/trace/kprobes.html[kretprobe]:::
        Entry, exit, or both of a Linux kernel function.
+
When you create such an instrumentation point, you set the memory
address or symbol name of its function.

User space::
    LTTng tracepoint:::
        A statically defined point in the source code of a C/$$C++$$
        application/library using the
        <<lttng-ust,LTTng-UST>> macros.

`java.util.logging`, Apache log4j, and Python::
    Java or Python logging statement:::
        A method call on a Java or Python logger attached to an
        LTTng-UST handler.

See ``<<list-instrumentation-points,List the available instrumentation
points>>'' to learn how to list available Linux kernel, user space, and
logging instrumentation points.


[[trigger]]
=== Trigger

A _trigger_ associates a condition to one or more actions.

When the condition of a trigger is satisfied, LTTng attempts to execute
its actions.

As of LTTng{nbsp}{revision}, the available trigger conditions and
actions are:

Conditions::
+
* The consumed buffer size of a given <<tracing-session,recording
  session>> becomes greater than some value.

* The buffer usage of a given <<channel,channel>> becomes greater than
  some value.

* The buffer usage of a given channel becomes less than some value.

* There's an ongoing <<session-rotation,recording session rotation>>.

* A recording session rotation becomes completed.

* An <<add-event-rule-matches-trigger,event rule matches>> an event.

Actions::
+
* <<trigger-event-notif,Send a notification>> to a user application.
* <<basic-tracing-session-control,Start>> a given recording session.
* <<basic-tracing-session-control,Stop>> a given recording session.
* <<session-rotation,Archive the current trace chunk>> of a given
  recording session (rotate).
* <<taking-a-snapshot,Take a snapshot>> of a given recording session.

A trigger belongs to a <<lttng-sessiond,session daemon>>, not to a
specific recording session. For a given session daemon, each Unix user has
its own, private triggers. Note, however, that the `root` Unix user may,
for the root session daemon:

* Add a trigger as another Unix user.

* List all the triggers, regardless of their owner.

* Remove a trigger which belongs to another Unix user.

For a given session daemon and Unix user, a trigger has a unique name.


[[tracing-session]]
=== Recording session

A _recording session_ (named ``tracing session'' prior to
LTTng{nbsp}2.13) is a stateful dialogue between you and a
<<lttng-sessiond,session daemon>> for everything related to
<<event,event recording>>.

Everything that you do when you control LTTng tracers to record events
happens within a recording session. In particular, a recording session:

* Has its own name, unique for a given session daemon.

* Has its own set of trace files, if any.

* Has its own state of activity (started or stopped).
+
An active recording session is an implicit <<event,recording event rule>>
condition.

* Has its own <<tracing-session-mode,mode>> (local, network streaming,
  snapshot, or live).

* Has its own <<channel,channels>> to which are attached their own
  recording event rules.

* Has its own <<pid-tracking,process attribute inclusion sets>>.

[role="img-100"]
.A _recording session_ contains <<channel,channels>> that are members of <<domain,tracing domains>> and contain <<event,recording event rules>>.
image::concepts.png[]

Those attributes and objects are completely isolated between different
recording sessions.

A recording session is like an
https://en.wikipedia.org/wiki/Automated_teller_machine[ATM] session: the
operations you do on the banking system through the ATM don't alter the
data of other users of the same system. In the case of the ATM, a
session lasts as long as your bank card is inside. In the case of LTTng,
a recording session lasts from the man:lttng-create(1) command to the
man:lttng-destroy(1) command.

[role="img-100"]
.Each Unix user has its own set of recording sessions.
image::many-sessions.png[]

A recording session belongs to a <<lttng-sessiond,session daemon>>. For a
given session daemon, each Unix user has its own, private recording
sessions. Note, however, that the `root` Unix user may operate on or
destroy another user's recording session.


[[tracing-session-mode]]
==== Recording session mode

LTTng offers four recording session modes:

[[local-mode]]Local mode::
    Write the trace data to the local file system.

[[net-streaming-mode]]Network streaming mode::
    Send the trace data over the network to a listening
    <<lttng-relayd,relay daemon>>.

[[snapshot-mode]]Snapshot mode::
    Only write the trace data to the local file system or send it to a
    listening relay daemon when LTTng <<taking-a-snapshot,takes a
    snapshot>>.
+
LTTng forces all the <<channel,channels>>
to be created to be configured to be snapshot-ready.
+
LTTng takes a snapshot of such a recording session when:
+
--
* You run the man:lttng-snapshot(1) command.

* LTTng executes a `snapshot-session` <<trigger,trigger>> action.
--

[[live-mode]]Live mode::
    Send the trace data over the network to a listening relay daemon
    for <<lttng-live,live reading>>.
+
An LTTng live reader (for example, man:babeltrace2(1)) can connect to
the same relay daemon to receive trace data while the recording session is
active.


[[domain]]
=== Tracing domain

A _tracing domain_ identifies a type of LTTng tracer.

A tracing domain has its own properties and features.

There are currently five available tracing domains:

* Linux kernel
* User space
* `java.util.logging` (JUL)
* log4j
* Python

You must specify a tracing domain to target a type of LTTng tracer when
using some <<lttng-cli,cmd:lttng>> commands to avoid ambiguity. For
example, because the Linux kernel and user space tracing domains support
named tracepoints as <<event-rule,instrumentation points>>, you need to
specify a tracing domain when you <<enabling-disabling-events,create
an event rule>> because both tracing domains could have tracepoints
sharing the same name.

You can create <<channel,channels>> in the Linux kernel and user space
tracing domains. The other tracing domains have a single, default
channel.


[[channel]]
=== Channel and ring buffer

A _channel_ is an object which is responsible for a set of
_ring buffers_.

Each ring buffer is divided into multiple _sub-buffers_. When a
<<event,recording event rule>>
matches an event, LTTng can record it to one or more sub-buffers of one
or more channels.

When you <<enabling-disabling-channels,create a channel>>, you set its
final attributes, that is:

* Its <<channel-buffering-schemes,buffering scheme>>.

* What to do <<channel-overwrite-mode-vs-discard-mode,when there's no
  space left>> for a new event record because all sub-buffers are full.

* The <<channel-subbuf-size-vs-subbuf-count,size of each ring buffer and
  how many sub-buffers>> a ring buffer has.

* The <<tracefile-rotation,size of each trace file LTTng writes for this
  channel and the maximum count>> of trace files.

* The periods of its <<channel-read-timer,read>>,
  <<channel-switch-timer,switch>>, and <<channel-monitor-timer,monitor>>
  timers.

* For a Linux kernel channel: its output type.
+
See the opt:lttng-enable-channel(1):--output option of the
man:lttng-enable-channel(1) command.

* For a user space channel: the value of its
  <<blocking-timeout-example,blocking timeout>>.

A channel is always associated to a <<domain,tracing domain>>. The
`java.util.logging` (JUL), log4j, and Python tracing domains each have a
default channel which you can't configure.

A channel owns <<event,recording event rules>>.


[[channel-buffering-schemes]]
==== Buffering scheme

A channel has at least one ring buffer _per CPU_. LTTng always records
an event to the ring buffer dedicated to the CPU which emits it.

The buffering scheme of a user space channel determines what has its own
set of per-CPU ring buffers:

Per-user buffering::
  Allocate one set of ring buffers--one per CPU--shared by all the
  instrumented processes of:
    If your Unix user is `root`:::
      Each Unix user.
+
--
[role="img-100"]
.Per-user buffering scheme (recording session belongs to the `root` Unix user).
image::per-user-buffering-root.png[]
--

    Otherwise:::
      Your Unix user.
+
--
[role="img-100"]
.Per-user buffering scheme (recording session belongs to the `Bob` Unix user).
image::per-user-buffering.png[]
--

Per-process buffering::
  Allocate one set of ring buffers--one per CPU--for each
  instrumented process of:
    If your Unix user is `root`:::
      All Unix users.
+
--
[role="img-100"]
.Per-process buffering scheme (recording session belongs to the `root` Unix user).
image::per-process-buffering-root.png[]
--

    Otherwise:::
      Your Unix user.
+
--
[role="img-100"]
.Per-process buffering scheme (recording session belongs to the `Bob` Unix user).
image::per-process-buffering.png[]
--

The per-process buffering scheme tends to consume more memory than the
per-user option because systems generally have more instrumented
processes than Unix users running instrumented processes. However, the
per-process buffering scheme ensures that one process having a high
event throughput won't fill all the shared sub-buffers of the same Unix
user, only its own.

The buffering scheme of a Linux kernel channel is always to allocate a
single set of ring buffers for the whole system. This scheme is similar
to the per-user option, but with a single, global user ``running'' the
kernel.


[[channel-overwrite-mode-vs-discard-mode]]
==== Event record loss mode

When LTTng emits an event, LTTng can record it to a specific, available
sub-buffer within the ring buffers of specific channels. When there's no
space left in a sub-buffer, the tracer marks it as consumable and
another, available sub-buffer starts receiving the following event
records. An LTTng <<lttng-consumerd,consumer daemon>> eventually
consumes the marked sub-buffer, which returns to the available state.

[NOTE]
[role="docsvg-channel-subbuf-anim"]
====
{note-no-anim}
====

In an ideal world, sub-buffers are consumed faster than they're filled.
In the real world, however, all sub-buffers can be full at some point,
leaving no space to record the following events.

In an ideal world, sub-buffers are consumed faster than they're filled,
as it's the case in the previous animation. In the real world,
however, all sub-buffers can be full at some point, leaving no space to
record the following events.

By default, <<lttng-modules,LTTng-modules>> and <<lttng-ust,LTTng-UST>>
are _non-blocking_ tracers: when there's no available sub-buffer to
record an event, it's acceptable to lose event records when the
alternative would be to cause substantial delays in the execution of the
instrumented application. LTTng privileges performance over integrity;
it aims at perturbing the instrumented application as little as possible
in order to make the detection of subtle race conditions and rare
interrupt cascades possible.

Since LTTng{nbsp}2.10, the LTTng user space tracer, LTTng-UST, supports
a _blocking mode_. See the <<blocking-timeout-example,blocking timeout
example>> to learn how to use the blocking mode.

When it comes to losing event records because there's no available
sub-buffer, or because the blocking timeout of
the channel is reached, the _event record loss mode_ of the channel
determines what to do. The available event record loss modes are:

[[discard-mode]]Discard mode::
    Drop the newest event records until a sub-buffer becomes available.
+
This is the only available mode when you specify a blocking timeout.
+
With this mode, LTTng increments a count of lost event records when an
event record is lost and saves this count to the trace. A trace reader
can use the saved discarded event record count of the trace to decide
whether or not to perform some analysis even if trace data is known to
be missing.

[[overwrite-mode]]Overwrite mode::
    Clear the sub-buffer containing the oldest event records and start
    writing the newest event records there.
+
This mode is sometimes called _flight recorder mode_ because it's
similar to a https://en.wikipedia.org/wiki/Flight_recorder[flight
recorder]: always keep a fixed amount of the latest data. It's also
similar to the roll mode of an oscilloscope.
+
Since LTTng{nbsp}2.8, with this mode, LTTng writes to a given sub-buffer
its sequence number within its data stream. With a <<local-mode,local>>,
<<net-streaming-mode,network streaming>>, or <<live-mode,live>> recording
session, a trace reader can use such sequence numbers to report lost
packets. A trace reader can use the saved discarded sub-buffer (packet)
count of the trace to decide whether or not to perform some analysis
even if trace data is known to be missing.
+
With this mode, LTTng doesn't write to the trace the exact number of
lost event records in the lost sub-buffers.

Which mechanism you should choose depends on your context: prioritize
the newest or the oldest event records in the ring buffer?

Beware that, in overwrite mode, the tracer abandons a _whole sub-buffer_
as soon as a there's no space left for a new event record, whereas in
discard mode, the tracer only discards the event record that doesn't
fit.

There are a few ways to decrease your probability of losing event
records. The ``<<channel-subbuf-size-vs-subbuf-count,Sub-buffer size and
count>>'' section shows how to fine-tune the sub-buffer size and count
of a channel to virtually stop losing event records, though at the cost
of greater memory usage.


[[channel-subbuf-size-vs-subbuf-count]]
==== Sub-buffer size and count

A channel has one or more ring buffer for each CPU of the target system.

See the ``<<channel-buffering-schemes,Buffering scheme>>'' section to
learn how many ring buffers of a given channel are dedicated to each CPU
depending on its buffering scheme.

Set the size of each sub-buffer the ring buffers of a channel contain
and how many there are
when you <<enabling-disabling-channels,create it>>.

Note that LTTng switching the current sub-buffer of a ring buffer
(marking a full one as consumable and switching to an available one for
LTTng to record the next events) introduces noticeable CPU overhead.
Knowing this, the following list presents a few practical situations
along with how to configure the sub-buffer size and count for them:

High event throughput::
    In general, prefer large sub-buffers to lower the risk of losing
    event records.
+
Having larger sub-buffers also ensures a lower sub-buffer switching
frequency.
+
The sub-buffer count is only meaningful if you create the channel in
<<overwrite-mode,overwrite mode>>: in this case, if LTTng overwrites a
sub-buffer, then the other sub-buffers are left unaltered.

Low event throughput::
    In general, prefer smaller sub-buffers since the risk of losing
    event records is low.
+
Because LTTng emits events less frequently, the sub-buffer switching
frequency should remain low and therefore the overhead of the tracer
shouldn't be a problem.

Low memory system::
    If your target system has a low memory limit, prefer fewer first,
    then smaller sub-buffers.
+
Even if the system is limited in memory, you want to keep the
sub-buffers as large as possible to avoid a high sub-buffer switching
frequency.

Note that LTTng uses https://diamon.org/ctf/[CTF] as its trace format,
which means event record data is very compact. For example, the average
LTTng kernel event record weights about 32{nbsp}bytes. Therefore, a
sub-buffer size of 1{nbsp}MiB is considered large.

The previous scenarios highlight the major trade-off between a few large
sub-buffers and more, smaller sub-buffers: sub-buffer switching
frequency vs. how many event records are lost in overwrite mode.
Assuming a constant event throughput and using the overwrite mode, the
two following configurations have the same ring buffer total size:

[NOTE]
[role="docsvg-channel-subbuf-size-vs-count-anim"]
====
{note-no-anim}
====

Two sub-buffers of 4{nbsp}MiB each::
    Expect a very low sub-buffer switching frequency, but if LTTng
    ever needs to overwrite a sub-buffer, half of the event records so
    far (4{nbsp}MiB) are definitely lost.

Eight sub-buffers of 1{nbsp}MiB each::
    Expect four times the tracer overhead of the configuration above,
    but if LTTng needs to overwrite a sub-buffer, only the eighth of
    event records so far (1{nbsp}MiB) are definitely lost.

In <<discard-mode,discard mode>>, the sub-buffer count parameter is
pointless: use two sub-buffers and set their size according to your
requirements.


[[tracefile-rotation]]
==== Maximum trace file size and count (trace file rotation)

By default, trace files can grow as large as needed.

Set the maximum size of each trace file that LTTng writes of a given
channel when you <<enabling-disabling-channels,create it>>.

When the size of a trace file reaches the fixed maximum size of the
channel, LTTng creates another file to contain the next event records.
LTTng appends a file count to each trace file name in this case.

If you set the trace file size attribute when you create a channel, the
maximum number of trace files that LTTng creates is _unlimited_ by
default. To limit them, set a maximum number of trace files. When the
number of trace files reaches the fixed maximum count of the channel,
LTTng overwrites the oldest trace file. This mechanism is called _trace
file rotation_.

[IMPORTANT]
====
Even if you don't limit the trace file count, always assume that LTTng
manages all the trace files of the recording session.

In other words, there's no safe way to know if LTTng still holds a given
trace file open with the trace file rotation feature.

The only way to obtain an unmanaged, self-contained LTTng trace before
you <<creating-destroying-tracing-sessions,destroy the recording session>>
is with the <<session-rotation,recording session rotation>> feature, which
is available since LTTng{nbsp}2.11.
====


[[channel-timers]]
==== Timers

Each channel can have up to three optional timers:

[[channel-switch-timer]]Switch timer::
    When this timer expires, a sub-buffer switch happens: for each ring
    buffer of the channel, LTTng marks the current sub-buffer as
    consumable and _switches_ to an available one to record the next
    events.
+
[NOTE]
[role="docsvg-channel-switch-timer"]
====
{note-no-anim}
====
+
A switch timer is useful to ensure that LTTng consumes and commits trace
data to trace files or to a distant <<lttng-relayd,relay daemon>>
periodically in case of a low event throughput.
+
Such a timer is also convenient when you use large
<<channel-subbuf-size-vs-subbuf-count,sub-buffers>> to cope with a
sporadic high event throughput, even if the throughput is otherwise low.
+
Set the period of the switch timer of a channel when you
<<enabling-disabling-channels,create it>> with
the opt:lttng-enable-channel(1):--switch-timer option.

[[channel-read-timer]]Read timer::
    When this timer expires, LTTng checks for full, consumable
    sub-buffers.
+
By default, the LTTng tracers use an asynchronous message mechanism to
signal a full sub-buffer so that a <<lttng-consumerd,consumer daemon>>
can consume it.
+
When such messages must be avoided, for example in real-time
applications, use this timer instead.
+
Set the period of the read timer of a channel when you
<<enabling-disabling-channels,create it>> with the
opt:lttng-enable-channel(1):--read-timer option.

[[channel-monitor-timer]]Monitor timer::
    When this timer expires, the consumer daemon samples some channel
    statistics to evaluate the following <<trigger,trigger>>
    conditions:
+
--
. The consumed buffer size of a given <<tracing-session,recording
  session>> becomes greater than some value.
. The buffer usage of a given channel becomes greater than some value.
. The buffer usage of a given channel becomes less than some value.
--
+
If you disable the monitor timer of a channel{nbsp}__C__:
+
--
* The consumed buffer size value of the recording session of{nbsp}__C__
  could be wrong for trigger condition type{nbsp}1: the consumed buffer
  size of{nbsp}__C__ won't be part of the grand total.

* The buffer usage trigger conditions (types{nbsp}2 and{nbsp}3)
  for{nbsp}__C__ will never be satisfied.
--
+
Set the period of the monitor timer of a channel when you
<<enabling-disabling-channels,create it>> with the
opt:lttng-enable-channel(1):--monitor-timer option.


[[event]]
=== Recording event rule and event record

A _recording event rule_ is a specific type of <<event-rule,event rule>>
of which the action is to serialize and record the matched event as an
_event record_.

Set the explicit conditions of a recording event rule when you
<<enabling-disabling-events,create it>>. A recording event rule also has
the following implicit conditions:

* The recording event rule itself is enabled.
+
A recording event rule is enabled on creation.

* The <<channel,channel>> to which the recording event rule is attached
  is enabled.
+
A channel is enabled on creation.

* The <<tracing-session,recording session>> of the recording event rule is
  <<basic-tracing-session-control,active>> (started).
+
A recording session is inactive (stopped) on creation.

* The process for which LTTng creates an event to match is
  <<pid-tracking,allowed to record events>>.
+
All processes are allowed to record events on recording session
creation.

You always attach a recording event rule to a channel, which belongs to
a recording session, when you create it.

When a recording event rule{nbsp}__ER__ matches an event{nbsp}__E__,
LTTng attempts to serialize and record{nbsp}__E__ to one of the
available sub-buffers of the channel to which{nbsp}__E__ is attached.

When multiple matching recording event rules are attached to the same
channel, LTTng attempts to serialize and record the matched event
_once_. In the following example, the second recording event rule is
redundant when both are enabled:

[role="term"]
----
$ lttng enable-event --userspace hello:world
$ lttng enable-event --userspace hello:world --loglevel=INFO
----

[role="img-100"]
.Logical path from an instrumentation point to an event record.
image::event-rule.png[]

As of LTTng{nbsp}{revision}, you cannot remove a recording event
rule: it exists as long as its recording session exists.


[[plumbing]]
== Components of noch:{LTTng}

The second _T_ in _LTTng_ stands for _toolkit_: it would be wrong
to call LTTng a simple _tool_ since it's composed of multiple
interacting components.

This section describes those components, explains their respective
roles, and shows how they connect together to form the LTTng ecosystem.

The following diagram shows how the most important components of LTTng
interact with user applications, the Linux kernel, and you:

[role="img-100"]
.Control and trace data paths between LTTng components.
image::plumbing.png[]

The LTTng project integrates:

LTTng-tools::
    Libraries and command-line interface to control recording sessions:
+
* <<lttng-sessiond,Session daemon>> (man:lttng-sessiond(8)).
* <<lttng-consumerd,Consumer daemon>> (cmd:lttng-consumerd).
* <<lttng-relayd,Relay daemon>> (man:lttng-relayd(8)).
* <<liblttng-ctl-lttng,Tracing control library>> (`liblttng-ctl`).
* <<lttng-cli,Tracing control command-line tool>> (man:lttng(1)).
* <<persistent-memory-file-systems,`lttng-crash` command-line tool>>
  (man:lttng-crash(1)).

LTTng-UST::
    Libraries and Java/Python packages to instrument and trace user
    applications:
+
* <<lttng-ust,User space tracing library>> (`liblttng-ust`) and its
  headers to instrument and trace any native user application.
* <<prebuilt-ust-helpers,Preloadable user space tracing helpers>>:
** `liblttng-ust-libc-wrapper`
** `liblttng-ust-pthread-wrapper`
** `liblttng-ust-cyg-profile`
** `liblttng-ust-cyg-profile-fast`
** `liblttng-ust-dl`
* <<lttng-ust-agents,LTTng-UST Java agent>> to instrument and trace
  Java applications using `java.util.logging` or
  Apache log4j{nbsp}1.2 logging.
* <<lttng-ust-agents,LTTng-UST Python agent>> to instrument
  Python applications using the standard `logging` package.

LTTng-modules::
    <<lttng-modules,Linux kernel modules>> to instrument and trace the
    kernel:
+
* LTTng kernel tracer module.
* Recording ring buffer kernel modules.
* Probe kernel modules.
* LTTng logger kernel module.


[[lttng-cli]]
=== Tracing control command-line interface

The _man:lttng(1) command-line tool_ is the standard user interface to
control LTTng <<tracing-session,recording sessions>>.

The cmd:lttng tool is part of LTTng-tools.

The cmd:lttng tool is linked with
<<liblttng-ctl-lttng,`liblttng-ctl`>> to communicate with
one or more <<lttng-sessiond,session daemons>> behind the scenes.

The cmd:lttng tool has a Git-like interface:

[role="term"]
----
$ lttng [GENERAL OPTIONS] <COMMAND> [COMMAND OPTIONS]
----

The ``<<controlling-tracing,Tracing control>>'' section explores the
available features of LTTng through its cmd:lttng tool.


[[liblttng-ctl-lttng]]
=== Tracing control library

[role="img-100"]
.The tracing control library.
image::plumbing-liblttng-ctl.png[]

The _LTTng control library_, `liblttng-ctl`, is used to communicate with
a <<lttng-sessiond,session daemon>> using a C{nbsp}API that hides the
underlying details of the protocol.

`liblttng-ctl` is part of LTTng-tools.

The <<lttng-cli,cmd:lttng command-line tool>> is linked with
`liblttng-ctl`.

Use `liblttng-ctl` in C or $$C++$$ source code by including its
``master'' header:

[source,c]
----
#include <lttng/lttng.h>
----

As of LTTng{nbsp}{revision}, the best available developer documentation
for `liblttng-ctl` is its installed header files. Functions and
structures are documented with header comments.


[[lttng-ust]]
=== User space tracing library

[role="img-100"]
.The user space tracing library.
image::plumbing-liblttng-ust.png[]

The _user space tracing library_, `liblttng-ust` (see man:lttng-ust(3)),
is the LTTng user space tracer.

`liblttng-ust` receives commands from a <<lttng-sessiond,session
daemon>>, for example to allow specific instrumentation points to emit
LTTng <<event-rule,events>>, and writes event records to <<channel,ring
buffers>> shared with a <<lttng-consumerd,consumer daemon>>.

`liblttng-ust` is part of LTTng-UST.

`liblttng-ust` can also send asynchronous messages to the session daemon
when it emits an event. This supports the ``event rule matches''
<<trigger,trigger>> condition feature (see
“<<add-event-rule-matches-trigger,Add an ``event rule matches'' trigger
to a session daemon>>”).

Public C{nbsp}header files are installed beside `liblttng-ust` to
instrument any <<c-application,C or $$C++$$ application>>.

<<lttng-ust-agents,LTTng-UST agents>>, which are regular Java and Python
packages, use their own <<tracepoint-provider,tracepoint provider
package>> which is linked with `liblttng-ust`.

An application or library doesn't have to initialize `liblttng-ust`
manually: its constructor does the necessary tasks to register the
application to a session daemon. The initialization phase also
configures instrumentation points depending on the <<event-rule,event
rules>> that you already created.


[[lttng-ust-agents]]
=== User space tracing agents

[role="img-100"]
.The user space tracing agents.
image::plumbing-lttng-ust-agents.png[]

The _LTTng-UST Java and Python agents_ are regular Java and Python
packages which add LTTng tracing capabilities to the
native logging frameworks.

The LTTng-UST agents are part of LTTng-UST.

In the case of Java, the
https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[`java.util.logging`
core logging facilities] and
https://logging.apache.org/log4j/1.2/[Apache log4j{nbsp}1.2] are supported.
Note that Apache Log4j{nbsp}2 isn't supported.

In the case of Python, the standard
https://docs.python.org/3/library/logging.html[`logging`] package
is supported. Both Python{nbsp}2 and Python{nbsp}3 modules can import the
LTTng-UST Python agent package.

The applications using the LTTng-UST agents are in the
`java.util.logging` (JUL), log4j, and Python <<domain,tracing domains>>.

Both agents use the same mechanism to convert log statements to LTTng
events. When an agent initializes, it creates a log handler that
attaches to the root logger. The agent also registers to a
<<lttng-sessiond,session daemon>>. When the user application executes a
log statement, the root logger passes it to the log handler of the
agent. The custom log handler of the agent calls a native function in a
tracepoint provider package shared library linked with
<<lttng-ust,`liblttng-ust`>>, passing the formatted log message and
other fields, like its logger name and its log level. This native
function contains a user space instrumentation point, therefore tracing
the log statement.

The log level condition of a <<event,recording event rule>> is
considered when tracing a Java or a Python application, and it's
compatible with the standard `java.util.logging`, log4j, and Python log
levels.


[[lttng-modules]]
=== LTTng kernel modules

[role="img-100"]
.The LTTng kernel modules.
image::plumbing-lttng-modules.png[]

The _LTTng kernel modules_ are a set of Linux kernel modules
which implement the kernel tracer of the LTTng project.

The LTTng kernel modules are part of LTTng-modules.

The LTTng kernel modules include:

* A set of _probe_ modules.
+
Each module attaches to a specific subsystem
of the Linux kernel using its tracepoint instrument points.
+
There are also modules to attach to the entry and return points of the
Linux system call functions.

* _Ring buffer_ modules.
+
A ring buffer implementation is provided as kernel modules. The LTTng
kernel tracer writes to ring buffers; a
<<lttng-consumerd,consumer daemon>> reads from ring buffers.

* The _LTTng kernel tracer_ module.
* The <<proc-lttng-logger-abi,_LTTng logger_>> module.
+
The LTTng logger module implements the special path:{/proc/lttng-logger}
(and path:{/dev/lttng-logger}, since LTTng{nbsp}2.11) files so that any
executable can generate LTTng events by opening those files and
writing to them.

The LTTng kernel tracer can also send asynchronous messages to the
<<lttng-sessiond,session daemon>> when it emits an event.
This supports the ``event rule matches''
<<trigger,trigger>> condition feature (see
“<<add-event-rule-matches-trigger,Add an ``event rule matches'' trigger
to a session daemon>>”).

Generally, you don't have to load the LTTng kernel modules manually
(using man:modprobe(8), for example): a root session daemon loads the
necessary modules when starting. If you have extra probe modules, you
can specify to load them to the session daemon on the command line
(see the opt:lttng-sessiond(8):--extra-kmod-probes option).

The LTTng kernel modules are installed in
+/usr/lib/modules/__release__/extra+ by default, where +__release__+ is
the kernel release (output of `uname --kernel-release`).


[[lttng-sessiond]]
=== Session daemon

[role="img-100"]
.The session daemon.
image::plumbing-sessiond.png[]

The _session daemon_, man:lttng-sessiond(8), is a
https://en.wikipedia.org/wiki/Daemon_(computing)[daemon] which:

* Manages <<tracing-session,recording sessions>>.

* Controls the various components (like tracers and
  <<lttng-consumerd,consumer daemons>>) of LTTng.

* Sends <<notif-trigger-api,asynchronous notifications>> to user
  applications.

The session daemon is part of LTTng-tools.

The session daemon sends control requests to and receives control
responses from:

* The <<lttng-ust,user space tracing library>>.
+
Any instance of the user space tracing library first registers to
a session daemon. Then, the session daemon can send requests to
this instance, such as:
+
--
** Get the list of tracepoints.
** Share a <<event,recording event rule>> so that the user space tracing
   library can decide whether or not a given tracepoint can emit events.
   Amongst the possible conditions of a recording event rule is a filter
   expression which `liblttng-ust` evaluates before it emits an event.
** Share <<channel,channel>> attributes and ring buffer locations.
--
+
The session daemon and the user space tracing library use a Unix
domain socket to communicate.

* The <<lttng-ust-agents,user space tracing agents>>.
+
Any instance of a user space tracing agent first registers to
a session daemon. Then, the session daemon can send requests to
this instance, such as:
+
--
** Get the list of loggers.
** Enable or disable a specific logger.
--
+
The session daemon and the user space tracing agent use a TCP connection
to communicate.

* The <<lttng-modules,LTTng kernel tracer>>.
* The <<lttng-consumerd,consumer daemon>>.
+
The session daemon sends requests to the consumer daemon to instruct
it where to send the trace data streams, amongst other information.

* The <<lttng-relayd,relay daemon>>.

The session daemon receives commands from the
<<liblttng-ctl-lttng,tracing control library>>.

The session daemon can receive asynchronous messages from the
<<lttng-ust,user space>> and <<lttng-modules,kernel>> tracers
when they emit events. This supports the ``event rule matches''
<<trigger,trigger>> condition feature (see
“<<add-event-rule-matches-trigger,Add an ``event rule matches'' trigger
to a session daemon>>”).

The root session daemon loads the appropriate
<<lttng-modules,LTTng kernel modules>> on startup. It also spawns
one or more <<lttng-consumerd,consumer daemons>> as soon as you create
a <<event,recording event rule>>.

The session daemon doesn't send and receive trace data: this is the
role of the <<lttng-consumerd,consumer daemon>> and
<<lttng-relayd,relay daemon>>. It does, however, generate the
https://diamon.org/ctf/[CTF] metadata stream.

Each Unix user can have its own session daemon instance. The
recording sessions which different session daemons manage are completely
independent.

The root user's session daemon is the only one which is
allowed to control the LTTng kernel tracer, and its spawned consumer
daemon is the only one which is allowed to consume trace data from the
LTTng kernel tracer. Note, however, that any Unix user which is a member
of the <<tracing-group,tracing group>> is allowed
to create <<channel,channels>> in the
Linux kernel <<domain,tracing domain>>, and therefore to use the Linux
kernel LTTng tracer.

The <<lttng-cli,cmd:lttng command-line tool>> automatically starts a
session daemon when using its `create` command if none is currently
running. You can also start the session daemon manually.


[[lttng-consumerd]]
=== Consumer daemon

[role="img-100"]
.The consumer daemon.
image::plumbing-consumerd.png[]

The _consumer daemon_, cmd:lttng-consumerd, is a
https://en.wikipedia.org/wiki/Daemon_(computing)[daemon] which shares
ring buffers with user applications or with the LTTng kernel modules to
collect trace data and send it to some location (file system or to a
<<lttng-relayd,relay daemon>> over the network).

The consumer daemon is part of LTTng-tools.

You don't start a consumer daemon manually: a consumer daemon is always
spawned by a <<lttng-sessiond,session daemon>> as soon as you create a
<<event,recording event rule>>, that is, before you start recording. When
you kill its owner session daemon, the consumer daemon also exits
because it's the child process of the session daemon. Command-line
options of man:lttng-sessiond(8) target the consumer daemon process.

There are up to two running consumer daemons per Unix user, whereas only
one session daemon can run per user. This is because each process can be
either 32-bit or 64-bit: if the target system runs a mixture of 32-bit
and 64-bit processes, it's more efficient to have separate
corresponding 32-bit and 64-bit consumer daemons. The root user is an
exception: it can have up to _three_ running consumer daemons: 32-bit
and 64-bit instances for its user applications, and one more
reserved for collecting kernel trace data.


[[lttng-relayd]]
=== Relay daemon

[role="img-100"]
.The relay daemon.
image::plumbing-relayd.png[]

The _relay daemon_, man:lttng-relayd(8), is a
https://en.wikipedia.org/wiki/Daemon_(computing)[daemon] acting as a bridge
between remote session and consumer daemons, local trace files, and a
remote live trace reader.

The relay daemon is part of LTTng-tools.

The main purpose of the relay daemon is to implement a receiver of
<<sending-trace-data-over-the-network,trace data over the network>>.
This is useful when the target system doesn't have much file system
space to write trace files locally.

The relay daemon is also a server to which a
<<lttng-live,live trace reader>> can
connect. The live trace reader sends requests to the relay daemon to
receive trace data as the target system records events. The
communication protocol is named _LTTng live_; it's used over TCP
connections.

Note that you can start the relay daemon on the target system directly.
This is the setup of choice when the use case is to view/analyze events
as the target system records them without the need of a remote system.


[[instrumenting]]
== [[using-lttng]]Instrumentation

There are many examples of tracing and monitoring in our everyday life:

* You have access to real-time and historical weather reports and
  forecasts thanks to weather stations installed around the country.
* You know your heart is safe thanks to an electrocardiogram.
* You make sure not to drive your car too fast and to have enough fuel
  to reach your destination thanks to gauges visible on your dashboard.

All the previous examples have something in common: they rely on
**instruments**. Without the electrodes attached to the surface of your
body skin, cardiac monitoring is futile.

LTTng, as a tracer, is no different from those real life examples. If
you're about to trace a software system or, in other words, record its
history of execution, you better have **instrumentation points** in the
subject you're tracing, that is, the actual software system.

<<instrumentation-point-types,Various ways>> were developed to
instrument a piece of software for LTTng tracing. The most
straightforward one is to manually place static instrumentation points,
called _tracepoints_, in the source code of the application. The Linux
kernel <<domain,tracing domain>> also makes it possible to dynamically
add instrumentation points.

If you're only interested in tracing the Linux kernel, your
instrumentation needs are probably already covered by the built-in
<<lttng-modules,Linux kernel instrumentation points>> of LTTng. You may
also wish to have LTTng trace a user application which is already
instrumented for LTTng tracing. In such cases, skip this whole section
and read the topics of the ``<<controlling-tracing,Tracing control>>''
section.

Many methods are available to instrument a piece of software for LTTng
tracing:

* <<c-application,Instrument a C/$$C++$$ user application>>.
* <<prebuilt-ust-helpers,Load a prebuilt user space tracing helper>>.
* <<java-application,Instrument a Java application>>.
* <<python-application,Instrument a Python application>>.
* <<proc-lttng-logger-abi,Use the LTTng logger>>.
* <<instrumenting-linux-kernel,Instrument a Linux kernel image or module>>.


[[c-application]]
=== [[cxx-application]]Instrument a C/$$C++$$ user application

The high level procedure to instrument a C or $$C++$$ user application
with the <<lttng-ust,LTTng user space tracing library>>, `liblttng-ust`,
is:

. <<tracepoint-provider,Create the source files of a tracepoint provider
  package>>.

. <<probing-the-application-source-code,Add tracepoints to
  the source code of the application>>.

. <<building-tracepoint-providers-and-user-application,Build and link
  a tracepoint provider package and the user application>>.

If you need quick, man:printf(3)-like instrumentation, skip those steps
and use <<tracef,`lttng_ust_tracef()`>> or
<<tracelog,`lttng_ust_tracelog()`>> instead.

IMPORTANT: You need to <<installing-lttng,install>> LTTng-UST to
instrument a user application with `liblttng-ust`.


[[tracepoint-provider]]
==== Create the source files of a tracepoint provider package

A _tracepoint provider_ is a set of compiled functions which provide
**tracepoints** to an application, the type of instrumentation point
which LTTng-UST provides.

Those functions can make LTTng emit events with user-defined fields and
serialize those events as event records to one or more LTTng-UST
<<channel,channel>> sub-buffers. The `lttng_ust_tracepoint()` macro,
which you <<probing-the-application-source-code,insert in the source
code of a user application>>, calls those functions.

A _tracepoint provider package_ is an object file (`.o`) or a shared
library (`.so`) which contains one or more tracepoint providers. Its
source files are:

* One or more <<tpp-header,tracepoint provider header>> (`.h`).
* A <<tpp-source,tracepoint provider package source>> (`.c`).

A tracepoint provider package is dynamically linked with `liblttng-ust`,
the LTTng user space tracer, at run time.

[role="img-100"]
.User application linked with `liblttng-ust` and containing a tracepoint provider.
image::ust-app.png[]

NOTE: If you need quick, man:printf(3)-like instrumentation, skip
creating and using a tracepoint provider and use
<<tracef,`lttng_ust_tracef()`>> or <<tracelog,`lttng_ust_tracelog()`>>
instead.


[[tpp-header]]
===== Create a tracepoint provider header file template

A _tracepoint provider header file_ contains the tracepoint definitions
of a tracepoint provider.

To create a tracepoint provider header file:

. Start from this template:
+
--
[source,c]
.Tracepoint provider header file template (`.h` file extension).
----
#undef LTTNG_UST_TRACEPOINT_PROVIDER
#define LTTNG_UST_TRACEPOINT_PROVIDER provider_name

#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>

/*
 * Use LTTNG_UST_TRACEPOINT_EVENT(), LTTNG_UST_TRACEPOINT_EVENT_CLASS(),
 * LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(), and
 * LTTNG_UST_TRACEPOINT_LOGLEVEL() here.
 */

#endif /* _TP_H */

#include <lttng/tracepoint-event.h>
----
--
+
Replace:
+
* +__provider_name__+ with the name of your tracepoint provider.
* `"tp.h"` with the name of your tracepoint provider header file.

. Below the `#include <lttng/tracepoint.h>` line, put your
  <<defining-tracepoints,tracepoint definitions>>.

Your tracepoint provider name must be unique amongst all the possible
tracepoint provider names used on the same target system. We suggest to
include the name of your project or company in the name, for example,
`org_lttng_my_project_tpp`.


[[defining-tracepoints]]
===== Create a tracepoint definition

A _tracepoint definition_ defines, for a given tracepoint:

* Its **input arguments**.
+
They're the macro parameters that the `lttng_ust_tracepoint()` macro
accepts for this particular tracepoint in the source code of the user
application.

* Its **output event fields**.
+
They're the sources of event fields that form the payload of any event
that the execution of the `lttng_ust_tracepoint()` macro emits for this
particular tracepoint.

Create a tracepoint definition with the
`LTTNG_UST_TRACEPOINT_EVENT()` macro below the `#include <lttng/tracepoint.h>`
line in the
<<tpp-header,tracepoint provider header file template>>.

The syntax of the `LTTNG_UST_TRACEPOINT_EVENT()` macro is:

[source,c]
.`LTTNG_UST_TRACEPOINT_EVENT()` macro syntax.
----
LTTNG_UST_TRACEPOINT_EVENT(
    /* Tracepoint provider name */
    provider_name,

    /* Tracepoint name */
    tracepoint_name,

    /* Input arguments */
    LTTNG_UST_TP_ARGS(
        arguments
    ),

    /* Output event fields */
    LTTNG_UST_TP_FIELDS(
        fields
    )
)
----

Replace:

* +__provider_name__+ with your tracepoint provider name.
* +__tracepoint_name__+ with your tracepoint name.
* +__arguments__+ with the <<tpp-def-input-args,input arguments>>.
* +__fields__+ with the <<tpp-def-output-fields,output event field>>
  definitions.

The full name of this tracepoint is `provider_name:tracepoint_name`.

[IMPORTANT]
.Event name length limitation
====
The concatenation of the tracepoint provider name and the tracepoint
name must not exceed **254{nbsp}characters**. If it does, the
instrumented application compiles and runs, but LTTng throws multiple
warnings and you could experience serious issues.
====

[[tpp-def-input-args]]The syntax of the `LTTNG_UST_TP_ARGS()` macro is:

[source,c]
.`LTTNG_UST_TP_ARGS()` macro syntax.
----
LTTNG_UST_TP_ARGS(
    type, arg_name
)
----

Replace:

* +__type__+ with the C{nbsp}type of the argument.
* +__arg_name__+ with the argument name.

You can repeat +__type__+ and +__arg_name__+ up to 10{nbsp}times to have
more than one argument.

.`LTTNG_UST_TP_ARGS()` usage with three arguments.
====
[source,c]
----
LTTNG_UST_TP_ARGS(
    int, count,
    float, ratio,
    const char*, query
)
----
====

The `LTTNG_UST_TP_ARGS()` and `LTTNG_UST_TP_ARGS(void)` forms are valid
to create a tracepoint definition with no input arguments.

[[tpp-def-output-fields]]The `LTTNG_UST_TP_FIELDS()` macro contains a
list of `lttng_ust_field_*()` macros. Each `lttng_ust_field_*()` macro
defines one event field. See man:lttng-ust(3) for a complete description
of the available `lttng_ust_field_*()` macros. A `lttng_ust_field_*()`
macro specifies the type, size, and byte order of one event field.

Each `lttng_ust_field_*()` macro takes an _argument expression_
parameter. This is a C{nbsp}expression that the tracer evaluates at the
`lttng_ust_tracepoint()` macro site in the source code of the
application. This expression provides the source of data of a field. The
argument expression can include input argument names listed in the
`LTTNG_UST_TP_ARGS()` macro.

Each `lttng_ust_field_*()` macro also takes a _field name_ parameter.
Field names must be unique within a given tracepoint definition.

Here's a complete tracepoint definition example:

.Tracepoint definition.
====
The following tracepoint definition defines a tracepoint which takes
three input arguments and has four output event fields.

[source,c]
----
#include "my-custom-structure.h"

LTTNG_UST_TRACEPOINT_EVENT(
    my_provider,
    my_tracepoint,
    LTTNG_UST_TP_ARGS(
        const struct my_custom_structure *, my_custom_structure,
        float, ratio,
        const char *, query
    ),
    LTTNG_UST_TP_FIELDS(
        lttng_ust_field_string(query_field, query)
        lttng_ust_field_float(double, ratio_field, ratio)
        lttng_ust_field_integer(int, recv_size,
                                my_custom_structure->recv_size)
        lttng_ust_field_integer(int, send_size,
                                my_custom_structure->send_size)
    )
)
----

Refer to this tracepoint definition with the `lttng_ust_tracepoint()`
macro in the source code of your application like this:

[source,c]
----
lttng_ust_tracepoint(my_provider, my_tracepoint,
                     my_structure, some_ratio, the_query);
----
====

NOTE: The LTTng-UST tracer only evaluates the arguments of a tracepoint
at run time when such a tracepoint _could_ emit an event. See
<<event-creation-emission-opti,this note>> to learn more.


[[using-tracepoint-classes]]
===== Use a tracepoint class

A _tracepoint class_ is a class of tracepoints which share the same
output event field definitions. A _tracepoint instance_ is one
instance of such a defined tracepoint class, with its own tracepoint
name.

The <<defining-tracepoints,`LTTNG_UST_TRACEPOINT_EVENT()` macro>> is
actually a shorthand which defines both a tracepoint class and a
tracepoint instance at the same time.

When you build a tracepoint provider package, the C or $$C++$$ compiler
creates one serialization function for each **tracepoint class**. A
serialization function is responsible for serializing the event fields
of a tracepoint to a sub-buffer when recording.

For various performance reasons, when your situation requires multiple
tracepoint definitions with different names, but with the same event
fields, we recommend that you manually create a tracepoint class and
instantiate as many tracepoint instances as needed. One positive effect
of such a design, amongst other advantages, is that all tracepoint
instances of the same tracepoint class reuse the same serialization
function, thus reducing
https://en.wikipedia.org/wiki/Cache_pollution[cache pollution].

.Use a tracepoint class and tracepoint instances.
====
Consider the following three tracepoint definitions:

[source,c]
----
LTTNG_UST_TRACEPOINT_EVENT(
    my_app,
    get_account,
    LTTNG_UST_TP_ARGS(
        int, userid,
        size_t, len
    ),
    LTTNG_UST_TP_FIELDS(
        lttng_ust_field_integer(int, userid, userid)
        lttng_ust_field_integer(size_t, len, len)
    )
)

LTTNG_UST_TRACEPOINT_EVENT(
    my_app,
    get_settings,
    LTTNG_UST_TP_ARGS(
        int, userid,
        size_t, len
    ),
    LTTNG_UST_TP_FIELDS(
        lttng_ust_field_integer(int, userid, userid)
        lttng_ust_field_integer(size_t, len, len)
    )
)

LTTNG_UST_TRACEPOINT_EVENT(
    my_app,
    get_transaction,
    LTTNG_UST_TP_ARGS(
        int, userid,
        size_t, len
    ),
    LTTNG_UST_TP_FIELDS(
        lttng_ust_field_integer(int, userid, userid)
        lttng_ust_field_integer(size_t, len, len)
    )
)
----

In this case, we create three tracepoint classes, with one implicit
tracepoint instance for each of them: `get_account`, `get_settings`, and
`get_transaction`. However, they all share the same event field names
and types. Hence three identical, yet independent serialization
functions are created when you build the tracepoint provider package.

A better design choice is to define a single tracepoint class and three
tracepoint instances:

[source,c]
----
/* The tracepoint class */
LTTNG_UST_TRACEPOINT_EVENT_CLASS(
    /* Tracepoint class provider name */
    my_app,

    /* Tracepoint class name */
    my_class,

    /* Input arguments */
    LTTNG_UST_TP_ARGS(
        int, userid,
        size_t, len
    ),

    /* Output event fields */
    LTTNG_UST_TP_FIELDS(
        lttng_ust_field_integer(int, userid, userid)
        lttng_ust_field_integer(size_t, len, len)
    )
)

/* The tracepoint instances */
LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(
    /* Tracepoint class provider name */
    my_app,

    /* Tracepoint class name */
    my_class,

    /* Instance provider name */
    my_app,

    /* Tracepoint name */
    get_account,

    /* Input arguments */
    LTTNG_UST_TP_ARGS(
        int, userid,
        size_t, len
    )
)
LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(
    my_app,
    my_class,
    get_settings,
    LTTNG_UST_TP_ARGS(
        int, userid,
        size_t, len
    )
)
LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(
    my_app,
    my_class,
    get_transaction,
    LTTNG_UST_TP_ARGS(
        int, userid,
        size_t, len
    )
)
----
====

The tracepoint class and instance provider names must be the same if the
`LTTNG_UST_TRACEPOINT_EVENT_CLASS()` and
`LTTNG_UST_TRACEPOINT_EVENT_INSTANCE()` expansions are part of the same
translation unit. See man:lttng-ust(3) to learn more.


[[assigning-log-levels]]
===== Assign a log level to a tracepoint definition

Assign a _log level_ to a <<defining-tracepoints,tracepoint definition>>
with the `LTTNG_UST_TRACEPOINT_LOGLEVEL()` macro.

Assigning different levels of severity to tracepoint definitions can be
useful: when you <<enabling-disabling-events,create a recording event
rule>>, you can target tracepoints having a log level at least as severe
as a specific value.

The concept of LTTng-UST log levels is similar to the levels found
in typical logging frameworks:

* In a logging framework, the log level is given by the function
  or method name you use at the log statement site: `debug()`,
  `info()`, `warn()`, `error()`, and so on.

* In LTTng-UST, you statically assign the log level to a tracepoint
  definition; any `lttng_ust_tracepoint()` macro invocation which refers
  to this definition has this log level.

You must use `LTTNG_UST_TRACEPOINT_LOGLEVEL()` _after_ the
<<defining-tracepoints,`LTTNG_UST_TRACEPOINT_EVENT()`>> or
<<using-tracepoint-classes,`LTTNG_UST_TRACEPOINT_INSTANCE()`>> macro for
a given tracepoint.

The syntax of the `LTTNG_UST_TRACEPOINT_LOGLEVEL()` macro is:

[source,c]
.`LTTNG_UST_TRACEPOINT_LOGLEVEL()` macro syntax.
----
LTTNG_UST_TRACEPOINT_LOGLEVEL(provider_name, tracepoint_name, log_level)
----

Replace:

* +__provider_name__+ with the tracepoint provider name.
* +__tracepoint_name__+ with the tracepoint name.
* +__log_level__+ with the log level to assign to the tracepoint
  definition named +__tracepoint_name__+ in the +__provider_name__+
  tracepoint provider.
+
See man:lttng-ust(3) for a list of available log level names.

.Assign the `LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_UNIT` log level to a
tracepoint definition.
====
[source,c]
----
/* Tracepoint definition */
LTTNG_UST_TRACEPOINT_EVENT(
    my_app,
    get_transaction,
    LTTNG_UST_TP_ARGS(
        int, userid,
        size_t, len
    ),
    LTTNG_UST_TP_FIELDS(
        lttng_ust_field_integer(int, userid, userid)
        lttng_ust_field_integer(size_t, len, len)
    )
)

/* Log level assignment */
LTTNG_UST_TRACEPOINT_LOGLEVEL(my_app, get_transaction,
                              LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_UNIT)
----
====


[[tpp-source]]
===== Create a tracepoint provider package source file

A _tracepoint provider package source file_ is a C source file which
includes a <<tpp-header,tracepoint provider header file>> to expand its
macros into event serialization and other functions.

Use the following tracepoint provider package source file template:

[source,c]
.Tracepoint provider package source file template.
----
#define LTTNG_UST_TRACEPOINT_CREATE_PROBES

#include "tp.h"
----

Replace `tp.h` with the name of your <<tpp-header,tracepoint provider
header file>> name. You may also include more than one tracepoint
provider header file here to create a tracepoint provider package
holding more than one tracepoint providers.


[[probing-the-application-source-code]]
==== Add tracepoints to the source code of an application

Once you <<tpp-header,create a tracepoint provider header file>>, use
the `lttng_ust_tracepoint()` macro in the source code of your
application to insert the tracepoints that this header
<<defining-tracepoints,defines>>.

The `lttng_ust_tracepoint()` macro takes at least two parameters: the
tracepoint provider name and the tracepoint name. The corresponding
tracepoint definition defines the other parameters.

.`lttng_ust_tracepoint()` usage.
====
The following <<defining-tracepoints,tracepoint definition>> defines a
tracepoint which takes two input arguments and has two output event
fields.

[source,c]
.Tracepoint provider header file.
----
#include "my-custom-structure.h"

LTTNG_UST_TRACEPOINT_EVENT(
    my_provider,
    my_tracepoint,
    LTTNG_UST_TP_ARGS(
        int, argc,
        const char *, cmd_name
    ),
    LTTNG_UST_TP_FIELDS(
        lttng_ust_field_string(cmd_name, cmd_name)
        lttng_ust_field_integer(int, number_of_args, argc)
    )
)
----

Refer to this tracepoint definition with the `lttng_ust_tracepoint()`
macro in the source code of your application like this:

[source,c]
.Application source file.
----
#include "tp.h"

int main(int argc, char* argv[])
{
    lttng_ust_tracepoint(my_provider, my_tracepoint, argc, argv[0]);
    return 0;
}
----

Note how the source code of the application includes
the tracepoint provider header file containing the tracepoint
definitions to use, path:{tp.h}.
====

.`lttng_ust_tracepoint()` usage with a complex tracepoint definition.
====
Consider this complex tracepoint definition, where multiple event
fields refer to the same input arguments in their argument expression
parameter:

[source,c]
.Tracepoint provider header file.
----
/* For `struct stat` */
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

LTTNG_UST_TRACEPOINT_EVENT(
    my_provider,
    my_tracepoint,
    LTTNG_UST_TP_ARGS(
        int, my_int_arg,
        char *, my_str_arg,
        struct stat *, st
    ),
    LTTNG_UST_TP_FIELDS(
        lttng_ust_field_integer(int, my_constant_field, 23 + 17)
        lttng_ust_field_integer(int, my_int_arg_field, my_int_arg)
        lttng_ust_field_integer(int, my_int_arg_field2,
                                my_int_arg * my_int_arg)
        lttng_ust_field_integer(int, sum4_field,
                                my_str_arg[0] + my_str_arg[1] +
                                my_str_arg[2] + my_str_arg[3])
        lttng_ust_field_string(my_str_arg_field, my_str_arg)
        lttng_ust_field_integer_hex(off_t, size_field, st->st_size)
        lttng_ust_field_float(double, size_dbl_field, (double) st->st_size)
        lttng_ust_field_sequence_text(char, half_my_str_arg_field,
                                      my_str_arg, size_t,
                                      strlen(my_str_arg) / 2)
    )
)
----

Refer to this tracepoint definition with the `lttng_ust_tracepoint()`
macro in the source code of your application like this:

[source,c]
.Application source file.
----
#define LTTNG_UST_TRACEPOINT_DEFINE
#include "tp.h"

int main(void)
{
    struct stat s;

    stat("/etc/fstab", &s);
    lttng_ust_tracepoint(my_provider, my_tracepoint, 23,
                         "Hello, World!", &s);

    return 0;
}
----

If you look at the event record that LTTng writes when recording this
program, assuming the file size of path:{/etc/fstab} is 301{nbsp}bytes,
it should look like this:

.Event record fields
|====
|Field name              |Field value
|`my_constant_field`     |40
|`my_int_arg_field`      |23
|`my_int_arg_field2`     |529
|`sum4_field`            |389
|`my_str_arg_field`      |`Hello, World!`
|`size_field`            |0x12d
|`size_dbl_field`        |301.0
|`half_my_str_arg_field` |`Hello,`
|====
====

Sometimes, the arguments you pass to `lttng_ust_tracepoint()` are
expensive to evaluate--they use the call stack, for example. To avoid
this computation when LTTng wouldn't emit any event anyway, use the
`lttng_ust_tracepoint_enabled()` and `lttng_ust_do_tracepoint()` macros.

The syntax of the `lttng_ust_tracepoint_enabled()` and
`lttng_ust_do_tracepoint()` macros is:

[source,c]
.`lttng_ust_tracepoint_enabled()` and `lttng_ust_do_tracepoint()` macros syntax.
----
lttng_ust_tracepoint_enabled(provider_name, tracepoint_name)

lttng_ust_do_tracepoint(provider_name, tracepoint_name, ...)
----

Replace:

* +__provider_name__+ with the tracepoint provider name.
* +__tracepoint_name__+ with the tracepoint name.

`lttng_ust_tracepoint_enabled()` returns a non-zero value if executing
the tracepoint named `tracepoint_name` from the provider named
`provider_name` _could_ make LTTng emit an event, depending on the
payload of said event.

`lttng_ust_do_tracepoint()` is like `lttng_ust_tracepoint()`, except
that it doesn't check what `lttng_ust_tracepoint_enabled()` checks.
Using `lttng_ust_tracepoint()` with `lttng_ust_tracepoint_enabled()` is
dangerous because `lttng_ust_tracepoint()` also contains the
`lttng_ust_tracepoint_enabled()` check; therefore, a race condition is
possible in this situation:

[source,c]
.Possible race condition when using `lttng_ust_tracepoint_enabled()` with `lttng_ust_tracepoint()`.
----
if (lttng_ust_tracepoint_enabled(my_provider, my_tracepoint)) {
    stuff = prepare_stuff();
}

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

If `lttng_ust_tracepoint_enabled()` is false, but would be true after
the conditional block, then `stuff` isn't prepared: the emitted event
will either contain wrong data, or the whole application could crash
(with a segmentation fault, for example).

NOTE: Neither `lttng_ust_tracepoint_enabled()` nor
`lttng_ust_do_tracepoint()` have an `STAP_PROBEV()` call. If you need
it, you must emit this call yourself.


[[building-tracepoint-providers-and-user-application]]
==== Build and link a tracepoint provider package and an application

Once you have one or more <<tpp-header,tracepoint provider header
files>> and a <<tpp-source,tracepoint provider package source file>>,
create the tracepoint provider package by compiling its source
file. From here, multiple build and run scenarios are possible. The
following table shows common application and library configurations
along with the required command lines to achieve them.

In the following diagrams, we use the following file names:

`app`::
  Executable application.

`app.o`::
  Application object file.

`tpp.o`::
  Tracepoint provider package object file.

`tpp.a`::
  Tracepoint provider package archive file.

`libtpp.so`::
  Tracepoint provider package shared object file.

`emon.o`::
  User library object file.

`libemon.so`::
  User library shared object file.

We use the following symbols in the diagrams of table below:

[role="img-100"]
.Symbols used in the build scenario diagrams.
image::ust-sit-symbols.png[]

We assume that path:{.} is part of the env:LD_LIBRARY_PATH environment
variable in the following instructions.

[role="growable ust-scenarios",cols="asciidoc,asciidoc"]
.Common tracepoint provider package scenarios.
|====
|Scenario |Instructions

|
The instrumented application is statically linked with
the tracepoint provider package object.

image::ust-sit+app-linked-with-tp-o+app-instrumented.png[]

|
include::../common/ust-sit-step-tp-o.txt[]

To build the instrumented application:

. In path:{app.c}, before including path:{tpp.h}, add the following line:
+
--
[source,c]
----
#define LTTNG_UST_TRACEPOINT_DEFINE
----
--

. Compile the application source file:
+
--
[role="term"]
----
$ gcc -c app.c
----
--

. Build the application:
+
--
[role="term"]
----
$ gcc -o app app.o tpp.o -llttng-ust -ldl
----
--

To run the instrumented application:

* Start the application:
+
--
[role="term"]
----
$ ./app
----
--

|
The instrumented application is statically linked with the
tracepoint provider package archive file.

image::ust-sit+app-linked-with-tp-a+app-instrumented.png[]

|
To create the tracepoint provider package archive file:

. Compile the <<tpp-source,tracepoint provider package source file>>:
+
--
[role="term"]
----
$ gcc -I. -c tpp.c
----
--

. Create the tracepoint provider package archive file:
+
--
[role="term"]
----
$ ar rcs tpp.a tpp.o
----
--

To build the instrumented application:

. In path:{app.c}, before including path:{tpp.h}, add the following line:
+
--
[source,c]
----
#define LTTNG_UST_TRACEPOINT_DEFINE
----
--

. Compile the application source file:
+
--
[role="term"]
----
$ gcc -c app.c
----
--

. Build the application:
+
--
[role="term"]
----
$ gcc -o app app.o tpp.a -llttng-ust -ldl
----
--

To run the instrumented application:

* Start the application:
+
--
[role="term"]
----
$ ./app
----
--

|
The instrumented application is linked with the tracepoint provider
package shared object.

image::ust-sit+app-linked-with-tp-so+app-instrumented.png[]

|
include::../common/ust-sit-step-tp-so.txt[]

To build the instrumented application:

. In path:{app.c}, before including path:{tpp.h}, add the following line:
+
--
[source,c]
----
#define LTTNG_UST_TRACEPOINT_DEFINE
----
--

. Compile the application source file:
+
--
[role="term"]
----
$ gcc -c app.c
----
--

. Build the application:
+
--
[role="term"]
----
$ gcc -o app app.o -ldl -L. -ltpp
----
--

To run the instrumented application:

* Start the application:
+
--
[role="term"]
----
$ ./app
----
--

|
The tracepoint provider package shared object is preloaded before the
instrumented application starts.

image::ust-sit+tp-so-preloaded+app-instrumented.png[]

|
include::../common/ust-sit-step-tp-so.txt[]

To build the instrumented application:

. In path:{app.c}, before including path:{tpp.h}, add the
  following lines:
+
--
[source,c]
----
#define LTTNG_UST_TRACEPOINT_DEFINE
#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE
----
--

. Compile the application source file:
+
--
[role="term"]
----
$ gcc -c app.c
----
--

. Build the application:
+
--
[role="term"]
----
$ gcc -o app app.o -ldl
----
--

To run the instrumented application with tracing support:

* Preload the tracepoint provider package shared object and
  start the application:
+
--
[role="term"]
----
$ LD_PRELOAD=./libtpp.so ./app
----
--

To run the instrumented application without tracing support:

* Start the application:
+
--
[role="term"]
----
$ ./app
----
--

|
The instrumented application dynamically loads the tracepoint provider
package shared object.

image::ust-sit+app-dlopens-tp-so+app-instrumented.png[]

|
include::../common/ust-sit-step-tp-so.txt[]

To build the instrumented application:

. In path:{app.c}, before including path:{tpp.h}, add the
  following lines:
+
--
[source,c]
----
#define LTTNG_UST_TRACEPOINT_DEFINE
#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE
----
--

. Compile the application source file:
+
--
[role="term"]
----
$ gcc -c app.c
----
--

. Build the application:
+
--
[role="term"]
----
$ gcc -o app app.o -ldl
----
--

To run the instrumented application:

* Start the application:
+
--
[role="term"]
----
$ ./app
----
--

|
The application is linked with the instrumented user library.

The instrumented user library is statically linked with the tracepoint
provider package object file.

image::ust-sit+app-linked-with-lib+lib-linked-with-tp-o+lib-instrumented.png[]

|
include::../common/ust-sit-step-tp-o-fpic.txt[]

To build the instrumented user library:

. In path:{emon.c}, before including path:{tpp.h}, add the
  following line:
+
--
[source,c]
----
#define LTTNG_UST_TRACEPOINT_DEFINE
----
--

. Compile the user library source file:
+
--
[role="term"]
----
$ gcc -I. -fpic -c emon.c
----
--

. Build the user library shared object:
+
--
[role="term"]
----
$ gcc -shared -o libemon.so emon.o tpp.o -llttng-ust -ldl
----
--

To build the application:

. Compile the application source file:
+
--
[role="term"]
----
$ gcc -c app.c
----
--

. Build the application:
+
--
[role="term"]
----
$ gcc -o app app.o -L. -lemon
----
--

To run the application:

* Start the application:
+
--
[role="term"]
----
$ ./app
----
--

|
The application is linked with the instrumented user library.

The instrumented user library is linked with the tracepoint provider
package shared object.

image::ust-sit+app-linked-with-lib+lib-linked-with-tp-so+lib-instrumented.png[]

|
include::../common/ust-sit-step-tp-so.txt[]

To build the instrumented user library:

. In path:{emon.c}, before including path:{tpp.h}, add the
  following line:
+
--
[source,c]
----
#define LTTNG_UST_TRACEPOINT_DEFINE
----
--

. Compile the user library source file:
+
--
[role="term"]
----
$ gcc -I. -fpic -c emon.c
----
--

. Build the user library shared object:
+
--
[role="term"]
----
$ gcc -shared -o libemon.so emon.o -ldl -L. -ltpp
----
--

To build the application:

. Compile the application source file:
+
--
[role="term"]
----
$ gcc -c app.c
----
--

. Build the application:
+
--
[role="term"]
----
$ gcc -o app app.o -L. -lemon
----
--

To run the application:

* Start the application:
+
--
[role="term"]
----
$ ./app
----
--

|
The tracepoint provider package shared object is preloaded before the
application starts.

The application is linked with the instrumented user library.

image::ust-sit+tp-so-preloaded+app-linked-with-lib+lib-instrumented.png[]

|
include::../common/ust-sit-step-tp-so.txt[]

To build the instrumented user library:

. In path:{emon.c}, before including path:{tpp.h}, add the
  following lines:
+
--
[source,c]
----
#define LTTNG_UST_TRACEPOINT_DEFINE
#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE
----
--

. Compile the user library source file:
+
--
[role="term"]
----
$ gcc -I. -fpic -c emon.c
----
--

. Build the user library shared object:
+
--
[role="term"]
----
$ gcc -shared -o libemon.so emon.o -ldl
----
--

To build the application:

. Compile the application source file:
+
--
[role="term"]
----
$ gcc -c app.c
----
--

. Build the application:
+
--
[role="term"]
----
$ gcc -o app app.o -L. -lemon
----
--

To run the application with tracing support:

* Preload the tracepoint provider package shared object and
  start the application:
+
--
[role="term"]
----
$ LD_PRELOAD=./libtpp.so ./app
----
--

To run the application without tracing support:

* Start the application:
+
--
[role="term"]
----
$ ./app
----
--

|
The application is linked with the instrumented user library.

The instrumented user library dynamically loads the tracepoint provider
package shared object.

image::ust-sit+app-linked-with-lib+lib-dlopens-tp-so+lib-instrumented.png[]

|
include::../common/ust-sit-step-tp-so.txt[]

To build the instrumented user library:

. In path:{emon.c}, before including path:{tpp.h}, add the
  following lines:
+
--
[source,c]
----
#define LTTNG_UST_TRACEPOINT_DEFINE
#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE
----
--

. Compile the user library source file:
+
--
[role="term"]
----
$ gcc -I. -fpic -c emon.c
----
--

. Build the user library shared object:
+
--
[role="term"]
----
$ gcc -shared -o libemon.so emon.o -ldl
----
--

To build the application:

. Compile the application source file:
+
--
[role="term"]
----
$ gcc -c app.c
----
--

. Build the application:
+
--
[role="term"]
----
$ gcc -o app app.o -L. -lemon
----
--

To run the application:

* Start the application:
+
--
[role="term"]
----
$ ./app
----
--

|
The application dynamically loads the instrumented user library.

The instrumented user library is linked with the tracepoint provider
package shared object.

image::ust-sit+app-dlopens-lib+lib-linked-with-tp-so+lib-instrumented.png[]

|
include::../common/ust-sit-step-tp-so.txt[]

To build the instrumented user library:

. In path:{emon.c}, before including path:{tpp.h}, add the
  following line:
+
--
[source,c]
----
#define LTTNG_UST_TRACEPOINT_DEFINE
----
--

. Compile the user library source file:
+
--
[role="term"]
----
$ gcc -I. -fpic -c emon.c
----
--

. Build the user library shared object:
+
--
[role="term"]
----
$ gcc -shared -o libemon.so emon.o -ldl -L. -ltpp
----
--

To build the application:

. Compile the application source file:
+
--
[role="term"]
----
$ gcc -c app.c
----
--

. Build the application:
+
--
[role="term"]
----
$ gcc -o app app.o -ldl -L. -lemon
----
--

To run the application:

* Start the application:
+
--
[role="term"]
----
$ ./app
----
--

|
The application dynamically loads the instrumented user library.

The instrumented user library dynamically loads the tracepoint provider
package shared object.

image::ust-sit+app-dlopens-lib+lib-dlopens-tp-so+lib-instrumented.png[]

|
include::../common/ust-sit-step-tp-so.txt[]

To build the instrumented user library:

. In path:{emon.c}, before including path:{tpp.h}, add the
  following lines:
+
--
[source,c]
----
#define LTTNG_UST_TRACEPOINT_DEFINE
#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE
----
--

. Compile the user library source file:
+
--
[role="term"]
----
$ gcc -I. -fpic -c emon.c
----
--

. Build the user library shared object:
+
--
[role="term"]
----
$ gcc -shared -o libemon.so emon.o -ldl
----
--

To build the application:

. Compile the application source file:
+
--
[role="term"]
----
$ gcc -c app.c
----
--

. Build the application:
+
--
[role="term"]
----
$ gcc -o app app.o -ldl -L. -lemon
----
--

To run the application:

* Start the application:
+
--
[role="term"]
----
$ ./app
----
--

|
The tracepoint provider package shared object is preloaded before the
application starts.

The application dynamically loads the instrumented user library.

image::ust-sit+tp-so-preloaded+app-dlopens-lib+lib-instrumented.png[]

|
include::../common/ust-sit-step-tp-so.txt[]

To build the instrumented user library:

. In path:{emon.c}, before including path:{tpp.h}, add the
  following lines:
+
--
[source,c]
----
#define LTTNG_UST_TRACEPOINT_DEFINE
#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE
----
--

. Compile the user library source file:
+
--
[role="term"]
----
$ gcc -I. -fpic -c emon.c
----
--

. Build the user library shared object:
+
--
[role="term"]
----
$ gcc -shared -o libemon.so emon.o -ldl
----
--

To build the application:

. Compile the application source file:
+
--
[role="term"]
----
$ gcc -c app.c
----
--

. Build the application:
+
--
[role="term"]
----
$ gcc -o app app.o -L. -lemon
----
--

To run the application with tracing support:

* Preload the tracepoint provider package shared object and
  start the application:
+
--
[role="term"]
----
$ LD_PRELOAD=./libtpp.so ./app
----
--

To run the application without tracing support:

* Start the application:
+
--
[role="term"]
----
$ ./app
----
--

|
The application is statically linked with the tracepoint provider
package object file.

The application is linked with the instrumented user library.

image::ust-sit+app-linked-with-tp-o+app-linked-with-lib+lib-instrumented.png[]

|
include::../common/ust-sit-step-tp-o.txt[]

To build the instrumented user library:

. In path:{emon.c}, before including path:{tpp.h}, add the
  following line:
+
--
[source,c]
----
#define LTTNG_UST_TRACEPOINT_DEFINE
----
--

. Compile the user library source file:
+
--
[role="term"]
----
$ gcc -I. -fpic -c emon.c
----
--

. Build the user library shared object:
+
--
[role="term"]
----
$ gcc -shared -o libemon.so emon.o
----
--

To build the application:

. Compile the application source file:
+
--
[role="term"]
----
$ gcc -c app.c
----
--

. Build the application:
+
--
[role="term"]
----
$ gcc -o app app.o tpp.o -llttng-ust -ldl -L. -lemon
----
--

To run the instrumented application:

* Start the application:
+
--
[role="term"]
----
$ ./app
----
--

|
The application is statically linked with the tracepoint provider
package object file.

The application dynamically loads the instrumented user library.

image::ust-sit+app-linked-with-tp-o+app-dlopens-lib+lib-instrumented.png[]

|
include::../common/ust-sit-step-tp-o.txt[]

To build the application:

. In path:{app.c}, before including path:{tpp.h}, add the following line:
+
--
[source,c]
----
#define LTTNG_UST_TRACEPOINT_DEFINE
----
--

. Compile the application source file:
+
--
[role="term"]
----
$ gcc -c app.c
----
--

. Build the application:
+
--
[role="term"]
----
$ gcc -Wl,--export-dynamic -o app app.o tpp.o \
      -llttng-ust -ldl
----
--
+
The `--export-dynamic` option passed to the linker is necessary for the
dynamically loaded library to ``see'' the tracepoint symbols defined in
the application.

To build the instrumented user library:

. Compile the user library source file:
+
--
[role="term"]
----
$ gcc -I. -fpic -c emon.c
----
--

. Build the user library shared object:
+
--
[role="term"]
----
$ gcc -shared -o libemon.so emon.o
----
--

To run the application:

* Start the application:
+
--
[role="term"]
----
$ ./app
----
--
|====


[[using-lttng-ust-with-daemons]]
===== Use noch:{LTTng-UST} with daemons

If your instrumented application calls man:fork(2), man:clone(2),
or BSD's man:rfork(2), without a following man:exec(3)-family
system call, you must preload the path:{liblttng-ust-fork.so} shared
object when you start the application.

[role="term"]
----
$ LD_PRELOAD=liblttng-ust-fork.so ./my-app
----

If your tracepoint provider package is
a shared library which you also preload, you must put both
shared objects in env:LD_PRELOAD:

[role="term"]
----
$ LD_PRELOAD=liblttng-ust-fork.so:/path/to/tp.so ./my-app
----


[role="since-2.9"]
[[liblttng-ust-fd]]
===== Use noch:{LTTng-UST} with applications which close file descriptors that don't belong to them

If your instrumented application closes one or more file descriptors
which it did not open itself, you must preload the
path:{liblttng-ust-fd.so} shared object when you start the application:

[role="term"]
----
$ LD_PRELOAD=liblttng-ust-fd.so ./my-app
----

Typical use cases include closing all the file descriptors after
man:fork(2) or man:rfork(2) and buggy applications doing
``double closes''.


[[lttng-ust-pkg-config]]
===== Use noch:{pkg-config}

On some distributions, LTTng-UST ships with a
https://www.freedesktop.org/wiki/Software/pkg-config/[pkg-config]
metadata file. If this is your case, then use cmd:pkg-config to
build an application on the command line:

[role="term"]
----
$ gcc -o my-app my-app.o tp.o $(pkg-config --cflags --libs lttng-ust)
----


[[instrumenting-32-bit-app-on-64-bit-system]]
===== [[advanced-instrumenting-techniques]]Build a 32-bit instrumented application for a 64-bit target system

In order to trace a 32-bit application running on a 64-bit system,
LTTng must use a dedicated 32-bit
<<lttng-consumerd,consumer daemon>>.

The following steps show how to build and install a 32-bit consumer
daemon, which is _not_ part of the default 64-bit LTTng build, how to
build and install the 32-bit LTTng-UST libraries, and how to build and
link an instrumented 32-bit application in that context.

To build a 32-bit instrumented application for a 64-bit target system,
assuming you have a fresh target system with no installed Userspace RCU
or LTTng packages:

. Download, build, and install a 32-bit version of Userspace RCU:
+
--
[role="term"]
----
$ cd $(mktemp -d) &&
  wget https://lttng.org/files/urcu/userspace-rcu-latest-0.13.tar.bz2 &&
  tar -xf userspace-rcu-latest-0.13.tar.bz2 &&
  cd userspace-rcu-0.13.* &&
  ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 &&
  make &&
  sudo make install &&
  sudo ldconfig
----
--

. Using the package manager of your distribution, or from source,
  install the 32-bit versions of the following dependencies of
  LTTng-tools and LTTng-UST:
+
--
* https://sourceforge.net/projects/libuuid/[libuuid]
* https://directory.fsf.org/wiki/Popt[popt]
* https://www.xmlsoft.org/[libxml2]
* **Optional**: https://github.com/numactl/numactl[numactl]
--

. Download, build, and install a 32-bit version of the latest
  LTTng-UST{nbsp}{revision}:
+
--
[role="term"]
----
$ cd $(mktemp -d) &&
  wget https://lttng.org/files/lttng-ust/lttng-ust-latest-2.13.tar.bz2 &&
  tar -xf lttng-ust-latest-2.13.tar.bz2 &&
  cd lttng-ust-2.13.* &&
  ./configure --libdir=/usr/local/lib32 \
              CFLAGS=-m32 CXXFLAGS=-m32 \
              LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' &&
  make &&
  sudo make install &&
  sudo ldconfig
----
--
+
Add `--disable-numa` to `./configure` if you don't have
https://github.com/numactl/numactl[numactl].
+
[NOTE]
====
Depending on your distribution, 32-bit libraries could be installed at a
different location than `/usr/lib32`. For example, Debian is known to
install some 32-bit libraries in `/usr/lib/i386-linux-gnu`.

In this case, make sure to set `LDFLAGS` to all the
relevant 32-bit library paths, for example:

[role="term"]
----
$ LDFLAGS='-L/usr/lib/i386-linux-gnu -L/usr/lib32'
----
====

. Download the latest LTTng-tools{nbsp}{revision}, build, and install
  the 32-bit consumer daemon:
+
--
[role="term"]
----
$ cd $(mktemp -d) &&
  wget https://lttng.org/files/lttng-tools/lttng-tools-latest-2.13.tar.bz2 &&
  tar -xf lttng-tools-latest-2.13.tar.bz2 &&
  cd lttng-tools-2.13.* &&
  ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 CXXFLAGS=-m32 \
              LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' \
              --disable-bin-lttng --disable-bin-lttng-crash \
              --disable-bin-lttng-relayd --disable-bin-lttng-sessiond &&
  make &&
  cd src/bin/lttng-consumerd &&
  sudo make install &&
  sudo ldconfig
----
--

. From your distribution or from source, <<installing-lttng,install>>
  the 64-bit versions of LTTng-UST and Userspace RCU.

. Download, build, and install the 64-bit version of the
  latest LTTng-tools{nbsp}{revision}:
+
--
[role="term"]
----
$ cd $(mktemp -d) &&
  wget https://lttng.org/files/lttng-tools/lttng-tools-latest-2.13.tar.bz2 &&
  tar -xf lttng-tools-latest-2.13.tar.bz2 &&
  cd lttng-tools-2.13.* &&
  ./configure --with-consumerd32-libdir=/usr/local/lib32 \
              --with-consumerd32-bin=/usr/local/lib32/lttng/libexec/lttng-consumerd &&
  make &&
  sudo make install &&
  sudo ldconfig
----
--

. Pass the following options to man:gcc(1), man:g++(1), or man:clang(1)
  when linking your 32-bit application:
+
----
-m32 -L/usr/lib32 -L/usr/local/lib32 \
-Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32
----
+
For example, let's rebuild the quick start example in
``<<tracing-your-own-user-application,Record user application events>>''
as an instrumented 32-bit application:
+
--
[role="term"]
----
$ gcc -m32 -c -I. hello-tp.c
$ gcc -m32 -c hello.c
$ gcc -m32 -o hello hello.o hello-tp.o \
      -L/usr/lib32 -L/usr/local/lib32 \
      -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32 \
      -llttng-ust -ldl
----
--

No special action is required to execute the 32-bit application and
for LTTng to trace it: use the command-line man:lttng(1) tool as usual.


[role="since-2.5"]
[[tracef]]
==== Use `lttng_ust_tracef()`

man:lttng_ust_tracef(3) is a small LTTng-UST API designed for quick,
man:printf(3)-like instrumentation without the burden of
<<tracepoint-provider,creating>> and
<<building-tracepoint-providers-and-user-application,building>>
a tracepoint provider package.

To use `lttng_ust_tracef()` in your application:

. In the C or $$C++$$ source files where you need to use
  `lttng_ust_tracef()`, include `<lttng/tracef.h>`:
+
--
[source,c]
----
#include <lttng/tracef.h>
----
--

. In the source code of the application, use `lttng_ust_tracef()` like
  you would use man:printf(3):
+
--
[source,c]
----
    /* ... */

    lttng_ust_tracef("my message: %d (%s)", my_integer, my_string);

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

. Link your application with `liblttng-ust`:
+
--
[role="term"]
----
$ gcc -o app app.c -llttng-ust
----
--

To record the events that `lttng_ust_tracef()` calls emit:

* <<enabling-disabling-events,Create a recording event rule>> which
  matches user space events named `lttng_ust_tracef:*`:
+
--
[role="term"]
----
$ lttng enable-event --userspace 'lttng_ust_tracef:*'
----
--

[IMPORTANT]
.Limitations of `lttng_ust_tracef()`
====
The `lttng_ust_tracef()` utility function was developed to make user
space tracing super simple, albeit with notable disadvantages compared
to <<defining-tracepoints,user-defined tracepoints>>:

* All the created events have the same tracepoint provider and
  tracepoint names, respectively `lttng_ust_tracef` and `event`.
* There's no static type checking.
* The only event record field you actually get, named `msg`, is a string
  potentially containing the values you passed to `lttng_ust_tracef()`
  using your own format string. This also means that you can't filter
  events with a custom expression at run time because there are no
  isolated fields.
* Since `lttng_ust_tracef()` uses the man:vasprintf(3) function of the
  C{nbsp}standard library behind the scenes to format the strings at run
  time, its expected performance is lower than with user-defined
  tracepoints, which don't require a conversion to a string.

Taking this into consideration, `lttng_ust_tracef()` is useful for some
quick prototyping and debugging, but you shouldn't consider it for any
permanent and serious applicative instrumentation.
====


[role="since-2.7"]
[[tracelog]]
==== Use `lttng_ust_tracelog()`

The man:tracelog(3) API is very similar to
<<tracef,`lttng_ust_tracef()`>>, with the difference that it accepts an
additional log level parameter.

The goal of `lttng_ust_tracelog()` is to ease the migration from logging
to tracing.

To use `lttng_ust_tracelog()` in your application:

. In the C or $$C++$$ source files where you need to use `tracelog()`,
  include `<lttng/tracelog.h>`:
+
--
[source,c]
----
#include <lttng/tracelog.h>
----
--

. In the source code of the application, use `lttng_ust_tracelog()` like
  you would use man:printf(3), except for the first parameter which is
  the log level:
+
--
[source,c]
----
    /* ... */

    tracelog(LTTNG_UST_TRACEPOINT_LOGLEVEL_WARNING,
            "my message: %d (%s)", my_integer, my_string);

    /* ... */
----
--
+
See man:lttng-ust(3) for a list of available log level names.

. Link your application with `liblttng-ust`:
+
--
[role="term"]
----
$ gcc -o app app.c -llttng-ust
----
--

To record the events that `lttng_ust_tracelog()` calls emit with a log
level _at least as severe as_ a specific log level:

* <<enabling-disabling-events,Create a recording event rule>> which
  matches user space tracepoint events named `lttng_ust_tracelog:*` and
  with some minimum level of severity:
+
--
[role="term"]
----
$ lttng enable-event --userspace 'lttng_ust_tracelog:*' \
                     --loglevel=WARNING
----
--

To record the events that `lttng_ust_tracelog()` calls emit with a
_specific log level_:

* Create a recording event rule which matches tracepoint events named
  `lttng_ust_tracelog:*` and with a specific log level:
+
--
[role="term"]
----
$ lttng enable-event --userspace 'lttng_ust_tracelog:*' \
                     --loglevel-only=INFO
----
--


[[prebuilt-ust-helpers]]
=== Load a prebuilt user space tracing helper

The LTTng-UST package provides a few helpers in the form of preloadable
shared objects which automatically instrument system functions and
calls.

The helper shared objects are normally found in dir:{/usr/lib}. If you
built LTTng-UST <<building-from-source,from source>>, they're probably
located in dir:{/usr/local/lib}.

The installed user space tracing helpers in LTTng-UST{nbsp}{revision}
are:

path:{liblttng-ust-libc-wrapper.so}::
path:{liblttng-ust-pthread-wrapper.so}::
  <<liblttng-ust-libc-pthread-wrapper,C{nbsp}standard library
  memory and POSIX threads function tracing>>.

path:{liblttng-ust-cyg-profile.so}::
path:{liblttng-ust-cyg-profile-fast.so}::
  <<liblttng-ust-cyg-profile,Function entry and exit tracing>>.

path:{liblttng-ust-dl.so}::
  <<liblttng-ust-dl,Dynamic linker tracing>>.

To use a user space tracing helper with any user application:

* Preload the helper shared object when you start the application:
+
--
[role="term"]
----
$ LD_PRELOAD=liblttng-ust-libc-wrapper.so my-app
----
--
+
You can preload more than one helper:
+
--
[role="term"]
----
$ LD_PRELOAD=liblttng-ust-libc-wrapper.so:liblttng-ust-dl.so my-app
----
--


[role="since-2.3"]
[[liblttng-ust-libc-pthread-wrapper]]
==== Instrument C standard library memory and POSIX threads functions

The path:{liblttng-ust-libc-wrapper.so} and
path:{liblttng-ust-pthread-wrapper.so} helpers
add instrumentation to some C standard library and POSIX
threads functions.

[role="growable"]
.Functions instrumented by preloading path:{liblttng-ust-libc-wrapper.so}.
|====
|TP provider name |TP name |Instrumented function

.6+|`lttng_ust_libc` |`malloc`         |man:malloc(3)
                     |`calloc`         |man:calloc(3)
                     |`realloc`        |man:realloc(3)
                     |`free`           |man:free(3)
                     |`memalign`       |man:memalign(3)
                     |`posix_memalign` |man:posix_memalign(3)
|====

[role="growable"]
.Functions instrumented by preloading path:{liblttng-ust-pthread-wrapper.so}.
|====
|TP provider name |TP name |Instrumented function

.4+|`lttng_ust_pthread` |`pthread_mutex_lock_req` |man:pthread_mutex_lock(3p) (request time)
                        |`pthread_mutex_lock_acq` |man:pthread_mutex_lock(3p) (acquire time)
                        |`pthread_mutex_trylock`  |man:pthread_mutex_trylock(3p)
                        |`pthread_mutex_unlock`   |man:pthread_mutex_unlock(3p)
|====

When you preload the shared object, it replaces the functions listed
in the previous tables by wrappers which contain tracepoints and call
the replaced functions.


[[liblttng-ust-cyg-profile]]
==== Instrument function entry and exit

The path:{liblttng-ust-cyg-profile*.so} helpers can add instrumentation
to the entry and exit points of functions.

man:gcc(1) and man:clang(1) have an option named
https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html[`-finstrument-functions`]
which generates instrumentation calls for entry and exit to functions.
The LTTng-UST function tracing helpers,
path:{liblttng-ust-cyg-profile.so} and
path:{liblttng-ust-cyg-profile-fast.so}, take advantage of this feature
to add tracepoints to the two generated functions (which contain
`cyg_profile` in their names, hence the name of the helper).

To use the LTTng-UST function tracing helper, the source files to
instrument must be built using the `-finstrument-functions` compiler
flag.

There are two versions of the LTTng-UST function tracing helper:

* **path:{liblttng-ust-cyg-profile-fast.so}** is a lightweight variant
  that you should only use when it can be _guaranteed_ that the
  complete event stream is recorded without any lost event record.
  Any kind of duplicate information is left out.
+
Assuming no event record is lost, having only the function addresses on
entry is enough to create a call graph, since an event record always
contains the ID of the CPU that generated it.
+
Use a tool like man:addr2line(1) to convert function addresses back to
source file names and line numbers.

* **path:{liblttng-ust-cyg-profile.so}** is a more robust variant
which also works in use cases where event records might get discarded or
not recorded from application startup.
In these cases, the trace analyzer needs more information to be
able to reconstruct the program flow.

See man:lttng-ust-cyg-profile(3) to learn more about the instrumentation
points of this helper.

All the tracepoints that this helper provides have the log level
`LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_FUNCTION` (see man:lttng-ust(3)).

TIP: It's sometimes a good idea to limit the number of source files that
you compile with the `-finstrument-functions` option to prevent LTTng
from writing an excessive amount of trace data at run time. When using
man:gcc(1), use the
`-finstrument-functions-exclude-function-list` option to avoid
instrument entries and exits of specific function names.


[role="since-2.4"]
[[liblttng-ust-dl]]
==== Instrument the dynamic linker

The path:{liblttng-ust-dl.so} helper adds instrumentation to the
man:dlopen(3) and man:dlclose(3) function calls.

See man:lttng-ust-dl(3) to learn more about the instrumentation points
of this helper.


[role="since-2.4"]
[[java-application]]
=== Instrument a Java application

You can instrument any Java application which uses one of the following
logging frameworks:

* The https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[**`java.util.logging`**]
  (JUL) core logging facilities.

* https://logging.apache.org/log4j/1.2/[**Apache log4j{nbsp}1.2**], since
  LTTng{nbsp}2.6. Note that Apache Log4j{nbsp}2 isn't supported.

[role="img-100"]
.LTTng-UST Java agent imported by a Java application.
image::java-app.png[]

Note that the methods described below are new in LTTng{nbsp}2.8.
Previous LTTng versions use another technique.

NOTE: We use https://openjdk.java.net/[OpenJDK]{nbsp}8 for development
and https://ci.lttng.org/[continuous integration], thus this version is
directly supported. However, the LTTng-UST Java agent is also tested
with OpenJDK{nbsp}7.


[role="since-2.8"]
[[jul]]
==== Use the LTTng-UST Java agent for `java.util.logging`

To use the LTTng-UST Java agent in a Java application which uses
`java.util.logging` (JUL):

. In the source code of the Java application, import the LTTng-UST log
  handler package for `java.util.logging`:
+
--
[source,java]
----
import org.lttng.ust.agent.jul.LttngLogHandler;
----
--

. Create an LTTng-UST `java.util.logging` log handler:
+
--
[source,java]
----
Handler lttngUstLogHandler = new LttngLogHandler();
----
--

. Add this handler to the `java.util.logging` loggers which should emit
  LTTng events:
+
--
[source,java]
----
Logger myLogger = Logger.getLogger("some-logger");

myLogger.addHandler(lttngUstLogHandler);
----
--

. Use `java.util.logging` log statements and configuration as usual.
  The loggers with an attached LTTng-UST log handler can emit
  LTTng events.

. Before exiting the application, remove the LTTng-UST log handler from
  the loggers attached to it and call its `close()` method:
+
--
[source,java]
----
myLogger.removeHandler(lttngUstLogHandler);
lttngUstLogHandler.close();
----
--
+
This isn't strictly necessary, but it's recommended for a clean
disposal of the resources of the handler.

. Include the common and JUL-specific JAR files of the LTTng-UST Java agent,
  path:{lttng-ust-agent-common.jar} and path:{lttng-ust-agent-jul.jar},
  in the
  https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class
  path] when you build the Java application.
+
The JAR files are typically located in dir:{/usr/share/java}.
+
IMPORTANT: The LTTng-UST Java agent must be
<<installing-lttng,installed>> for the logging framework your
application uses.

.Use the LTTng-UST Java agent for `java.util.logging`.
====
[source,java]
.path:{Test.java}
----
import java.io.IOException;
import java.util.logging.Handler;
import java.util.logging.Logger;
import org.lttng.ust.agent.jul.LttngLogHandler;

public class Test
{
    private static final int answer = 42;

    public static void main(String[] argv) throws Exception
    {
        // Create a logger
        Logger logger = Logger.getLogger("jello");

        // Create an LTTng-UST log handler
        Handler lttngUstLogHandler = new LttngLogHandler();

        // Add the LTTng-UST log handler to our logger
        logger.addHandler(lttngUstLogHandler);

        // Log at will!
        logger.info("some info");
        logger.warning("some warning");
        Thread.sleep(500);
        logger.finer("finer information; the answer is " + answer);
        Thread.sleep(123);
        logger.severe("error!");

        // Not mandatory, but cleaner
        logger.removeHandler(lttngUstLogHandler);
        lttngUstLogHandler.close();
    }
}
----

Build this example:

[role="term"]
----
$ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java
----

<<creating-destroying-tracing-sessions,Create a recording session>>,
<<enabling-disabling-events,create a recording event rule>> matching JUL
events named `jello`, and <<basic-tracing-session-control,start
recording>>:

[role="term"]
----
$ lttng create
$ lttng enable-event --jul jello
$ lttng start
----

Run the compiled class:

[role="term"]
----
$ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test
----

<<basic-tracing-session-control,Stop recording>> and inspect the
recorded events:

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

In the resulting trace, an <<event,event record>> which a Java
application using `java.util.logging` generated is named
`lttng_jul:event` and has the following fields:

`msg`::
  Log record message.

`logger_name`::
  Logger name.

`class_name`::
  Name of the class in which the log statement was executed.

`method_name`::
  Name of the method in which the log statement was executed.

`long_millis`::
  Logging time (timestamp in milliseconds).

`int_loglevel`::
  Log level integer value.

`int_threadid`::
  ID of the thread in which the log statement was executed.

Use the opt:lttng-enable-event(1):--loglevel or
opt:lttng-enable-event(1):--loglevel-only option of the
man:lttng-enable-event(1) command to target a range of
`java.util.logging` log levels or a specific `java.util.logging` log
level.


[role="since-2.8"]
[[log4j]]
==== Use the LTTng-UST Java agent for Apache log4j

To use the LTTng-UST Java agent in a Java application which uses
Apache log4j{nbsp}1.2:

. In the source code of the Java application, import the LTTng-UST log
  appender package for Apache log4j:
+
--
[source,java]
----
import org.lttng.ust.agent.log4j.LttngLogAppender;
----
--

. Create an LTTng-UST log4j log appender:
+
--
[source,java]
----
Appender lttngUstLogAppender = new LttngLogAppender();
----
--

. Add this appender to the log4j loggers which should emit LTTng events:
+
--
[source,java]
----
Logger myLogger = Logger.getLogger("some-logger");

myLogger.addAppender(lttngUstLogAppender);
----
--

. Use Apache log4j log statements and configuration as usual. The
  loggers with an attached LTTng-UST log appender can emit LTTng events.

. Before exiting the application, remove the LTTng-UST log appender from
  the loggers attached to it and call its `close()` method:
+
--
[source,java]
----
myLogger.removeAppender(lttngUstLogAppender);
lttngUstLogAppender.close();
----
--
+
This isn't strictly necessary, but it's recommended for a clean
disposal of the resources of the appender.

. Include the common and log4j-specific JAR
  files of the LTTng-UST Java agent, path:{lttng-ust-agent-common.jar} and
  path:{lttng-ust-agent-log4j.jar}, in the
  https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class
  path] when you build the Java application.
+
The JAR files are typically located in dir:{/usr/share/java}.
+
IMPORTANT: The LTTng-UST Java agent must be
<<installing-lttng,installed>> for the logging framework your
application uses.

.Use the LTTng-UST Java agent for Apache log4j.
====
[source,java]
.path:{Test.java}
----
import org.apache.log4j.Appender;
import org.apache.log4j.Logger;
import org.lttng.ust.agent.log4j.LttngLogAppender;

public class Test
{
    private static final int answer = 42;

    public static void main(String[] argv) throws Exception
    {
        // Create a logger
        Logger logger = Logger.getLogger("jello");

        // Create an LTTng-UST log appender
        Appender lttngUstLogAppender = new LttngLogAppender();

        // Add the LTTng-UST log appender to our logger
        logger.addAppender(lttngUstLogAppender);

        // Log at will!
        logger.info("some info");
        logger.warn("some warning");
        Thread.sleep(500);
        logger.debug("debug information; the answer is " + answer);
        Thread.sleep(123);
        logger.fatal("error!");

        // Not mandatory, but cleaner
        logger.removeAppender(lttngUstLogAppender);
        lttngUstLogAppender.close();
    }
}

----

Build this example (`$LOG4JPATH` is the path to the Apache log4j JAR
file):

[role="term"]
----
$ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH Test.java
----

<<creating-destroying-tracing-sessions,Create a recording session>>,
<<enabling-disabling-events,create a recording event rule>> matching
log4j events named `jello`, and <<basic-tracing-session-control,start
recording>>:

[role="term"]
----
$ lttng create
$ lttng enable-event --log4j jello
$ lttng start
----

Run the compiled class:

[role="term"]
----
$ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH:. Test
----

<<basic-tracing-session-control,Stop recording>> and inspect the
recorded events:

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

In the resulting trace, an <<event,event record>> which a Java
application using log4j generated is named `lttng_log4j:event` and
has the following fields:

`msg`::
  Log record message.

`logger_name`::
  Logger name.

`class_name`::
  Name of the class in which the log statement was executed.

`method_name`::
  Name of the method in which the log statement was executed.

`filename`::
  Name of the file in which the executed log statement is located.

`line_number`::
  Line number at which the log statement was executed.

`timestamp`::
  Logging timestamp.

`int_loglevel`::
  Log level integer value.

`thread_name`::
  Name of the Java thread in which the log statement was executed.

Use the opt:lttng-enable-event(1):--loglevel or
opt:lttng-enable-event(1):--loglevel-only option of the
man:lttng-enable-event(1) command to target a range of Apache log4j
log levels or a specific log4j log level.


[role="since-2.8"]
[[java-application-context]]
==== Provide application-specific context fields in a Java application

A Java application-specific context field is a piece of state which
the Java application provides. You can <<adding-context,add>> such
a context field to be recorded, using the
man:lttng-add-context(1) command, to each <<event,event record>>
which the log statements of this application produce.

For example, a given object might have a current request ID variable.
You can create a context information retriever for this object and
assign a name to this current request ID. You can then, using the
man:lttng-add-context(1) command, add this context field by name so that
LTTng writes it to the event records of a given `java.util.logging` or
log4j <<channel,channel>>.

To provide application-specific context fields in a Java application:

. In the source code of the Java application, import the LTTng-UST
  Java agent context classes and interfaces:
+
--
[source,java]
----
import org.lttng.ust.agent.context.ContextInfoManager;
import org.lttng.ust.agent.context.IContextInfoRetriever;
----
--

. Create a context information retriever class, that is, a class which
  implements the `IContextInfoRetriever` interface:
+
--
[source,java]
----
class MyContextInfoRetriever implements IContextInfoRetriever
{
    @Override
    public Object retrieveContextInfo(String key)
    {
        if (key.equals("intCtx")) {
            return (short) 17;
        } else if (key.equals("strContext")) {
            return "context value!";
        } else {
            return null;
        }
    }
}
----
--
+
This `retrieveContextInfo()` method is the only member of the
`IContextInfoRetriever` interface. Its role is to return the current
value of a state by name to create a context field. The names of the
context fields and which state variables they return depends on your
specific scenario.
+
All primitive types and objects are supported as context fields.
When `retrieveContextInfo()` returns an object, the context field
serializer calls its `toString()` method to add a string field to
event records. The method can also return `null`, which means that
no context field is available for the required name.

. Register an instance of your context information retriever class to
  the context information manager singleton:
+
--
[source,java]
----
IContextInfoRetriever cir = new MyContextInfoRetriever();
ContextInfoManager cim = ContextInfoManager.getInstance();
cim.registerContextInfoRetriever("retrieverName", cir);
----
--

. Before exiting the application, remove your context information
  retriever from the context information manager singleton:
+
--
[source,java]
----
ContextInfoManager cim = ContextInfoManager.getInstance();
cim.unregisterContextInfoRetriever("retrieverName");
----
--
+
This isn't strictly necessary, but it's recommended for a clean
disposal of some resources of the manager.

. Build your Java application with LTTng-UST Java agent support as
  usual, following the procedure for either the
  <<jul,`java.util.logging`>> or <<log4j,Apache log4j>> framework.

.Provide application-specific context fields in a Java application.
====
[source,java]
.path:{Test.java}
----
import java.util.logging.Handler;
import java.util.logging.Logger;
import org.lttng.ust.agent.jul.LttngLogHandler;
import org.lttng.ust.agent.context.ContextInfoManager;
import org.lttng.ust.agent.context.IContextInfoRetriever;

public class Test
{
    // Our context information retriever class
    private static class MyContextInfoRetriever
    implements IContextInfoRetriever
    {
        @Override
        public Object retrieveContextInfo(String key) {
            if (key.equals("intCtx")) {
                return (short) 17;
            } else if (key.equals("strContext")) {
                return "context value!";
            } else {
                return null;
            }
        }
    }

    private static final int answer = 42;

    public static void main(String args[]) throws Exception
    {
        // Get the context information manager instance
        ContextInfoManager cim = ContextInfoManager.getInstance();

        // Create and register our context information retriever
        IContextInfoRetriever cir = new MyContextInfoRetriever();
        cim.registerContextInfoRetriever("myRetriever", cir);

        // Create a logger
        Logger logger = Logger.getLogger("jello");

        // Create an LTTng-UST log handler
        Handler lttngUstLogHandler = new LttngLogHandler();

        // Add the LTTng-UST log handler to our logger
        logger.addHandler(lttngUstLogHandler);

        // Log at will!
        logger.info("some info");
        logger.warning("some warning");
        Thread.sleep(500);
        logger.finer("finer information; the answer is " + answer);
        Thread.sleep(123);
        logger.severe("error!");

        // Not mandatory, but cleaner
        logger.removeHandler(lttngUstLogHandler);
        lttngUstLogHandler.close();
        cim.unregisterContextInfoRetriever("myRetriever");
    }
}
----

Build this example:

[role="term"]
----
$ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java
----

<<creating-destroying-tracing-sessions,Create a recording session>> and
<<enabling-disabling-events,create a recording event rule>> matching
`java.util.logging` events named `jello`:

[role="term"]
----
$ lttng create
$ lttng enable-event --jul jello
----

<<adding-context,Add the application-specific context fields>> to be
recorded to the event records of the `java.util.logging` channel:

[role="term"]
----
$ lttng add-context --jul --type='$app.myRetriever:intCtx'
$ lttng add-context --jul --type='$app.myRetriever:strContext'
----

<<basic-tracing-session-control,Start recording>>:

[role="term"]
----
$ lttng start
----

Run the compiled class:

[role="term"]
----
$ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test
----

<<basic-tracing-session-control,Stop recording>> and inspect the
recorded events:

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


[role="since-2.7"]
[[python-application]]
=== Instrument a Python application

You can instrument a Python{nbsp}2 or Python{nbsp}3 application which
uses the standard
https://docs.python.org/3/library/logging.html[`logging`] package.

Each log statement creates an LTTng event once the application module
imports the <<lttng-ust-agents,LTTng-UST Python agent>> package.

[role="img-100"]
.A Python application importing the LTTng-UST Python agent.
image::python-app.png[]

To use the LTTng-UST Python agent:

. In the source code of the Python application, import the LTTng-UST
  Python agent:
+
--
[source,python]
----
import lttngust
----
--
+
The LTTng-UST Python agent automatically adds its logging handler to the
root logger at import time.
+
A log statement that the application executes before this import doesn't
create an LTTng event.
+
IMPORTANT: The LTTng-UST Python agent must be
<<installing-lttng,installed>>.

. Use log statements and logging configuration as usual.
  Since the LTTng-UST Python agent adds a handler to the _root_
  logger, any log statement from any logger can emit an LTTng event.

.Use the LTTng-UST Python agent.
====
[source,python]
.path:{test.py}
----
import lttngust
import logging
import time


def example():
    logging.basicConfig()
    logger = logging.getLogger('my-logger')

    while True:
        logger.debug('debug message')
        logger.info('info message')
        logger.warn('warn message')
        logger.error('error message')
        logger.critical('critical message')
        time.sleep(1)


if __name__ == '__main__':
    example()
----

NOTE: `logging.basicConfig()`, which adds to the root logger a basic
logging handler which prints to the standard error stream, isn't
strictly required for LTTng-UST tracing to work, but in versions of
Python preceding{nbsp}3.2, you could see a warning message which
indicates that no handler exists for the logger `my-logger`.

<<creating-destroying-tracing-sessions,Create a recording session>>,
<<enabling-disabling-events,create a recording event rule>> matching
Python logging events named `my-logger`, and
<<basic-tracing-session-control,start recording>>:

[role="term"]
----
$ lttng create
$ lttng enable-event --python my-logger
$ lttng start
----

Run the Python script:

[role="term"]
----
$ python test.py
----

<<basic-tracing-session-control,Stop recording>> and inspect the
recorded events:

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

In the resulting trace, an <<event,event record>> which a Python
application generated is named `lttng_python:event` and has the
following fields:

`asctime`::
  Logging time (string).

`msg`::
  Log record message.

`logger_name`::
  Logger name.

`funcName`::
  Name of the function in which the log statement was executed.

`lineno`::
  Line number at which the log statement was executed.

`int_loglevel`::
  Log level integer value.

`thread`::
  ID of the Python thread in which the log statement was executed.

`threadName`::
  Name of the Python thread in which the log statement was executed.

Use the opt:lttng-enable-event(1):--loglevel or
opt:lttng-enable-event(1):--loglevel-only option of the
man:lttng-enable-event(1) command to target a range of Python log levels
or a specific Python log level.

When an application imports the LTTng-UST Python agent, the agent tries
to register to a <<lttng-sessiond,session daemon>>. Note that you must
<<start-sessiond,start the session daemon>> _before_ you run the Python
application. If a session daemon is found, the agent tries to register
to it during five seconds, after which the application continues
without LTTng tracing support. Override this timeout value with
the env:LTTNG_UST_PYTHON_REGISTER_TIMEOUT environment variable
(milliseconds).

If the session daemon stops while a Python application with an imported
LTTng-UST Python agent runs, the agent retries to connect and to
register to a session daemon every three seconds. Override this
delay with the env:LTTNG_UST_PYTHON_REGISTER_RETRY_DELAY environment
variable.


[role="since-2.5"]
[[proc-lttng-logger-abi]]
=== Use the LTTng logger

The `lttng-tracer` Linux kernel module, part of
<<lttng-modules,LTTng-modules>>, creates the special LTTng logger files
path:{/proc/lttng-logger} and path:{/dev/lttng-logger} (since
LTTng{nbsp}2.11) when it's loaded. Any application can write text data
to any of those files to create one or more LTTng events.

[role="img-100"]
.An application writes to the LTTng logger file to create one or more LTTng events.
image::lttng-logger.png[]

The LTTng logger is the quickest method--not the most efficient,
however--to add instrumentation to an application. It's designed
mostly to instrument shell scripts:

[role="term"]
----
$ echo "Some message, some $variable" > /dev/lttng-logger
----

Any event that the LTTng logger creates is named `lttng_logger` and
belongs to the Linux kernel <<domain,tracing domain>>. However, unlike
other instrumentation points in the kernel tracing domain, **any Unix
user** can <<enabling-disabling-events,create a recording event rule>>
which matches events named `lttng_logger`, not only the root user or
users in the <<tracing-group,tracing group>>.

To use the LTTng logger:

* From any application, write text data to the path:{/dev/lttng-logger}
  file.

The `msg` field of `lttng_logger` event records contains the
recorded message.

NOTE: The maximum message length of an LTTng logger event is
1024{nbsp}bytes. Writing more than this makes the LTTng logger emit more
than one event to contain the remaining data.

You shouldn't use the LTTng logger to trace a user application which you
can instrument in a more efficient way, namely:

* <<c-application,C and $$C++$$ applications>>.
* <<java-application,Java applications>>.
* <<python-application,Python applications>>.

.Use the LTTng logger.
====
[source,bash]
.path:{test.bash}
----
echo 'Hello, World!' > /dev/lttng-logger
sleep 2
df --human-readable --print-type / > /dev/lttng-logger
----

<<creating-destroying-tracing-sessions,Create a recording session>>,
<<enabling-disabling-events,create a recording event rule>> matching
Linux kernel tracepoint events named `lttng_logger`, and
<<basic-tracing-session-control,start recording>>:

[role="term"]
----
$ lttng create
$ lttng enable-event --kernel lttng_logger
$ lttng start
----

Run the Bash script:

[role="term"]
----
$ bash test.bash
----

<<basic-tracing-session-control,Stop recording>> and inspect the recorded
events:

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


[[instrumenting-linux-kernel]]
=== Instrument a Linux kernel image or module

NOTE: This section shows how to _add_ instrumentation points to the
Linux kernel. The subsystems of the kernel are already thoroughly
instrumented at strategic points for LTTng when you
<<installing-lttng,install>> the <<lttng-modules,LTTng-modules>>
package.


[[linux-add-lttng-layer]]
==== [[instrumenting-linux-kernel-itself]][[mainline-trace-event]][[lttng-adaptation-layer]]Add an LTTng layer to an existing ftrace tracepoint

This section shows how to add an LTTng layer to existing ftrace
instrumentation using the `TRACE_EVENT()` API.

This section doesn't document the `TRACE_EVENT()` macro. Read the
following articles to learn more about this API:

* https://lwn.net/Articles/379903/[Using the TRACE_EVENT() macro (Part{nbsp}1)]
* https://lwn.net/Articles/381064/[Using the TRACE_EVENT() macro (Part{nbsp}2)]
* https://lwn.net/Articles/383362/[Using the TRACE_EVENT() macro (Part{nbsp}3)]

The following procedure assumes that your ftrace tracepoints are
correctly defined in their own header and that they're created in
one source file using the `CREATE_TRACE_POINTS` definition.

To add an LTTng layer over an existing ftrace tracepoint:

. Make sure the following kernel configuration options are
  enabled:
+
--
* `CONFIG_MODULES`
* `CONFIG_KALLSYMS`
* `CONFIG_HIGH_RES_TIMERS`
* `CONFIG_TRACEPOINTS`
--

. Build the Linux source tree with your custom ftrace tracepoints.
. Boot the resulting Linux image on your target system.
+
Confirm that the tracepoints exist by looking for their names in the
dir:{/sys/kernel/debug/tracing/events/subsys} directory, where `subsys`
is your subsystem name.

. Get a copy of the latest LTTng-modules{nbsp}{revision}:
+
--
[role="term"]
----
$ cd $(mktemp -d) &&
  wget https://lttng.org/files/lttng-modules/lttng-modules-latest-2.13.tar.bz2 &&
  tar -xf lttng-modules-latest-2.13.tar.bz2 &&
  cd lttng-modules-2.13.*
----
--

. In dir:{instrumentation/events/lttng-module}, relative to the root
  of the LTTng-modules source tree, create a header file named
  +__subsys__.h+ for your custom subsystem +__subsys__+ and write your
  LTTng-modules tracepoint definitions using the LTTng-modules
  macros in it.
+
Start with this template:
+
--
[source,c]
.path:{instrumentation/events/lttng-module/my_subsys.h}
----
#undef TRACE_SYSTEM
#define TRACE_SYSTEM my_subsys

#if !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ)
#define _LTTNG_MY_SUBSYS_H

#include "../../../probes/lttng-tracepoint-event.h"
#include <linux/tracepoint.h>

LTTNG_TRACEPOINT_EVENT(
    /*
     * Format is identical to the TRACE_EVENT() version for the three
     * following macro parameters:
     */
    my_subsys_my_event,
    TP_PROTO(int my_int, const char *my_string),
    TP_ARGS(my_int, my_string),

    /* LTTng-modules specific macros */
    TP_FIELDS(
        ctf_integer(int, my_int_field, my_int)
        ctf_string(my_bar_field, my_bar)
    )
)

#endif /* !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ) */

#include "../../../probes/define_trace.h"
----
--
+
The entries in the `TP_FIELDS()` section are the list of fields for the
LTTng tracepoint. This is similar to the `TP_STRUCT__entry()` part of
the `TRACE_EVENT()` ftrace macro.
+
See ``<<lttng-modules-tp-fields,Tracepoint fields macros>>'' for a
complete description of the available `ctf_*()` macros.

. Create the kernel module C{nbsp}source file of the LTTng-modules
  probe, +probes/lttng-probe-__subsys__.c+, where +__subsys__+ is your
  subsystem name:
+
--
[source,c]
.path:{probes/lttng-probe-my-subsys.c}
----
#include <linux/module.h>
#include "../lttng-tracer.h"

/*
 * Build-time verification of mismatch between mainline
 * TRACE_EVENT() arguments and the LTTng-modules adaptation
 * layer LTTNG_TRACEPOINT_EVENT() arguments.
 */
#include <trace/events/my_subsys.h>

/* Create LTTng tracepoint probes */
#define LTTNG_PACKAGE_BUILD
#define CREATE_TRACE_POINTS
#define TRACE_INCLUDE_PATH ../instrumentation/events/lttng-module

#include "../instrumentation/events/lttng-module/my_subsys.h"

MODULE_LICENSE("GPL and additional rights");
MODULE_AUTHOR("Your name <your-email>");
MODULE_DESCRIPTION("LTTng my_subsys probes");
MODULE_VERSION(__stringify(LTTNG_MODULES_MAJOR_VERSION) "."
    __stringify(LTTNG_MODULES_MINOR_VERSION) "."
    __stringify(LTTNG_MODULES_PATCHLEVEL_VERSION)
    LTTNG_MODULES_EXTRAVERSION);
----
--

. Edit path:{probes/KBuild} and add your new kernel module object
  next to the existing ones:
+
--
[source,make]
.path:{probes/KBuild}
----
# ...

obj-m += lttng-probe-module.o
obj-m += lttng-probe-power.o

obj-m += lttng-probe-my-subsys.o

# ...
----
--

. Build and install the LTTng kernel modules:
+
--
[role="term"]
----
$ make KERNELDIR=/path/to/linux
# make modules_install && depmod -a
----
--
+
Replace `/path/to/linux` with the path to the Linux source tree where
you defined and used tracepoints with the `TRACE_EVENT()` ftrace macro.

Note that you can also use the
<<lttng-tracepoint-event-code,`LTTNG_TRACEPOINT_EVENT_CODE()` macro>>
instead of `LTTNG_TRACEPOINT_EVENT()` to use custom local variables and
C{nbsp}code that need to be executed before LTTng records the event
fields.

The best way to learn how to use the previous LTTng-modules macros is to
inspect the existing LTTng-modules tracepoint definitions in the
dir:{instrumentation/events/lttng-module} header files. Compare them
with the Linux kernel mainline versions in the
dir:{include/trace/events} directory of the Linux source tree.


[role="since-2.7"]
[[lttng-tracepoint-event-code]]
===== Use custom C code to access the data for tracepoint fields

Although we recommended to always use the
<<lttng-adaptation-layer,`LTTNG_TRACEPOINT_EVENT()`>> macro to describe
the arguments and fields of an LTTng-modules tracepoint when possible,
sometimes you need a more complex process to access the data that the
tracer records as event record fields. In other words, you need local
variables and multiple C{nbsp}statements instead of simple
argument-based expressions that you pass to the
<<lttng-modules-tp-fields,`ctf_*()` macros of `TP_FIELDS()`>>.

Use the `LTTNG_TRACEPOINT_EVENT_CODE()` macro instead of
`LTTNG_TRACEPOINT_EVENT()` to declare custom local variables and define
a block of C{nbsp}code to be executed before LTTng records the fields.
The structure of this macro is:

[source,c]
.`LTTNG_TRACEPOINT_EVENT_CODE()` macro syntax.
----
LTTNG_TRACEPOINT_EVENT_CODE(
    /*
     * Format identical to the LTTNG_TRACEPOINT_EVENT()
     * version for the following three macro parameters:
     */
    my_subsys_my_event,
    TP_PROTO(int my_int, const char *my_string),
    TP_ARGS(my_int, my_string),

    /* Declarations of custom local variables */
    TP_locvar(
        int a = 0;
        unsigned long b = 0;
        const char *name = "(undefined)";
        struct my_struct *my_struct;
    ),

    /*
     * Custom code which uses both tracepoint arguments
     * (in TP_ARGS()) and local variables (in TP_locvar()).
     *
     * Local variables are actually members of a structure pointed
     * to by the special variable tp_locvar.
     */
    TP_code(
        if (my_int) {
            tp_locvar->a = my_int + 17;
            tp_locvar->my_struct = get_my_struct_at(tp_locvar->a);
            tp_locvar->b = my_struct_compute_b(tp_locvar->my_struct);
            tp_locvar->name = my_struct_get_name(tp_locvar->my_struct);
            put_my_struct(tp_locvar->my_struct);

            if (tp_locvar->b) {
                tp_locvar->a = 1;
            }
        }
    ),

    /*
     * Format identical to the LTTNG_TRACEPOINT_EVENT()
     * version for this, except that tp_locvar members can be
     * used in the argument expression parameters of
     * the ctf_*() macros.
     */
    TP_FIELDS(
        ctf_integer(unsigned long, my_struct_b, tp_locvar->b)
        ctf_integer(int, my_struct_a, tp_locvar->a)
        ctf_string(my_string_field, my_string)
        ctf_string(my_struct_name, tp_locvar->name)
    )
)
----

IMPORTANT: The C code defined in `TP_code()` must not have any side
effects when executed. In particular, the code must not allocate
memory or get resources without deallocating this memory or putting
those resources afterwards.


[[instrumenting-linux-kernel-tracing]]
==== Load and unload a custom probe kernel module

You must load a <<lttng-adaptation-layer,created LTTng-modules probe
kernel module>> in the kernel before it can emit LTTng events.

To load the default probe kernel modules and a custom probe kernel
module:

* Use the opt:lttng-sessiond(8):--extra-kmod-probes option to give extra
  probe modules to load when starting a root <<lttng-sessiond,session
  daemon>>:
+
--
.Load the `my_subsys`, `usb`, and the default probe modules.
====
[role="term"]
----
# lttng-sessiond --extra-kmod-probes=my_subsys,usb
----
====
--
+
You only need to pass the subsystem name, not the whole kernel module
name.

To load _only_ a given custom probe kernel module:

* Use the opt:lttng-sessiond(8):--kmod-probes option to give the probe
  modules to load when starting a root session daemon:
+
--
.Load only the `my_subsys` and `usb` probe modules.
====
[role="term"]
----
# lttng-sessiond --kmod-probes=my_subsys,usb
----
====
--

To confirm that a probe module is loaded:

* Use man:lsmod(8):
+
--
[role="term"]
----
$ lsmod | grep lttng_probe_usb
----
--

To unload the loaded probe modules:

* Kill the session daemon with `SIGTERM`:
+
--
[role="term"]
----
# pkill lttng-sessiond
----
--
+
You can also use the `--remove` option of man:modprobe(8) if the session
daemon terminates abnormally.


[[controlling-tracing]]
== Tracing control

Once an application or a Linux kernel is <<instrumenting,instrumented>>
for LTTng tracing, you can _trace_ it.

In the LTTng context, _tracing_ means making sure that LTTng attempts to
execute some action(s) when a CPU executes an instrumentation point.

This section is divided in topics on how to use the various
<<plumbing,components of LTTng>>, in particular the
<<lttng-cli,cmd:lttng command-line tool>>, to _control_ the LTTng
daemons and tracers.

NOTE: In the following subsections, we refer to an man:lttng(1) command
using its man page name. For example, instead of ``Run the `create`
command to'', we write ``Run the man:lttng-create(1) command to''.


[[start-sessiond]]
=== Start a session daemon

In some situations, you need to run a <<lttng-sessiond,session daemon>>
(man:lttng-sessiond(8)) _before_ you can use the man:lttng(1)
command-line tool.

You will see the following error when you run a command while no session
daemon is running:

----
Error: No session daemon is available
----

The only command that automatically runs a session daemon is
man:lttng-create(1), which you use to
<<creating-destroying-tracing-sessions,create a recording session>>. While
this could be your most used first operation, sometimes it's not. Some
examples are:

* <<list-instrumentation-points,List the available instrumentation points>>.
* <<saving-loading-tracing-session,Load a recording session configuration>>.
* <<add-event-rule-matches-trigger,Add a trigger>>.

All the examples above don't require a recording session to operate on.

[[tracing-group]] Each Unix user can have its own running session daemon
to use the user space LTTng tracer. The session daemon that the `root`
user starts is the only one allowed to control the LTTng kernel tracer.
Members of the Unix _tracing group_ may connect to and control the root
session daemon, even for user space tracing. See the ``Session daemon
connection'' section of man:lttng(1) to learn more about the Unix
tracing group.

To start a user session daemon:

* Run man:lttng-sessiond(8):
+
--
[role="term"]
----
$ lttng-sessiond --daemonize
----
--

To start the root session daemon:

* Run man:lttng-sessiond(8) as the `root` user:
+
--
[role="term"]
----
# lttng-sessiond --daemonize
----
--

In both cases, remove the opt:lttng-sessiond(8):--daemonize option to
start the session daemon in foreground.

To stop a session daemon, kill its process (see man:kill(1)) with the
standard `TERM` signal.

Note that some Linux distributions could manage the LTTng session daemon
as a service. In this case, we suggest that you use the service manager
to start, restart, and stop session daemons.


[[creating-destroying-tracing-sessions]]
=== Create and destroy a recording session

Many LTTng control operations happen in the scope of a
<<tracing-session,recording session>>, which is the dialogue between the
<<lttng-sessiond,session daemon>> and you for everything related to
<<event,event recording>>.

To create a recording session with a generated name:

* Use the man:lttng-create(1) command:
+
--
[role="term"]
----
$ lttng create
----
--

The name of the created recording session is `auto` followed by the
creation date.

To create a recording session with a specific name:

* Use the optional argument of the man:lttng-create(1) command:
+
--
[role="term"]
----
$ lttng create SESSION
----
--
+
Replace +__SESSION__+ with your specific recording session name.

In <<local-mode,local mode>>, LTTng writes the traces of a recording
session to the +$LTTNG_HOME/lttng-traces/__NAME__-__DATE__-__TIME__+
directory by default, where +__NAME__+ is the name of the recording
session. Note that the env:LTTNG_HOME environment variable defaults to
`$HOME` if not set.

To output LTTng traces to a non-default location:

* Use the opt:lttng-create(1):--output option of the man:lttng-create(1)
  command:
+
--
[role="term"]
----
$ lttng create my-session --output=/tmp/some-directory
----
--

You may create as many recording sessions as you wish.

To list all the existing recording sessions for your Unix user, or for
all users if your Unix user is `root`:

* Use the man:lttng-list(1) command:
+
--
[role="term"]
----
$ lttng list
----
--

[[cur-tracing-session]]When you create a recording session, the
man:lttng-create(1) command sets it as the _current recording session_.
The following man:lttng(1) commands operate on the current recording
session when you don't specify one:

[role="list-3-cols"]
* man:lttng-add-context(1)
* man:lttng-clear(1)
* man:lttng-destroy(1)
* man:lttng-disable-channel(1)
* man:lttng-disable-event(1)
* man:lttng-disable-rotation(1)
* man:lttng-enable-channel(1)
* man:lttng-enable-event(1)
* man:lttng-enable-rotation(1)
* man:lttng-load(1)
* man:lttng-regenerate(1)
* man:lttng-rotate(1)
* man:lttng-save(1)
* man:lttng-snapshot(1)
* man:lttng-start(1)
* man:lttng-status(1)
* man:lttng-stop(1)
* man:lttng-track(1)
* man:lttng-untrack(1)
* man:lttng-view(1)

To change the current recording session:

* Use the man:lttng-set-session(1) command:
+
--
[role="term"]
----
$ lttng set-session SESSION
----
--
+
Replace +__SESSION__+ with the name of the new current recording session.

When you're done recording in a given recording session, destroy it.
This operation frees the resources taken by the recording session to
destroy; it doesn't destroy the trace data that LTTng wrote for this
recording session (see ``<<clear,Clear a recording session>>'' for one
way to do this).

To destroy the current recording session:

* Use the man:lttng-destroy(1) command:
+
--
[role="term"]
----
$ lttng destroy
----
--

The man:lttng-destroy(1) command also runs the man:lttng-stop(1) command
implicitly (see ``<<basic-tracing-session-control,Start and stop a
recording session>>''). You need to stop recording to make LTTng flush the
remaining trace data and make the trace readable.


[[list-instrumentation-points]]
=== List the available instrumentation points

The <<lttng-sessiond,session daemon>> can query the running instrumented
user applications and the Linux kernel to get a list of available
instrumentation points:

* LTTng tracepoints and system calls for the Linux kernel
  <<domain,tracing domain>>.

* LTTng tracepoints for the user space tracing domain.

To list the available instrumentation points:

. <<start-sessiond,Make sure>> there's a running
  <<lttng-sessiond,session daemon>> to which your Unix user can
  connect.

. Use the man:lttng-list(1) command with the option of the requested
  tracing domain amongst:
+
--
opt:lttng-list(1):--kernel::
    Linux kernel tracepoints.
+
Your Unix user must be `root`, or it must be a member of the Unix
<<tracing-group,tracing group>>.

opt:lttng-list(1):--kernel with opt:lttng-list(1):--syscall::
    Linux kernel system calls.
+
Your Unix user must be `root`, or it must be a member of the Unix
<<tracing-group,tracing group>>.

opt:lttng-list(1):--userspace::
    User space tracepoints.

opt:lttng-list(1):--jul::
   `java.util.logging` loggers.

opt:lttng-list(1):--log4j::
    Apache log4j loggers.

opt:lttng-list(1):--python::
    Python loggers.
--

.List the available user space tracepoints.
====
[role="term"]
----
$ lttng list --userspace
----
====

.List the available Linux kernel system calls.
====
[role="term"]
----
$ lttng list --kernel --syscall
----
====


[[enabling-disabling-events]]
=== Create and enable a recording event rule

Once you <<creating-destroying-tracing-sessions,create a recording
session>>, you can create <<event,recording event rules>> with the
man:lttng-enable-event(1) command.

The man:lttng-enable-event(1) command always attaches an event rule to a
<<channel,channel>> on creation. The command can create a _default
channel_, named `channel0`, for you. The man:lttng-enable-event(1)
command reuses the default channel each time you run it for the same
tracing domain and session.

A recording event rule is always enabled at creation time.

The following examples show how to combine the command-line arguments of
the man:lttng-enable-event(1) command to create simple to more complex
recording event rules within the <<cur-tracing-session,current recording
session>>.

.Create a recording event rule matching specific Linux kernel tracepoint events (default channel).
====
[role="term"]
----
# lttng enable-event --kernel sched_switch
----
====

.Create a recording event rule matching Linux kernel system call events with four specific names (default channel).
====
[role="term"]
----
# lttng enable-event --kernel --syscall open,write,read,close
----
====

.Create recording event rules matching tracepoint events which satisfy a filter expressions (default channel).
====
[role="term"]
----
# lttng enable-event --kernel sched_switch --filter='prev_comm == "bash"'
----

[role="term"]
----
# lttng enable-event --kernel --all \
                     --filter='$ctx.tid == 1988 || $ctx.tid == 1534'
----

[role="term"]
----
$ lttng enable-event --jul my_logger \
                     --filter='$app.retriever:cur_msg_id > 3'
----

IMPORTANT: Make sure to always single-quote the filter string when you
run man:lttng(1) from a shell.

See also ``<<pid-tracking,Allow specific processes to record events>>''
which offers another, more efficient filtering mechanism for process ID,
user ID, and group ID attributes.
====

.Create a recording event rule matching any user space event from the `my_app` tracepoint provider and with a log level range (default channel).
====
[role="term"]
----
$ lttng enable-event --userspace my_app:'*' --loglevel=INFO
----

IMPORTANT: Make sure to always single-quote the wildcard character when
you run man:lttng(1) from a shell.
====

.Create a recording event rule matching user space events named specifically, but with name exclusions (default channel).
====
[role="term"]
----
$ lttng enable-event --userspace my_app:'*' \
                     --exclude=my_app:set_user,my_app:handle_sig
----
====

.Create a recording event rule matching any Apache log4j event with a specific log level (default channel).
====
[role="term"]
----
$ lttng enable-event --log4j --all --loglevel-only=WARN
----
====

.Create a recording event rule, attached to a specific channel, and matching user space tracepoint events named `my_app:my_tracepoint`.
====
[role="term"]
----
$ lttng enable-event --userspace my_app:my_tracepoint \
                     --channel=my-channel
----
====

.Create a recording event rule matching user space probe events for the `malloc` function entry in path:{/usr/lib/libc.so.6}:
====
[role="term"]
----
# lttng enable-event --kernel \
                     --userspace-probe=/usr/lib/libc.so.6:malloc \
                     libc_malloc
----
====

.Create a recording event rule matching user space probe events for the `server`/`accept_request` https://www.sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps[USDT probe] in path:{/usr/bin/serv}:
====
[role="term"]
----
# lttng enable-event --kernel \
                     --userspace-probe=sdt:serv:server:accept_request \
                     server_accept_request
----
====

The recording event rules of a given channel form a whitelist: as soon
as an event rule matches an event, LTTng emits it _once_ and therefore
<<channel-overwrite-mode-vs-discard-mode,can>> record it. For example,
the following rules both match user space tracepoint events named
`my_app:my_tracepoint` with an `INFO` log level:

[role="term"]
----
$ lttng enable-event --userspace my_app:my_tracepoint
$ lttng enable-event --userspace my_app:my_tracepoint \
                     --loglevel=INFO
----

The second recording event rule is redundant: the first one includes the
second one.


[[disable-event-rule]]
=== Disable a recording event rule

To disable a <<event,recording event rule>> that you
<<enabling-disabling-events,created>> previously, use the
man:lttng-disable-event(1) command.

man:lttng-disable-event(1) can only find recording event rules to
disable by their <<instrumentation-point-types,instrumentation point
type>> and event name conditions. Therefore, you cannot disable
recording event rules having a specific instrumentation point log level
condition, for example.

LTTng doesn't emit (and, therefore, won't record) an event which only
_disabled_ recording event rules match.

.Disable event rules matching Python logging events from the `my-logger` logger (default <<channel,channel>>, <<cur-tracing-session,current recording session>>).
====
[role="term"]
----
$ lttng disable-event --python my-logger
----
====

.Disable event rules matching all `java.util.logging` events (default channel, recording session `my-session`).
====
[role="term"]
----
$ lttng disable-event --jul --session=my-session '*'
----
====

.Disable _all_ the Linux kernel recording event rules (channel `my-chan`, current recording session).
====
The opt:lttng-disable-event(1):--all-events option isn't, like the
opt:lttng-enable-event(1):--all option of the man:lttng-enable-event(1)
command, an alias for the event name globbing pattern `*`: it disables
_all_ the recording event rules of a given channel.

[role="term"]
----
# lttng disable-event --kernel --channel=my-chan --all-events
----
====

NOTE: You can't _remove_ a recording event rule once you create it.


[[status]]
=== Get the status of a recording session

To get the status of the <<cur-tracing-session,current recording
session>>, that is, its parameters, its channels, recording event rules,
and their attributes:

* Use the man:lttng-status(1) command:
+
--
[role="term"]
----
$ lttng status
----
--

To get the status of any recording session:

* Use the man:lttng-list(1) command with the name of the recording
  session:
+
--
[role="term"]
----
$ lttng list SESSION
----
--
+
Replace +__SESSION__+ with the recording session name.


[[basic-tracing-session-control]]
=== Start and stop a recording session

Once you <<creating-destroying-tracing-sessions,create a recording
session>> and <<enabling-disabling-events,create one or more recording
event rules>>, you can start and stop the tracers for this recording
session.

To start the <<cur-tracing-session,current recording session>>:

* Use the man:lttng-start(1) command:
+
--
[role="term"]
----
$ lttng start
----
--

LTTng is flexible: you can launch user applications before or after you
start the tracers. An LTTng tracer only <<event,records an event>> if a
recording event rule matches it, which means the tracer is active.

The `start-session` <<trigger,trigger>> action can also start a recording
session.

To stop the current recording session:

* Use the man:lttng-stop(1) command:
+
--
[role="term"]
----
$ lttng stop
----
--
+
If there were <<channel-overwrite-mode-vs-discard-mode,lost event
records>> or lost sub-buffers since the last time you ran
man:lttng-start(1), the man:lttng-stop(1) command prints corresponding
warnings.

IMPORTANT: You need to stop recording to make LTTng flush the remaining
trace data and make the trace readable. Note that the
man:lttng-destroy(1) command (see
``<<creating-destroying-tracing-sessions,Create and destroy a recording
session>>'') also runs the man:lttng-stop(1) command implicitly.

The `stop-session` <<trigger,trigger>> action can also stop a recording
session.

[role="since-2.12"]
[[clear]]
=== Clear a recording session

You might need to remove all the current tracing data of one or more
<<tracing-session,recording sessions>> between multiple attempts to
reproduce a problem without interrupting the LTTng recording activity.

To clear the tracing data of the
<<cur-tracing-session,current recording session>>:

* Use the man:lttng-clear(1) command:
+
--
[role="term"]
----
$ lttng clear
----
--

To clear the tracing data of all the recording sessions:

* Use the `lttng clear` command with its opt:lttng-clear(1):--all
  option:
+
--
[role="term"]
----
$ lttng clear --all
----
--


[[enabling-disabling-channels]]
=== Create a channel

Once you <<creating-destroying-tracing-sessions,create a recording
session>>, you can create a <<channel,channel>> with the
man:lttng-enable-channel(1) command.

Note that LTTng can automatically create a default channel when you
<<enabling-disabling-events,create a recording event rule>>.
Therefore, you only need to create a channel when you need non-default
attributes.

Specify each non-default channel attribute with a command-line
option when you run the man:lttng-enable-channel(1) command.

You can only create a custom channel in the Linux kernel and user space
<<domain,tracing domains>>: the Java/Python logging tracing domains have
their own default channel which LTTng automatically creates when you
<<enabling-disabling-events,create a recording event rule>>.

[IMPORTANT]
====
As of LTTng{nbsp}{revision}, you may _not_ perform the
following operations with the man:lttng-enable-channel(1) command:

* Change an attribute of an existing channel.

* Enable a disabled channel once its recording session has been
  <<basic-tracing-session-control,active>> at least once.

* Create a channel once its recording session has been active at
  least once.

* Create a user space channel with a given
  <<channel-buffering-schemes,buffering scheme>> and create a second
  user space channel with a different buffering scheme in the same
  recording session.
====

The following examples show how to combine the command-line options of
the man:lttng-enable-channel(1) command to create simple to more complex
channels within the <<cur-tracing-session,current recording session>>.

.Create a Linux kernel channel with default attributes.
====
[role="term"]
----
# lttng enable-channel --kernel my-channel
----
====

.Create a user space channel with four sub-buffers or 1{nbsp}MiB each, per CPU, per instrumented process.
====
[role="term"]
----
$ lttng enable-channel --userspace --num-subbuf=4 --subbuf-size=1M \
                       --buffers-pid my-channel
----
====

.[[blocking-timeout-example]]Create a default user space channel with an infinite blocking timeout.
====
<<creating-destroying-tracing-sessions,Create a recording session>>,
create the channel, <<enabling-disabling-events,create a recording event
rule>>, and <<basic-tracing-session-control,start recording>>:

[role="term"]
----
$ lttng create
$ lttng enable-channel --userspace --blocking-timeout=inf blocking-chan
$ lttng enable-event --userspace --channel=blocking-chan --all
$ lttng start
----

Run an application instrumented with LTTng-UST tracepoints and allow it
to block:

[role="term"]
----
$ LTTNG_UST_ALLOW_BLOCKING=1 my-app
----
====

.Create a Linux kernel channel which rotates eight trace files of 4{nbsp}MiB each for each stream.
====
[role="term"]
----
# lttng enable-channel --kernel --tracefile-count=8 \
                       --tracefile-size=4194304 my-channel
----
====

.Create a user space channel in <<overwrite-mode,overwrite>> (or ``flight recorder'') mode.
====
[role="term"]
----
$ lttng enable-channel --userspace --overwrite my-channel
----
====

.<<enabling-disabling-events,Create>> the same <<event,recording event rule>> attached to two different channels.
====
[role="term"]
----
$ lttng enable-event --userspace --channel=my-channel app:tp
$ lttng enable-event --userspace --channel=other-channel app:tp
----

When a CPU executes the `app:tp` <<c-application,user space
tracepoint>>, the two recording event rules above match the created
event, making LTTng emit the event. Because the recording event rules
are not attached to the same channel, LTTng records the event twice.
====


[[disable-channel]]
=== Disable a channel

To disable a specific channel that you
<<enabling-disabling-channels,created>> previously, use the
man:lttng-disable-channel(1) command.

.Disable a specific Linux kernel channel (<<cur-tracing-session,current recording session>>).
====
[role="term"]
----
# lttng disable-channel --kernel my-channel
----
====

An enabled channel is an implicit <<event,recording event rule>>
condition.

NOTE: As of LTTng{nbsp}{revision}, you may _not_ enable a disabled
channel once its recording session has been
<<basic-tracing-session-control,started>> at least once.


[[adding-context]]
=== Add context fields to be recorded to the event records of a channel

<<event,Event record>> fields in trace files provide important
information about previously emitted events, but sometimes some external
context may help you solve a problem faster.

Examples of context fields are:

* The **process ID**, **thread ID**, **process name**, and
  **process priority** of the thread from which LTTng emits the event.

* The **hostname** of the system on which LTTng emits the event.

* The Linux kernel and user call stacks (since LTTng{nbsp}2.11).

* The current values of many possible **performance counters** using
  perf, for example:

** CPU cycles, stalled cycles, idle cycles, and the other cycle types.
** Cache misses.
** Branch instructions, misses, and loads.
** CPU faults.

* Any state defined at the application level (supported for the
  `java.util.logging` and Apache log4j <<domain,tracing domains>>).

To get the full list of available context fields:

* Use the opt:lttng-add-context(1):--list option of the
  man:lttng-add-context(1) command:
+
[role="term"]
----
$ lttng add-context --list
----

.Add context fields to be recorded to the event records of all the <<channel,channels>> of the <<cur-tracing-session,current recording session>>.
====
The following command line adds the virtual process identifier and the
per-thread CPU cycles count fields to all the user space channels of the
current recording session.

[role="term"]
----
$ lttng add-context --userspace --type=vpid --type=perf:thread:cpu-cycles
----
====

.Add performance counter context fields by raw ID
====
See man:lttng-add-context(1) for the exact format of the context field
type, which is partly compatible with the format used in
man:perf-record(1).

[role="term"]
----
# lttng add-context --userspace --type=perf:thread:raw:r0110:test
# lttng add-context --kernel --type=perf:cpu:raw:r0013c:x86unhalted
----
====

.Add context fields to be recorded to the event records of a specific channel.
====
The following command line adds the thread identifier and user call
stack context fields to the Linux kernel channel named `my-channel` of
the <<cur-tracing-session,current recording session>>.

[role="term"]
----
# lttng add-context --kernel --channel=my-channel \
                    --type=tid --type=callstack-user
----
====

.Add an <<java-application-context,application-specific context field>> to be recorded to the event records of a specific channel.
====
The following command line makes sure LTTng writes the `cur_msg_id`
context field of the `retriever` context retriever to all the Java
logging <<event,event records>> of the channel named `my-channel`:

[role="term"]
----
# lttng add-context --kernel --channel=my-channel \
                    --type='$app:retriever:cur_msg_id'
----

IMPORTANT: Make sure to always single-quote the `$` character when you
run man:lttng-add-context(1) from a shell.
====

NOTE: You can't undo what the man:lttng-add-context(1) command does.


[role="since-2.7"]
[[pid-tracking]]
=== Allow specific processes to record events

It's often useful to only allow processes with specific attributes to
record events. For example, you may wish to record all the system calls
which a given process makes (à la man:strace(1)).

The man:lttng-track(1) and man:lttng-untrack(1) commands serve this
purpose. Both commands operate on _inclusion sets_ of process
attributes. The available process attribute types are:

Linux kernel <<domain,tracing domain>>::
+
* Process ID (PID).

* Virtual process ID (VPID).
+
This is the PID as seen by the application.

* Unix user ID (UID).

* Virtual Unix user ID (VUID).
+
This is the UID as seen by the application.

* Unix group ID (GID).

* Virtual Unix group ID (VGID).
+
This is the GID as seen by the application.

User space tracing domain::
+
* VPID
* VUID
* VGID

A <<tracing-session,recording session>> has nine process
attribute inclusion sets: six for the Linux kernel <<domain,tracing domain>>
and three for the user space tracing domain.

For a given recording session, a process{nbsp}__P__ is allowed to record
LTTng events for a given <<domain,tracing domain>>{nbsp}__D__ if _all_
the attributes of{nbsp}__P__ are part of the inclusion sets
of{nbsp}__D__.

Whether a process is allowed or not to record LTTng events is an
implicit condition of all <<event,recording event rules>>. Therefore, if
LTTng creates an event{nbsp}__E__ for a given process, but this process
may not record events, then no recording event rule matches{nbsp}__E__,
which means LTTng won't emit and record{nbsp}__E__.

When you <<creating-destroying-tracing-sessions,create a recording
session>>, all its process attribute inclusion sets contain all the
possible values. In other words, all processes are allowed to record
events.

Add values to an inclusion set with the man:lttng-track(1) command and
remove values with the man:lttng-untrack(1) command.

[NOTE]
====
The process attribute values are _numeric_.

Should a process with a given ID (part of an inclusion set), for
example, exit, and then a new process be given this same ID, then the
latter would also be allowed to record events.

With the man:lttng-track(1) command, you can add Unix user and group
_names_ to the user and group inclusion sets: the
<<lttng-sessiond,session daemon>> finds the corresponding UID, VUID,
GID, or VGID once on _addition_ to the inclusion set. This means that if
you rename the user or group after you run the man:lttng-track(1)
command, its user/group ID remains part of the inclusion sets.
====

.Allow processes to record events based on their virtual process ID (VPID).
====
For the sake of the following example, assume the target system has
16{nbsp}possible VPIDs.

When you
<<creating-destroying-tracing-sessions,create a recording session>>,
the user space VPID inclusion set contains _all_ the possible VPIDs:

[role="img-100"]
.The VPID inclusion set is full.
image::track-all.png[]

When the inclusion set is full and you run the man:lttng-track(1)
command to specify some VPIDs, LTTng:

. Clears the inclusion set.
. Adds the specific VPIDs to the inclusion set.

After:

[role="term"]
----
$ lttng track --userspace --vpid=3,4,7,10,13
----

the VPID inclusion set is:

[role="img-100"]
.The VPID inclusion set contains the VPIDs 3, 4, 7, 10, and 13.
image::track-3-4-7-10-13.png[]

Add more VPIDs to the inclusion set afterwards:

[role="term"]
----
$ lttng track --userspace --vpid=1,15,16
----

The result is:

[role="img-100"]
.VPIDs 1, 15, and 16 are added to the inclusion set.
image::track-1-3-4-7-10-13-15-16.png[]

The man:lttng-untrack(1) command removes entries from process attribute
inclusion sets. Given the previous example, the following command:

[role="term"]
----
$ lttng untrack --userspace --vpid=3,7,10,13
----

leads to this VPID inclusion set:

[role="img-100"]
.VPIDs 3, 7, 10, and 13 are removed from the inclusion set.
image::track-1-4-15-16.png[]

You can make the VPID inclusion set full again with the
opt:lttng-track(1):--all option:

[role="term"]
----
$ lttng track --userspace --vpid --all
----

The result is, again:

[role="img-100"]
.The VPID inclusion set is full.
image::track-all.png[]
====

.Allow specific processes to record events based on their user ID (UID).
====
A typical use case with process attribute inclusion sets is to start
with an empty inclusion set, then <<basic-tracing-session-control,start
the tracers>>, and finally add values manually while the tracers are
active.

Use the opt:lttng-untrack(1):--all option of the
man:lttng-untrack(1) command to clear the inclusion set after you
<<creating-destroying-tracing-sessions,create a recording session>>, for
example (with UIDs):

[role="term"]
----
# lttng untrack --kernel --uid --all
----

gives:

[role="img-100"]
.The UID inclusion set is empty.
image::untrack-all.png[]

If the LTTng tracer runs with this inclusion set configuration, it
records no events within the <<cur-tracing-session,current recording
session>> because no processes is allowed to do so. Use the
man:lttng-track(1) command as usual to add specific values to the UID
inclusion set when you need to, for example:

[role="term"]
----
# lttng track --kernel --uid=http,11
----

Result:

[role="img-100"]
.UIDs 6 (`http`) and 11 are part of the UID inclusion set.
image::track-6-11.png[]
====


[role="since-2.5"]
[[saving-loading-tracing-session]]
=== Save and load recording session configurations

Configuring a <<tracing-session,recording session>> can be long. Some of
the tasks involved are:

* <<enabling-disabling-channels,Create channels>> with
  specific attributes.

* <<adding-context,Add context fields>> to be recorded to the
  <<event,event records>> of specific channels.

* <<enabling-disabling-events,Create recording event rules>> with
  specific log level, filter, and other conditions.

If you use LTTng to solve real world problems, chances are you have to
record events using the same recording session setup over and over,
modifying a few variables each time in your instrumented program or
environment.

To avoid constant recording session reconfiguration, the man:lttng(1)
command-line tool can save and load recording session configurations
to/from XML files.

To save a given recording session configuration:

* Use the man:lttng-save(1) command:
+
--
[role="term"]
----
$ lttng save SESSION
----
--
+
Replace +__SESSION__+ with the name of the recording session to save.

LTTng saves recording session configurations to
dir:{$LTTNG_HOME/.lttng/sessions} by default. Note that the
env:LTTNG_HOME environment variable defaults to `$HOME` if not set. See
man:lttng-save(1) to learn more about the recording session configuration
output path.

LTTng saves all configuration parameters, for example:

* The recording session name.
* The trace data output path.
* The <<channel,channels>>, with their state and all their attributes.
* The context fields you added to channels.
* The <<event,recording event rules>> with their state and conditions.

To load a recording session:

* Use the man:lttng-load(1) command:
+
--
[role="term"]
----
$ lttng load SESSION
----
--
+
Replace +__SESSION__+ with the name of the recording session to load.

When LTTng loads a configuration, it restores your saved recording session
as if you just configured it manually.

You can also save and load many sessions at a time; see
man:lttng-save(1) and man:lttng-load(1) to learn more.


[[sending-trace-data-over-the-network]]
=== Send trace data over the network

LTTng can send the recorded trace data of a <<tracing-session,recording
session>> to a remote system over the network instead of writing it to
the local file system.

To send the trace data over the network:

. On the _remote_ system (which can also be the target system),
  start an LTTng <<lttng-relayd,relay daemon>> (man:lttng-relayd(8)):
+
--
[role="term"]
----
$ lttng-relayd
----
--

. On the _target_ system, create a recording session
  <<net-streaming-mode,configured>> to send trace data over the network:
+
--
[role="term"]
----
$ lttng create my-session --set-url=net://remote-system
----
--
+
Replace +__remote-system__+ with the host name or IP address of the
remote system. See man:lttng-create(1) for the exact URL format.

. On the target system, use the man:lttng(1) command-line tool as usual.
+
When recording is <<basic-tracing-session-control,active>>, the
<<lttng-consumerd,consumer daemon>> of the target sends the contents of
<<channel,sub-buffers>> to the remote relay daemon instead of flushing
them to the local file system. The relay daemon writes the received
packets to its local file system.

See the ``Output directory'' section of man:lttng-relayd(8) to learn
where a relay daemon writes its received trace data.


[role="since-2.4"]
[[lttng-live]]
=== View events as LTTng records them (noch:{LTTng} live)

_LTTng live_ is a network protocol implemented by the
<<lttng-relayd,relay daemon>> (man:lttng-relayd(8)) to allow compatible
trace readers to display or analyze <<event,event records>> as LTTng
records events on the target system while recording is
<<basic-tracing-session-control,active>>.

The relay daemon creates a _tee_: it forwards the trace data to both the
local file system and to connected live readers:

[role="img-90"]
.The relay daemon creates a _tee_, forwarding the trace data to both trace files and a connected live reader.
image::live.png[]

To use LTTng live:

. On the _target system_, create a <<tracing-session,recording session>>
  in _live mode_:
+
--
[role="term"]
----
$ lttng create my-session --live
----
--
+
This operation spawns a local relay daemon.

. Start the live reader and configure it to connect to the relay daemon.
+
For example, with man:babeltrace2(1):
+
--
[role="term"]
----
$ babeltrace2 net://localhost/host/HOSTNAME/my-session
----
--
+
Replace +__HOSTNAME__+ with the host name of the target system.

. Configure the recording session as usual with the man:lttng(1)
  command-line tool, and <<basic-tracing-session-control,start recording>>.

List the available live recording sessions with man:babeltrace2(1):

[role="term"]
----
$ babeltrace2 net://localhost
----

You can start the relay daemon on another system. In this case, you need
to specify the URL of the relay daemon when you
<<creating-destroying-tracing-sessions,create the recording session>> with
the opt:lttng-create(1):--set-url option of the man:lttng-create(1)
command. You also need to replace +__localhost__+ in the procedure above
with the host name of the system on which the relay daemon runs.


[role="since-2.3"]
[[taking-a-snapshot]]
=== Take a snapshot of the current sub-buffers of a recording session

The normal behavior of LTTng is to append full sub-buffers to growing
trace data files. This is ideal to keep a full history of the events
which the target system emitted, but it can represent too much data in
some situations.

For example, you may wish to have LTTng record your application
continuously until some critical situation happens, in which case you
only need the latest few recorded events to perform the desired
analysis, not multi-gigabyte trace files.

With the man:lttng-snapshot(1) command, you can take a _snapshot_ of the
current <<channel,sub-buffers>> of a given <<tracing-session,recording
session>>. LTTng can write the snapshot to the local file system or send
it over the network.

[role="img-100"]
.A snapshot is a copy of the current sub-buffers, which LTTng does _not_ clear after the operation.
image::snapshot.png[]

The snapshot feature of LTTng is similar to how a
https://en.wikipedia.org/wiki/Flight_recorder[flight recorder] or the
``roll'' mode of an oscilloscope work.

TIP: If you wish to create unmanaged, self-contained, non-overlapping
trace chunk archives instead of a simple copy of the current
sub-buffers, see the <<session-rotation,recording session rotation>>
feature (available since LTTng{nbsp}2.11).

To take a snapshot of the <<cur-tracing-session,current recording
session>>:

. Create a recording session in <<snapshot-mode,snapshot mode>>:
+
--
[role="term"]
----
$ lttng create my-session --snapshot
----
--
+
The <<channel-overwrite-mode-vs-discard-mode,event record loss mode>> of
<<channel,channels>> created in this mode is automatically set to
<<overwrite-mode,_overwrite_>>.

. Configure the recording session as usual with the man:lttng(1)
  command-line tool, and <<basic-tracing-session-control,start
  recording>>.

. **Optional**: When you need to take a snapshot,
  <<basic-tracing-session-control,stop recording>>.
+
You can take a snapshot when the tracers are active, but if you stop
them first, you're guaranteed that the trace data in the sub-buffers
doesn't change before you actually take the snapshot.

. Take a snapshot:
+
--
[role="term"]
----
$ lttng snapshot record --name=my-first-snapshot
----
--
+
LTTng writes the current sub-buffers of all the channels of the
<<cur-tracing-session,current recording session>> to
trace files on the local file system. Those trace files have
`my-first-snapshot` in their name.

There's no difference between the format of a normal trace file and the
format of a snapshot: LTTng trace readers also support LTTng snapshots.

By default, LTTng writes snapshot files to the path shown by

[role="term"]
----
$ lttng snapshot list-output
----

You can change this path or decide to send snapshots over the network
using either:

. An output path or URL that you specify when you
  <<creating-destroying-tracing-sessions,create the recording session>>.

. A snapshot output path or URL that you add using the
  `add-output` action of the man:lttng-snapshot(1) command.

. An output path or URL that you provide directly to the
  `record` action of the man:lttng-snapshot(1) command.

Method{nbsp}3 overrides method{nbsp}2, which overrides method 1. When
you specify a URL, a <<lttng-relayd,relay daemon>> must listen on a
remote system (see ``<<sending-trace-data-over-the-network,Send trace
data over the network>>'').

The `snapshot-session` <<trigger,trigger>> action can also take
a recording session snapshot.


[role="since-2.11"]
[[session-rotation]]
=== Archive the current trace chunk (rotate a recording session)

The <<taking-a-snapshot,snapshot user guide>> shows how to dump the
current sub-buffers of a recording session to the file system or send them
over the network. When you take a snapshot, LTTng doesn't clear the ring
buffers of the recording session: if you take another snapshot immediately
after, both snapshots could contain overlapping trace data.

Inspired by https://en.wikipedia.org/wiki/Log_rotation[log rotation],
_recording session rotation_ is a feature which appends the content of the
ring buffers to what's already on the file system or sent over the
network since the creation of the recording session or since the last
rotation, and then clears those ring buffers to avoid trace data
overlaps.

What LTTng is about to write when performing a recording session rotation
is called the _current trace chunk_. When LTTng writes or sends over the
network this current trace chunk, it becomes a _trace chunk archive_.
Therefore, a recording session rotation operation _archives_ the current
trace chunk.

[role="img-100"]
.A recording session rotation operation _archives_ the current trace chunk.
image::rotation.png[]

A trace chunk archive is a self-contained LTTng trace which LTTng
doesn't manage anymore: you can read it, modify it, move it, or remove
it.

As of LTTng{nbsp}{revision}, there are three methods to perform a
recording session rotation:

* <<immediate-rotation,Immediately>>.

* With a <<rotation-schedule,rotation schedule>>.

* Through the execution of a `rotate-session` <<trigger,trigger>>
  action.

[[immediate-rotation]]To perform an immediate rotation of the
<<cur-tracing-session,current recording session>>:

. <<creating-destroying-tracing-sessions,Create a recording session>> in
  <<local-mode,local mode>> or <<net-streaming-mode,network streaming
  mode>> (only those two recording session modes support recording session
  rotation):
+
--
[role="term"]
----
# lttng create my-session
----
--

. <<enabling-disabling-events,Create one or more recording event rules>>
  and <<basic-tracing-session-control,start recording>>:
+
--
[role="term"]
----
# lttng enable-event --kernel sched_'*'
# lttng start
----
--

. When needed, immediately rotate the current recording session:
+
--
[role="term"]
----
# lttng rotate
----
--
+
The man:lttng-rotate(1) command prints the path to the created trace
chunk archive. See its manual page to learn about the format of trace
chunk archive directory names.
+
Perform other immediate rotations while the recording session is active.
It's guaranteed that all the trace chunk archives don't contain
overlapping trace data. You can also perform an immediate rotation once
you have <<basic-tracing-session-control,stopped>> the recording session.

. When you're done recording,
  <<creating-destroying-tracing-sessions,destroy the current recording
  session>>:
+
--
[role="term"]
----
# lttng destroy
----
--
+
The recording session destruction operation creates one last trace chunk
archive from the current trace chunk.

[[rotation-schedule]]A recording session rotation schedule is a planned
rotation which LTTng performs automatically based on one of the
following conditions:

* A timer with a configured period expires.

* The total size of the _flushed_ part of the current trace chunk
  becomes greater than or equal to a configured value.

To schedule a rotation of the <<cur-tracing-session,current recording
session>>, set a _rotation schedule_:

. <<creating-destroying-tracing-sessions,Create a recording session>> in
  <<local-mode,local mode>> or <<net-streaming-mode,network streaming
  mode>> (only those two creation modes support recording session
  rotation):
+
--
[role="term"]
----
# lttng create my-session
----
--

. <<enabling-disabling-events,Create one or more recording event rules>>:
+
--
[role="term"]
----
# lttng enable-event --kernel sched_'*'
----
--

. Set a recording session rotation schedule:
+
--
[role="term"]
----
# lttng enable-rotation --timer=10s
----
--
+
In this example, we set a rotation schedule so that LTTng performs a
recording session rotation every ten seconds.
+
See man:lttng-enable-rotation(1) to learn more about other ways to set a
rotation schedule.

. <<basic-tracing-session-control,Start recording>>:
+
--
[role="term"]
----
# lttng start
----
--
+
LTTng performs recording session rotations automatically while the
recording session is active thanks to the rotation schedule.

. When you're done recording,
  <<creating-destroying-tracing-sessions,destroy the current recording
  session>>:
+
--
[role="term"]
----
# lttng destroy
----
--
+
The recording session destruction operation creates one last trace chunk
archive from the current trace chunk.

Unset a recording session rotation schedule with the
man:lttng-disable-rotation(1) command.


[role="since-2.13"]
[[add-event-rule-matches-trigger]]
=== Add an ``event rule matches'' trigger to a session daemon

With the man:lttng-add-trigger(1) command, you can add a
<<trigger,trigger>> to a <<lttng-sessiond,session daemon>>.

A trigger associates an LTTng tracing condition to one or more actions:
when the condition is satisfied, LTTng attempts to execute the actions.

A trigger doesn't need any <<tracing-session,recording session>> to exist:
it belongs to a session daemon.

As of LTTng{nbsp}{revision}, many condition types are available through
the <<liblttng-ctl-lttng,`liblttng-ctl`>> C{nbsp}API, but the
man:lttng-add-trigger(1) command only accepts the ``event rule matches''
condition.

An ``event rule matches'' condition is satisfied when its event rule
matches an event.

Unlike a <<event,recording event rule>>, the event rule of an
``event rule matches'' trigger condition has no implicit conditions,
that is:

* It has no enabled/disabled state.
* It has no attached <<channel,channel>>.
* It doesn't belong to a <<tracing-session,recording session>>.

Both the man:lttng-add-trigger(1) and man:lttng-enable-event(1) commands
accept command-line arguments to specify an <<event-rule,event rule>>.
That being said, the former is a more recent command and therefore
follows the common event rule specification format (see
man:lttng-event-rule(7)).

.Start a <<tracing-session,recording session>> when an event rule matches.
====
This example shows how to add the following trigger to the root
<<lttng-sessiond,session daemon>>:

Condition::
    An event rule matches a Linux kernel system call event of which the
    name starts with `exec` and `*/ls` matches the `filename` payload
    field.
+
With such an event rule, LTTng emits an event when the cmd:ls program
starts.

Action::
    <<basic-tracing-session-control,Start the recording session>>
    named `pitou`.

To add such a trigger to the root session daemon:

. **If there's no currently running LTTng root session daemon**, start
  one:
+
[role="term"]
----
# lttng-sessiond --daemonize
----

. <<creating-destroying-tracing-sessions,Create a recording session>>
  named `pitou` and
  <<enabling-disabling-events,create a recording event rule>> matching
  all the system call events:
+
[role="term"]
----
# lttng create pitou
# lttng enable-event --kernel --syscall --all
----

. Add the trigger to the root session daemon:
+
[role="term"]
----
# lttng add-trigger --condition=event-rule-matches \
                    --type=syscall --name='exec*' \
                    --filter='filename == "*/ls"' \
                    --action=start-session pitou
----
+
Confirm that the trigger exists with the man:lttng-list-triggers(1)
command:
+
[role="term"]
----
# lttng list-triggers
----

. Make sure the `pitou` recording session is still inactive (stopped):
+
[role="term"]
----
# lttng list pitou
----
+
The first line should be something like:
+
----
Recording session pitou: [inactive]
----

Run the cmd:ls program to fire the LTTng trigger above:

[role="term"]
----
$ ls ~
----

At this point, the `pitou` recording session should be active
(started). Confirm this with the man:lttng-list(1) command again:

[role="term"]
----
# lttng list pitou
----

The first line should now look like:

----
Recording session pitou: [active]
----

This line confirms that the LTTng trigger you added fired, therefore
starting the `pitou` recording session.
====

.[[trigger-event-notif]]Send a notification to a user application when an event rule matches.
====
This example shows how to add the following trigger to the root
<<lttng-sessiond,session daemon>>:

Condition::
    An event rule matches a Linux kernel tracepoint event named
    `sched_switch` and of which the value of the `next_comm` payload
    field is `bash`.
+
With such an event rule, LTTng emits an event when Linux gives access to
the processor to a process named `bash`.

Action::
    Send an LTTng notification to a user application.

Moreover, we'll specify a _capture descriptor_ with the
`event-rule-matches` trigger condition so that the user application can
get the value of a specific `sched_switch` event payload field.

First, write and build the user application:

. Create the C{nbsp}source file of the application:
+
--
[source,c]
.path:{notif-app.c}
----
#include <stdlib.h>
#include <stdio.h>
#include <stdbool.h>
#include <assert.h>
#include <string.h>
#include <lttng/lttng.h>

/*
 * Subscribes to notifications, through the notification channel
 * `notification_channel`, which match the condition of the trigger
 * named `trigger_name`.
 *
 * Returns `true` on success.
 */
static bool subscribe(struct lttng_notification_channel *notification_channel,
                      const char *trigger_name)
{
    const struct lttng_condition *condition = NULL;
    struct lttng_triggers *triggers = NULL;
    unsigned int trigger_count;
    unsigned int i;
    enum lttng_error_code error_code;
    enum lttng_trigger_status trigger_status;
    bool ret = false;

    /* Get all LTTng triggers */
    error_code = lttng_list_triggers(&triggers);
    assert(error_code == LTTNG_OK);

    /* Get the number of triggers */
    trigger_status = lttng_triggers_get_count(triggers, &trigger_count);
    assert(trigger_status == LTTNG_TRIGGER_STATUS_OK);

    /* Find the trigger named `trigger_name` */
    for (i = 0; i < trigger_count; i++) {
        const struct lttng_trigger *trigger;
        const char *this_trigger_name;

        trigger = lttng_triggers_get_at_index(triggers, i);
        trigger_status = lttng_trigger_get_name(trigger, &this_trigger_name);
        assert(trigger_status == LTTNG_TRIGGER_STATUS_OK);

        if (strcmp(this_trigger_name, trigger_name) == 0) {
            /* Trigger found: subscribe with its condition */
            enum lttng_notification_channel_status notification_channel_status;

            notification_channel_status = lttng_notification_channel_subscribe(
                notification_channel,
                lttng_trigger_get_const_condition(trigger));
            assert(notification_channel_status ==
                   LTTNG_NOTIFICATION_CHANNEL_STATUS_OK);
            ret = true;
            break;
        }
    }

    lttng_triggers_destroy(triggers);
    return ret;
}

/*
 * Handles the evaluation `evaluation` of a single notification.
 */
static void handle_evaluation(const struct lttng_evaluation *evaluation)
{
    enum lttng_evaluation_status evaluation_status;
    const struct lttng_event_field_value *array_field_value;
    const struct lttng_event_field_value *string_field_value;
    enum lttng_event_field_value_status event_field_value_status;
    const char *string_field_string_value;

    /* Get the value of the first captured (string) field */
    evaluation_status = lttng_evaluation_event_rule_matches_get_captured_values(
        evaluation, &array_field_value);
    assert(evaluation_status == LTTNG_EVALUATION_STATUS_OK);
    event_field_value_status =
        lttng_event_field_value_array_get_element_at_index(
            array_field_value, 0, &string_field_value);
    assert(event_field_value_status == LTTNG_EVENT_FIELD_VALUE_STATUS_OK);
    assert(lttng_event_field_value_get_type(string_field_value) ==
           LTTNG_EVENT_FIELD_VALUE_TYPE_STRING);
    event_field_value_status = lttng_event_field_value_string_get_value(
            string_field_value, &string_field_string_value);
    assert(event_field_value_status == LTTNG_EVENT_FIELD_VALUE_STATUS_OK);

    /* Print the string value of the field */
    puts(string_field_string_value);
}

int main(int argc, char *argv[])
{
    int exit_status = EXIT_SUCCESS;
    struct lttng_notification_channel *notification_channel;
    enum lttng_notification_channel_status notification_channel_status;
    const struct lttng_condition *condition;
    const char *trigger_name;
    bool subscribe_res;

    assert(argc >= 2);
    trigger_name = argv[1];

    /*
     * Create a notification channel.
     *
     * A notification channel connects the user application to the LTTng
     * session daemon.
     *
     * You can use this notification channel to listen to various types
     * of notifications.
     */
    notification_channel = lttng_notification_channel_create(
        lttng_session_daemon_notification_endpoint);
    assert(notification_channel);

    /*
     * Subscribe to notifications which match the condition of the
     * trigger named `trigger_name`.
     */
    if (!subscribe(notification_channel, trigger_name)) {
        fprintf(stderr,
                "Error: Failed to subscribe to notifications (trigger `%s`).\n",
                trigger_name);
        exit_status = EXIT_FAILURE;
        goto end;
    }

    /*
     * Notification loop.
     *
     * Put this in a dedicated thread to avoid blocking the main thread.
     */
    while (true) {
        struct lttng_notification *notification;
        enum lttng_notification_channel_status status;
        const struct lttng_evaluation *notification_evaluation;

        /* Receive the next notification */
        status = lttng_notification_channel_get_next_notification(
            notification_channel, &notification);

        switch (status) {
        case LTTNG_NOTIFICATION_CHANNEL_STATUS_OK:
            break;
        case LTTNG_NOTIFICATION_CHANNEL_STATUS_NOTIFICATIONS_DROPPED:
            /*
             * The session daemon can drop notifications if a receiving
             * application doesn't consume the notifications fast
             * enough.
             */
            continue;
        case LTTNG_NOTIFICATION_CHANNEL_STATUS_CLOSED:
            /*
             * The session daemon closed the notification channel.
             *
             * This is typically caused by a session daemon shutting
             * down.
             */
            goto end;
        default:
            /* Unhandled conditions or errors */
            exit_status = EXIT_FAILURE;
            goto end;
        }

        /*
         * Handle the condition evaluation.
         *
         * A notification provides, amongst other things:
         *
         * * The condition that caused LTTng to send this notification.
         *
         * * The condition evaluation, which provides more specific
         *   information on the evaluation of the condition.
         */
        handle_evaluation(lttng_notification_get_evaluation(notification));

        /* Destroy the notification object */
        lttng_notification_destroy(notification);
    }

end:
    lttng_notification_channel_destroy(notification_channel);
    return exit_status;
}
----
--
+
This application prints the first captured string field value of the
condition evaluation of each LTTng notification it receives.

. Build the `notif-app` application,
  using https://www.freedesktop.org/wiki/Software/pkg-config/[pkg-config]
  to provide the right compiler and linker flags:
+
--
[role="term"]
----
$ gcc -o notif-app notif-app.c $(pkg-config --cflags --libs lttng-ctl)
----
--

Now, to add the trigger to the root session daemon:

[start=3]
. **If there's no currently running LTTng root session daemon**, start
  one:
+
[role="term"]
----
# lttng-sessiond --daemonize
----

. Add the trigger, naming it `sched-switch-notif`, to the root
  session daemon:
+
[role="term"]
----
# lttng add-trigger --name=sched-switch-notif \
                    --condition=event-rule-matches \
                    --type=kernel --name=sched_switch \
                    --filter='next_comm == "bash"' --capture=prev_comm \
                    --action=notify
----
+
Confirm that the `sched-switch-notif` trigger exists with the
man:lttng-list-triggers(1) command:
+
[role="term"]
----
# lttng list-triggers
----

Run the cmd:notif-app application, passing the name of the trigger
of which to watch the notifications:

[role="term"]
----
# ./notif-app sched-switch-notif
----

Now, in an interactive Bash, type a few keys to fire the
`sched-switch-notif` trigger. Watch the `notif-app` application print
the previous process names.
====

[role="since-2.6"]
[[mi]]
=== Use the machine interface

With any command of the man:lttng(1) command-line tool, set the
opt:lttng(1):--mi option to `xml` (before the command name) to get an
XML machine interface output, for example:

[role="term"]
----
$ lttng --mi=xml list my-session
----

A schema definition (XSD) is
https://github.com/lttng/lttng-tools/blob/stable-{revision}/src/common/mi-lttng-4.0.xsd[available]
to ease the integration with external tools as much as possible.


[role="since-2.8"]
[[metadata-regenerate]]
=== Regenerate the metadata of an LTTng trace

An LTTng trace, which is a https://diamon.org/ctf[CTF] trace, has both
data stream files and a metadata stream file. This metadata file
contains, amongst other things, information about the offset of the
clock sources which LTTng uses to assign timestamps to <<event,event
records>> when recording.

If, once a <<tracing-session,recording session>> is
<<basic-tracing-session-control,started>>, a major
https://en.wikipedia.org/wiki/Network_Time_Protocol[NTP] correction
happens, the clock offset of the trace also needs to be updated. Use
the `metadata` item of the man:lttng-regenerate(1) command to do so.

The main use case of this command is to allow a system to boot with
an incorrect wall time and have LTTng trace it before its wall time
is corrected. Once the system is known to be in a state where its
wall time is correct, you can run `lttng regenerate metadata`.

To regenerate the metadata stream files of the
<<cur-tracing-session,current recording session>>:

* Use the `metadata` item of the man:lttng-regenerate(1) command:
+
--
[role="term"]
----
$ lttng regenerate metadata
----
--


[role="since-2.9"]
[[regenerate-statedump]]
=== Regenerate the state dump event records of a recording session

The LTTng kernel and user space tracers generate state dump
<<event,event records>> when the application starts or when you
<<basic-tracing-session-control,start a recording session>>.

An analysis can use the state dump event records to set an initial state
before it builds the rest of the state from the subsequent event
records. http://tracecompass.org/[Trace Compass] and
https://github.com/lttng/lttng-analyses[LTTng analyses] are notable
examples of applications which use the state dump of an LTTng trace.

When you <<taking-a-snapshot,take a snapshot>>, it's possible that the
state dump event records aren't included in the snapshot trace files
because they were recorded to a <<channel,sub-buffer>> that has been
consumed or <<overwrite-mode,overwritten>> already.

Use the `statedump` item of the man:lttng-regenerate(1) command to emit
and record the state dump events again.

To regenerate the state dump of the <<cur-tracing-session,current
recording session>>, provided you created it in <<snapshot-mode,snapshot
mode>>, before you take a snapshot:

. Use the `statedump` item of the man:lttng-regenerate(1) command:
+
--
[role="term"]
----
$ lttng regenerate statedump
----
--

. <<basic-tracing-session-control,Stop the recording session>>:
+
--
[role="term"]
----
$ lttng stop
----
--

. <<taking-a-snapshot,Take a snapshot>>:
+
--
[role="term"]
----
$ lttng snapshot record --name=my-snapshot
----
--

Depending on the event throughput, you should run steps{nbsp}1
and{nbsp}2 as closely as possible.

[NOTE]
====
To record the state dump events, you need to
<<enabling-disabling-events,create recording event rules>> which enable
them:

* The names of LTTng-UST state dump tracepoints start with
  `lttng_ust_statedump:`.

* The names of LTTng-modules state dump tracepoints start with
  `lttng_statedump_`.
====


[role="since-2.7"]
[[persistent-memory-file-systems]]
=== Record trace data on persistent memory file systems

https://en.wikipedia.org/wiki/Non-volatile_random-access_memory[Non-volatile
random-access memory] (NVRAM) is random-access memory that retains its
information when power is turned off (non-volatile). Systems with such
memory can store data structures in RAM and retrieve them after a
reboot, without flushing to typical _storage_.

Linux supports NVRAM file systems thanks to either
http://pramfs.sourceforge.net/[PRAMFS] or
https://www.kernel.org/doc/Documentation/filesystems/dax.txt[DAX]{nbsp}+{nbsp}http://lkml.iu.edu/hypermail/linux/kernel/1504.1/03463.html[pmem]
(requires Linux{nbsp}4.1+).

This section doesn't describe how to operate such file systems; we
assume that you have a working persistent memory file system.

When you <<creating-destroying-tracing-sessions,create a recording
session>>, you can specify the path of the shared memory holding the
sub-buffers. If you specify a location on an NVRAM file system, then you
can retrieve the latest recorded trace data when the system reboots
after a crash.

To record trace data on a persistent memory file system and retrieve the
trace data after a system crash:

. Create a recording session with a <<channel,sub-buffer>> shared memory
  path located on an NVRAM file system:
+
--
[role="term"]
----
$ lttng create my-session --shm-path=/path/to/shm/on/nvram
----
--

. Configure the recording session as usual with the man:lttng(1)
  command-line tool, and <<basic-tracing-session-control,start
  recording>>.

. After a system crash, use the man:lttng-crash(1) command-line tool to
  read the trace data recorded on the NVRAM file system:
+
--
[role="term"]
----
$ lttng-crash /path/to/shm/on/nvram
----
--

The binary layout of the ring buffer files isn't exactly the same as the
trace files layout. This is why you need to use man:lttng-crash(1)
instead of some standard LTTng trace reader.

To convert the ring buffer files to LTTng trace files:

* Use the opt:lttng-crash(1):--extract option of man:lttng-crash(1):
+
--
[role="term"]
----
$ lttng-crash --extract=/path/to/trace /path/to/shm/on/nvram
----
--


[role="since-2.10"]
[[notif-trigger-api]]
=== Get notified when the buffer usage of a channel is too high or too low

With the notification and <<trigger,trigger>> C{nbsp}API of
<<liblttng-ctl-lttng,`liblttng-ctl`>>, LTTng can notify your user
application when the buffer usage of one or more <<channel,channels>>
becomes too low or too high.

Use this API and enable or disable <<event,recording event rules>> while
a recording session <<basic-tracing-session-control,is active>> to avoid
<<channel-overwrite-mode-vs-discard-mode,discarded event records>>, for
example.

.Send a notification to a user application when the buffer usage of an LTTng channel is too high.
====
In this example, we create and build an application which gets notified
when the buffer usage of a specific LTTng channel is higher than
75{nbsp}%.

We only print that it's the case in this example, but we could as well
use the `liblttng-ctl` C{nbsp}API to <<enabling-disabling-events,disable
recording event rules>> when this happens, for example.

. Create the C{nbsp}source file of the application:
+
--
[source,c]
.path:{notif-app.c}
----
#include <stdlib.h>
#include <stdio.h>
#include <assert.h>
#include <lttng/lttng.h>

int main(int argc, char *argv[])
{
    int exit_status = EXIT_SUCCESS;
    struct lttng_notification_channel *notification_channel;
    struct lttng_condition *condition;
    struct lttng_action *action;
    struct lttng_trigger *trigger;
    const char *recording_session_name;
    const char *channel_name;

    assert(argc >= 3);
    recording_session_name = argv[1];
    channel_name = argv[2];

    /*
     * Create a notification channel.
     *
     * A notification channel connects the user application to the LTTng
     * session daemon.
     *
     * You can use this notification channel to listen to various types
     * of notifications.
     */
    notification_channel = lttng_notification_channel_create(
        lttng_session_daemon_notification_endpoint);

    /*
     * Create a "buffer usage becomes greater than" condition.
     *
     * In this case, the condition is satisfied when the buffer usage
     * becomes greater than or equal to 75 %.
     *
     * We create the condition for a specific recording session name,
     * channel name, and for the user space tracing domain.
     *
     * The following condition types also exist:
     *
     * * The buffer usage of a channel becomes less than a given value.
     *
     * * The consumed data size of a recording session becomes greater
     *   than a given value.
     *
     * * A recording session rotation becomes ongoing.
     *
     * * A recording session rotation becomes completed.
     *
     * * A given event rule matches an event.
     */
    condition = lttng_condition_buffer_usage_high_create();
    lttng_condition_buffer_usage_set_threshold_ratio(condition, .75);
    lttng_condition_buffer_usage_set_session_name(condition,
                                                  recording_session_name);
    lttng_condition_buffer_usage_set_channel_name(condition,
                                                  channel_name);
    lttng_condition_buffer_usage_set_domain_type(condition,
                                                 LTTNG_DOMAIN_UST);

    /*
     * Create an action (receive a notification) to execute when the
     * condition created above is satisfied.
     */
    action = lttng_action_notify_create();

    /*
     * Create a trigger.
     *
     * A trigger associates a condition to an action: LTTng executes
     * the action when the condition is satisfied.
     */
    trigger = lttng_trigger_create(condition, action);

    /* Register the trigger to the LTTng session daemon. */
    lttng_register_trigger(trigger);

    /*
     * Now that we have registered a trigger, LTTng will send a
     * notification every time its condition is met through a
     * notification channel.
     *
     * To receive this notification, we must subscribe to notifications
     * which match the same condition.
     */
    lttng_notification_channel_subscribe(notification_channel,
                                         condition);

    /*
     * Notification loop.
     *
     * Put this in a dedicated thread to avoid blocking the main thread.
     */
    for (;;) {
        struct lttng_notification *notification;
        enum lttng_notification_channel_status status;
        const struct lttng_evaluation *notification_evaluation;
        const struct lttng_condition *notification_condition;
        double buffer_usage;

        /* Receive the next notification. */
        status = lttng_notification_channel_get_next_notification(
            notification_channel, &notification);

        switch (status) {
        case LTTNG_NOTIFICATION_CHANNEL_STATUS_OK:
            break;
        case LTTNG_NOTIFICATION_CHANNEL_STATUS_NOTIFICATIONS_DROPPED:
            /*
             * The session daemon can drop notifications if a monitoring
             * application isn't consuming the notifications fast
             * enough.
             */
            continue;
        case LTTNG_NOTIFICATION_CHANNEL_STATUS_CLOSED:
            /*
             * The session daemon closed the notification channel.
             *
             * This is typically caused by a session daemon shutting
             * down.
             */
            goto end;
        default:
            /* Unhandled conditions or errors. */
            exit_status = EXIT_FAILURE;
            goto end;
        }

        /*
         * A notification provides, amongst other things:
         *
         * * The condition that caused LTTng to send this notification.
         *
         * * The condition evaluation, which provides more specific
         *   information on the evaluation of the condition.
         *
         * The condition evaluation provides the buffer usage
         * value at the moment the condition was satisfied.
         */
        notification_condition = lttng_notification_get_condition(
            notification);
        notification_evaluation = lttng_notification_get_evaluation(
            notification);

        /* We're subscribed to only one condition. */
        assert(lttng_condition_get_type(notification_condition) ==
            LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH);

        /*
         * Get the exact sampled buffer usage from the condition
         * evaluation.
         */
        lttng_evaluation_buffer_usage_get_usage_ratio(
            notification_evaluation, &buffer_usage);

        /*
         * At this point, instead of printing a message, we could do
         * something to reduce the buffer usage of the channel, like
         * disable specific events, for example.
         */
        printf("Buffer usage is %f %% in recording session \"%s\", "
               "user space channel \"%s\".\n", buffer_usage * 100,
               recording_session_name, channel_name);

        /* Destroy the notification object. */
        lttng_notification_destroy(notification);
    }

end:
    lttng_action_destroy(action);
    lttng_condition_destroy(condition);
    lttng_trigger_destroy(trigger);
    lttng_notification_channel_destroy(notification_channel);
    return exit_status;
}
----
--

. Build the `notif-app` application, linking it with `liblttng-ctl`:
+
--
[role="term"]
----
$ gcc -o notif-app notif-app.c $(pkg-config --cflags --libs lttng-ctl)
----
--

. <<creating-destroying-tracing-sessions,Create a recording session>>,
  <<enabling-disabling-events,create a recording event rule>> matching
  all the user space tracepoint events, and
  <<basic-tracing-session-control,start recording>>:
+
--
[role="term"]
----
$ lttng create my-session
$ lttng enable-event --userspace --all
$ lttng start
----
--
+
If you create the channel manually with the man:lttng-enable-channel(1)
command, you can set its <<channel-monitor-timer,monitor timer>> to
control how frequently LTTng samples the current values of the channel
properties to evaluate user conditions.

. Run the `notif-app` application.
+
This program accepts the <<tracing-session,recording session>> and
user space channel names as its two first arguments. The channel
which LTTng automatically creates with the man:lttng-enable-event(1)
command above is named `channel0`:
+
--
[role="term"]
----
$ ./notif-app my-session channel0
----
--

. In another terminal, run an application with a very high event
  throughput so that the 75{nbsp}% buffer usage condition is reached.
+
In the first terminal, the application should print lines like this:
+
----
Buffer usage is 81.45197 % in recording session "my-session", user space
channel "channel0".
----
+
If you don't see anything, try to make the threshold of the condition in
path:{notif-app.c} lower (0.1{nbsp}%, for example), and then rebuild the
`notif-app` application (step{nbsp}2) and run it again (step{nbsp}4).
====


[[reference]]
== Reference

[[lttng-modules-ref]]
=== noch:{LTTng-modules}


[role="since-2.9"]
[[lttng-tracepoint-enum]]
==== `LTTNG_TRACEPOINT_ENUM()` usage

Use the `LTTNG_TRACEPOINT_ENUM()` macro to define an enumeration:

[source,c]
----
LTTNG_TRACEPOINT_ENUM(name, TP_ENUM_VALUES(entries))
----

Replace:

* `name` with the name of the enumeration (C identifier, unique
  amongst all the defined enumerations).
* `entries` with a list of enumeration entries.

The available enumeration entry macros are:

+ctf_enum_value(__name__, __value__)+::
  Entry named +__name__+ mapped to the integral value +__value__+.

+ctf_enum_range(__name__, __begin__, __end__)+::
  Entry named +__name__+ mapped to the range of integral values between
  +__begin__+ (included) and +__end__+ (included).

+ctf_enum_auto(__name__)+::
  Entry named +__name__+ mapped to the integral value following the
  last mapping value.
+
The last value of a `ctf_enum_value()` entry is its +__value__+
parameter.
+
The last value of a `ctf_enum_range()` entry is its +__end__+ parameter.
+
If `ctf_enum_auto()` is the first entry in the list, its integral
value is 0.

Use the `ctf_enum()` <<lttng-modules-tp-fields,field definition macro>>
to use a defined enumeration as a tracepoint field.

.Define an enumeration with `LTTNG_TRACEPOINT_ENUM()`.
====
[source,c]
----
LTTNG_TRACEPOINT_ENUM(
    my_enum,
    TP_ENUM_VALUES(
        ctf_enum_auto("AUTO: EXPECT 0")
        ctf_enum_value("VALUE: 23", 23)
        ctf_enum_value("VALUE: 27", 27)
        ctf_enum_auto("AUTO: EXPECT 28")
        ctf_enum_range("RANGE: 101 TO 303", 101, 303)
        ctf_enum_auto("AUTO: EXPECT 304")
    )
)
----
====


[role="since-2.7"]
[[lttng-modules-tp-fields]]
==== Tracepoint fields macros (for `TP_FIELDS()`)

[[tp-fast-assign]][[tp-struct-entry]]The available macros to define
tracepoint fields, which must be listed within `TP_FIELDS()` in
`LTTNG_TRACEPOINT_EVENT()`, are:

[role="func-desc growable",cols="asciidoc,asciidoc"]
.Available macros to define LTTng-modules tracepoint fields
|====
|Macro |Description and parameters

|
+ctf_integer(__t__, __n__, __e__)+

+ctf_integer_nowrite(__t__, __n__, __e__)+

+ctf_user_integer(__t__, __n__, __e__)+

+ctf_user_integer_nowrite(__t__, __n__, __e__)+
|
Standard integer, displayed in base{nbsp}10.

+__t__+::
  Integer C type (`int`, `long`, `size_t`, ...).

+__n__+::
  Field name.

+__e__+::
  Argument expression.

|
+ctf_integer_hex(__t__, __n__, __e__)+

+ctf_user_integer_hex(__t__, __n__, __e__)+
|
Standard integer, displayed in base{nbsp}16.

+__t__+::
  Integer C type.

+__n__+::
  Field name.

+__e__+::
  Argument expression.

|+ctf_integer_oct(__t__, __n__, __e__)+
|
Standard integer, displayed in base{nbsp}8.

+__t__+::
  Integer C type.

+__n__+::
  Field name.

+__e__+::
  Argument expression.

|
+ctf_integer_network(__t__, __n__, __e__)+

+ctf_user_integer_network(__t__, __n__, __e__)+
|
Integer in network byte order (big-endian), displayed in base{nbsp}10.

+__t__+::
  Integer C type.

+__n__+::
  Field name.

+__e__+::
  Argument expression.

|
+ctf_integer_network_hex(__t__, __n__, __e__)+

+ctf_user_integer_network_hex(__t__, __n__, __e__)+
|
Integer in network byte order, displayed in base{nbsp}16.

+__t__+::
  Integer C type.

+__n__+::
  Field name.

+__e__+::
  Argument expression.

|
+ctf_enum(__N__, __t__, __n__, __e__)+

+ctf_enum_nowrite(__N__, __t__, __n__, __e__)+

+ctf_user_enum(__N__, __t__, __n__, __e__)+

+ctf_user_enum_nowrite(__N__, __t__, __n__, __e__)+
|
Enumeration.

+__N__+::
  Name of a <<lttng-tracepoint-enum,previously defined enumeration>>.

+__t__+::
  Integer C type (`int`, `long`, `size_t`, ...).

+__n__+::
  Field name.

+__e__+::
  Argument expression.

|
+ctf_string(__n__, __e__)+

+ctf_string_nowrite(__n__, __e__)+

+ctf_user_string(__n__, __e__)+

+ctf_user_string_nowrite(__n__, __e__)+
|
Null-terminated string; undefined behavior if +__e__+ is `NULL`.

+__n__+::
  Field name.

+__e__+::
  Argument expression.

|
+ctf_array(__t__, __n__, __e__, __s__)+

+ctf_array_nowrite(__t__, __n__, __e__, __s__)+

+ctf_user_array(__t__, __n__, __e__, __s__)+

+ctf_user_array_nowrite(__t__, __n__, __e__, __s__)+
|
Statically-sized array of integers.

+__t__+::
  Array element C type.

+__n__+::
  Field name.

+__e__+::
  Argument expression.

+__s__+::
  Number of elements.

|
+ctf_array_bitfield(__t__, __n__, __e__, __s__)+

+ctf_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+

+ctf_user_array_bitfield(__t__, __n__, __e__, __s__)+

+ctf_user_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+
|
Statically-sized array of bits.

The type of +__e__+ must be an integer type. +__s__+ is the number
of elements of such type in +__e__+, not the number of bits.

+__t__+::
  Array element C type.

+__n__+::
  Field name.

+__e__+::
  Argument expression.

+__s__+::
  Number of elements.

|
+ctf_array_text(__t__, __n__, __e__, __s__)+

+ctf_array_text_nowrite(__t__, __n__, __e__, __s__)+

+ctf_user_array_text(__t__, __n__, __e__, __s__)+

+ctf_user_array_text_nowrite(__t__, __n__, __e__, __s__)+
|
Statically-sized array, printed as text.

The string doesn't need to be null-terminated.

+__t__+::
  Array element C type (always `char`).

+__n__+::
  Field name.

+__e__+::
  Argument expression.

+__s__+::
  Number of elements.

|
+ctf_sequence(__t__, __n__, __e__, __T__, __E__)+

+ctf_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+

+ctf_user_sequence(__t__, __n__, __e__, __T__, __E__)+

+ctf_user_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+
|
Dynamically-sized array of integers.

The type of +__E__+ must be unsigned.

+__t__+::
  Array element C type.

+__n__+::
  Field name.

+__e__+::
  Argument expression.

+__T__+::
  Length expression C type.

+__E__+::
  Length expression.

|
+ctf_sequence_hex(__t__, __n__, __e__, __T__, __E__)+

+ctf_user_sequence_hex(__t__, __n__, __e__, __T__, __E__)+
|
Dynamically-sized array of integers, displayed in base{nbsp}16.

The type of +__E__+ must be unsigned.

+__t__+::
  Array element C type.

+__n__+::
  Field name.

+__e__+::
  Argument expression.

+__T__+::
  Length expression C type.

+__E__+::
  Length expression.

|+ctf_sequence_network(__t__, __n__, __e__, __T__, __E__)+
|
Dynamically-sized array of integers in network byte order (big-endian),
displayed in base{nbsp}10.

The type of +__E__+ must be unsigned.

+__t__+::
  Array element C type.

+__n__+::
  Field name.

+__e__+::
  Argument expression.

+__T__+::
  Length expression C type.

+__E__+::
  Length expression.

|
+ctf_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+

+ctf_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+

+ctf_user_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+

+ctf_user_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+
|
Dynamically-sized array of bits.

The type of +__e__+ must be an integer type. +__s__+ is the number
of elements of such type in +__e__+, not the number of bits.

The type of +__E__+ must be unsigned.

+__t__+::
  Array element C type.

+__n__+::
  Field name.

+__e__+::
  Argument expression.

+__T__+::
  Length expression C type.

+__E__+::
  Length expression.

|
+ctf_sequence_text(__t__, __n__, __e__, __T__, __E__)+

+ctf_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+

+ctf_user_sequence_text(__t__, __n__, __e__, __T__, __E__)+

+ctf_user_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+
|
Dynamically-sized array, displayed as text.

The string doesn't need to be null-terminated.

The type of +__E__+ must be unsigned.

The behaviour is undefined if +__e__+ is `NULL`.

+__t__+::
  Sequence element C type (always `char`).

+__n__+::
  Field name.

+__e__+::
  Argument expression.

+__T__+::
  Length expression C type.

+__E__+::
  Length expression.
|====

Use the `_user` versions when the argument expression, `e`, is
a user space address. In the cases of `ctf_user_integer*()` and
`ctf_user_float*()`, `&e` must be a user space address, thus `e` must
be addressable.

The `_nowrite` versions omit themselves from the trace data, but are
otherwise identical. This means LTTng won't write the `_nowrite` fields
to the recorded trace. Their primary purpose is to make some of the
event context available to the <<enabling-disabling-events,recording
event rule filters>> without having to commit the data to
<<channel,sub-buffers>>.


[[glossary]]
== Glossary

Terms related to LTTng and to tracing in general:

[[def-action]]action::
  The part of a <<def-trigger,trigger>> which LTTng executes when the
  trigger <<def-condition,condition>> is satisfied.

Babeltrace::
  The https://diamon.org/babeltrace[Babeltrace] project, which includes:
+
* The
  https://babeltrace.org/docs/v2.0/man1/babeltrace2.1/[cmd:babeltrace2]
  command-line interface.
* The libbabeltrace2 library which offers a
  https://babeltrace.org/docs/v2.0/libbabeltrace2/[C API].
* https://babeltrace.org/docs/v2.0/python/bt2/[Python{nbsp}3 bindings].
* Plugins.

[[def-buffering-scheme]]<<channel-buffering-schemes,buffering scheme>>::
  A layout of <<def-sub-buffer,sub-buffers>> applied to a given channel.

[[def-channel]]<<channel,channel>>::
  An entity which is responsible for a set of
  <<def-ring-buffer,ring buffers>>.
+
<<def-recording-event-rule,Recording event rules>> are always attached
to a specific channel.

clock::
  A source of time for a <<def-tracer,tracer>>.

[[def-condition]]condition::
  The part of a <<def-trigger,trigger>> which must be satisfied for
  LTTng to attempt to execute the trigger <<def-action,actions>>.

[[def-consumer-daemon]]<<lttng-consumerd,consumer daemon>>::
  A program which is responsible for consuming the full
  <<def-sub-buffer,sub-buffers>> and write them to a file system or
  send them over the network.

[[def-current-trace-chunk]]current trace chunk::
  A <<def-trace-chunk,trace chunk>> which includes the current content
  of all the <<def-sub-buffer,sub-buffers>> of the
  <<def-tracing-session,recording session>> and the stream files
  produced since the latest event amongst:
+
* The creation of the recording session.
* The last <<def-tracing-session-rotation,recording session rotation>>, if
  any.

<<channel-overwrite-mode-vs-discard-mode,discard mode>>::
  The <<def-event-record-loss-mode,event record loss mode>> in which
  the <<def-tracer,tracer>> _discards_ new <<def-event-record,event
  records>> when there's no <<def-sub-buffer,sub-buffer>> space left to
  store them.

[[def-event]]event::
  The execution of an <<def-instrumentation-point,instrumentation
  point>>, like a <<def-tracepoint,tracepoint>> that you manually place
  in some source code, or a Linux kprobe.
+
When an instrumentation point is executed, LTTng creates an event.
+
When an <<def-event-rule,event rule>> matches the event,
<<def-lttng,LTTng>> executes some action, for example:
+
* Record its payload to a <<def-sub-buffer,sub-buffer>> as an
  <<def-event-record,event record>>.
* Attempt to execute the user-defined actions of a
  <<def-trigger,trigger>> with an
  <<add-event-rule-matches-trigger,``event rule matches''>> condition.

[[def-event-name]]event name::
  The name of an <<def-event,event>>, which is also the name of the
  <<def-event-record,event record>>.
+
This is also called the _instrumentation point name_.

[[def-event-record]]event record::
  A record (binary serialization), in a <<def-trace,trace>>, of the
  payload of an <<def-event,event>>.
+
The payload of an event record has zero or more _fields_.

[[def-event-record-loss-mode]]<<channel-overwrite-mode-vs-discard-mode,event record loss mode>>::
  The mechanism by which event records of a given
  <<def-channel,channel>> are lost (not recorded) when there's no
  <<def-sub-buffer,sub-buffer>> space left to store them.

[[def-event-rule]]<<event-rule,event rule>>::
  Set of conditions which an <<def-event,event>> must satisfy
  for LTTng to execute some action.
+
An event rule is said to _match_ events, like a
https://en.wikipedia.org/wiki/Regular_expression[regular expression]
matches strings.
+
A <<def-recording-event-rule,recording event rule>> is a specific type
of event rule of which the action is to <<def-record,record>> the event
to a <<def-sub-buffer,sub-buffer>>.

[[def-incl-set]]inclusion set::
  In the <<pid-tracking,process attribute inclusion set>> context: a
  set of <<def-proc-attr,process attributes>> of a given type.

<<instrumenting,instrumentation>>::
  The use of <<def-lttng,LTTng>> probes to make a kernel or
  <<def-user-application,user application>> traceable.

[[def-instrumentation-point]]instrumentation point::
  A point in the execution path of a kernel or
  <<def-user-application,user application>> which, when executed,
  create an <<def-event,event>>.

instrumentation point name::
  See _<<def-event-name,event name>>_.

`java.util.logging`::
  The
  https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[core logging facilities]
  of the Java platform.

log4j::
  A https://logging.apache.org/log4j/1.2/[logging library] for Java
  developed by the Apache Software Foundation.

log level::
  Level of severity of a log statement or user space
  <<def-instrumentation-point,instrumentation point>>.

[[def-lttng]]LTTng::
  The _Linux Trace Toolkit: next generation_ project.

<<lttng-cli,cmd:lttng>>::
  A command-line tool provided by the <<def-lttng-tools,LTTng-tools>>
  project which you can use to send and receive control messages to and
  from a <<def-session-daemon,session daemon>>.

LTTng analyses::
  The https://github.com/lttng/lttng-analyses[LTTng analyses] project,
  which is a set of analyzing programs that you can use to obtain a
  higher level view of an <<def-lttng,LTTng>> <<def-trace,trace>>.

cmd:lttng-consumerd::
  The name of the <<def-consumer-daemon,consumer daemon>> program.

cmd:lttng-crash::
  A utility provided by the <<def-lttng-tools,LTTng-tools>> project
  which can convert <<def-ring-buffer,ring buffer>> files (usually
  <<persistent-memory-file-systems,saved on a persistent memory file
  system>>) to <<def-trace,trace>> files.
+
See man:lttng-crash(1).

LTTng Documentation::
  This document.

<<lttng-live,LTTng live>>::
  A communication protocol between the <<lttng-relayd,relay daemon>> and
  live readers which makes it possible to show or analyze
  <<def-event-record,event records>> ``live'', as they're received by
  the <<def-relay-daemon,relay daemon>>.

<<lttng-modules,LTTng-modules>>::
  The https://github.com/lttng/lttng-modules[LTTng-modules] project,
  which contains the Linux kernel modules to make the Linux kernel
  <<def-instrumentation-point,instrumentation points>> available for
  <<def-lttng,LTTng>> tracing.

cmd:lttng-relayd::
  The name of the <<def-relay-daemon,relay daemon>> program.

cmd:lttng-sessiond::
  The name of the <<def-session-daemon,session daemon>> program.

[[def-lttng-tools]]LTTng-tools::
  The https://github.com/lttng/lttng-tools[LTTng-tools] project, which
  contains the various programs and libraries used to
  <<controlling-tracing,control tracing>>.

[[def-lttng-ust]]<<lttng-ust,LTTng-UST>>::
  The https://github.com/lttng/lttng-ust[LTTng-UST] project, which
  contains libraries to instrument
  <<def-user-application,user applications>>.

<<lttng-ust-agents,LTTng-UST Java agent>>::
  A Java package provided by the <<def-lttng-ust,LTTng-UST>> project to
  allow the LTTng instrumentation of `java.util.logging` and Apache
  log4j{nbsp}1.2 logging statements.

<<lttng-ust-agents,LTTng-UST Python agent>>::
  A Python package provided by the <<def-lttng-ust,LTTng-UST>> project
  to allow the <<def-lttng,LTTng>> instrumentation of Python logging
  statements.

<<channel-overwrite-mode-vs-discard-mode,overwrite mode>>::
  The <<def-event-record-loss-mode,event record loss mode>> in which new
  <<def-event-record,event records>> _overwrite_ older event records
  when there's no <<def-sub-buffer,sub-buffer>> space left to store
  them.

<<channel-buffering-schemes,per-process buffering>>::
  A <<def-buffering-scheme,buffering scheme>> in which each instrumented
  process has its own <<def-sub-buffer,sub-buffers>> for a given user
  space <<def-channel,channel>>.

<<channel-buffering-schemes,per-user buffering>>::
  A <<def-buffering-scheme,buffering scheme>> in which all the processes
  of a Unix user share the same <<def-sub-buffer,sub-buffers>> for a
  given user space <<def-channel,channel>>.

[[def-proc-attr]]process attribute::
  In the <<pid-tracking,process attribute inclusion set>> context:
+
* A process ID.
* A virtual process ID.
* A Unix user ID.
* A virtual Unix user ID.
* A Unix group ID.
* A virtual Unix group ID.

record (_noun_)::
  See <<def-event-record,_event record_>>.

[[def-record]]record (_verb_)::
  Serialize the binary payload of an <<def-event,event>> to a
  <<def-sub-buffer,sub-buffer>>.

[[def-recording-event-rule]]<<event,recording event rule>>::
  Specific type of <<def-event-rule,event rule>> of which the action is
  to <<def-record,record>> the matched event to a
  <<def-sub-buffer,sub-buffer>>.

[[def-tracing-session]][[def-recording-session]]<<tracing-session,recording session>>::
  A stateful dialogue between you and a <<lttng-sessiond,session daemon>>.

[[def-tracing-session-rotation]]<<session-rotation,recording session rotation>>::
  The action of archiving the
  <<def-current-trace-chunk,current trace chunk>> of a
  <<def-tracing-session,recording session>>.

[[def-relay-daemon]]<<lttng-relayd,relay daemon>>::
  A process which is responsible for receiving the <<def-trace,trace>>
  data which a distant <<def-consumer-daemon,consumer daemon>> sends.

[[def-ring-buffer]]ring buffer::
  A set of <<def-sub-buffer,sub-buffers>>.

rotation::
  See _<<def-tracing-session-rotation,recording session rotation>>_.

[[def-session-daemon]]<<lttng-sessiond,session daemon>>::
  A process which receives control commands from you and orchestrates
  the <<def-tracer,tracers>> and various <<def-lttng,LTTng>> daemons.

<<taking-a-snapshot,snapshot>>::
  A copy of the current data of all the <<def-sub-buffer,sub-buffers>>
  of a given <<def-tracing-session,recording session>>, saved as
  <<def-trace,trace>> files.

[[def-sub-buffer]]sub-buffer::
  One part of an <<def-lttng,LTTng>> <<def-ring-buffer,ring buffer>>
  which contains <<def-event-record,event records>>.

timestamp::
  The time information attached to an <<def-event,event>> when LTTng
  creates it.

[[def-trace]]trace (_noun_)::
  A set of:
+
* One https://diamon.org/ctf/[CTF] metadata stream file.
* One or more CTF data stream files which are the concatenations of one
  or more flushed <<def-sub-buffer,sub-buffers>>.

[[def-trace-verb]]trace (_verb_)::
  From the perspective of a <<def-tracer,tracer>>: attempt to execute
  one or more actions when emitting an <<def-event,event>> in an
  application or in a system.

[[def-trace-chunk]]trace chunk::
  A self-contained <<def-trace,trace>> which is part of a
  <<def-tracing-session,recording session>>. Each
  <<def-tracing-session-rotation, recording session rotation>> produces a
  <<def-trace-chunk-archive,trace chunk archive>>.

[[def-trace-chunk-archive]]trace chunk archive::
  The result of a <<def-tracing-session-rotation, recording session
  rotation>>.
+
<<def-lttng,LTTng>> doesn't manage any trace chunk archive, even if its
containing <<def-tracing-session,recording session>> is still active: you
are free to read it, modify it, move it, or remove it.

Trace Compass::
  The http://tracecompass.org[Trace Compass] project and application.

[[def-tracepoint]]tracepoint::
  An instrumentation point using the tracepoint mechanism of the Linux
  kernel or of <<def-lttng-ust,LTTng-UST>>.

tracepoint definition::
  The definition of a single <<def-tracepoint,tracepoint>>.

tracepoint name::
  The name of a <<def-tracepoint,tracepoint>>.

[[def-tracepoint-provider]]tracepoint provider::
  A set of functions providing <<def-tracepoint,tracepoints>> to an
  instrumented <<def-user-application,user application>>.
+
Not to be confused with a <<def-tracepoint-provider-package,tracepoint
provider package>>: many tracepoint providers can exist within a
tracepoint provider package.

[[def-tracepoint-provider-package]]tracepoint provider package::
  One or more <<def-tracepoint-provider,tracepoint providers>> compiled
  as an https://en.wikipedia.org/wiki/Object_file[object file] or as a
  link:https://en.wikipedia.org/wiki/Library_(computing)#Shared_libraries[shared
  library].

[[def-tracer]]tracer::
  A piece of software which executes some action when it emits
  an <<def-event,event>>, like <<def-record,record>> it to some
  buffer.

<<domain,tracing domain>>::
  A type of LTTng <<def-tracer,tracer>>.

<<tracing-group,tracing group>>::
  The Unix group which a Unix user can be part of to be allowed to
  control the Linux kernel LTTng <<def-tracer,tracer>>.

[[def-trigger]]<<trigger,trigger>>::
  A <<def-condition,condition>>-<<def-action,actions>> pair; when the
  condition of a trigger is satisfied, LTTng attempts to execute its
  actions.

[[def-user-application]]user application::
  An application (program or library) running in user space, as opposed
  to a Linux kernel module, for example.