2.8-2.12: "lost packets" is only meaningful for non-snapshot modes

[lttng-docs.git] / 2.10 / lttng-docs-2.10.txt

The LTTng Documentation
=======================
Philippe Proulx <pproulx@efficios.com>
v2.10, 25 February 2021


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


include::../common/warning-not-maintained.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.
+
You can 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.
+
You can skip this section if you already properly installed LTTng on
your target system.

* **<<getting-started,Quick start>>** is a concise guide to
  getting 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.
+
You can 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.
+
Instrumenting source code is essential to provide a meaningful
source of events.
+
You can skip this section if you do not 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 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 {revision}?

LTTng{nbsp}{revision} bears the name _KeKriek_. From
http://brasseriedunham.com/[Brasserie Dunham], the _**KeKriek**_ is a
sour mashed golden wheat ale fermented with local sour cherries from
Tougas orchards. Fresh sweet cherry notes with some tartness, lively
carbonation with a dry finish.

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

* **Tracing control**:
** You can put more than one wildcard special character (`*`), and not
   only at the end, when you <<enabling-disabling-events,create an event
   rule>>, in both the instrumentation point name and the literal
   strings of
   link:/man/1/lttng-enable-event/v{revision}/#doc-filter-syntax[filter expressions]:
+
--
[role="term"]
----
# lttng enable-event --kernel 'x86_*_local_timer_*' \
                   --filter='name == "*a*b*c*d*e" && count >= 23'
----
--
+
--
[role="term"]
----
$ lttng enable-event --userspace '*_my_org:*msg*'
----
--

** New trigger and notification API for
   <<liblttng-ctl-lttng,`liblttng-ctl`>>. This new subsystem allows you
   to register triggers which emit a notification when a given
   condition is satisfied. As of LTTng{nbsp}{revision}, only
   <<channel,channel>> buffer usage conditions are available.
   Documentation is available in the
   https://github.com/lttng/lttng-tools/tree/stable-{revision}/include/lttng[`liblttng-ctl`
   header files] and in
   <<notif-trigger-api,Get notified when a channel's buffer usage is too
   high or too low>>.

** You can now embed the whole textual LTTng-tools man pages into the
   executables at build time with the `--enable-embedded-help`
   configuration option. Thanks to this option, you don't need the
   http://www.methods.co.nz/asciidoc/[AsciiDoc] and
   https://directory.fsf.org/wiki/Xmlto[xmlto] tools at build time, and
   a manual pager at run time, to get access to this documentation.

* **User space tracing**:
** New blocking mode: an LTTng-UST tracepoint can now block until
   <<channel,sub-buffer>> space is available instead of discarding event
   records in <<channel-overwrite-mode-vs-discard-mode,discard mode>>.
   With this feature, you can be sure that no event records are
   discarded during your application's execution at the expense of
   performance.
+
For example, the following command lines create a user space tracing
channel with an infinite blocking timeout and run an application
instrumented with LTTng-UST which is explicitly allowed to block:
+
--
[role="term"]
----
$ lttng create
$ lttng enable-channel --userspace --blocking-timeout=inf blocking-channel
$ lttng enable-event --userspace --channel=blocking-channel --all
$ lttng start
$ LTTNG_UST_ALLOW_BLOCKING=1 my-app
----
--
+
See the complete <<blocking-timeout-example,blocking timeout example>>.

* **Linux kernel tracing**:
** Linux 4.10, 4.11, and 4.12 support.
** The thread state dump events recorded by LTTng-modules now contain
   the task's CPU identifier. This improves the precision of the
   scheduler model for analyses.
** Extended man:socketpair(2) system call tracing data.


[[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 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 software. This
is where tracing comes in handy.

_Tracing_ is a technique used to understand what goes on in a running
software system. The 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 trace user applications and the operating system 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 is 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] is a port of
  Sun Microsystems's 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] is 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]
  is 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] is
  a performance analyzing tool for Linux which supports hardware
  performance counters, tracepoints, as well as other counters and
  types of probes. perf's controlling utility is the cmd:perf command
  line/curses tool.
* http://linux.die.net/man/1/strace[strace]
  is 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.
* http://www.sysdig.org/[sysdig], like SystemTap, uses scripts to
  analyze Linux kernel events. You write scripts, or _chisels_ in
  sysdig's jargon, in Lua and sysdig executes them while the system is
  being traced or afterwards. sysdig's interface is the cmd:sysdig
  command-line tool as well as the curses-based cmd:csysdig tool.
* https://sourceware.org/systemtap/[SystemTap] is 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. SystemTap's primary user interface 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
http://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 years of active open source
development by a community of passionate developers.
LTTng{nbsp}{revision} 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 tracing sessions, enable
and disable events on the fly, filter events efficiently with custom
user expressions, start and stop tracing, and much more. LTTng can
record the traces on the file system or send them over the network, and
keep them totally or partially. You can view the traces once tracing
becomes inactive or in real-time.

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


[[installing-lttng]]
== Installation

include::../common/warning-no-installation.txt[]

**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
tracing, enable and disable 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). Note
that:

* You only need to install LTTng-modules if you intend to trace the
  Linux kernel.
* You only need to install LTTng-UST if you intend to trace user
  applications.


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

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

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

. Download, build, and install the latest LTTng-modules{nbsp}{revision}:
+
--
[role="term"]
----
$ cd $(mktemp -d) &&
wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.10.tar.bz2 &&
tar -xf lttng-modules-latest-2.10.tar.bz2 &&
cd lttng-modules-2.10.* &&
make &&
sudo make modules_install &&
sudo depmod -a
----
--

. Download, build, and install the latest LTTng-UST{nbsp}{revision}:
+
--
[role="term"]
----
$ cd $(mktemp -d) &&
wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.10.tar.bz2 &&
tar -xf lttng-ust-latest-2.10.tar.bz2 &&
cd lttng-ust-2.10.* &&
./configure &&
make &&
sudo make install &&
sudo ldconfig
----
--
+
--
[IMPORTANT]
.Java and Python application tracing
====
If you need to instrument and 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 trace <<python-application,Python
applications>>, pass the `--enable-python-agent` option to the
`configure` script. You can set the `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 http://lttng.org/files/lttng-tools/lttng-tools-latest-2.10.tar.bz2 &&
tar -xf lttng-tools-latest-2.10.tar.bz2 &&
cd lttng-tools-2.10.* &&
./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 in a specific directory. This can be useful to test
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,Trace the Linux kernel>>.
. <<tracing-your-own-user-application,Trace a user application>> written
  in C.
. <<viewing-and-analyzing-your-traces,View and analyze the
  recorded events>>.


[[tracing-the-linux-kernel]]
=== Trace the Linux kernel

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

. Create a <<tracing-session,tracing session>> which writes its 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,event rules>> which match the desired instrumentation
  point 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
----
--
+
You can also create an event rule which matches _all_ the Linux kernel
tracepoints (this will generate a lot of data when tracing):
+
--
[role="term"]
----
# lttng enable-event --kernel --all
----
--

. <<basic-tracing-session-control,Start tracing>>:
+
--
[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
  tracing session:
+
--
[role="term"]
----
# lttng destroy
----
--
+
The man:lttng-destroy(1) command does not destroy the trace data; it
only destroys the state of the tracing 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 tracing
session>>). You need to stop tracing 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]]
=== Trace a user application

This section steps you through a simple example to trace a
_Hello world_ program written in 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 TRACEPOINT_PROVIDER
#define TRACEPOINT_PROVIDER hello_world

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

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

#include <lttng/tracepoint.h>

TRACEPOINT_EVENT(
    hello_world,
    my_first_tracepoint,
    TP_ARGS(
        int, my_integer_arg,
        char*, my_string_arg
    ),
    TP_FIELDS(
        ctf_string(my_string_field, my_string_arg)
        ctf_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 TRACEPOINT_CREATE_PROBES
#define 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[])
{
    int x;

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

    /*
     * The following getchar() call is only placed here for the purpose
     * of this demonstration, to pause the application in order for
     * you to have time to list its tracepoints. It is not
     * needed otherwise.
     */
    getchar();

    /*
     * A 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
     * NOT strings: they are in fact parts of variables that the
     * macros in hello-tp.h create.
     */
    tracepoint(hello_world, my_first_tracepoint, 23, "hi there!");

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

    puts("Quitting now!");
    tracepoint(hello_world, my_first_tracepoint, x * x, "x^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"]
.User space tracing tutorial's build steps.
image::ust-flow.png[]

To trace 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 that a session daemon might already be running, for example as
a service that the distribution's service manager 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,tracing session>>:
+
--
[role="term"]
----
$ lttng create my-user-space-session
----
--

. Create an <<event,event rule>> which matches the
  `hello_world:my_first_tracepoint` event name:
+
--
[role="term"]
----
$ lttng enable-event --userspace hello_world:my_first_tracepoint
----
--

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

. Go back to the running `hello` application and press Enter. The
  program executes all `tracepoint()` instrumentation points and exits.
. <<creating-destroying-tracing-sessions,Destroy>> the current
  tracing session:
+
--
[role="term"]
----
$ lttng destroy
----
--
+
The man:lttng-destroy(1) command does not destroy the trace data; it
only destroys the state of the tracing 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 tracing
session>>). You need to stop tracing to make LTTng flush the remaining
trace data and make the trace readable.

By default, LTTng saves the traces in
+$LTTNG_HOME/lttng-traces/__name__-__date__-__time__+,
where +__name__+ is the tracing session name. The
env:LTTNG_HOME environment variable defaults to `$HOME` if not set.

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


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

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

Many tools are available to read LTTng traces:

* **cmd:babeltrace** is a command-line utility which converts trace
  formats; it supports the format that LTTng produces, CTF, as well as a
  basic text output which can be ++grep++ed. The cmd:babeltrace command
  is part of the http://diamon.org/babeltrace[Babeltrace] project.
* Babeltrace also includes
  **https://www.python.org/[Python] bindings** so
  that you can easily open and read an LTTng trace with your own script,
  benefiting from the power of Python.
* http://tracecompass.org/[**Trace Compass**]
  is a graphical user interface for viewing and analyzing any type of
  logs or traces, including LTTng's.
* https://github.com/lttng/lttng-analyses[**LTTng analyses**] is a
  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 the traces recorded during the previous
tutorials were saved 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:babeltrace command-line tool

The simplest way to list all the recorded events of a trace is to pass
its path to cmd:babeltrace with no options:

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

cmd:babeltrace finds all traces recursively within the given path and
prints all their events, merging them in chronological order.

You can pipe the output of cmd:babeltrace into a tool like man:grep(1) for
further filtering:

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

You can pipe the output of cmd:babeltrace into a tool like man:wc(1) to
count the recorded events:

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


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

The <<viewing-and-analyzing-your-traces-bt,text output of cmd:babeltrace>>
is useful to isolate events 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 ships with Python 3 bindings which makes 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 5 running processes
on CPU 0 during the whole trace:

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


def top5proc():
    if len(sys.argv) != 2:
        msg = 'Usage: python3 {} TRACEPATH'.format(sys.argv[0])
        print(msg, file=sys.stderr)
        return False

    # A trace collection contains one or more traces
    col = babeltrace.TraceCollection()

    # Add the trace provided by the user (LTTng traces always have
    # the 'ctf' format)
    if col.add_trace(sys.argv[1], 'ctf') is None:
        raise RuntimeError('Cannot add trace')

    # This counter dict contains execution times:
    #
    #   task command name -> total execution time (ns)
    exec_times = Counter()

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

    # Iterate on events
    for event in col.events:
        # Keep only `sched_switch` events
        if event.name != 'sched_switch':
            continue

        # Keep only events which happened on CPU 0
        if event['cpu_id'] != 0:
            continue

        # Event timestamp
        cur_ts = event.timestamp

        if last_ts is None:
            # We start here
            last_ts = cur_ts

        # Previous task command (short) name
        prev_comm = event['prev_comm']

        # Initialize entry in our dict if not yet done
        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

    # Display top 5
    for name, ns in exec_times.most_common(5):
        s = ns / 1000000000
        print('{:20}{} s'.format(name, s))

    return True


if __name__ == '__main__':
    sys.exit(0 if top5proc() else 1)
----

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 0 on Linux; since we
weren't using the CPU that much when tracing, 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>>.
Understanding how those objects relate to eachother is key in mastering
the toolkit.

The core concepts are:

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


[[tracing-session]]
=== Tracing session

A _tracing session_ is a stateful dialogue between you and
a <<lttng-sessiond,session daemon>>. You can
<<creating-destroying-tracing-sessions,create a new tracing
session>> with the `lttng create` command.

Anything that you do when you control LTTng tracers happens within a
tracing session. In particular, a tracing session:

* Has its own name.
* Has its own set of trace files.
* Has its own state of activity (started or stopped).
* Has its own <<tracing-session-mode,mode>> (local, network streaming,
  snapshot, or live).
* Has its own <<channel,channels>> which have their own
  <<event,event rules>>.

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

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

A tracing session is analogous to a cash machine session:
the operations you do on the banking system through the cash machine do
not alter the data of other users of the same system. In the case of
the cash machine, a session lasts as long as your bank card is inside.
In the case of LTTng, a tracing session lasts from the `lttng create`
command to the `lttng destroy` command.

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


[[tracing-session-mode]]
==== Tracing session mode

LTTng can send the generated trace data to different locations. The
_tracing session mode_ dictates where to send it. The following modes
are available in LTTng{nbsp}{revision}:

[[local-mode]]Local mode::
  LTTng writes the traces to the file system of the machine being traced
  (target system).

[[net-streaming-mode]]Network streaming mode::
  LTTng sends the traces over the network to a
  <<lttng-relayd,relay daemon>> running on a remote system.

Snapshot mode::
  LTTng does not write the traces by default. Instead, you can request
  LTTng to <<taking-a-snapshot,take a snapshot>>, that is, a copy of the
  current tracing buffers, and to write it to the target's file system
  or to send it over the network to a <<lttng-relayd,relay daemon>>
  running on a remote system.

[[live-mode]]Live mode::
  This mode is similar to the network streaming mode, but a live
  trace viewer can connect to the distant relay daemon to
  <<lttng-live,view event records as LTTng generates them>> by
  the tracers.


[[domain]]
=== Tracing domain

A _tracing domain_ is a namespace for event sources. 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 when using some commands to avoid
ambiguity. For example, since all the domains support named tracepoints
as event sources (instrumentation points that you manually insert in the
source code), you need to specify a tracing domain when
<<enabling-disabling-events,creating an event rule>> because all the
tracing domains could have tracepoints with the same names.

Some features are reserved to specific tracing domains. Dynamic function
entry and return instrumentation points, for example, are currently only
supported in the Linux kernel tracing domain, but support for other
tracing domains could be added in the future.

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 an LTTng
tracer emits an event, it can record it to one or more
sub-buffers. The attributes of a channel determine what to do when
there's no space left for a new event record because all sub-buffers
are full, where to send a full sub-buffer, and other behaviours.

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 cannot configure.

A channel also owns <<event,event rules>>. When an LTTng tracer emits
an event, it records it to the sub-buffers of all
the enabled channels with a satisfied event rule, as long as those
channels are part of active <<tracing-session,tracing sessions>>.


[[channel-buffering-schemes]]
==== Per-user vs. per-process buffering schemes

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

Two _buffering schemes_ are available when you
<<enabling-disabling-channels,create a channel>> in the
user space <<domain,tracing domain>>:

Per-user buffering::
  Allocate one set of ring buffers--one per CPU--shared by all the
  instrumented processes of each Unix user.
+
--
[role="img-100"]
.Per-user buffering scheme.
image::per-user-buffering.png[]
--

Per-process buffering::
  Allocate one set of ring buffers--one per CPU--for each
  instrumented process.
+
--
[role="img-100"]
.Per-process buffering scheme.
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
user, only its own.

The Linux kernel tracing domain has only one available buffering scheme
which is 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]]
==== Overwrite vs. discard event loss modes

When an event occurs, LTTng records it to a specific sub-buffer (yellow
arc in the following animation) of a specific channel's ring buffer.
When there's no space left in a sub-buffer, the tracer marks it as
consumable (red) and another, empty sub-buffer starts receiving the
following event records. A <<lttng-consumerd,consumer daemon>>
eventually consumes the marked sub-buffer (returns to white).

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

In an ideal world, sub-buffers are consumed faster than they are filled,
as is 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 and LTTng-UST are _non-blocking_ tracers: when
no empty sub-buffer is available, it is acceptable to lose event records
when the alternative would be to cause substantial delays in the
instrumented application's execution. LTTng privileges performance over
integrity; it aims at perturbing the traced system as little as possible
in order to make tracing of subtle race conditions and rare interrupt
cascades possible.

Starting from 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 no empty sub-buffer is
available, or because the <<opt-blocking-timeout,blocking timeout>> is
reached, the channel's _event loss mode_ determines what to do. The
available event loss modes are:

Discard mode::
  Drop the newest event records until a the tracer releases a
  sub-buffer.
+
This is the only available mode when you specify a
<<opt-blocking-timeout,blocking timeout>>.

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.

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.

In discard mode, LTTng increments a count of lost event records when an
event record is lost and saves this count to the trace. Since
LTTng{nbsp}2.8, in overwrite 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>>
<<tracing-session,tracing session>>, a trace reader can use such
sequence numbers to report lost packets. In overwrite mode, LTTng
doesn't write to the trace the exact number of lost event records in
those lost sub-buffers.

Trace analyses can use saved discarded event record and sub-buffer
(packet) counts of the trace to decide whether or not to perform the
analyses even if trace data is known to be missing.

There are a few ways to decrease your probability of losing event
records.
<<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>> shows
how you can fine-tune the sub-buffer count and size 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 count and size

When you <<enabling-disabling-channels,create a channel>>, you can
set its number of sub-buffers and their size.

Note that there is noticeable CPU overhead introduced when
switching sub-buffers (marking a full one as consumable and switching
to an empty one for the following events to be recorded). Knowing this,
the following list presents a few practical situations along with how
to configure the sub-buffer count and size for them:

* **High event throughput**: In general, prefer bigger sub-buffers to
  lower the risk of losing event records.
+
Having bigger sub-buffers also ensures a lower
<<channel-switch-timer,sub-buffer switching frequency>>.
+
The number of sub-buffers is only meaningful if you create the channel
in overwrite mode: in this case, if a sub-buffer overwrite happens, 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 events occur less frequently, the sub-buffer switching frequency
should remain low and thus the tracer's overhead should not 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 big as possible to avoid a high sub-buffer switching
frequency.

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

The previous situations highlight the major trade-off between a few big
sub-buffers and more, smaller sub-buffers: sub-buffer switching
frequency vs. how much data is 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}
====

* **2 sub-buffers of 4{nbsp}MiB each**: Expect a very low sub-buffer
  switching frequency, but if a sub-buffer overwrite happens, half of
  the event records so far (4{nbsp}MiB) are definitely lost.
* **8 sub-buffers of 1{nbsp}MiB each**: Expect 4{nbsp}times the tracer's
  overhead as the previous configuration, but if a sub-buffer
  overwrite happens, only the eighth of event records so far are
  definitely lost.

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


[[channel-switch-timer]]
==== Switch timer period

The _switch timer period_ is an important configurable attribute of
a channel to ensure periodic sub-buffer flushing.

When the _switch timer_ expires, a sub-buffer switch happens. You can
set the switch timer period attribute when you
<<enabling-disabling-channels,create a channel>> to ensure that event
data is consumed and committed to trace files or to a distant relay
daemon periodically in case of a low event throughput.

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

This attribute is also convenient when you use big sub-buffers to cope
with a sporadic high event throughput, even if the throughput is
normally low.


[[channel-read-timer]]
==== Read timer period

By default, the LTTng tracers use a notification mechanism to signal a
full sub-buffer so that a consumer daemon can consume it. When such
notifications must be avoided, for example in real-time applications,
you can use the channel's _read timer_ instead. When the read timer
fires, the <<lttng-consumerd,consumer daemon>> checks for full,
consumable sub-buffers.


[[tracefile-rotation]]
==== Trace file count and size

By default, trace files can grow as large as needed. You can set the
maximum size of each trace file that a channel writes when you
<<enabling-disabling-channels,create a channel>>. When the size of
a trace file reaches the channel's fixed maximum size, 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, you can also set a maximum number of trace
files. When the number of trace files reaches the channel's fixed
maximum count, the oldest trace file is overwritten. This mechanism is
called _trace file rotation_.


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

An _event rule_ is a set of conditions which must be **all** satisfied
for LTTng to record an occuring event.

You set the conditions when you <<enabling-disabling-events,create
an event rule>>.

You always attach an event rule to <<channel,channel>> when you create
it.

When an event passes the conditions of an event rule, LTTng records it
in one of the attached channel's sub-buffers.

The available conditions, as of LTTng{nbsp}{revision}, are:

* The event rule _is enabled_.
* The instrumentation point's type _is{nbsp}T_.
* The instrumentation point's name (sometimes called _event name_)
  _matches{nbsp}N_, but _is not{nbsp}E_.
* The instrumentation point's log level _is as severe as{nbsp}L_, or
  _is exactly{nbsp}L_.
* The fields of the event's payload _satisfy_ a filter
  expression{nbsp}__F__.

As you can see, all the conditions but the dynamic filter are related to
the event rule's status or to the instrumentation point, not to the
occurring events. This is why, without a filter, checking if an event
passes an event rule is not a dynamic task: when you create or modify an
event rule, all the tracers of its tracing domain enable or disable the
instrumentation points themselves once. This is possible because the
attributes of an instrumentation point (type, name, and log level) are
defined statically. In other words, without a dynamic filter, the tracer
_does not evaluate_ the arguments of an instrumentation point unless it
matches an enabled event rule.

Note that, for LTTng to record an event, the <<channel,channel>> to
which a matching event rule is attached must also be enabled, and the
tracing session owning this channel must be active.

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

.Event, event record, or event rule?
****
With so many similar terms, it's easy to get confused.

An **event** is the consequence of the execution of an _instrumentation
point_, like a tracepoint that you manually place in some source code,
or a Linux kernel KProbe. An event is said to _occur_ at a specific
time. Different actions can be taken upon the occurrence of an event,
like record the event's payload to a buffer.

An **event record** is the representation of an event in a sub-buffer. A
tracer is responsible for capturing the payload of an event, current
context variables, the event's ID, and the event's timestamp. LTTng
can append this sub-buffer to a trace file.

An **event rule** is a set of conditions which must all be satisfied for
LTTng to record an occuring event. Events still occur without
satisfying event rules, but LTTng does not record them.
****


[[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 is 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 incorporates:

* **LTTng-tools**: Libraries and command-line interface to
  control tracing 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)).
* **LTTng-UST**: Libraries and Java/Python packages to 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`
** User space tracepoint provider source files generator command-line
   tool (man:lttng-gen-tp(1)).
** <<lttng-ust-agents,LTTng-UST Java agent>> to instrument and trace
   Java applications using `java.util.logging` or
   Apache log4j 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 trace
  the kernel.
** LTTng kernel tracer module.
** Tracing ring buffer kernel modules.
** Probe kernel modules.
** LTTng logger kernel module.


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

[role="img-100"]
.The tracing control command-line interface.
image::plumbing-lttng-cli.png[]

The _man:lttng(1) command-line tool_ is the standard user interface to
control LTTng <<tracing-session,tracing 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 using the 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 API that hides the
underlying protocol's details. `liblttng-ctl` is part of LTTng-tools.

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

You can use `liblttng-ctl` in C or $$C++$$ source code by including its
"master" header:

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

Some objects are referenced by name (C string), such as tracing
sessions, but most of them require to create a handle first using
`lttng_create_handle()`.

The best available developer documentation for `liblttng-ctl` is, as of
LTTng{nbsp}{revision}, its installed header files. Every function and
structure is thoroughly documented.


[[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. It receives commands from a
<<lttng-sessiond,session daemon>>, for example to
enable and disable specific instrumentation points, and writes event
records to ring buffers shared with a
<<lttng-consumerd,consumer daemon>>.
`liblttng-ust` is part of LTTng-UST.

Public C 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 library providing tracepoints which is
linked with `liblttng-ust`.

An application or library does not have to initialize `liblttng-ust`
manually: its constructor does the necessary tasks to properly register
to a session daemon. The initialization phase also enables the
instrumentation points matching the <<event,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 1.2] are supported.
Note that Apache Log4{nbsp}2 is not supported.

In the case of Python, the standard
https://docs.python.org/3/library/logging.html[`logging`] package
is supported. Both Python 2 and Python 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 trace the log statements. When an
agent is initialized, it creates a log handler that attaches to the root
logger. The agent also registers to a <<lttng-sessiond,session daemon>>.
When the application executes a log statement, it is passed to the
agent's log handler by the root logger. The agent's log handler 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, hence tracing the
log statement.

The log level condition of an
<<event,event rule>> is considered when tracing
a Java or a Python application, and it's compatible with the standard
JUL, 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 the ring buffer; a
<<lttng-consumerd,consumer daemon>> reads from the ring buffer.

* The _LTTng kernel tracer_ module.
* The _LTTng logger_ module.
+
The LTTng logger module implements the special path:{/proc/lttng-logger}
file so that any executable can generate LTTng events by opening and
writing to this file.
+
See <<proc-lttng-logger-abi,LTTng logger>>.

Generally, you do not have to load the LTTng kernel modules manually
(using man:modprobe(8), for example): a root <<lttng-sessiond,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.

The LTTng kernel modules are installed in
+/usr/lib/modules/__release__/extra+ by default, where +__release__+ is
the kernel release (see `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 daemon responsible for
managing tracing sessions and for controlling the various components of
LTTng. 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 an <<event,event rule>> so that the user space tracing library
   can enable or disable tracepoints. Amongst the possible conditions
   of an event rule is a filter expression which `liblttng-ust` evalutes
   when an event occurs.
** Share <<channel,channel>> attributes and ring buffer locations.
--
+
The session daemon and the user space tracing library use a Unix
domain socket for their communication.

* 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
for their communication.

* 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 root session daemon loads the appropriate
<<lttng-modules,LTTng kernel modules>> on startup. It also spawns
a <<lttng-consumerd,consumer daemon>> as soon as you create
an <<event,event rule>>.

The session daemon does not 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
http://diamon.org/ctf/[CTF] metadata stream.

Each Unix user can have its own session daemon instance. The
tracing sessions managed by different session daemons 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 thus to trace the Linux
kernel.

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 daemon which shares
ring buffers with user applications or with the LTTng kernel modules to
collect trace data and send it to some location (on disk or to a
<<lttng-relayd,relay daemon>> over the network). The consumer daemon
is part of LTTng-tools.

You do not start a consumer daemon manually: a consumer daemon is always
spawned by a <<lttng-sessiond,session daemon>> as soon as you create an
<<event,event rule>>, that is, before you start tracing. When you kill
its owner session daemon, the consumer daemon also exits because it is
the session daemon's child process. 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 is 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 daemon acting as a bridge
between remote session and consumer daemons, local trace files, and a
remote live trace viewer. 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 does not have much file system
space to record trace files locally.

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

Various ways were developed to instrument a piece of software for LTTng
tracing. The most straightforward one is to manually place
instrumentation points, called _tracepoints_, in the software's source
code. It is also possible to add instrumentation points dynamically in
the Linux kernel <<domain,tracing domain>>.

If you're only interested in tracing the Linux kernel, your
instrumentation needs are probably already covered by LTTng's built-in
<<lttng-modules,Linux kernel tracepoints>>. You may also wish to trace a
user application which is already instrumented for LTTng tracing.
In such cases, you can 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. They are:

* <<c-application,User space instrumentation for C and $$C++$$
  applications>>.
* <<prebuilt-ust-helpers,Prebuilt user space tracing helpers>>.
* <<java-application,User space Java agent>>.
* <<python-application,User space Python agent>>.
* <<proc-lttng-logger-abi,LTTng logger>>.
* <<instrumenting-linux-kernel,LTTng kernel tracepoints>>.


[[c-application]]
=== [[cxx-application]]User space instrumentation for C and $$C++$$ applications

The 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 application's source code>>.
. <<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, you can skip
those steps and use <<tracef,`tracef()`>> or <<tracelog,`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
supported by LTTng-UST. Those functions can emit events with
user-defined fields and serialize those events as event records to one
or more LTTng-UST <<channel,channel>> sub-buffers. The `tracepoint()`
macro, which you <<probing-the-application-source-code,insert in a user
application's source code>>, 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, you can
skip creating and using a tracepoint provider and use
<<tracef,`tracef()`>> or <<tracelog,`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 TRACEPOINT_PROVIDER
#define TRACEPOINT_PROVIDER provider_name

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

#if !defined(_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
#define _TP_H

#include <lttng/tracepoint.h>

/*
 * Use TRACEPOINT_EVENT(), TRACEPOINT_EVENT_CLASS(),
 * TRACEPOINT_EVENT_INSTANCE(), and 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`.

TIP: [[lttng-gen-tp]]You can use the man:lttng-gen-tp(1) tool to create
this boilerplate for you. When using cmd:lttng-gen-tp, all you need to
write are the <<defining-tracepoints,tracepoint definitions>>.


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

A _tracepoint definition_ defines, for a given tracepoint:

* Its **input arguments**. They are the macro parameters that the
  `tracepoint()` macro accepts for this particular tracepoint
  in the user application's source code.
* Its **output event fields**. They are the sources of event fields
  that form the payload of any event that the execution of the
  `tracepoint()` macro emits for this particular tracepoint.

You can create a tracepoint definition by using the
`TRACEPOINT_EVENT()` macro below the `#include <lttng/tracepoint.h>`
line in the
<<tpp-header,tracepoint provider header file template>>.

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

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

    /* Tracepoint name */
    tracepoint_name,

    /* Input arguments */
    TP_ARGS(
        arguments
    ),

    /* Output event fields */
    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.

This tracepoint emits events named `provider_name:tracepoint_name`.

[IMPORTANT]
.Event name's length limitation
====
The concatenation of the tracepoint provider name and the
tracepoint name must not exceed **254 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 `TP_ARGS()` macro is:

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

Replace:

* `type` with the C type of the argument.
* `arg_name` with the argument name.

You can repeat `type` and `arg_name` up to 10 times to have
more than one argument.

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

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

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

Each `ctf_*()` macro takes an _argument expression_ parameter. This is a
C expression that the tracer evalutes at the `tracepoint()` macro site
in the application's source code. This expression provides a field's
source of data. The argument expression can include input argument names
listed in the `TP_ARGS()` macro.

Each `ctf_*()` 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"

TRACEPOINT_EVENT(
    my_provider,
    my_tracepoint,
    TP_ARGS(
        const struct my_custom_structure*, my_custom_structure,
        float, ratio,
        const char*, query
    ),
    TP_FIELDS(
        ctf_string(query_field, query)
        ctf_float(double, ratio_field, ratio)
        ctf_integer(int, recv_size, my_custom_structure->recv_size)
        ctf_integer(int, send_size, my_custom_structure->send_size)
    )
)
----

You can refer to this tracepoint definition with the `tracepoint()`
macro in your application's source code like this:

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

NOTE: The LTTng tracer only evaluates tracepoint arguments at run time
if they satisfy an enabled <<event,event rule>>.


[[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,`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 tracing.

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]
----
TRACEPOINT_EVENT(
    my_app,
    get_account,
    TP_ARGS(
        int, userid,
        size_t, len
    ),
    TP_FIELDS(
        ctf_integer(int, userid, userid)
        ctf_integer(size_t, len, len)
    )
)

TRACEPOINT_EVENT(
    my_app,
    get_settings,
    TP_ARGS(
        int, userid,
        size_t, len
    ),
    TP_FIELDS(
        ctf_integer(int, userid, userid)
        ctf_integer(size_t, len, len)
    )
)

TRACEPOINT_EVENT(
    my_app,
    get_transaction,
    TP_ARGS(
        int, userid,
        size_t, len
    ),
    TP_FIELDS(
        ctf_integer(int, userid, userid)
        ctf_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 */
TRACEPOINT_EVENT_CLASS(
    /* Tracepoint provider name */
    my_app,

    /* Tracepoint class name */
    my_class,

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

    /* Output event fields */
    TP_FIELDS(
        ctf_integer(int, userid, userid)
        ctf_integer(size_t, len, len)
    )
)

/* The tracepoint instances */
TRACEPOINT_EVENT_INSTANCE(
    /* Tracepoint provider name */
    my_app,

    /* Tracepoint class name */
    my_class,

    /* Tracepoint name */
    get_account,

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


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

You can assign an optional _log level_ to a
<<defining-tracepoints,tracepoint definition>>.

Assigning different levels of severity to tracepoint definitions can
be useful: when you <<enabling-disabling-events,create an event rule>>,
you can target tracepoints having a log level 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 `tracepoint()` macro invocation which refers to
  this definition has this log level.

You can assign a log level to a tracepoint definition with the
`TRACEPOINT_LOGLEVEL()` macro. You must use this macro _after_ the
<<defining-tracepoints,`TRACEPOINT_EVENT()`>> or
<<using-tracepoint-classes,`TRACEPOINT_INSTANCE()`>> macro for a given
tracepoint.

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

[source,c]
.`TRACEPOINT_LOGLEVEL()` macro syntax.
----
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 `TRACE_DEBUG_UNIT` log level to a tracepoint definition.
====
[source,c]
----
/* Tracepoint definition */
TRACEPOINT_EVENT(
    my_app,
    get_transaction,
    TP_ARGS(
        int, userid,
        size_t, len
    ),
    TP_FIELDS(
        ctf_integer(int, userid, userid)
        ctf_integer(size_t, len, len)
    )
)

/* Log level assignment */
TRACEPOINT_LOGLEVEL(my_app, get_transaction, TRACE_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.

You can always use the following tracepoint provider package source
file template:

[source,c]
.Tracepoint provider package source file template.
----
#define 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 an application's source code

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

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

.`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"

TRACEPOINT_EVENT(
    my_provider,
    my_tracepoint,
    TP_ARGS(
        int, argc,
        const char*, cmd_name
    ),
    TP_FIELDS(
        ctf_string(cmd_name, cmd_name)
        ctf_integer(int, number_of_args, argc)
    )
)
----

You can refer to this tracepoint definition with the `tracepoint()`
macro in your application's source code like this:

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

int main(int argc, char* argv[])
{
    tracepoint(my_provider, my_tracepoint, argc, argv[0]);

    return 0;
}
----

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

.`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>

TRACEPOINT_EVENT(
    my_provider,
    my_tracepoint,
    TP_ARGS(
        int, my_int_arg,
        char*, my_str_arg,
        struct stat*, st
    ),
    TP_FIELDS(
        ctf_integer(int, my_constant_field, 23 + 17)
        ctf_integer(int, my_int_arg_field, my_int_arg)
        ctf_integer(int, my_int_arg_field2, my_int_arg * my_int_arg)
        ctf_integer(int, sum4_field, my_str_arg[0] + my_str_arg[1] +
                                     my_str_arg[2] + my_str_arg[3])
        ctf_string(my_str_arg_field, my_str_arg)
        ctf_integer_hex(off_t, size_field, st->st_size)
        ctf_float(double, size_dbl_field, (double) st->st_size)
        ctf_sequence_text(char, half_my_str_arg_field, my_str_arg,
                          size_t, strlen(my_str_arg) / 2)
    )
)
----

You can refer to this tracepoint definition with the `tracepoint()`
macro in your application's source code like this:

[source,c]
.Application's source file.
----
#define TRACEPOINT_DEFINE
#include "tp.h"

int main(void)
{
    struct stat s;

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

    return 0;
}
----

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

.Event record fields
|====
|Field's name            |Field's 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 `tracepoint()` are expensive to
compute--they use the call stack, for example. To avoid this
computation when the tracepoint is disabled, you can use the
`tracepoint_enabled()` and `do_tracepoint()` macros.

The syntax of the `tracepoint_enabled()` and `do_tracepoint()` macros
is:

[source,c]
.`tracepoint_enabled()` and `do_tracepoint()` macros syntax.
----
tracepoint_enabled(provider_name, tracepoint_name)
do_tracepoint(provider_name, tracepoint_name, ...)
----

Replace:

* `provider_name` with the tracepoint provider name.
* `tracepoint_name` with the tracepoint name.

`tracepoint_enabled()` returns a non-zero value if the tracepoint named
`tracepoint_name` from the provider named `provider_name` is enabled
**at run time**.

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

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

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

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

NOTE: Neither `tracepoint_enabled()` nor `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>>,
you can 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's 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 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 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 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 TRACEPOINT_DEFINE
#define 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.

See the <<dlclose-warning,warning about `dlclose()`>>.

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 TRACEPOINT_DEFINE
#define 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 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 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 TRACEPOINT_DEFINE
#define 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.

See the <<dlclose-warning,warning about `dlclose()`>>.

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 TRACEPOINT_DEFINE
#define 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.

See the <<dlclose-warning,warning about `dlclose()`>>.

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 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.

See the <<dlclose-warning,warning about `dlclose()`>>.

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 TRACEPOINT_DEFINE
#define 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 TRACEPOINT_DEFINE
#define 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 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 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
----
--
|====

[[dlclose-warning]]
[IMPORTANT]
.Do not use man:dlclose(3) on a tracepoint provider package
====
Never use man:dlclose(3) on any shared object which:

* Is linked with, statically or dynamically, a tracepoint provider
  package.
* Calls man:dlopen(3) itself to dynamically open a tracepoint provider
  package shared object.

This is currently considered **unsafe** due to a lack of reference
counting from LTTng-UST to the shared object.

A known workaround (available since glibc 2.2) is to use the
`RTLD_NODELETE` flag when calling man:dlopen(3) initially. This has the
effect of not unloading the loaded shared object, even if man:dlclose(3)
is called.

You can also preload the tracepoint provider package shared object with
the env:LD_PRELOAD environment variable to overcome this limitation.
====


[[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 you can 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 http://lttng.org/files/urcu/userspace-rcu-latest-0.9.tar.bz2 &&
tar -xf userspace-rcu-latest-0.9.tar.bz2 &&
cd userspace-rcu-0.9.* &&
./configure --libdir=/usr/local/lib32 CFLAGS=-m32 &&
make &&
sudo make install &&
sudo ldconfig
----
--

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

. Download, build, and install a 32-bit version of the latest
  LTTng-UST{nbsp}{revision}:
+
--
[role="term"]
----
$ cd $(mktemp -d) &&
wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.10.tar.bz2 &&
tar -xf lttng-ust-latest-2.10.tar.bz2 &&
cd lttng-ust-2.10.* &&
./configure --libdir=/usr/local/lib32 \
            CFLAGS=-m32 CXXFLAGS=-m32 \
            LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' &&
make &&
sudo make install &&
sudo ldconfig
----
--
+
[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 http://lttng.org/files/lttng-tools/lttng-tools-latest-2.10.tar.bz2 &&
tar -xf lttng-tools-latest-2.10.tar.bz2 &&
cd lttng-tools-2.10.* &&
./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 http://lttng.org/files/lttng-tools/lttng-tools-latest-2.10.tar.bz2 &&
tar -xf lttng-tools-latest-2.10.tar.bz2 &&
cd lttng-tools-2.10.* &&
./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,Trace a user application>> 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
to trace it: use the command-line man:lttng(1) tool as usual.


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

man: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 `tracef()` in your application:

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

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

    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 trace the events that `tracef()` calls emit:

* <<enabling-disabling-events,Create an event rule>> which matches the
  `lttng_ust_tracef:*` event name:
+
--
[role="term"]
----
$ lttng enable-event --userspace 'lttng_ust_tracef:*'
----
--

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

Taking this into consideration, `tracef()` is useful for some quick
prototyping and debugging, but you should not consider it for any
permanent and serious applicative instrumentation.
====


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

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

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

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

    tracelog(TRACE_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 trace the events that `tracelog()` calls emit with a log level
_as severe as_ a specific log level:

* <<enabling-disabling-events,Create an event rule>> which matches the
  `lttng_ust_tracelog:*` event name and a minimum level
  of severity:
+
--
[role="term"]
----
$ lttng enable-event --userspace 'lttng_ust_tracelog:*'
                   --loglevel=TRACE_WARNING
----
--

To trace the events that `tracelog()` calls emit with a
_specific log level_:

* Create an event rule which matches the `lttng_ust_tracelog:*`
  event name and a specific log level:
+
--
[role="term"]
----
$ lttng enable-event --userspace 'lttng_ust_tracelog:*'
                   --loglevel-only=TRACE_INFO
----
--


[[prebuilt-ust-helpers]]
=== Prebuilt user space tracing helpers

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 are 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 helper's name).

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.
+
You can 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 `TRACE_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), you can 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]]
=== User space Java agent

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.
* http://logging.apache.org/log4j/1.2/[**Apache log4j 1.2**], since
  LTTng 2.6. Note that Apache Log4j{nbsp}2 is not 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 http://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 Java application's source code, import the LTTng-UST
  log handler package for `java.util.logging`:
+
--
[source,java]
----
import org.lttng.ust.agent.jul.LttngLogHandler;
----
--

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

. Add this handler to the JUL 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 is not strictly necessary, but it is recommended for a clean
disposal of the handler's resources.

. Include the LTTng-UST Java agent's common and JUL-specific JAR files,
  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 tracing session>>,
<<enabling-disabling-events,create an event rule>> matching the
`jello` JUL logger, and <<basic-tracing-session-control,start tracing>>:

[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 tracing>> and inspect the
recorded events:

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

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

`msg`::
  Log record's 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.

You can 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 JUL log levels
or a specific JUL 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 1.2:

. In the Java application's source code, 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 is not strictly necessary, but it is recommended for a clean
disposal of the appender's resources.

. Include the LTTng-UST Java agent's common and log4j-specific JAR
  files, 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 tracing session>>,
<<enabling-disabling-events,create an event rule>> matching the
`jello` log4j logger, and <<basic-tracing-session-control,start tracing>>:

[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 tracing>> and inspect the
recorded events:

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

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

`msg`::
  Log record's 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.

You can 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 provided
by the application which <<adding-context,you can add>>, using the
man:lttng-add-context(1) command, to each <<event,event record>>
produced by the log statements of this application.

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 to
the JUL or log4j <<channel,channel>>.

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

. In the Java application's source code, 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 is not strictly necessary, but it is recommended for a clean
disposal of some manager's resources.

. Build your Java application with LTTng-UST Java agent support as
  usual, following the procedure for either the <<jul,JUL>> 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 tracing session>>
and <<enabling-disabling-events,create an event rule>> matching the
`jello` JUL logger:

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

<<adding-context,Add the application-specific context fields>> to the
JUL channel:

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

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

[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 tracing>> and inspect the
recorded events:

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


[role="since-2.7"]
[[python-application]]
=== User space Python agent

You can instrument a Python 2 or Python 3 application which uses the
standard https://docs.python.org/3/library/logging.html[`logging`]
package.

Each log statement emits 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 Python application's source code, 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.
+
Any log statement that the application executes before this import does
not emit 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, you can trace any log statement from any logger.

.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, is not
strictly required for LTTng-UST tracing to work, but in versions of
Python preceding 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 tracing session>>,
<<enabling-disabling-events,create an event rule>> matching the
`my-logger` Python logger, and <<basic-tracing-session-control,start
tracing>>:

[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 tracing>> and inspect the recorded
events:

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

In the resulting trace, an <<event,event record>> generated by a Python
application is named `lttng_python:event` and has the following fields:

`asctime`::
  Logging time (string).

`msg`::
  Log record's 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.

You can 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 5{nbsp}seconds, after which the application continues
without LTTng tracing support. You can 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 3{nbsp}seconds. You can override this
delay with the env:LTTNG_UST_PYTHON_REGISTER_RETRY_DELAY environment
variable.


[role="since-2.5"]
[[proc-lttng-logger-abi]]
=== LTTng logger

The `lttng-tracer` Linux kernel module, part of
<<lttng-modules,LTTng-modules>>, creates the special LTTng logger file
path:{/proc/lttng-logger} when it's loaded. Any application can write
text data to this file to emit an LTTng event.

[role="img-100"]
.An application writes to the LTTng logger file to emit an LTTng event.
image::lttng-logger.png[]

The LTTng logger is the quickest method--not the most efficient,
however--to add instrumentation to an application. It is designed
mostly to instrument shell scripts:

[role="term"]
----
$ echo "Some message, some $variable" > /proc/lttng-logger
----

Any event that the LTTng logger emits 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 an event rule>> which
matches its event name, 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:{/proc/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 should not use the LTTng logger to trace a user application which
can be instrumented 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!' > /proc/lttng-logger
sleep 2
df --human-readable --print-type / > /proc/lttng-logger
----

<<creating-destroying-tracing-sessions,Create a tracing session>>,
<<enabling-disabling-events,create an event rule>> matching the
`lttng_logger` Linux kernel tracepoint, and
<<basic-tracing-session-control,start tracing>>:

[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 tracing>> and inspect the recorded
events:

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


[[instrumenting-linux-kernel]]
=== LTTng kernel tracepoints

NOTE: This section shows how to _add_ instrumentation points to the
Linux kernel. The kernel's subsystems are already thoroughly
instrumented at strategic places for LTTng when you
<<installing-lttng,install>> the <<lttng-modules,LTTng-modules>>
package.

////
There are two methods to instrument the Linux kernel:

. <<linux-add-lttng-layer,Add an LTTng layer>> over an existing ftrace
  tracepoint which uses the `TRACE_EVENT()` API.
+
Choose this if you want to instrumentation a Linux kernel tree with an
instrumentation point compatible with ftrace, perf, and SystemTap.

. Use an <<linux-lttng-tracepoint-event,LTTng-only approach>> to
  instrument an out-of-tree kernel module.
+
Choose this if you don't need ftrace, perf, or SystemTap support.
////


[[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 does not document the `TRACE_EVENT()` macro. You can
read the following articles to learn more about this API:

* http://lwn.net/Articles/379903/[Using the TRACE_EVENT() macro (Part 1)]
* http://lwn.net/Articles/381064/[Using the TRACE_EVENT() macro (Part 2)]
* http://lwn.net/Articles/383362/[Using the TRACE_EVENT() macro (Part 3)]

The following procedure assumes that your ftrace tracepoints are
correctly defined in their own header and that they are 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's name.

. Get a copy of the latest LTTng-modules{nbsp}{revision}:
+
--
[role="term"]
----
$ cd $(mktemp -d) &&
wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.10.tar.bz2 &&
tar -xf lttng-modules-latest-2.10.tar.bz2 &&
cd lttng-modules-2.10.*
----
--

. 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 TRACE_EVENT()'s 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
ftrace's `TRACE_EVENT()` macro.
+
See <<lttng-modules-tp-fields,Tracepoint fields macros>> for a
complete description of the available `ctf_*()` macros.

. Create the LTTng-modules probe's kernel module C source file,
  +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 ftrace's `TRACE_EVENT()` 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 code that need to be executed before the event fields are recorded.

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()`>>.

You can 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 man:modprobe(8)'s `--remove` option 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.

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 use _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 tracing session>>. While
this is most of the time the first operation that you do, sometimes it's
not. Some examples are:

* <<list-instrumentation-points,List the available instrumentation points>>.
* <<saving-loading-tracing-session,Load a tracing session configuration>>.

[[tracing-group]] Each Unix user must have its own running session
daemon to trace user applications. The session daemon that the root user
starts is the only one allowed to control the LTTng kernel tracer. Users
that are part of the _tracing group_ can control the root session
daemon. The default tracing group name is `tracing`; you can set it to
something else with the opt:lttng-sessiond(8):--group option when you
start the root session daemon.

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, use man:kill(1) on its process ID (standard
`TERM` signal).

Note that some Linux distributions could manage the LTTng session daemon
as a service. In this case, you should use the service manager to
start, restart, and stop session daemons.


[[creating-destroying-tracing-sessions]]
=== Create and destroy a tracing session

Almost all the LTTng control operations happen in the scope of
a <<tracing-session,tracing session>>, which is the dialogue between the
<<lttng-sessiond,session daemon>> and you.

To create a tracing session with a generated name:

* Use the man:lttng-create(1) command:
+
--
[role="term"]
----
$ lttng create
----
--

The created tracing session's name is `auto` followed by the
creation date.

To create a tracing session with a specific name:

* Use the optional argument of the man:lttng-create(1) command:
+
--
[role="term"]
----
$ lttng create my-session
----
--
+
Replace `my-session` with the specific tracing session name.

LTTng appends the creation date to the created tracing session's name.

LTTng writes the traces of a tracing session in
+$LTTNG_HOME/lttng-trace/__name__+ by default, where +__name__+ is the
name of the tracing session. Note that the env:LTTNG_HOME environment
variable defaults to `$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 tracing sessions as you wish.

To list all the existing tracing sessions for your Unix user:

* Use the man:lttng-list(1) command:
+
--
[role="term"]
----
$ lttng list
----
--

When you create a tracing session, it is set as the _current tracing
session_. The following man:lttng(1) commands operate on the current
tracing session when you don't specify one:

[role="list-3-cols"]
* `add-context`
* `destroy`
* `disable-channel`
* `disable-event`
* `enable-channel`
* `enable-event`
* `load`
* `regenerate`
* `save`
* `snapshot`
* `start`
* `stop`
* `track`
* `untrack`
* `view`

To change the current tracing session:

* Use the man:lttng-set-session(1) command:
+
--
[role="term"]
----
$ lttng set-session new-session
----
--
+
Replace `new-session` by the name of the new current tracing session.

When you are done tracing in a given tracing session, you can destroy
it. This operation frees the resources taken by the tracing session
to destroy; it does not destroy the trace data that LTTng wrote for
this tracing session.

To destroy the current tracing 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
tracing session>>). You need to stop tracing 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. For the Linux kernel <<domain,tracing domain>>,
they are tracepoints and system calls. For the user space tracing
domain, they are tracepoints. For the other tracing domains, they are
logger names.

To list the available instrumentation points:

* Use the man:lttng-list(1) command with the requested tracing domain's
  option amongst:
+
--
* opt:lttng-list(1):--kernel: Linux kernel tracepoints (your Unix user
  must be a root user, or it must be a member of the
  <<tracing-group,tracing group>>).
* opt:lttng-list(1):--kernel with opt:lttng-list(1):--syscall: Linux
  kernel system calls (your Unix user must be a root user, or it must be
  a member of the 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 call tracepoints.
====
[role="term"]
----
$ lttng list --kernel --syscall
----
====


[[enabling-disabling-events]]
=== Create and enable an event rule

Once you <<creating-destroying-tracing-sessions,create a tracing
session>>, you can create <<event,event rules>> with the
man:lttng-enable-event(1) command.

You specify each condition with a command-line option. The available
condition options are shown in the following table.

[role="growable",cols="asciidoc,asciidoc,default"]
.Condition command-line options for the man:lttng-enable-event(1) command.
|====
|Option |Description |Applicable tracing domains

|
One of:

. `--syscall`
. +--probe=__ADDR__+
. +--function=__ADDR__+

|
Instead of using the default _tracepoint_ instrumentation type, use:

. A Linux system call.
. A Linux https://lwn.net/Articles/132196/[KProbe] (symbol or address).
. The entry and return points of a Linux function (symbol or address).

|Linux kernel.

|First positional argument.

|
Tracepoint or system call name. In the case of a Linux KProbe or
function, this is a custom name given to the event rule. With the
JUL, log4j, and Python domains, this is a logger name.

With a tracepoint, logger, or system call name, you can use the special
`*` globbing character to match anything (for example, `sched_*`,
`my_comp*:*msg_*`).

|All.

|
One of:

. +--loglevel=__LEVEL__+
. +--loglevel-only=__LEVEL__+

|
. Match only tracepoints or log statements with a logging level at
  least as severe as +__LEVEL__+.
. Match only tracepoints or log statements with a logging level
  equal to +__LEVEL__+.

See man:lttng-enable-event(1) for the list of available logging level
names.

|User space, JUL, log4j, and Python.

|+--exclude=__EXCLUSIONS__+

|
When you use a `*` character at the end of the tracepoint or logger
name (first positional argument), exclude the specific names in the
comma-delimited list +__EXCLUSIONS__+.

|
User space, JUL, log4j, and Python.

|+--filter=__EXPR__+

|
Match only events which satisfy the expression +__EXPR__+.

See man:lttng-enable-event(1) to learn more about the syntax of a
filter expression.

|All.

|====

You attach an event rule to a <<channel,channel>> on creation. If you do
not specify the channel with the opt:lttng-enable-event(1):--channel
option, and if the event rule to create is the first in its
<<domain,tracing domain>> for a given tracing session, then LTTng
creates a _default channel_ for you. This default channel is reused in
subsequent invocations of the man:lttng-enable-event(1) command for the
same tracing domain.

An event rule is always enabled at creation time.

The following examples show how you can combine the previous
command-line options to create simple to more complex event rules.

.Create an event rule targetting a Linux kernel tracepoint (default channel).
====
[role="term"]
----
$ lttng enable-event --kernel sched_switch
----
====

.Create an event rule matching four Linux kernel system calls (default channel).
====
[role="term"]
----
$ lttng enable-event --kernel --syscall open,write,read,close
----
====

.Create event rules matching tracepoints with 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 quote the filter string when you
use man:lttng(1) from a shell.
====

.Create an event rule matching any user space tracepoint of a given tracepoint provider with a log level range (default channel).
====
[role="term"]
----
$ lttng enable-event --userspace my_app:'*' --loglevel=TRACE_INFO
----

IMPORTANT: Make sure to always quote the wildcard character when you
use man:lttng(1) from a shell.
====

.Create an event rule matching multiple Python loggers with a wildcard and with exclusions (default channel).
====
[role="term"]
----
$ lttng enable-event --python my-app.'*' \
                   --exclude='my-app.module,my-app.hello'
----
====

.Create an event rule matching any Apache log4j logger with a specific log level (default channel).
====
[role="term"]
----
$ lttng enable-event --log4j --all --loglevel-only=LOG4J_WARN
----
====

.Create an event rule attached to a specific channel matching a specific user space tracepoint provider and tracepoint.
====
[role="term"]
----
$ lttng enable-event --userspace my_app:my_tracepoint --channel=my-channel
----
====

The event rules of a given channel form a whitelist: as soon as an
emitted event passes one of them, LTTng can record the event. For
example, an event named `my_app:my_tracepoint` emitted from a user space
tracepoint with a `TRACE_ERROR` log level passes both of the following
rules:

[role="term"]
----
$ lttng enable-event --userspace my_app:my_tracepoint
$ lttng enable-event --userspace my_app:my_tracepoint \
                   --loglevel=TRACE_INFO
----

The second event rule is redundant: the first one includes
the second one.


[[disable-event-rule]]
=== Disable an event rule

To disable an event rule that you <<enabling-disabling-events,created>>
previously, use the man:lttng-disable-event(1) command. This command
disables _all_ the event rules (of a given tracing domain and channel)
which match an instrumentation point. The other conditions are not
supported as of LTTng{nbsp}{revision}.

The LTTng tracer does not record an emitted event which passes
a _disabled_ event rule.

.Disable an event rule matching a Python logger (default channel).
====
[role="term"]
----
$ lttng disable-event --python my-logger
----
====

.Disable an event rule matching all `java.util.logging` loggers (default channel).
====
[role="term"]
----
$ lttng disable-event --jul '*'
----
====

.Disable _all_ the event rules of the default channel.
====
The opt:lttng-disable-event(1):--all-events option is not, like the
opt:lttng-enable-event(1):--all option of man:lttng-enable-event(1), the
equivalent of the event name `*` (wildcard): it disables _all_ the event
rules of a given channel.

[role="term"]
----
$ lttng disable-event --jul --all-events
----
====

NOTE: You cannot delete an event rule once you create it.


[[status]]
=== Get the status of a tracing session

To get the status of the current tracing session, that is, its
parameters, its channels, event rules, and their attributes:

* Use the man:lttng-status(1) command:
+
--
[role="term"]
----
$ lttng status
----
--
+

To get the status of any tracing session:

* Use the man:lttng-list(1) command with the tracing session's name:
+
--
[role="term"]
----
$ lttng list my-session
----
--
+
Replace `my-session` with the desired tracing session's name.


[[basic-tracing-session-control]]
=== Start and stop a tracing session

Once you <<creating-destroying-tracing-sessions,create a tracing
session>> and
<<enabling-disabling-events,create one or more event rules>>,
you can start and stop the tracers for this tracing session.

To start tracing in the current tracing session:

* Use the man:lttng-start(1) command:
+
--
[role="term"]
----
$ lttng start
----
--

LTTng is very flexible: you can launch user applications before
or after the you start the tracers. The tracers only record the events
if they pass enabled event rules and if they occur while the tracers are
started.

To stop tracing in the current tracing 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), warnings are printed when you run the
man:lttng-stop(1) command.

IMPORTANT: You need to stop tracing 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 tracing
session>>) also runs the man:lttng-stop(1) command implicitly.


[[enabling-disabling-channels]]
=== Create a channel

Once you create a tracing session, you can create a <<channel,channel>>
with the man:lttng-enable-channel(1) command.

Note that LTTng automatically creates a default channel when, for a
given <<domain,tracing domain>>, no channels exist and you
<<enabling-disabling-events,create>> the first event rule. This default
channel is named `channel0` and its attributes are set to reasonable
values. Therefore, you only need to create a channel when you need
non-default attributes.

You specify each non-default channel attribute with a command-line
option when you use the man:lttng-enable-channel(1) command. The
available command-line options are:

[role="growable",cols="asciidoc,asciidoc"]
.Command-line options for the man:lttng-enable-channel(1) command.
|====
|Option |Description

|`--overwrite`

|
Use the _overwrite_
<<channel-overwrite-mode-vs-discard-mode,event loss mode>> instead of
the default _discard_ mode.

|`--buffers-pid` (user space tracing domain only)

|
Use the per-process <<channel-buffering-schemes,buffering scheme>>
instead of the default per-user buffering scheme.

|+--subbuf-size=__SIZE__+

|
Allocate sub-buffers of +__SIZE__+ bytes (power of two), for each CPU,
either for each Unix user (default), or for each instrumented process.

See <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>>.

|+--num-subbuf=__COUNT__+

|
Allocate +__COUNT__+ sub-buffers (power of two), for each CPU, either
for each Unix user (default), or for each instrumented process.

See <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>>.

|+--tracefile-size=__SIZE__+

|
Set the maximum size of each trace file that this channel writes within
a stream to +__SIZE__+ bytes instead of no maximum.

See <<tracefile-rotation,Trace file count and size>>.

|+--tracefile-count=__COUNT__+

|
Limit the number of trace files that this channel creates to
+__COUNT__+ channels instead of no limit.

See <<tracefile-rotation,Trace file count and size>>.

|+--switch-timer=__PERIODUS__+

|
Set the <<channel-switch-timer,switch timer period>>
to +__PERIODUS__+{nbsp}µs.

|+--read-timer=__PERIODUS__+

|
Set the <<channel-read-timer,read timer period>>
to +__PERIODUS__+{nbsp}µs.

|[[opt-blocking-timeout]]+--blocking-timeout=__TIMEOUTUS__+

|
Set the timeout of user space applications which load LTTng-UST
in blocking mode to +__TIMEOUTUS__+:

0 (default)::
    Never block (non-blocking mode).

`inf`::
    Block forever until space is available in a sub-buffer to record
    the event.

__n__, a positive value::
    Wait for at most __n__ µs when trying to write into a sub-buffer.

Note that, for this option to have any effect on an instrumented
user space application, you need to run the application with a set
env:LTTNG_UST_ALLOW_BLOCKING environment variable.

|+--output=__TYPE__+ (Linux kernel tracing domain only)

|
Set the channel's output type to +__TYPE__+, either `mmap` or `splice`.

|====

You can only create a channel in the Linux kernel and user space
<<domain,tracing domains>>: other tracing domains have their own channel
created on the fly when <<enabling-disabling-events,creating event
rules>>.

[IMPORTANT]
====
Because of a current LTTng limitation, you must create all channels
_before_ you <<basic-tracing-session-control,start tracing>> in a given
tracing session, that is, before the first time you run
man:lttng-start(1).

Since LTTng automatically creates a default channel when you use the
man:lttng-enable-event(1) command with a specific tracing domain, you
cannot, for example, create a Linux kernel event rule, start tracing,
and then create a user space event rule, because no user space channel
exists yet and it's too late to create one.

For this reason, make sure to configure your channels properly
before starting the tracers for the first time!
====

The following examples show how you can combine the previous
command-line options to create simple to more complex channels.

.Create a Linux kernel channel with default attributes.
====
[role="term"]
----
$ lttng enable-channel --kernel my-channel
----
====

.Create a user space channel with 4 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 tracing-session>>,
create the channel, <<enabling-disabling-events,create an event rule>>,
and <<basic-tracing-session-control,start tracing>>:

[role="term"]
----
$ lttng create
$ lttng enable-channel --userspace --blocking-timeout=inf blocking-channel
$ lttng enable-event --userspace --channel=blocking-channel --all
$ lttng start
----

Run an application instrumented with LTTng-UST and allow it to block:

[role="term"]
----
$ LTTNG_UST_ALLOW_BLOCKING=1 my-app
----
====

.Create a Linux kernel channel which rotates 8 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 (or _flight recorder_) mode.
====
[role="term"]
----
$ lttng enable-channel --userspace --overwrite my-channel
----
====

You can <<enabling-disabling-events,create>> the same event rule in
two different channels:

[role="term"]
----
$ lttng enable-event --userspace --channel=my-channel app:tp
$ lttng enable-event --userspace --channel=other-channel app:tp
----

If both channels are enabled, when a tracepoint named `app:tp` is
reached, LTTng records two events, one for each channel.


[[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.
====
[role="term"]
----
$ lttng disable-channel --kernel my-channel
----
====

The state of a channel precedes the individual states of event rules
attached to it: event rules which belong to a disabled channel, even if
they are enabled, are also considered disabled.


[[adding-context]]
=== Add context fields to a channel

Event record fields in trace files provide important information about
events that occured previously, 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 in which the event occurs.
* The **hostname** of the system on which the event occurs.
* 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 context defined at the application level (supported for the
  JUL and log4j <<domain,tracing domains>>).

To get the full list of available context fields, see
`lttng add-context --list`. Some context fields are reserved for a
specific <<domain,tracing domain>> (Linux kernel or user space).

You add context fields to <<channel,channels>>. All the events
that a channel with added context fields records contain those fields.

To add context fields to one or all the channels of a given tracing
session:

* Use the man:lttng-add-context(1) command.

.Add context fields to all the channels of the current tracing 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 tracing 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 a context field to a specific channel.
====
The following command line adds the thread identifier context field
to the Linux kernel channel named `my-channel` in the current
tracing session.

[role="term"]
----
$ lttng add-context --kernel --channel=my-channel --type=tid
----
====

.Add an application-specific context field to a specific channel.
====
The following command line adds the `cur_msg_id` context field of the
`retriever` context retriever for all the instrumented
<<java-application,Java applications>> recording <<event,event records>>
in 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 quote the `$` character when you
use man:lttng-add-context(1) from a shell.
====

NOTE: You cannot remove context fields from a channel once you add it.


[role="since-2.7"]
[[pid-tracking]]
=== Track process IDs

It's often useful to allow only specific process IDs (PIDs) to emit
events. For example, you may wish to record all the system calls made by
a given process (à la http://linux.die.net/man/1/strace[strace]).

The man:lttng-track(1) and man:lttng-untrack(1) commands serve this
purpose. Both commands operate on a whitelist of process IDs. You _add_
entries to this whitelist with the man:lttng-track(1) command and remove
entries with the man:lttng-untrack(1) command. Any process which has one
of the PIDs in the whitelist is allowed to emit LTTng events which pass
an enabled <<event,event rule>>.

NOTE: The PID tracker tracks the _numeric process IDs_. Should a
process with a given tracked ID exit and another process be given this
ID, then the latter would also be allowed to emit events.

.Track and untrack process IDs.
====
For the sake of the following example, assume the target system has 16
possible PIDs.

When you
<<creating-destroying-tracing-sessions,create a tracing session>>,
the whitelist contains all the possible PIDs:

[role="img-100"]
.All PIDs are tracked.
image::track-all.png[]

When the whitelist is full and you use the man:lttng-track(1) command to
specify some PIDs to track, LTTng first clears the whitelist, then it
tracks the specific PIDs. After:

[role="term"]
----
$ lttng track --pid=3,4,7,10,13
----

the whitelist is:

[role="img-100"]
.PIDs 3, 4, 7, 10, and 13 are tracked.
image::track-3-4-7-10-13.png[]

You can add more PIDs to the whitelist afterwards:

[role="term"]
----
$ lttng track --pid=1,15,16
----

The result is:

[role="img-100"]
.PIDs 1, 15, and 16 are added to the whitelist.
image::track-1-3-4-7-10-13-15-16.png[]

The man:lttng-untrack(1) command removes entries from the PID tracker's
whitelist. Given the previous example, the following command:

[role="term"]
----
$ lttng untrack --pid=3,7,10,13
----

leads to this whitelist:

[role="img-100"]
.PIDs 3, 7, 10, and 13 are removed from the whitelist.
image::track-1-4-15-16.png[]

LTTng can track all possible PIDs again using the
opt:lttng-track(1):--all option:

[role="term"]
----
$ lttng track --pid --all
----

The result is, again:

[role="img-100"]
.All PIDs are tracked.
image::track-all.png[]
====

.Track only specific PIDs
====
A very typical use case with PID tracking is to start with an empty
whitelist, then <<basic-tracing-session-control,start the tracers>>, and
then add PIDs manually while tracers are active. You can accomplish this
by using the opt:lttng-untrack(1):--all option of the
man:lttng-untrack(1) command to clear the whitelist after you
<<creating-destroying-tracing-sessions,create a tracing session>>:

[role="term"]
----
$ lttng untrack --pid --all
----

gives:

[role="img-100"]
.No PIDs are tracked.
image::untrack-all.png[]

If you trace with this whitelist configuration, the tracer records no
events for this <<domain,tracing domain>> because no processes are
tracked. You can use the man:lttng-track(1) command as usual to track
specific PIDs, for example:

[role="term"]
----
$ lttng track --pid=6,11
----

Result:

[role="img-100"]
.PIDs 6 and 11 are tracked.
image::track-6-11.png[]
====


[role="since-2.5"]
[[saving-loading-tracing-session]]
=== Save and load tracing session configurations

Configuring a <<tracing-session,tracing session>> can be long. Some of
the tasks involved are:

* <<enabling-disabling-channels,Create channels>> with
  specific attributes.
* <<adding-context,Add context fields>> to specific channels.
* <<enabling-disabling-events,Create event rules>> with specific log
  level and filter conditions.

If you use LTTng to solve real world problems, chances are you have to
record events using the same tracing session setup over and over,
modifying a few variables each time in your instrumented program
or environment. To avoid constant tracing session reconfiguration,
the man:lttng(1) command-line tool can save and load tracing session
configurations to/from XML files.

To save a given tracing session configuration:

* Use the man:lttng-save(1) command:
+
--
[role="term"]
----
$ lttng save my-session
----
--
+
Replace `my-session` with the name of the tracing session to save.

LTTng saves tracing session configurations to
dir:{$LTTNG_HOME/.lttng/sessions} by default. Note that the
env:LTTNG_HOME environment variable defaults to `$HOME` if not set. Use
the opt:lttng-save(1):--output-path option to change this destination
directory.

LTTng saves all configuration parameters, for example:

* The tracing session name.
* The trace data output path.
* The channels with their state and all their attributes.
* The context fields you added to channels.
* The event rules with their state, log level and filter conditions.

To load a tracing session:

* Use the man:lttng-load(1) command:
+
--
[role="term"]
----
$ lttng load my-session
----
--
+
Replace `my-session` with the name of the tracing session to load.

When LTTng loads a configuration, it restores your saved tracing session
as if you just configured it manually.

See man:lttng(1) for the complete list of command-line options. You
can also save and load all many sessions at a time, and decide in which
directory to output the XML files.


[[sending-trace-data-over-the-network]]
=== Send trace data over the network

LTTng can send the recorded trace data 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 tracing session configured to
  send trace data over the network:
+
--
[role="term"]
----
$ lttng create my-session --set-url=net://remote-system
----
--
+
Replace `remote-system` by 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 tracing is active, the target's consumer daemon sends sub-buffers
  to the relay daemon running on the remote system instead of flushing
  them to the local file system. The relay daemon writes the received
  packets to the local file system.

The relay daemon writes trace files to
+$LTTNG_HOME/lttng-traces/__hostname__/__session__+ by default, where
+__hostname__+ is the host name of the target system and +__session__+
is the tracing session name. Note that the env:LTTNG_HOME environment
variable defaults to `$HOME` if not set. Use the
opt:lttng-relayd(8):--output option of man:lttng-relayd(8) to write
trace files to another base directory.


[role="since-2.4"]
[[lttng-live]]
=== View events as LTTng emits 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 viewers to
display events as LTTng emits them on the target system while tracing is
active.

The relay daemon creates a _tee_: it forwards the trace data to both
the local file system and to connected live viewers:

[role="img-90"]
.The relay daemon creates a _tee_, forwarding the trace data to both trace files and a connected live viewer.
image::live.png[]

To use LTTng live:

. On the _target system_, create a <<tracing-session,tracing session>>
  in _live mode_:
+
--
[role="term"]
----
$ lttng create my-session --live
----
--
+
This spawns a local relay daemon.

. Start the live viewer and configure it to connect to the relay
  daemon. For example, with http://diamon.org/babeltrace[Babeltrace]:
+
--
[role="term"]
----
$ babeltrace --input-format=lttng-live \
           net://localhost/host/hostname/my-session
----
--
+
Replace:
+
--
* `hostname` with the host name of the target system.
* `my-session` with the name of the tracing session to view.
--

. Configure the tracing session as usual with the man:lttng(1)
  command-line tool, and <<basic-tracing-session-control,start tracing>>.

You can list the available live tracing sessions with Babeltrace:

[role="term"]
----
$ babeltrace --input-format=lttng-live net://localhost
----

You can start the relay daemon on another system. In this case, you need
to specify the relay daemon's URL when you create the tracing session
with the opt:lttng-create(1):--set-url option. You also need to replace
`localhost` in the procedure above with the host name of the system on
which the relay daemon is running.

See man:lttng-create(1) and man:lttng-relayd(8) for the complete list of
command-line options.


[role="since-2.3"]
[[taking-a-snapshot]]
=== Take a snapshot of the current sub-buffers of a tracing 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
that occurred on the target system, but it can
represent too much data in some situations. For example, you may wish
to trace 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 sub-buffers of a given <<tracing-session,tracing session>>.
LTTng can write the snapshot to the local file system or send it over
the network.

To take a snapshot:

. Create a tracing session in _snapshot mode_:
+
--
[role="term"]
----
$ lttng create my-session --snapshot
----
--
+
The <<channel-overwrite-mode-vs-discard-mode,event loss mode>> of
<<channel,channels>> created in this mode is automatically set to
_overwrite_ (flight recorder mode).

. Configure the tracing session as usual with the man:lttng(1)
  command-line tool, and <<basic-tracing-session-control,start tracing>>.

. **Optional**: When you need to take a snapshot,
  <<basic-tracing-session-control,stop tracing>>.
+
You can take a snapshot when the tracers are active, but if you stop
them first, you are sure that the data in the sub-buffers does not
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 current tracing
session's channels to trace files on the local file system. Those trace
files have `my-first-snapshot` in their name.

There is no difference between the format of a normal trace file and the
format of a snapshot: viewers of LTTng traces also support LTTng
snapshots.

By default, LTTng writes snapshot files to the path shown by
`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 create the
  tracing session.
. An snapshot output path or URL that you add using
  `lttng snapshot add-output`
. An output path or URL that you provide directly to the
  `lttng snapshot record` command.

Method 3 overrides method 2, which overrides method 1. When you
specify a URL, a relay daemon must listen on a remote system (see
<<sending-trace-data-over-the-network,Send trace data over the network>>).


[role="since-2.6"]
[[mi]]
=== Use the machine interface

With any command of the man:lttng(1) command-line tool, you can 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 enable-event --kernel --syscall open
----

A schema definition (XSD) is
https://github.com/lttng/lttng-tools/blob/stable-2.10/src/common/mi-lttng-3.0.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 http://diamon.org/ctf[CTF] trace, has both
data stream files and a metadata file. This metadata file contains,
amongst other things, information about the offset of the clock sources
used to timestamp <<event,event records>> when tracing.

If, once a <<tracing-session,tracing session>> is
<<basic-tracing-session-control,started>>, a major
https://en.wikipedia.org/wiki/Network_Time_Protocol[NTP] correction
happens, the trace's clock offset also needs to be updated. You
can 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 trace it with LTTng before its wall time
is corrected. Once the system is known to be in a state where its
wall time is correct, it can run `lttng regenerate metadata`.

To regenerate the metadata of an LTTng trace:

* Use the `metadata` item of the man:lttng-regenerate(1) command:
+
--
[role="term"]
----
$ lttng regenerate metadata
----
--

[IMPORTANT]
====
`lttng regenerate metadata` has the following limitations:

* Tracing session <<creating-destroying-tracing-sessions,created>>
  in non-live mode.
* User space <<channel,channels>>, if any, are using
  <<channel-buffering-schemes,per-user buffering>>.
====


[role="since-2.9"]
[[regenerate-statedump]]
=== Regenerate the state dump of a tracing 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 tracing 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 following event records.
http://tracecompass.org/[Trace Compass] is a notable example of an
application which uses the state dump of an LTTng trace.

When you <<taking-a-snapshot,take a snapshot>>, it's possible that the
state dump event records are not included in the snapshot because they
were recorded to a sub-buffer that has been consumed or overwritten
already.

You can use the `lttng regenerate statedump` command to emit the state
dump event records again.

To regenerate the state dump of the current tracing session, provided
create it in 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 tracing 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 1 and 2
as closely as possible.

NOTE: To record the state dump events, you need to
<<enabling-disabling-events,create event rules>> which enable them.
LTTng-UST state dump tracepoints start with `lttng_ust_statedump:`.
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 4.1+).

This section does not describe how to operate such file systems;
we assume that you have a working persistent memory file system.

When you create a <<tracing-session,tracing 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 tracing session with a sub-buffer shared memory path located
  on an NVRAM file system:
+
--
[role="term"]
----
$ lttng create my-session --shm-path=/path/to/shm
----
--

. Configure the tracing session as usual with the man:lttng(1)
  command-line tool, and <<basic-tracing-session-control,start tracing>>.

. After a system crash, use the man:lttng-crash(1) command-line tool to
  view the trace data recorded on the NVRAM file system:
+
--
[role="term"]
----
$ lttng-crash /path/to/shm
----
--

The binary layout of the ring buffer files is not exactly the same as
the trace files layout. This is why you need to use man:lttng-crash(1)
instead of your preferred trace viewer directly.

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
----
--


[role="since-2.10"]
[[notif-trigger-api]]
=== Get notified when a channel's buffer usage is too high or too low

With LTTng's $$C/C++$$ notification and trigger API, your user
application can get notified when the buffer usage of one or more
<<channel,channels>> becomes too low or too high. You can use this API
and enable or disable <<event,event rules>> during tracing to avoid
<<channel-overwrite-mode-vs-discard-mode,discarded event records>>.

.Have a user application get notified when an LTTng channel's buffer usage 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 is the case in the example, but we
could as well use the API of <<liblttng-ctl-lttng,`liblttng-ctl`>> to
disable event rules when this happens.

. Create the application's C source file:
+
--
[source,c]
.path:{notif-app.c}
----
#include <stdio.h>
#include <assert.h>
#include <lttng/domain.h>
#include <lttng/action/action.h>
#include <lttng/action/notify.h>
#include <lttng/condition/condition.h>
#include <lttng/condition/buffer-usage.h>
#include <lttng/condition/evaluation.h>
#include <lttng/notification/channel.h>
#include <lttng/notification/notification.h>
#include <lttng/trigger/trigger.h>
#include <lttng/endpoint.h>

int main(int argc, char *argv[])
{
    int exit_status = 0;
    struct lttng_notification_channel *notification_channel;
    struct lttng_condition *condition;
    struct lttng_action *action;
    struct lttng_trigger *trigger;
    const char *tracing_session_name;
    const char *channel_name;

    assert(argc >= 3);
    tracing_session_name = argv[1];
    channel_name = argv[2];

    /*
     * Create a notification channel. A notification channel
     * connects the user application to the LTTng session daemon.
     * This notification channel can be used to listen to various
     * types of notifications.
     */
    notification_channel = lttng_notification_channel_create(
        lttng_session_daemon_notification_endpoint);

    /*
     * Create a "high buffer usage" condition. In this case, the
     * condition is reached when the buffer usage is greater than or
     * equal to 75 %. We create the condition for a specific tracing
     * session name, channel name, and for the user space tracing
     * domain.
     *
     * The "low buffer usage" condition type also exists.
     */
    condition = lttng_condition_buffer_usage_high_create();
    lttng_condition_buffer_usage_set_threshold_ratio(condition, .75);
    lttng_condition_buffer_usage_set_session_name(
        condition, tracing_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 (get a notification) to take when the
     * condition created above is reached.
     */
    action = lttng_action_notify_create();

    /*
     * Create a trigger. A trigger associates a condition to an
     * action: the action is executed when the condition is reached.
     */
    trigger = lttng_trigger_create(condition, action);

    /* Register the trigger to LTTng. */
    lttng_register_trigger(trigger);

    /*
     * Now that we have registered a trigger, a notification will be
     * emitted everytime its condition is met. To receive this
     * notification, we must subscribe to notifications that match
     * the same condition.
     */
    lttng_notification_channel_subscribe(notification_channel,
        condition);

    /*
     * Notification loop. You can 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 is not consuming the
             * notifications fast enough.
             */
            continue;
        case LTTNG_NOTIFICATION_CHANNEL_STATUS_CLOSED:
            /*
             * The notification channel has been closed by the
             * session daemon. This is typically caused by a session
             * daemon shutting down.
             */
            goto end;
        default:
            /* Unhandled conditions or errors. */
            exit_status = 1;
            goto end;
        }

        /*
         * A notification provides, amongst other things:
         *
         * * The condition that caused this notification to be
         *   emitted.
         * * 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 reached.
         */
        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 channel's buffer
         * usage, like disable specific events.
         */
        printf("Buffer usage is %f %% in tracing session \"%s\", "
            "user space channel \"%s\".\n", buffer_usage * 100,
            tracing_session_name, channel_name);
        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 to `liblttng-ctl`:
+
--
[role="term"]
----
$ gcc -o notif-app notif-app.c -llttng-ctl
----
--

. <<creating-destroying-tracing-sessions,Create a tracing session>>,
  <<enabling-disabling-events,create an event rule>> matching all the
  user space tracepoints, and
  <<basic-tracing-session-control,start tracing>>:
+
--
[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 control how frequently are the current values of the
channel's properties sampled to evaluate user conditions with the
opt:lttng-enable-channel(1):--monitor-timer option.

. Run the `notif-app` application. This program accepts the
  <<tracing-session,tracing session>> name and the user space channel
  name 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 tracing session "my-session", user space
channel "channel0".
----
+
If you don't see anything, try modifying the condition in
path:{notif-app.c} to a lower value (0.1, for example), rebuilding it
(step 2) and running it again (step 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's 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 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 16.

+__t__+::
  Integer C type.

+__n__+::
  Field name.

+__e__+::
  Argument expression.

|+ctf_integer_oct(__t__, __n__, __e__)+
|
Standard integer, displayed in base 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 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 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 does not 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 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 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 does not 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 session trace, but are
otherwise identical. This means the `_nowrite` fields won't be written
in the recorded trace. Their primary purpose is to make some
of the event context available to the
<<enabling-disabling-events,event filters>> without having to
commit the data to sub-buffers.


[[glossary]]
== Glossary

Terms related to LTTng and to tracing in general:

Babeltrace::
  The http://diamon.org/babeltrace[Babeltrace] project, which includes
  the cmd:babeltrace command, some libraries, and Python bindings.

<<channel-buffering-schemes,buffering scheme>>::
  A layout of sub-buffers applied to a given channel.

<<channel,channel>>::
  An entity which is responsible for a set of ring buffers.
+
<<event,Event rules>> are always attached to a specific channel.

clock::
  A reference of time for a tracer.

<<lttng-consumerd,consumer daemon>>::
  A process which is responsible for consuming the full sub-buffers
  and write them to a file system or send them over the network.

<<channel-overwrite-mode-vs-discard-mode,discard mode>>:: The event loss
  mode in which the tracer _discards_ new event records when there's no
  sub-buffer space left to store them.

event::
  The consequence of the execution of an instrumentation
  point, like a tracepoint that you manually place in some source code,
  or a Linux kernel KProbe.
+
An event is said to _occur_ at a specific time. Different actions can
be taken upon the occurrence of an event, like record the event's payload
to a sub-buffer.

<<channel-overwrite-mode-vs-discard-mode,event loss mode>>::
  The mechanism by which event records of a given channel are lost
  (not recorded) when there is no sub-buffer space left to store them.

[[def-event-name]]event name::
  The name of an event, which is also the name of the event record.
  This is also called the _instrumentation point name_.

event record::
  A record, in a trace, of the payload of an event which occured.

<<event,event rule>>::
  Set of conditions which must be satisfied for one or more occuring
  events to be recorded.

`java.util.logging`::
  Java platform's
  https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[core logging facilities].

<<instrumenting,instrumentation>>::
  The use of LTTng probes to make a piece of software traceable.

instrumentation point::
  A point in the execution path of a piece of software that, when
  reached by this execution, can emit an event.

instrumentation point name::
  See _<<def-event-name,event name>>_.

log4j::
  A http://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
  instrumentation point.

LTTng::
  The _Linux Trace Toolkit: next generation_ project.

<<lttng-cli,cmd:lttng>>::
  A command-line tool provided by the LTTng-tools project which you
  can use to send and receive control messages to and from a
  session daemon.

LTTng analyses::
  The https://github.com/lttng/lttng-analyses[LTTng analyses] project,
  which is a set of analyzing programs that are used to obtain a
  higher level view of an LTTng trace.

cmd:lttng-consumerd::
  The name of the consumer daemon program.

cmd:lttng-crash::
  A utility provided by the LTTng-tools project which can convert
  ring buffer files (usually
  <<persistent-memory-file-systems,saved on a persistent memory file system>>)
  to trace files.

LTTng Documentation::
  This document.

<<lttng-live,LTTng live>>::
  A communication protocol between the relay daemon and live viewers
  which makes it possible to see events "live", as they are received by
  the 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
  instrumentation points available for LTTng tracing.

cmd:lttng-relayd::
  The name of the relay daemon program.

cmd:lttng-sessiond::
  The name of the session daemon program.

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>>.

<<lttng-ust,LTTng-UST>>::
  The https://github.com/lttng/lttng-ust[LTTng-UST] project, which
  contains libraries to instrument user applications.

<<lttng-ust-agents,LTTng-UST Java agent>>::
  A Java package provided by the LTTng-UST project to allow the
  LTTng instrumentation of `java.util.logging` and Apache log4j 1.2
  logging statements.

<<lttng-ust-agents,LTTng-UST Python agent>>::
  A Python package provided by the LTTng-UST project to allow the
  LTTng instrumentation of Python logging statements.

<<channel-overwrite-mode-vs-discard-mode,overwrite mode>>::
  The event loss mode in which new event records overwrite older
  event records when there's no sub-buffer space left to store them.

<<channel-buffering-schemes,per-process buffering>>::
  A buffering scheme in which each instrumented process has its own
  sub-buffers for a given user space channel.

<<channel-buffering-schemes,per-user buffering>>::
  A buffering scheme in which all the processes of a Unix user share the
  same sub-buffer for a given user space channel.

<<lttng-relayd,relay daemon>>::
  A process which is responsible for receiving the trace data sent by
  a distant consumer daemon.

ring buffer::
  A set of sub-buffers.

<<lttng-sessiond,session daemon>>::
  A process which receives control commands from you and orchestrates
  the tracers and various LTTng daemons.

<<taking-a-snapshot,snapshot>>::
  A copy of the current data of all the sub-buffers of a given tracing
  session, saved as trace files.

sub-buffer::
  One part of an LTTng ring buffer which contains event records.

timestamp::
  The time information attached to an event when it is emitted.

trace (_noun_)::
  A set of files which are the concatenations of one or more
  flushed sub-buffers.

trace (_verb_)::
  The action of recording the events emitted by an application
  or by a system, or to initiate such recording by controlling
  a tracer.

Trace Compass::
  The http://tracecompass.org[Trace Compass] project and application.

tracepoint::
  An instrumentation point using the tracepoint mechanism of the Linux
  kernel or of LTTng-UST.

tracepoint definition::
  The definition of a single tracepoint.

tracepoint name::
  The name of a tracepoint.

tracepoint provider::
  A set of functions providing tracepoints to an instrumented user
  application.
+
Not to be confused with a _tracepoint provider package_: many tracepoint
providers can exist within a tracepoint provider package.

tracepoint provider package::
  One or more tracepoint providers compiled as an object file or as
  a shared library.

tracer::
  A software which records emitted events.

<<domain,tracing domain>>::
  A namespace for event sources.

<<tracing-group,tracing group>>::
  The Unix group in which a Unix user can be to be allowed to trace the
  Linux kernel.

<<tracing-session,tracing session>>::
  A stateful dialogue between you and a <<lttng-sessiond,session
  daemon>>.

user application::
  An application running in user space, as opposed to a Linux kernel
  module, for example.