--- /dev/null
+The LTTng Documentation
+=======================
+Philippe Proulx <pproulx@efficios.com>
+v2.13, 15 June 2021
+
+
+include::../common/copyright.txt[]
+
+
+include::../common/welcome.txt[]
+
+
+include::../common/audience.txt[]
+
+
+[[chapters]]
+=== What's in this documentation?
+
+The LTTng Documentation is divided into the following sections:
+
+* ``**<<nuts-and-bolts,Nuts and bolts>>**'' explains the
+ rudiments of software tracing and the rationale behind the
+ LTTng project.
++
+Skip this section if you’re familiar with software tracing and with the
+LTTng project.
+
+* ``**<<installing-lttng,Installation>>**'' describes the steps to
+ install the LTTng packages on common Linux distributions and from
+ their sources.
++
+Skip this section if you already properly installed LTTng on your target
+system.
+
+* ``**<<getting-started,Quick start>>**'' is a concise guide to
+ get started quickly with LTTng kernel and user space tracing.
++
+We recommend this section if you're new to LTTng or to software tracing
+in general.
++
+Skip this section if you're not new to LTTng.
+
+* ``**<<core-concepts,Core concepts>>**'' explains the concepts at
+ the heart of LTTng.
++
+It's a good idea to become familiar with the core concepts
+before attempting to use the toolkit.
+
+* ``**<<plumbing,Components of LTTng>>**'' describes the various
+ components of the LTTng machinery, like the daemons, the libraries,
+ and the command-line interface.
+
+* ``**<<instrumenting,Instrumentation>>**'' shows different ways to
+ instrument user applications and the Linux kernel for LTTng tracing.
++
+Instrumenting source code is essential to provide a meaningful
+source of events.
++
+Skip this section if you don't have a programming background.
+
+* ``**<<controlling-tracing,Tracing control>>**'' is divided into topics
+ which demonstrate how to use the vast array of features that
+ LTTng{nbsp}{revision} offers.
+
+* ``**<<reference,Reference>>**'' contains API reference tables.
+
+* ``**<<glossary,Glossary>>**'' is a specialized dictionary of terms
+ related to LTTng or to the field of software tracing.
+
+
+include::../common/convention.txt[]
+
+
+include::../common/acknowledgements.txt[]
+
+
+[[whats-new]]
+== What's new in LTTng{nbsp}{revision}?
+
+LTTng{nbsp}{revision} bears the name _Nordicité_, the product of a
+collaboration between https://champlibre.co/[Champ Libre] and
+https://champlibre.co/[Boréale]. This farmhouse IPA is brewed with
+https://en.wikipedia.org/wiki/Kveik[Kveik] yeast and Québec-grown
+barley, oats, and juniper branches. The result is a remarkable, fruity,
+hazy golden IPA that offers a balanced touch of resinous and woodsy
+bitterness.
+
+New features and changes in LTTng{nbsp}{revision}:
+
+General::
++
+* The LTTng trigger API of <<liblttng-ctl-lttng,`liblttng-ctl`>> now
+ offers the ``__event rule matches__'' condition (an <<event-rule,event
+ rule>> matches an event) as well as the following new actions:
++
+--
+* <<basic-tracing-session-control,Start or stop>> a recording session.
+* <<session-rotation,Archive the current trace chunk>> of a
+ recording session (rotate).
+* <<taking-a-snapshot,Take a snapshot>> of a recording session.
+--
++
+As a reminder, a <<trigger,trigger>> is a condition-actions pair. When
+the condition of a trigger is satisfied, LTTng attempts to execute its
+actions.
++
+This feature is also available with the new man:lttng-add-trigger(1),
+man:lttng-remove-trigger(1), and man:lttng-list-triggers(1)
+<<lttng-cli,cmd:lttng>> commands.
++
+Starting from LTTng{nbsp}{revision}, a trigger may have more than one
+action.
++
+See “<<add-event-rule-matches-trigger,Add an ``event rule matches''
+trigger to a session daemon>>” to learn more.
+
+* The LTTng <<lttng-ust,user space>> and <<lttng-modules,kernel>>
+ tracers offer the new namespace context field `time_ns`, which is the
+ inode number, in the proc file system, of the current clock namespace.
++
+See man:lttng-add-context(1), man:lttng-ust(3), and
+man:time_namespaces(7).
+
+* The link:/man[manual pages] of LTTng-tools now have a terminology and
+ style which match the LTTng Documentation, many fixes, more internal
+ and manual page links, clearer lists and procedures, superior
+ consistency, and usage examples.
++
+The new man:lttng-event-rule(7) manual page explains the new, common
+way to specify an event rule on the command line.
++
+The new man:lttng-concepts(7) manual page explains the core concepts of
+LTTng. Its contents is essentially the ``<<core-concepts,Core
+concepts>>'' section of this documentation, but more adapted to the
+manual page style.
+
+User space tracing::
++
+[IMPORTANT]
+====
+The major version part of the `liblttng-ust`
+https://en.wikipedia.org/wiki/Soname[soname] is bumped, which means you
+**must recompile** your instrumented applications/libraries and
+<<tracepoint-provider,tracepoint provider packages>> to use
+LTTng-UST{nbsp}{revision}.
+
+This change became a necessity to clean up the library and for
+`liblttng-ust` to stop exporting private symbols.
+
+Also, LTTng{nbsp}{revision} prepends the `lttng_ust_` and `LTTNG_UST_`
+prefix to all public macro/definition/function names to offer a
+consistent API namespace. The LTTng{nbsp}2.12 API is still available;
+see the ``Compatibility with previous APIs'' section of
+man:lttng-ust(3).
+====
++
+Other notable changes:
++
+* The `liblttng-ust` C{nbsp}API offers the new man:lttng_ust_vtracef(3)
+ and man:lttng_ust_vtracelog(3) macros which are to
+ man:lttng_ust_tracef(3) and man:lttng_ust_tracelog(3) what
+ man:vprintf(3) is to man:printf(3).
+
+* LTTng-UST now only depends on https://liburcu.org/[`liburcu`] at build
+ time, not at run time.
+
+Kernel tracing::
++
+* The preferred display base of event record integer fields which
+ contain memory addresses is now hexadecimal instead of decimal.
+
+* The `pid` field is removed from `lttng_statedump_file_descriptor`
+ event records and the `file_table_address` field is added.
++
+This new field is the address of the `files_struct` structure which
+contains the file descriptor.
++
+See the
+``https://github.com/lttng/lttng-modules/commit/e7a0ca7205fd4be7c829d171baa8823fe4784c90[statedump: introduce `file_table_address`]''
+patch to learn more.
+
+* The `flags` field of `syscall_entry_clone` event records is now a
+ structure containing two enumerations (exit signal and options).
++
+This change makes the flag values more readable and meaningful.
++
+See the
+``https://github.com/lttng/lttng-modules/commit/d775625e2ba4825b73b5897e7701ad6e2bdba115[syscalls: Make `clone()`'s `flags` field a 2 enum struct]''
+patch to learn more.
+
+* The memory footprint of the kernel tracer is improved: the latter only
+ generates metadata for the specific system call recording event rules
+ that you <<enabling-disabling-events,create>>.
+
+
+[[nuts-and-bolts]]
+== Nuts and bolts
+
+What is LTTng? As its name suggests, the _Linux Trace Toolkit: next
+generation_ is a modern toolkit for tracing Linux systems and
+applications. So your first question might be:
+**what is tracing?**
+
+
+[[what-is-tracing]]
+=== What is tracing?
+
+As the history of software engineering progressed and led to what
+we now take for granted--complex, numerous and
+interdependent software applications running in parallel on
+sophisticated operating systems like Linux--the authors of such
+components, software developers, began feeling a natural
+urge to have tools that would ensure the robustness and good performance
+of their masterpieces.
+
+One major achievement in this field is, inarguably, the
+https://www.gnu.org/software/gdb/[GNU debugger (GDB)],
+an essential tool for developers to find and fix bugs. But even the best
+debugger won't help make your software run faster, and nowadays, faster
+software means either more work done by the same hardware, or cheaper
+hardware for the same work.
+
+A _profiler_ is often the tool of choice to identify performance
+bottlenecks. Profiling is suitable to identify _where_ performance is
+lost in a given piece of software. The profiler outputs a profile, a
+statistical summary of observed events, which you may use to discover
+which functions took the most time to execute. However, a profiler won't
+report _why_ some identified functions are the bottleneck. Bottlenecks
+might only occur when specific conditions are met, conditions that are
+sometimes impossible to capture by a statistical profiler, or impossible
+to reproduce with an application altered by the overhead of an
+event-based profiler. For a thorough investigation of software
+performance issues, a history of execution is essential, with the
+recorded values of variables and context fields you choose, and with as
+little influence as possible on the instrumented application. This is
+where tracing comes in handy.
+
+_Tracing_ is a technique used to understand what goes on in a running
+software system. The piece of software used for tracing is called a
+_tracer_, which is conceptually similar to a tape recorder. When
+recording, specific instrumentation points placed in the software source
+code generate events that are saved on a giant tape: a _trace_ file. You
+can record user application and operating system events at the same
+time, opening the possibility of resolving a wide range of problems that
+would otherwise be extremely challenging.
+
+Tracing is often compared to _logging_. However, tracers and loggers are
+two different tools, serving two different purposes. Tracers are
+designed to record much lower-level events that occur much more
+frequently than log messages, often in the range of thousands per
+second, with very little execution overhead. Logging is more appropriate
+for a very high-level analysis of less frequent events: user accesses,
+exceptional conditions (errors and warnings, for example), database
+transactions, instant messaging communications, and such. Simply put,
+logging is one of the many use cases that can be satisfied with tracing.
+
+The list of recorded events inside a trace file can be read manually
+like a log file for the maximum level of detail, but it's generally
+much more interesting to perform application-specific analyses to
+produce reduced statistics and graphs that are useful to resolve a
+given problem. Trace viewers and analyzers are specialized tools
+designed to do this.
+
+In the end, this is what LTTng is: a powerful, open source set of
+tools to trace the Linux kernel and user applications at the same time.
+LTTng is composed of several components actively maintained and
+developed by its link:/community/#where[community].
+
+
+[[lttng-alternatives]]
+=== Alternatives to noch:{LTTng}
+
+Excluding proprietary solutions, a few competing software tracers
+exist for Linux:
+
+https://github.com/dtrace4linux/linux[dtrace4linux]::
+ A port of Sun Microsystems' DTrace to Linux.
++
+The cmd:dtrace tool interprets user scripts and is responsible for
+loading code into the Linux kernel for further execution and collecting
+the outputted data.
+
+https://en.wikipedia.org/wiki/Berkeley_Packet_Filter[eBPF]::
+ A subsystem in the Linux kernel in which a virtual machine can
+ execute programs passed from the user space to the kernel.
++
+You can attach such programs to tracepoints and kprobes thanks to a
+system call, and they can output data to the user space when executed
+thanks to different mechanisms (pipe, VM register values, and eBPF maps,
+to name a few).
+
+https://www.kernel.org/doc/Documentation/trace/ftrace.txt[ftrace]::
+ The de facto function tracer of the Linux kernel.
++
+Its user interface is a set of special files in sysfs.
+
+https://perf.wiki.kernel.org/[perf]::
+ A performance analysis tool for Linux which supports hardware
+ performance counters, tracepoints, as well as other counters and
+ types of probes.
++
+The controlling utility of perf is the cmd:perf command line/text UI
+tool.
+
+https://linux.die.net/man/1/strace[strace]::
+ A command-line utility which records system calls made by a
+ user process, as well as signal deliveries and changes of process
+ state.
++
+strace makes use of https://en.wikipedia.org/wiki/Ptrace[ptrace] to
+fulfill its function.
+
+https://www.sysdig.org/[sysdig]::
+ Like SystemTap, uses scripts to analyze Linux kernel events.
++
+You write scripts, or _chisels_ in the jargon of sysdig, in Lua and
+sysdig executes them while it traces the system or afterwards. The
+interface of sysdig is the cmd:sysdig command-line tool as well as the
+text UI-based cmd:csysdig tool.
+
+https://sourceware.org/systemtap/[SystemTap]::
+ A Linux kernel and user space tracer which uses custom user scripts
+ to produce plain text traces.
++
+SystemTap converts the scripts to the C language, and then compiles them
+as Linux kernel modules which are loaded to produce trace data. The
+primary user interface of SystemTap is the cmd:stap command-line tool.
+
+The main distinctive features of LTTng is that it produces correlated
+kernel and user space traces, as well as doing so with the lowest
+overhead amongst other solutions. It produces trace files in the
+https://diamon.org/ctf[CTF] format, a file format optimized
+for the production and analyses of multi-gigabyte data.
+
+LTTng is the result of more than 10{nbsp}years of active open source
+development by a community of passionate developers. LTTng is currently
+available on major desktop and server Linux distributions.
+
+The main interface for tracing control is a single command-line tool
+named cmd:lttng. The latter can create several recording sessions, enable
+and disable recording event rules on the fly, filter events efficiently
+with custom user expressions, start and stop tracing, and much more.
+LTTng can write the traces on the file system or send them over the
+network, and keep them totally or partially. You can make LTTng execute
+user-defined actions when LTTng emits an event. You can view the traces
+once tracing becomes inactive or as LTTng records events.
+
+<<installing-lttng,Install LTTng now>> and
+<<getting-started,start tracing>>!
+
+
+[[installing-lttng]]
+== Installation
+
+**LTTng** is a set of software <<plumbing,components>> which interact to
+<<instrumenting,instrument>> the Linux kernel and user applications, and
+to <<controlling-tracing,control tracing>> (start and stop
+recording, create recording event rules, and the rest). Those
+components are bundled into the following packages:
+
+LTTng-tools::
+ Libraries and command-line interface to control tracing.
+
+LTTng-modules::
+ Linux kernel modules to instrument and trace the kernel.
+
+LTTng-UST::
+ Libraries and Java/Python packages to instrument and trace user
+ applications.
+
+Most distributions mark the LTTng-modules and LTTng-UST packages as
+optional when installing LTTng-tools (which is always required). In the
+following sections, we always provide the steps to install all three,
+but note that:
+
+* You only need to install LTTng-modules if you intend to use
+ the Linux kernel LTTng tracer.
+
+* You only need to install LTTng-UST if you intend to use the user
+ space LTTng tracer.
+
+[NOTE]
+====
+As of 10{nbsp}June{nbsp}2021, LTTng{nbsp}{revision} is not yet available
+in any major non-enterprise Linux distribution.
+
+For https://www.redhat.com/[RHEL] and https://www.suse.com/[SLES]
+packages, see https://packages.efficios.com/[EfficiOS Enterprise
+Packages].
+
+For other distributions, <<building-from-source,build LTTng from
+source>>.
+====
+
+
+[[building-from-source]]
+=== Build from source
+
+To build and install LTTng{nbsp}{revision} from source:
+
+. Using the package manager of your distribution, or from source,
+ install the following dependencies of LTTng-tools and LTTng-UST:
++
+--
+* https://sourceforge.net/projects/libuuid/[libuuid]
+* https://directory.fsf.org/wiki/Popt[popt]
+* https://liburcu.org/[Userspace RCU]
+* http://www.xmlsoft.org/[libxml2]
+* **Optional**: https://github.com/numactl/numactl[numactl]
+--
+
+. Download, build, and install the latest LTTng-modules{nbsp}{revision}:
++
+--
+[role="term"]
+----
+$ cd $(mktemp -d) &&
+ wget https://lttng.org/files/lttng-modules/lttng-modules-latest-2.13.tar.bz2 &&
+ tar -xf lttng-modules-latest-2.13.tar.bz2 &&
+ cd lttng-modules-2.13.* &&
+ make &&
+ sudo make modules_install &&
+ sudo depmod -a
+----
+--
+
+. Download, build, and install the latest LTTng-UST{nbsp}{revision}:
++
+--
+[role="term"]
+----
+$ cd $(mktemp -d) &&
+ wget https://lttng.org/files/lttng-ust/lttng-ust-latest-2.13.tar.bz2 &&
+ tar -xf lttng-ust-latest-2.13.tar.bz2 &&
+ cd lttng-ust-2.13.* &&
+ ./configure &&
+ make &&
+ sudo make install &&
+ sudo ldconfig
+----
+--
++
+Add `--disable-numa` to `./configure` if you don't have
+https://github.com/numactl/numactl[numactl].
++
+--
+[IMPORTANT]
+.Java and Python application tracing
+====
+If you need to instrument and have LTTng trace <<java-application,Java
+applications>>, pass the `--enable-java-agent-jul`,
+`--enable-java-agent-log4j`, or `--enable-java-agent-all` options to the
+`configure` script, depending on which Java logging framework you use.
+
+If you need to instrument and have LTTng trace
+<<python-application,Python applications>>, pass the
+`--enable-python-agent` option to the `configure` script. You can set
+the env:PYTHON environment variable to the path to the Python interpreter
+for which to install the LTTng-UST Python agent package.
+====
+--
++
+--
+[NOTE]
+====
+By default, LTTng-UST libraries are installed to
+dir:{/usr/local/lib}, which is the de facto directory in which to
+keep self-compiled and third-party libraries.
+
+When <<building-tracepoint-providers-and-user-application,linking an
+instrumented user application with `liblttng-ust`>>:
+
+* Append `/usr/local/lib` to the env:LD_LIBRARY_PATH environment
+ variable.
+
+* Pass the `-L/usr/local/lib` and `-Wl,-rpath,/usr/local/lib` options to
+ man:gcc(1), man:g++(1), or man:clang(1).
+====
+--
+
+. Download, build, and install the latest LTTng-tools{nbsp}{revision}:
++
+--
+[role="term"]
+----
+$ cd $(mktemp -d) &&
+ wget https://lttng.org/files/lttng-tools/lttng-tools-latest-2.13.tar.bz2 &&
+ tar -xf lttng-tools-latest-2.13.tar.bz2 &&
+ cd lttng-tools-2.13.* &&
+ ./configure &&
+ make &&
+ sudo make install &&
+ sudo ldconfig
+----
+--
+
+TIP: The https://github.com/eepp/vlttng[vlttng tool] can do all the
+previous steps automatically for a given version of LTTng and confine
+the installed files to a specific directory. This can be useful to try
+LTTng without installing it on your system.
+
+
+[[getting-started]]
+== Quick start
+
+This is a short guide to get started quickly with LTTng kernel and user
+space tracing.
+
+Before you follow this guide, make sure to <<installing-lttng,install>>
+LTTng.
+
+This tutorial walks you through the steps to:
+
+. <<tracing-the-linux-kernel,Record Linux kernel events>>.
+
+. <<tracing-your-own-user-application,Record the events of a user
+ application>> written in C.
+
+. <<viewing-and-analyzing-your-traces,View and analyze the
+ recorded events>>.
+
+
+[[tracing-the-linux-kernel]]
+=== Record Linux kernel events
+
+NOTE: The following command lines start with the `#` prompt because you
+need root privileges to control the Linux kernel LTTng tracer. You can
+also control the kernel tracer as a regular user if your Unix user is a
+member of the <<tracing-group,tracing group>>.
+
+. Create a <<tracing-session,recording session>> to write LTTng traces
+ to dir:{/tmp/my-kernel-trace}:
++
+--
+[role="term"]
+----
+# lttng create my-kernel-session --output=/tmp/my-kernel-trace
+----
+--
+
+. List the available kernel tracepoints and system calls:
++
+--
+[role="term"]
+----
+# lttng list --kernel
+# lttng list --kernel --syscall
+----
+--
+
+. Create <<event,recording event rules>> which match events having
+ the desired names, for example the `sched_switch` and
+ `sched_process_fork` tracepoints, and the man:open(2) and man:close(2)
+ system calls:
++
+--
+[role="term"]
+----
+# lttng enable-event --kernel sched_switch,sched_process_fork
+# lttng enable-event --kernel --syscall open,close
+----
+--
++
+Create a recording event rule which matches _all_ the Linux kernel
+tracepoint events with the opt:lttng-enable-event(1):--all option
+(recording with such a recording event rule generates a lot of data):
++
+--
+[role="term"]
+----
+# lttng enable-event --kernel --all
+----
+--
+
+. <<basic-tracing-session-control,Start recording>>:
++
+--
+[role="term"]
+----
+# lttng start
+----
+--
+
+. Do some operation on your system for a few seconds. For example,
+ load a website, or list the files of a directory.
+
+. <<creating-destroying-tracing-sessions,Destroy>> the current
+ recording session:
++
+--
+[role="term"]
+----
+# lttng destroy
+----
+--
++
+The man:lttng-destroy(1) command doesn't destroy the trace data; it
+only destroys the state of the recording session.
++
+The man:lttng-destroy(1) command also runs the man:lttng-stop(1) command
+implicitly (see ``<<basic-tracing-session-control,Start and stop a
+recording session>>''). You need to stop recording to make LTTng flush
+the remaining trace data and make the trace readable.
+
+. For the sake of this example, make the recorded trace accessible to
+ the non-root users:
++
+--
+[role="term"]
+----
+# chown -R $(whoami) /tmp/my-kernel-trace
+----
+--
+
+See ``<<viewing-and-analyzing-your-traces,View and analyze the
+recorded events>>'' to view the recorded events.
+
+
+[[tracing-your-own-user-application]]
+=== Record user application events
+
+This section walks you through a simple example to record the events of
+a _Hello world_ program written in{nbsp}C.
+
+To create the traceable user application:
+
+. Create the tracepoint provider header file, which defines the
+ tracepoints and the events they can generate:
++
+--
+[source,c]
+.path:{hello-tp.h}
+----
+#undef LTTNG_UST_TRACEPOINT_PROVIDER
+#define LTTNG_UST_TRACEPOINT_PROVIDER hello_world
+
+#undef LTTNG_UST_TRACEPOINT_INCLUDE
+#define LTTNG_UST_TRACEPOINT_INCLUDE "./hello-tp.h"
+
+#if !defined(_HELLO_TP_H) || defined(LTTNG_UST_TRACEPOINT_HEADER_MULTI_READ)
+#define _HELLO_TP_H
+
+#include <lttng/tracepoint.h>
+
+LTTNG_UST_TRACEPOINT_EVENT(
+ hello_world,
+ my_first_tracepoint,
+ LTTNG_UST_TP_ARGS(
+ int, my_integer_arg,
+ char *, my_string_arg
+ ),
+ LTTNG_UST_TP_FIELDS(
+ lttng_ust_field_string(my_string_field, my_string_arg)
+ lttng_ust_field_integer(int, my_integer_field, my_integer_arg)
+ )
+)
+
+#endif /* _HELLO_TP_H */
+
+#include <lttng/tracepoint-event.h>
+----
+--
+
+. Create the tracepoint provider package source file:
++
+--
+[source,c]
+.path:{hello-tp.c}
+----
+#define LTTNG_UST_TRACEPOINT_CREATE_PROBES
+#define LTTNG_UST_TRACEPOINT_DEFINE
+
+#include "hello-tp.h"
+----
+--
+
+. Build the tracepoint provider package:
++
+--
+[role="term"]
+----
+$ gcc -c -I. hello-tp.c
+----
+--
+
+. Create the _Hello World_ application source file:
++
+--
+[source,c]
+.path:{hello.c}
+----
+#include <stdio.h>
+#include "hello-tp.h"
+
+int main(int argc, char *argv[])
+{
+ unsigned int i;
+
+ puts("Hello, World!\nPress Enter to continue...");
+
+ /*
+ * The following getchar() call only exists for the purpose of this
+ * demonstration, to pause the application in order for you to have
+ * time to list its tracepoints. You don't need it otherwise.
+ */
+ getchar();
+
+ /*
+ * An lttng_ust_tracepoint() call.
+ *
+ * Arguments, as defined in `hello-tp.h`:
+ *
+ * 1. Tracepoint provider name (required)
+ * 2. Tracepoint name (required)
+ * 3. `my_integer_arg` (first user-defined argument)
+ * 4. `my_string_arg` (second user-defined argument)
+ *
+ * Notice the tracepoint provider and tracepoint names are
+ * C identifiers, NOT strings: they're in fact parts of variables
+ * that the macros in `hello-tp.h` create.
+ */
+ lttng_ust_tracepoint(hello_world, my_first_tracepoint, 23,
+ "hi there!");
+
+ for (i = 0; i < argc; i++) {
+ lttng_ust_tracepoint(hello_world, my_first_tracepoint,
+ i, argv[i]);
+ }
+
+ puts("Quitting now!");
+ lttng_ust_tracepoint(hello_world, my_first_tracepoint,
+ i * i, "i^2");
+ return 0;
+}
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -c hello.c
+----
+--
+
+. Link the application with the tracepoint provider package,
+ `liblttng-ust` and `libdl`:
++
+--
+[role="term"]
+----
+$ gcc -o hello hello.o hello-tp.o -llttng-ust -ldl
+----
+--
+
+Here's the whole build process:
+
+[role="img-100"]
+.Build steps of the user space tracing tutorial.
+image::ust-flow.png[]
+
+To record the events of the user application:
+
+. Run the application with a few arguments:
++
+--
+[role="term"]
+----
+$ ./hello world and beyond
+----
+--
++
+You see:
++
+--
+----
+Hello, World!
+Press Enter to continue...
+----
+--
+
+. Start an LTTng <<lttng-sessiond,session daemon>>:
++
+--
+[role="term"]
+----
+$ lttng-sessiond --daemonize
+----
+--
++
+NOTE: A session daemon might already be running, for example as a
+service that the service manager of your distribution started.
+
+. List the available user space tracepoints:
++
+--
+[role="term"]
+----
+$ lttng list --userspace
+----
+--
++
+You see the `hello_world:my_first_tracepoint` tracepoint listed
+under the `./hello` process.
+
+. Create a <<tracing-session,recording session>>:
++
+--
+[role="term"]
+----
+$ lttng create my-user-space-session
+----
+--
+
+. Create a <<event,recording event rule>> which matches user space
+ tracepoint events named `hello_world:my_first_tracepoint`:
++
+--
+[role="term"]
+----
+$ lttng enable-event --userspace hello_world:my_first_tracepoint
+----
+--
+
+. <<basic-tracing-session-control,Start recording>>:
++
+--
+[role="term"]
+----
+$ lttng start
+----
+--
+
+. Go back to the running `hello` application and press **Enter**.
++
+The program executes all `lttng_ust_tracepoint()` instrumentation
+points, emitting events as the event rule you created in step{nbsp}5
+matches them, and
+exits.
+
+. <<creating-destroying-tracing-sessions,Destroy>> the current
+ recording session:
++
+--
+[role="term"]
+----
+$ lttng destroy
+----
+--
++
+The man:lttng-destroy(1) command doesn't destroy the trace data; it
+only destroys the state of the recording session.
++
+The man:lttng-destroy(1) command also runs the man:lttng-stop(1) command
+implicitly (see ``<<basic-tracing-session-control,Start and stop a
+recording session>>''). You need to stop recording to make LTTng flush
+the remaining trace data and make the trace readable.
+
+By default, LTTng saves the traces to the
++$LTTNG_HOME/lttng-traces/__NAME__-__DATE__-__TIME__+ directory, where
++__NAME__+ is the recording session name. The env:LTTNG_HOME environment
+variable defaults to `$HOME` if not set.
+
+
+[[viewing-and-analyzing-your-traces]]
+=== View and analyze the recorded events
+
+Once you have completed the <<tracing-the-linux-kernel,Record Linux
+kernel events>> and <<tracing-your-own-user-application,Record user
+application events>> tutorials, you can inspect the recorded events.
+
+There are many tools you can use to read LTTng traces:
+
+https://babeltrace.org/[Babeltrace{nbsp}2]::
+ A rich, flexible trace manipulation toolkit which includes
+ a versatile command-line interface
+ (man:babeltrace2(1)),
+ a https://babeltrace.org/docs/v2.0/libbabeltrace2/[C{nbsp}library],
+ and https://babeltrace.org/docs/v2.0/python/bt2/[Python{nbsp}3 bindings]
+ so that you can easily process or convert an LTTng trace with
+ your own script.
++
+The Babeltrace{nbsp}2 project ships with a plugin
+(man:babeltrace2-plugin-ctf(7)) which supports the format of the traces
+which LTTng produces, https://diamon.org/ctf/[CTF].
+
+http://tracecompass.org/[Trace Compass]::
+ A graphical user interface for viewing and analyzing any type of
+ logs or traces, including those of LTTng.
+
+https://github.com/lttng/lttng-analyses[LTTng analyses]::
+ An experimental project which includes many high-level analyses of
+ LTTng kernel traces, like scheduling statistics, interrupt
+ frequency distribution, top CPU usage, and more.
+
+NOTE: This section assumes that LTTng wrote the traces it recorded
+during the previous tutorials to their default location, in the
+dir:{$LTTNG_HOME/lttng-traces} directory. The env:LTTNG_HOME
+environment variable defaults to `$HOME` if not set.
+
+
+[[viewing-and-analyzing-your-traces-bt]]
+==== Use the cmd:babeltrace2 command-line tool
+
+The simplest way to list all the recorded events of an LTTng trace is to
+pass its path to man:babeltrace2(1), without options:
+
+[role="term"]
+----
+$ babeltrace2 ~/lttng-traces/my-user-space-session*
+----
+
+The cmd:babeltrace2 command finds all traces recursively within the
+given path and prints all their events, sorting them chronologically.
+
+Pipe the output of cmd:babeltrace2 into a tool like man:grep(1) for
+further filtering:
+
+[role="term"]
+----
+$ babeltrace2 /tmp/my-kernel-trace | grep _switch
+----
+
+Pipe the output of cmd:babeltrace2 into a tool like man:wc(1) to count
+the recorded events:
+
+[role="term"]
+----
+$ babeltrace2 /tmp/my-kernel-trace | grep _open | wc --lines
+----
+
+
+[[viewing-and-analyzing-your-traces-bt-python]]
+==== Use the Babeltrace{nbsp}2 Python bindings
+
+The <<viewing-and-analyzing-your-traces-bt,text output of
+cmd:babeltrace2>> is useful to isolate event records by simple matching
+using man:grep(1) and similar utilities. However, more elaborate
+filters, such as keeping only event records with a field value falling
+within a specific range, are not trivial to write using a shell.
+Moreover, reductions and even the most basic computations involving
+multiple event records are virtually impossible to implement.
+
+Fortunately, Babeltrace{nbsp}2 ships with
+https://babeltrace.org/docs/v2.0/python/bt2/[Python{nbsp}3 bindings]
+which make it easy to read the event records of an LTTng trace
+sequentially and compute the desired information.
+
+The following script accepts an LTTng Linux kernel trace path as its
+first argument and prints the short names of the top five running
+processes on CPU{nbsp}0 during the whole trace:
+
+[source,python]
+.path:{top5proc.py}
+----
+import bt2
+import sys
+import collections
+
+
+def top5proc():
+ # Get the trace path from the first command-line argument
+ it = bt2.TraceCollectionMessageIterator(sys.argv[1])
+
+ # This counter dictionary will hold execution times:
+ #
+ # Task command name -> Total execution time (ns)
+ exec_times = collections.Counter()
+
+ # This holds the last `sched_switch` timestamp
+ last_ts = None
+
+ for msg in it:
+ # We only care about event messages
+ if type(msg) is not bt2._EventMessageConst:
+ continue
+
+ # Event of the event message
+ event = msg.event
+
+ # Keep only `sched_switch` events
+ if event.cls.name != 'sched_switch':
+ continue
+
+ # Keep only records of events which LTTng emitted from CPU 0
+ if event.packet.context_field['cpu_id'] != 0:
+ continue
+
+ # Event timestamp (ns)
+ cur_ts = msg.default_clock_snapshot.ns_from_origin
+
+ if last_ts is None:
+ # Start here
+ last_ts = cur_ts
+
+ # (Short) name of the previous task command
+ prev_comm = str(event.payload_field['prev_comm'])
+
+ # Initialize an entry in our dictionary if not done yet
+ if prev_comm not in exec_times:
+ exec_times[prev_comm] = 0
+
+ # Compute previous command execution time
+ diff = cur_ts - last_ts
+
+ # Update execution time of this command
+ exec_times[prev_comm] += diff
+
+ # Update last timestamp
+ last_ts = cur_ts
+
+ # Print top 5
+ for name, ns in exec_times.most_common(5):
+ print('{:20}{} s'.format(name, ns / 1e9))
+
+
+if __name__ == '__main__':
+ top5proc()
+----
+
+Run this script:
+
+[role="term"]
+----
+$ python3 top5proc.py /tmp/my-kernel-trace/kernel
+----
+
+Output example:
+
+----
+swapper/0 48.607245889 s
+chromium 7.192738188 s
+pavucontrol 0.709894415 s
+Compositor 0.660867933 s
+Xorg.bin 0.616753786 s
+----
+
+Note that `swapper/0` is the ``idle'' process of CPU{nbsp}0 on Linux;
+since we weren't using the CPU that much when recording, its first
+position in the list makes sense.
+
+
+[[core-concepts]]
+== [[understanding-lttng]]Core concepts
+
+From a user's perspective, the LTTng system is built on a few concepts,
+or objects, on which the <<lttng-cli,cmd:lttng command-line tool>>
+operates by sending commands to the <<lttng-sessiond,session daemon>>
+(through <<liblttng-ctl-lttng,`liblttng-ctl`>>).
+
+Understanding how those objects relate to each other is key to master
+the toolkit.
+
+The core concepts of LTTng are:
+
+* <<"event-rule","Instrumentation point, event rule, and event">>
+* <<trigger,Trigger>>
+* <<tracing-session,Recording session>>
+* <<domain,Tracing domain>>
+* <<channel,Channel and ring buffer>>
+* <<event,Recording event rule and event record>>
+
+NOTE: The man:lttng-concepts(7) manual page also documents the core
+concepts of LTTng, with more links to other LTTng-tools manual pages.
+
+
+[[event-rule]]
+=== Instrumentation point, event rule, and event
+
+An _instrumentation point_ is a point, within a piece of software,
+which, when executed, creates an LTTng _event_.
+
+LTTng offers various <<instrumentation-point-types,types of
+instrumentation>>.
+
+An _event rule_ is a set of conditions to match a set of events.
+
+When LTTng creates an event{nbsp}__E__, an event rule{nbsp}__ER__ is
+said to __match__{nbsp}__E__ when{nbsp}__E__ satisfies _all_ the
+conditions of{nbsp}__ER__. This concept is similar to a
+https://en.wikipedia.org/wiki/Regular_expression[regular expression]
+which matches a set of strings.
+
+When an event rule matches an event, LTTng _emits_ the event, therefore
+attempting to execute one or more actions.
+
+[IMPORTANT]
+====
+[[event-creation-emission-opti]]The event creation and emission
+processes are documentation concepts to help understand the journey from
+an instrumentation point to the execution of actions.
+
+The actual creation of an event can be costly because LTTng needs to
+evaluate the arguments of the instrumentation point.
+
+In practice, LTTng implements various optimizations for the Linux kernel
+and user space <<domain,tracing domains>> to avoid actually creating an
+event when the tracer knows, thanks to properties which are independent
+from the event payload and current context, that it would never emit
+such an event. Those properties are:
+
+* The <<instrumentation-point-types,instrumentation point type>>.
+
+* The instrumentation point name.
+
+* The instrumentation point log level.
+
+* For a <<event,recording event rule>>:
+** The status of the rule itself.
+** The status of the <<channel,channel>>.
+** The activity of the <<tracing-session,recording session>>.
+** Whether or not the process for which LTTng would create the event is
+ <<pid-tracking,allowed to record events>>.
+
+In other words: if, for a given instrumentation point{nbsp}__IP__, the
+LTTng tracer knows that it would never emit an event,
+executing{nbsp}__IP__ represents a simple boolean variable check and,
+for a Linux kernel recording event rule, a few process attribute checks.
+====
+
+As of LTTng{nbsp}{revision}, there are two places where you can find an
+event rule:
+
+<<event,Recording event rule>>::
+ A specific type of event rule of which the action is to record the
+ matched event as an event record.
++
+See ``<<enabling-disabling-events,Create and enable a recording event
+rule>>'' to learn more.
+
+``Event rule matches'' <<trigger,trigger>> condition (since LTTng{nbsp}2.13)::
+ When the event rule of the trigger condition matches an event, LTTng
+ can execute user-defined actions such as sending an LTTng
+ notification,
+ <<basic-tracing-session-control,starting a recording session>>,
+ and more.
++
+See “<<add-event-rule-matches-trigger,Add an ``event rule matches''
+trigger to a session daemon>>” to learn more.
+
+For LTTng to emit an event{nbsp}__E__,{nbsp}__E__ must satisfy _all_ the
+basic conditions of an event rule{nbsp}__ER__, that is:
+
+* The instrumentation point from which LTTng
+ creates{nbsp}__E__ has a specific
+ <<instrumentation-point-types,type>>.
+
+* A pattern matches the name of{nbsp}__E__ while another pattern
+ doesn't.
+
+* The log level of the instrumentation point from which LTTng
+ creates{nbsp}__E__ is at least as severe as some value, or is exactly
+ some value.
+
+* The fields of the payload of{nbsp}__E__ and the current context fields
+ satisfy a filter expression.
+
+A <<event,recording event rule>> has additional, implicit conditions to
+satisfy.
+
+
+[[instrumentation-point-types]]
+==== Instrumentation point types
+
+As of LTTng{nbsp}{revision}, the available instrumentation point
+types are, depending on the <<domain,tracing domain>>:
+
+Linux kernel::
+ LTTng tracepoint:::
+ A statically defined point in the source code of the kernel
+ image or of a kernel module using the
+ <<lttng-modules,LTTng-modules>> macros.
+
+ Linux kernel system call:::
+ Entry, exit, or both of a Linux kernel system call.
+
+ Linux https://www.kernel.org/doc/html/latest/trace/kprobes.html[kprobe]:::
+ A single probe dynamically placed in the compiled kernel code.
++
+When you create such an instrumentation point, you set its memory
+address or symbol name.
+
+ Linux user space probe:::
+ A single probe dynamically placed at the entry of a compiled
+ user space application/library function through the kernel.
++
+When you create such an instrumentation point, you set:
++
+--
+With the ELF method::
+ Its application/library path and its symbol name.
+
+With the USDT method::
+ Its application/library path, its provider name, and its probe name.
++
+``USDT'' stands for _SystemTap User-level Statically Defined Tracing_,
+a http://dtrace.org/blogs/about/[DTrace]-style marker.
+--
++
+As of LTTng{nbsp}{revision}, LTTng only supports USDT probes which
+are _not_ reference-counted.
+
+ Linux https://www.kernel.org/doc/html/latest/trace/kprobes.html[kretprobe]:::
+ Entry, exit, or both of a Linux kernel function.
++
+When you create such an instrumentation point, you set the memory
+address or symbol name of its function.
+
+User space::
+ LTTng tracepoint:::
+ A statically defined point in the source code of a C/$$C++$$
+ application/library using the
+ <<lttng-ust,LTTng-UST>> macros.
+
+`java.util.logging`, Apache log4j, and Python::
+ Java or Python logging statement:::
+ A method call on a Java or Python logger attached to an
+ LTTng-UST handler.
+
+See ``<<list-instrumentation-points,List the available instrumentation
+points>>'' to learn how to list available Linux kernel, user space, and
+logging instrumentation points.
+
+
+[[trigger]]
+=== Trigger
+
+A _trigger_ associates a condition to one or more actions.
+
+When the condition of a trigger is satisfied, LTTng attempts to execute
+its actions.
+
+As of LTTng{nbsp}{revision}, the available trigger conditions and
+actions are:
+
+Conditions::
++
+* The consumed buffer size of a given <<tracing-session,recording
+ session>> becomes greater than some value.
+
+* The buffer usage of a given <<channel,channel>> becomes greater than
+ some value.
+
+* The buffer usage of a given channel becomes less than some value.
+
+* There's an ongoing <<session-rotation,recording session rotation>>.
+
+* A recording session rotation becomes completed.
+
+* An <<add-event-rule-matches-trigger,event rule matches>> an event.
+
+Actions::
++
+* <<trigger-event-notif,Send a notification>> to a user application.
+* <<basic-tracing-session-control,Start>> a given recording session.
+* <<basic-tracing-session-control,Stop>> a given recording session.
+* <<session-rotation,Archive the current trace chunk>> of a given
+ recording session (rotate).
+* <<taking-a-snapshot,Take a snapshot>> of a given recording session.
+
+A trigger belongs to a <<lttng-sessiond,session daemon>>, not to a
+specific recording session. For a given session daemon, each Unix user has
+its own, private triggers. Note, however, that the `root` Unix user may,
+for the root session daemon:
+
+* Add a trigger as another Unix user.
+
+* List all the triggers, regardless of their owner.
+
+* Remove a trigger which belongs to another Unix user.
+
+For a given session daemon and Unix user, a trigger has a unique name.
+
+
+[[tracing-session]]
+=== Recording session
+
+A _recording session_ (named ``tracing session'' prior to
+LTTng{nbsp}2.13) is a stateful dialogue between you and a
+<<lttng-sessiond,session daemon>> for everything related to
+<<event,event recording>>.
+
+Everything that you do when you control LTTng tracers to record events
+happens within a recording session. In particular, a recording session:
+
+* Has its own name, unique for a given session daemon.
+
+* Has its own set of trace files, if any.
+
+* Has its own state of activity (started or stopped).
++
+An active recording session is an implicit <<event,recording event rule>>
+condition.
+
+* Has its own <<tracing-session-mode,mode>> (local, network streaming,
+ snapshot, or live).
+
+* Has its own <<channel,channels>> to which are attached their own
+ recording event rules.
+
+* Has its own <<pid-tracking,process attribute inclusion sets>>.
+
+[role="img-100"]
+.A _recording session_ contains <<channel,channels>> that are members of <<domain,tracing domains>> and contain <<event,recording event rules>>.
+image::concepts.png[]
+
+Those attributes and objects are completely isolated between different
+recording sessions.
+
+A recording session is like an
+https://en.wikipedia.org/wiki/Automated_teller_machine[ATM] session: the
+operations you do on the banking system through the ATM don't alter the
+data of other users of the same system. In the case of the ATM, a
+session lasts as long as your bank card is inside. In the case of LTTng,
+a recording session lasts from the man:lttng-create(1) command to the
+man:lttng-destroy(1) command.
+
+[role="img-100"]
+.Each Unix user has its own set of recording sessions.
+image::many-sessions.png[]
+
+A recording session belongs to a <<lttng-sessiond,session daemon>>. For a
+given session daemon, each Unix user has its own, private recording
+sessions. Note, however, that the `root` Unix user may operate on or
+destroy another user's recording session.
+
+
+[[tracing-session-mode]]
+==== Recording session mode
+
+LTTng offers four recording session modes:
+
+[[local-mode]]Local mode::
+ Write the trace data to the local file system.
+
+[[net-streaming-mode]]Network streaming mode::
+ Send the trace data over the network to a listening
+ <<lttng-relayd,relay daemon>>.
+
+[[snapshot-mode]]Snapshot mode::
+ Only write the trace data to the local file system or send it to a
+ listening relay daemon when LTTng <<taking-a-snapshot,takes a
+ snapshot>>.
++
+LTTng forces all the <<channel,channels>>
+to be created to be configured to be snapshot-ready.
++
+LTTng takes a snapshot of such a recording session when:
++
+--
+* You run the man:lttng-snapshot(1) command.
+
+* LTTng executes a `snapshot-session` <<trigger,trigger>> action.
+--
+
+[[live-mode]]Live mode::
+ Send the trace data over the network to a listening relay daemon
+ for <<lttng-live,live reading>>.
++
+An LTTng live reader (for example, man:babeltrace2(1)) can connect to
+the same relay daemon to receive trace data while the recording session is
+active.
+
+
+[[domain]]
+=== Tracing domain
+
+A _tracing domain_ identifies a type of LTTng tracer.
+
+A tracing domain has its own properties and features.
+
+There are currently five available tracing domains:
+
+* Linux kernel
+* User space
+* `java.util.logging` (JUL)
+* log4j
+* Python
+
+You must specify a tracing domain to target a type of LTTng tracer when
+using some <<lttng-cli,cmd:lttng>> commands to avoid ambiguity. For
+example, because the Linux kernel and user space tracing domains support
+named tracepoints as <<event-rule,instrumentation points>>, you need to
+specify a tracing domain when you <<enabling-disabling-events,create
+an event rule>> because both tracing domains could have tracepoints
+sharing the same name.
+
+You can create <<channel,channels>> in the Linux kernel and user space
+tracing domains. The other tracing domains have a single, default
+channel.
+
+
+[[channel]]
+=== Channel and ring buffer
+
+A _channel_ is an object which is responsible for a set of
+_ring buffers_.
+
+Each ring buffer is divided into multiple _sub-buffers_. When a
+<<event,recording event rule>>
+matches an event, LTTng can record it to one or more sub-buffers of one
+or more channels.
+
+When you <<enabling-disabling-channels,create a channel>>, you set its
+final attributes, that is:
+
+* Its <<channel-buffering-schemes,buffering scheme>>.
+
+* What to do <<channel-overwrite-mode-vs-discard-mode,when there's no
+ space left>> for a new event record because all sub-buffers are full.
+
+* The <<channel-subbuf-size-vs-subbuf-count,size of each ring buffer and
+ how many sub-buffers>> a ring buffer has.
+
+* The <<tracefile-rotation,size of each trace file LTTng writes for this
+ channel and the maximum count>> of trace files.
+
+* The periods of its <<channel-read-timer,read>>,
+ <<channel-switch-timer,switch>>, and <<channel-monitor-timer,monitor>>
+ timers.
+
+* For a Linux kernel channel: its output type.
++
+See the opt:lttng-enable-channel(1):--output option of the
+man:lttng-enable-channel(1) command.
+
+* For a user space channel: the value of its
+ <<blocking-timeout-example,blocking timeout>>.
+
+A channel is always associated to a <<domain,tracing domain>>. The
+`java.util.logging` (JUL), log4j, and Python tracing domains each have a
+default channel which you can't configure.
+
+A channel owns <<event,recording event rules>>.
+
+
+[[channel-buffering-schemes]]
+==== Buffering scheme
+
+A channel has at least one ring buffer _per CPU_. LTTng always records
+an event to the ring buffer dedicated to the CPU which emits it.
+
+The buffering scheme of a user space channel determines what has its own
+set of per-CPU ring buffers:
+
+Per-user buffering::
+ Allocate one set of ring buffers--one per CPU--shared by all the
+ instrumented processes of:
+ If your Unix user is `root`:::
+ Each Unix user.
++
+--
+[role="img-100"]
+.Per-user buffering scheme (recording session belongs to the `root` Unix user).
+image::per-user-buffering-root.png[]
+--
+
+ Otherwise:::
+ Your Unix user.
++
+--
+[role="img-100"]
+.Per-user buffering scheme (recording session belongs to the `Bob` Unix user).
+image::per-user-buffering.png[]
+--
+
+Per-process buffering::
+ Allocate one set of ring buffers--one per CPU--for each
+ instrumented process of:
+ If your Unix user is `root`:::
+ All Unix users.
++
+--
+[role="img-100"]
+.Per-process buffering scheme (recording session belongs to the `root` Unix user).
+image::per-process-buffering-root.png[]
+--
+
+ Otherwise:::
+ Your Unix user.
++
+--
+[role="img-100"]
+.Per-process buffering scheme (recording session belongs to the `Bob` Unix user).
+image::per-process-buffering.png[]
+--
+
+The per-process buffering scheme tends to consume more memory than the
+per-user option because systems generally have more instrumented
+processes than Unix users running instrumented processes. However, the
+per-process buffering scheme ensures that one process having a high
+event throughput won't fill all the shared sub-buffers of the same Unix
+user, only its own.
+
+The buffering scheme of a Linux kernel channel is always to allocate a
+single set of ring buffers for the whole system. This scheme is similar
+to the per-user option, but with a single, global user ``running'' the
+kernel.
+
+
+[[channel-overwrite-mode-vs-discard-mode]]
+==== Event record loss mode
+
+When LTTng emits an event, LTTng can record it to a specific, available
+sub-buffer within the ring buffers of specific channels. When there's no
+space left in a sub-buffer, the tracer marks it as consumable and
+another, available sub-buffer starts receiving the following event
+records. An LTTng <<lttng-consumerd,consumer daemon>> eventually
+consumes the marked sub-buffer, which returns to the available state.
+
+[NOTE]
+[role="docsvg-channel-subbuf-anim"]
+====
+{note-no-anim}
+====
+
+In an ideal world, sub-buffers are consumed faster than they're filled.
+In the real world, however, all sub-buffers can be full at some point,
+leaving no space to record the following events.
+
+In an ideal world, sub-buffers are consumed faster than they're filled,
+as it's the case in the previous animation. In the real world,
+however, all sub-buffers can be full at some point, leaving no space to
+record the following events.
+
+By default, <<lttng-modules,LTTng-modules>> and <<lttng-ust,LTTng-UST>>
+are _non-blocking_ tracers: when there's no available sub-buffer to
+record an event, it's acceptable to lose event records when the
+alternative would be to cause substantial delays in the execution of the
+instrumented application. LTTng privileges performance over integrity;
+it aims at perturbing the instrumented application as little as possible
+in order to make the detection of subtle race conditions and rare
+interrupt cascades possible.
+
+Since LTTng{nbsp}2.10, the LTTng user space tracer, LTTng-UST, supports
+a _blocking mode_. See the <<blocking-timeout-example,blocking timeout
+example>> to learn how to use the blocking mode.
+
+When it comes to losing event records because there's no available
+sub-buffer, or because the blocking timeout of
+the channel is reached, the _event record loss mode_ of the channel
+determines what to do. The available event record loss modes are:
+
+[[discard-mode]]Discard mode::
+ Drop the newest event records until a sub-buffer becomes available.
++
+This is the only available mode when you specify a blocking timeout.
++
+With this mode, LTTng increments a count of lost event records when an
+event record is lost and saves this count to the trace. A trace reader
+can use the saved discarded event record count of the trace to decide
+whether or not to perform some analysis even if trace data is known to
+be missing.
+
+[[overwrite-mode]]Overwrite mode::
+ Clear the sub-buffer containing the oldest event records and start
+ writing the newest event records there.
++
+This mode is sometimes called _flight recorder mode_ because it's
+similar to a https://en.wikipedia.org/wiki/Flight_recorder[flight
+recorder]: always keep a fixed amount of the latest data. It's also
+similar to the roll mode of an oscilloscope.
++
+Since LTTng{nbsp}2.8, with this mode, LTTng writes to a given sub-buffer
+its sequence number within its data stream. With a <<local-mode,local>>,
+<<net-streaming-mode,network streaming>>, or <<live-mode,live>> recording
+session, a trace reader can use such sequence numbers to report lost
+packets. A trace reader can use the saved discarded sub-buffer (packet)
+count of the trace to decide whether or not to perform some analysis
+even if trace data is known to be missing.
++
+With this mode, LTTng doesn't write to the trace the exact number of
+lost event records in the lost sub-buffers.
+
+Which mechanism you should choose depends on your context: prioritize
+the newest or the oldest event records in the ring buffer?
+
+Beware that, in overwrite mode, the tracer abandons a _whole sub-buffer_
+as soon as a there's no space left for a new event record, whereas in
+discard mode, the tracer only discards the event record that doesn't
+fit.
+
+There are a few ways to decrease your probability of losing event
+records. The ``<<channel-subbuf-size-vs-subbuf-count,Sub-buffer size and
+count>>'' section shows how to fine-tune the sub-buffer size and count
+of a channel to virtually stop losing event records, though at the cost
+of greater memory usage.
+
+
+[[channel-subbuf-size-vs-subbuf-count]]
+==== Sub-buffer size and count
+
+A channel has one or more ring buffer for each CPU of the target system.
+
+See the ``<<channel-buffering-schemes,Buffering scheme>>'' section to
+learn how many ring buffers of a given channel are dedicated to each CPU
+depending on its buffering scheme.
+
+Set the size of each sub-buffer the ring buffers of a channel contain
+and how many there are
+when you <<enabling-disabling-channels,create it>>.
+
+Note that LTTng switching the current sub-buffer of a ring buffer
+(marking a full one as consumable and switching to an available one for
+LTTng to record the next events) introduces noticeable CPU overhead.
+Knowing this, the following list presents a few practical situations
+along with how to configure the sub-buffer size and count for them:
+
+High event throughput::
+ In general, prefer large sub-buffers to lower the risk of losing
+ event records.
++
+Having larger sub-buffers also ensures a lower sub-buffer switching
+frequency.
++
+The sub-buffer count is only meaningful if you create the channel in
+<<overwrite-mode,overwrite mode>>: in this case, if LTTng overwrites a
+sub-buffer, then the other sub-buffers are left unaltered.
+
+Low event throughput::
+ In general, prefer smaller sub-buffers since the risk of losing
+ event records is low.
++
+Because LTTng emits events less frequently, the sub-buffer switching
+frequency should remain low and therefore the overhead of the tracer
+shouldn't be a problem.
+
+Low memory system::
+ If your target system has a low memory limit, prefer fewer first,
+ then smaller sub-buffers.
++
+Even if the system is limited in memory, you want to keep the
+sub-buffers as large as possible to avoid a high sub-buffer switching
+frequency.
+
+Note that LTTng uses https://diamon.org/ctf/[CTF] as its trace format,
+which means event record data is very compact. For example, the average
+LTTng kernel event record weights about 32{nbsp}bytes. Therefore, a
+sub-buffer size of 1{nbsp}MiB is considered large.
+
+The previous scenarios highlight the major trade-off between a few large
+sub-buffers and more, smaller sub-buffers: sub-buffer switching
+frequency vs. how many event records are lost in overwrite mode.
+Assuming a constant event throughput and using the overwrite mode, the
+two following configurations have the same ring buffer total size:
+
+[NOTE]
+[role="docsvg-channel-subbuf-size-vs-count-anim"]
+====
+{note-no-anim}
+====
+
+Two sub-buffers of 4{nbsp}MiB each::
+ Expect a very low sub-buffer switching frequency, but if LTTng
+ ever needs to overwrite a sub-buffer, half of the event records so
+ far (4{nbsp}MiB) are definitely lost.
+
+Eight sub-buffers of 1{nbsp}MiB each::
+ Expect four times the tracer overhead of the configuration above,
+ but if LTTng needs to overwrite a sub-buffer, only the eighth of
+ event records so far (1{nbsp}MiB) are definitely lost.
+
+In <<discard-mode,discard mode>>, the sub-buffer count parameter is
+pointless: use two sub-buffers and set their size according to your
+requirements.
+
+
+[[tracefile-rotation]]
+==== Maximum trace file size and count (trace file rotation)
+
+By default, trace files can grow as large as needed.
+
+Set the maximum size of each trace file that LTTng writes of a given
+channel when you <<enabling-disabling-channels,create it>>.
+
+When the size of a trace file reaches the fixed maximum size of the
+channel, LTTng creates another file to contain the next event records.
+LTTng appends a file count to each trace file name in this case.
+
+If you set the trace file size attribute when you create a channel, the
+maximum number of trace files that LTTng creates is _unlimited_ by
+default. To limit them, set a maximum number of trace files. When the
+number of trace files reaches the fixed maximum count of the channel,
+LTTng overwrites the oldest trace file. This mechanism is called _trace
+file rotation_.
+
+[IMPORTANT]
+====
+Even if you don't limit the trace file count, always assume that LTTng
+manages all the trace files of the recording session.
+
+In other words, there's no safe way to know if LTTng still holds a given
+trace file open with the trace file rotation feature.
+
+The only way to obtain an unmanaged, self-contained LTTng trace before
+you <<creating-destroying-tracing-sessions,destroy the recording session>>
+is with the <<session-rotation,recording session rotation>> feature, which
+is available since LTTng{nbsp}2.11.
+====
+
+
+[[channel-timers]]
+==== Timers
+
+Each channel can have up to three optional timers:
+
+[[channel-switch-timer]]Switch timer::
+ When this timer expires, a sub-buffer switch happens: for each ring
+ buffer of the channel, LTTng marks the current sub-buffer as
+ consumable and _switches_ to an available one to record the next
+ events.
++
+[NOTE]
+[role="docsvg-channel-switch-timer"]
+====
+{note-no-anim}
+====
++
+A switch timer is useful to ensure that LTTng consumes and commits trace
+data to trace files or to a distant <<lttng-relayd,relay daemon>>
+periodically in case of a low event throughput.
++
+Such a timer is also convenient when you use large
+<<channel-subbuf-size-vs-subbuf-count,sub-buffers>> to cope with a
+sporadic high event throughput, even if the throughput is otherwise low.
++
+Set the period of the switch timer of a channel when you
+<<enabling-disabling-channels,create it>> with
+the opt:lttng-enable-channel(1):--switch-timer option.
+
+[[channel-read-timer]]Read timer::
+ When this timer expires, LTTng checks for full, consumable
+ sub-buffers.
++
+By default, the LTTng tracers use an asynchronous message mechanism to
+signal a full sub-buffer so that a <<lttng-consumerd,consumer daemon>>
+can consume it.
++
+When such messages must be avoided, for example in real-time
+applications, use this timer instead.
++
+Set the period of the read timer of a channel when you
+<<enabling-disabling-channels,create it>> with the
+opt:lttng-enable-channel(1):--read-timer option.
+
+[[channel-monitor-timer]]Monitor timer::
+ When this timer expires, the consumer daemon samples some channel
+ statistics to evaluate the following <<trigger,trigger>>
+ conditions:
++
+--
+. The consumed buffer size of a given <<tracing-session,recording
+ session>> becomes greater than some value.
+. The buffer usage of a given channel becomes greater than some value.
+. The buffer usage of a given channel becomes less than some value.
+--
++
+If you disable the monitor timer of a channel{nbsp}__C__:
++
+--
+* The consumed buffer size value of the recording session of{nbsp}__C__
+ could be wrong for trigger condition type{nbsp}1: the consumed buffer
+ size of{nbsp}__C__ won't be part of the grand total.
+
+* The buffer usage trigger conditions (types{nbsp}2 and{nbsp}3)
+ for{nbsp}__C__ will never be satisfied.
+--
++
+Set the period of the monitor timer of a channel when you
+<<enabling-disabling-channels,create it>> with the
+opt:lttng-enable-channel(1):--monitor-timer option.
+
+
+[[event]]
+=== Recording event rule and event record
+
+A _recording event rule_ is a specific type of <<event-rule,event rule>>
+of which the action is to serialize and record the matched event as an
+_event record_.
+
+Set the explicit conditions of a recording event rule when you
+<<enabling-disabling-events,create it>>. A recording event rule also has
+the following implicit conditions:
+
+* The recording event rule itself is enabled.
++
+A recording event rule is enabled on creation.
+
+* The <<channel,channel>> to which the recording event rule is attached
+ is enabled.
++
+A channel is enabled on creation.
+
+* The <<tracing-session,recording session>> of the recording event rule is
+ <<basic-tracing-session-control,active>> (started).
++
+A recording session is inactive (stopped) on creation.
+
+* The process for which LTTng creates an event to match is
+ <<pid-tracking,allowed to record events>>.
++
+All processes are allowed to record events on recording session
+creation.
+
+You always attach a recording event rule to a channel, which belongs to
+a recording session, when you create it.
+
+When a recording event rule{nbsp}__ER__ matches an event{nbsp}__E__,
+LTTng attempts to serialize and record{nbsp}__E__ to one of the
+available sub-buffers of the channel to which{nbsp}__E__ is attached.
+
+When multiple matching recording event rules are attached to the same
+channel, LTTng attempts to serialize and record the matched event
+_once_. In the following example, the second recording event rule is
+redundant when both are enabled:
+
+[role="term"]
+----
+$ lttng enable-event --userspace hello:world
+$ lttng enable-event --userspace hello:world --loglevel=INFO
+----
+
+[role="img-100"]
+.Logical path from an instrumentation point to an event record.
+image::event-rule.png[]
+
+As of LTTng{nbsp}{revision}, you cannot remove a recording event
+rule: it exists as long as its recording session exists.
+
+
+[[plumbing]]
+== Components of noch:{LTTng}
+
+The second _T_ in _LTTng_ stands for _toolkit_: it would be wrong
+to call LTTng a simple _tool_ since it's composed of multiple
+interacting components.
+
+This section describes those components, explains their respective
+roles, and shows how they connect together to form the LTTng ecosystem.
+
+The following diagram shows how the most important components of LTTng
+interact with user applications, the Linux kernel, and you:
+
+[role="img-100"]
+.Control and trace data paths between LTTng components.
+image::plumbing.png[]
+
+The LTTng project integrates:
+
+LTTng-tools::
+ Libraries and command-line interface to control recording sessions:
++
+* <<lttng-sessiond,Session daemon>> (man:lttng-sessiond(8)).
+* <<lttng-consumerd,Consumer daemon>> (cmd:lttng-consumerd).
+* <<lttng-relayd,Relay daemon>> (man:lttng-relayd(8)).
+* <<liblttng-ctl-lttng,Tracing control library>> (`liblttng-ctl`).
+* <<lttng-cli,Tracing control command-line tool>> (man:lttng(1)).
+* <<persistent-memory-file-systems,`lttng-crash` command-line tool>>
+ (man:lttng-crash(1)).
+
+LTTng-UST::
+ Libraries and Java/Python packages to instrument and trace user
+ applications:
++
+* <<lttng-ust,User space tracing library>> (`liblttng-ust`) and its
+ headers to instrument and trace any native user application.
+* <<prebuilt-ust-helpers,Preloadable user space tracing helpers>>:
+** `liblttng-ust-libc-wrapper`
+** `liblttng-ust-pthread-wrapper`
+** `liblttng-ust-cyg-profile`
+** `liblttng-ust-cyg-profile-fast`
+** `liblttng-ust-dl`
+* <<lttng-ust-agents,LTTng-UST Java agent>> to instrument and trace
+ Java applications using `java.util.logging` or
+ Apache log4j{nbsp}1.2 logging.
+* <<lttng-ust-agents,LTTng-UST Python agent>> to instrument
+ Python applications using the standard `logging` package.
+
+LTTng-modules::
+ <<lttng-modules,Linux kernel modules>> to instrument and trace the
+ kernel:
++
+* LTTng kernel tracer module.
+* Recording ring buffer kernel modules.
+* Probe kernel modules.
+* LTTng logger kernel module.
+
+
+[[lttng-cli]]
+=== Tracing control command-line interface
+
+The _man:lttng(1) command-line tool_ is the standard user interface to
+control LTTng <<tracing-session,recording sessions>>.
+
+The cmd:lttng tool is part of LTTng-tools.
+
+The cmd:lttng tool is linked with
+<<liblttng-ctl-lttng,`liblttng-ctl`>> to communicate with
+one or more <<lttng-sessiond,session daemons>> behind the scenes.
+
+The cmd:lttng tool has a Git-like interface:
+
+[role="term"]
+----
+$ lttng [GENERAL OPTIONS] <COMMAND> [COMMAND OPTIONS]
+----
+
+The ``<<controlling-tracing,Tracing control>>'' section explores the
+available features of LTTng through its cmd:lttng tool.
+
+
+[[liblttng-ctl-lttng]]
+=== Tracing control library
+
+[role="img-100"]
+.The tracing control library.
+image::plumbing-liblttng-ctl.png[]
+
+The _LTTng control library_, `liblttng-ctl`, is used to communicate with
+a <<lttng-sessiond,session daemon>> using a C{nbsp}API that hides the
+underlying details of the protocol.
+
+`liblttng-ctl` is part of LTTng-tools.
+
+The <<lttng-cli,cmd:lttng command-line tool>> is linked with
+`liblttng-ctl`.
+
+Use `liblttng-ctl` in C or $$C++$$ source code by including its
+``master'' header:
+
+[source,c]
+----
+#include <lttng/lttng.h>
+----
+
+As of LTTng{nbsp}{revision}, the best available developer documentation
+for `liblttng-ctl` is its installed header files. Functions and
+structures are documented with header comments.
+
+
+[[lttng-ust]]
+=== User space tracing library
+
+[role="img-100"]
+.The user space tracing library.
+image::plumbing-liblttng-ust.png[]
+
+The _user space tracing library_, `liblttng-ust` (see man:lttng-ust(3)),
+is the LTTng user space tracer.
+
+`liblttng-ust` receives commands from a <<lttng-sessiond,session
+daemon>>, for example to allow specific instrumentation points to emit
+LTTng <<event-rule,events>>, and writes event records to <<channel,ring
+buffers>> shared with a <<lttng-consumerd,consumer daemon>>.
+
+`liblttng-ust` is part of LTTng-UST.
+
+`liblttng-ust` can also send asynchronous messages to the session daemon
+when it emits an event. This supports the ``event rule matches''
+<<trigger,trigger>> condition feature (see
+“<<add-event-rule-matches-trigger,Add an ``event rule matches'' trigger
+to a session daemon>>”).
+
+Public C{nbsp}header files are installed beside `liblttng-ust` to
+instrument any <<c-application,C or $$C++$$ application>>.
+
+<<lttng-ust-agents,LTTng-UST agents>>, which are regular Java and Python
+packages, use their own <<tracepoint-provider,tracepoint provider
+package>> which is linked with `liblttng-ust`.
+
+An application or library doesn't have to initialize `liblttng-ust`
+manually: its constructor does the necessary tasks to register the
+application to a session daemon. The initialization phase also
+configures instrumentation points depending on the <<event-rule,event
+rules>> that you already created.
+
+
+[[lttng-ust-agents]]
+=== User space tracing agents
+
+[role="img-100"]
+.The user space tracing agents.
+image::plumbing-lttng-ust-agents.png[]
+
+The _LTTng-UST Java and Python agents_ are regular Java and Python
+packages which add LTTng tracing capabilities to the
+native logging frameworks.
+
+The LTTng-UST agents are part of LTTng-UST.
+
+In the case of Java, the
+https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[`java.util.logging`
+core logging facilities] and
+https://logging.apache.org/log4j/1.2/[Apache log4j{nbsp}1.2] are supported.
+Note that Apache Log4j{nbsp}2 isn't supported.
+
+In the case of Python, the standard
+https://docs.python.org/3/library/logging.html[`logging`] package
+is supported. Both Python{nbsp}2 and Python{nbsp}3 modules can import the
+LTTng-UST Python agent package.
+
+The applications using the LTTng-UST agents are in the
+`java.util.logging` (JUL), log4j, and Python <<domain,tracing domains>>.
+
+Both agents use the same mechanism to convert log statements to LTTng
+events. When an agent initializes, it creates a log handler that
+attaches to the root logger. The agent also registers to a
+<<lttng-sessiond,session daemon>>. When the user application executes a
+log statement, the root logger passes it to the log handler of the
+agent. The custom log handler of the agent calls a native function in a
+tracepoint provider package shared library linked with
+<<lttng-ust,`liblttng-ust`>>, passing the formatted log message and
+other fields, like its logger name and its log level. This native
+function contains a user space instrumentation point, therefore tracing
+the log statement.
+
+The log level condition of a <<event,recording event rule>> is
+considered when tracing a Java or a Python application, and it's
+compatible with the standard `java.util.logging`, log4j, and Python log
+levels.
+
+
+[[lttng-modules]]
+=== LTTng kernel modules
+
+[role="img-100"]
+.The LTTng kernel modules.
+image::plumbing-lttng-modules.png[]
+
+The _LTTng kernel modules_ are a set of Linux kernel modules
+which implement the kernel tracer of the LTTng project.
+
+The LTTng kernel modules are part of LTTng-modules.
+
+The LTTng kernel modules include:
+
+* A set of _probe_ modules.
++
+Each module attaches to a specific subsystem
+of the Linux kernel using its tracepoint instrument points.
++
+There are also modules to attach to the entry and return points of the
+Linux system call functions.
+
+* _Ring buffer_ modules.
++
+A ring buffer implementation is provided as kernel modules. The LTTng
+kernel tracer writes to ring buffers; a
+<<lttng-consumerd,consumer daemon>> reads from ring buffers.
+
+* The _LTTng kernel tracer_ module.
+* The <<proc-lttng-logger-abi,_LTTng logger_>> module.
++
+The LTTng logger module implements the special path:{/proc/lttng-logger}
+(and path:{/dev/lttng-logger}, since LTTng{nbsp}2.11) files so that any
+executable can generate LTTng events by opening those files and
+writing to them.
+
+The LTTng kernel tracer can also send asynchronous messages to the
+<<lttng-sessiond,session daemon>> when it emits an event.
+This supports the ``event rule matches''
+<<trigger,trigger>> condition feature (see
+“<<add-event-rule-matches-trigger,Add an ``event rule matches'' trigger
+to a session daemon>>”).
+
+Generally, you don't have to load the LTTng kernel modules manually
+(using man:modprobe(8), for example): a root session daemon loads the
+necessary modules when starting. If you have extra probe modules, you
+can specify to load them to the session daemon on the command line
+(see the opt:lttng-sessiond(8):--extra-kmod-probes option).
+
+The LTTng kernel modules are installed in
++/usr/lib/modules/__release__/extra+ by default, where +__release__+ is
+the kernel release (output of `uname --kernel-release`).
+
+
+[[lttng-sessiond]]
+=== Session daemon
+
+[role="img-100"]
+.The session daemon.
+image::plumbing-sessiond.png[]
+
+The _session daemon_, man:lttng-sessiond(8), is a
+https://en.wikipedia.org/wiki/Daemon_(computing)[daemon] which:
+
+* Manages <<tracing-session,recording sessions>>.
+
+* Controls the various components (like tracers and
+ <<lttng-consumerd,consumer daemons>>) of LTTng.
+
+* Sends <<notif-trigger-api,asynchronous notifications>> to user
+ applications.
+
+The session daemon is part of LTTng-tools.
+
+The session daemon sends control requests to and receives control
+responses from:
+
+* The <<lttng-ust,user space tracing library>>.
++
+Any instance of the user space tracing library first registers to
+a session daemon. Then, the session daemon can send requests to
+this instance, such as:
++
+--
+** Get the list of tracepoints.
+** Share a <<event,recording event rule>> so that the user space tracing
+ library can decide whether or not a given tracepoint can emit events.
+ Amongst the possible conditions of a recording event rule is a filter
+ expression which `liblttng-ust` evaluates before it emits an event.
+** Share <<channel,channel>> attributes and ring buffer locations.
+--
++
+The session daemon and the user space tracing library use a Unix
+domain socket to communicate.
+
+* The <<lttng-ust-agents,user space tracing agents>>.
++
+Any instance of a user space tracing agent first registers to
+a session daemon. Then, the session daemon can send requests to
+this instance, such as:
++
+--
+** Get the list of loggers.
+** Enable or disable a specific logger.
+--
++
+The session daemon and the user space tracing agent use a TCP connection
+to communicate.
+
+* The <<lttng-modules,LTTng kernel tracer>>.
+* The <<lttng-consumerd,consumer daemon>>.
++
+The session daemon sends requests to the consumer daemon to instruct
+it where to send the trace data streams, amongst other information.
+
+* The <<lttng-relayd,relay daemon>>.
+
+The session daemon receives commands from the
+<<liblttng-ctl-lttng,tracing control library>>.
+
+The session daemon can receive asynchronous messages from the
+<<lttng-ust,user space>> and <<lttng-modules,kernel>> tracers
+when they emit events. This supports the ``event rule matches''
+<<trigger,trigger>> condition feature (see
+“<<add-event-rule-matches-trigger,Add an ``event rule matches'' trigger
+to a session daemon>>”).
+
+The root session daemon loads the appropriate
+<<lttng-modules,LTTng kernel modules>> on startup. It also spawns
+one or more <<lttng-consumerd,consumer daemons>> as soon as you create
+a <<event,recording event rule>>.
+
+The session daemon doesn't send and receive trace data: this is the
+role of the <<lttng-consumerd,consumer daemon>> and
+<<lttng-relayd,relay daemon>>. It does, however, generate the
+https://diamon.org/ctf/[CTF] metadata stream.
+
+Each Unix user can have its own session daemon instance. The
+recording sessions which different session daemons manage are completely
+independent.
+
+The root user's session daemon is the only one which is
+allowed to control the LTTng kernel tracer, and its spawned consumer
+daemon is the only one which is allowed to consume trace data from the
+LTTng kernel tracer. Note, however, that any Unix user which is a member
+of the <<tracing-group,tracing group>> is allowed
+to create <<channel,channels>> in the
+Linux kernel <<domain,tracing domain>>, and therefore to use the Linux
+kernel LTTng tracer.
+
+The <<lttng-cli,cmd:lttng command-line tool>> automatically starts a
+session daemon when using its `create` command if none is currently
+running. You can also start the session daemon manually.
+
+
+[[lttng-consumerd]]
+=== Consumer daemon
+
+[role="img-100"]
+.The consumer daemon.
+image::plumbing-consumerd.png[]
+
+The _consumer daemon_, cmd:lttng-consumerd, is a
+https://en.wikipedia.org/wiki/Daemon_(computing)[daemon] which shares
+ring buffers with user applications or with the LTTng kernel modules to
+collect trace data and send it to some location (file system or to a
+<<lttng-relayd,relay daemon>> over the network).
+
+The consumer daemon is part of LTTng-tools.
+
+You don't start a consumer daemon manually: a consumer daemon is always
+spawned by a <<lttng-sessiond,session daemon>> as soon as you create a
+<<event,recording event rule>>, that is, before you start recording. When
+you kill its owner session daemon, the consumer daemon also exits
+because it's the child process of the session daemon. Command-line
+options of man:lttng-sessiond(8) target the consumer daemon process.
+
+There are up to two running consumer daemons per Unix user, whereas only
+one session daemon can run per user. This is because each process can be
+either 32-bit or 64-bit: if the target system runs a mixture of 32-bit
+and 64-bit processes, it's more efficient to have separate
+corresponding 32-bit and 64-bit consumer daemons. The root user is an
+exception: it can have up to _three_ running consumer daemons: 32-bit
+and 64-bit instances for its user applications, and one more
+reserved for collecting kernel trace data.
+
+
+[[lttng-relayd]]
+=== Relay daemon
+
+[role="img-100"]
+.The relay daemon.
+image::plumbing-relayd.png[]
+
+The _relay daemon_, man:lttng-relayd(8), is a
+https://en.wikipedia.org/wiki/Daemon_(computing)[daemon] acting as a bridge
+between remote session and consumer daemons, local trace files, and a
+remote live trace reader.
+
+The relay daemon is part of LTTng-tools.
+
+The main purpose of the relay daemon is to implement a receiver of
+<<sending-trace-data-over-the-network,trace data over the network>>.
+This is useful when the target system doesn't have much file system
+space to write trace files locally.
+
+The relay daemon is also a server to which a
+<<lttng-live,live trace reader>> can
+connect. The live trace reader sends requests to the relay daemon to
+receive trace data as the target system records events. The
+communication protocol is named _LTTng live_; it's used over TCP
+connections.
+
+Note that you can start the relay daemon on the target system directly.
+This is the setup of choice when the use case is to view/analyze events
+as the target system records them without the need of a remote system.
+
+
+[[instrumenting]]
+== [[using-lttng]]Instrumentation
+
+There are many examples of tracing and monitoring in our everyday life:
+
+* You have access to real-time and historical weather reports and
+ forecasts thanks to weather stations installed around the country.
+* You know your heart is safe thanks to an electrocardiogram.
+* You make sure not to drive your car too fast and to have enough fuel
+ to reach your destination thanks to gauges visible on your dashboard.
+
+All the previous examples have something in common: they rely on
+**instruments**. Without the electrodes attached to the surface of your
+body skin, cardiac monitoring is futile.
+
+LTTng, as a tracer, is no different from those real life examples. If
+you're about to trace a software system or, in other words, record its
+history of execution, you better have **instrumentation points** in the
+subject you're tracing, that is, the actual software system.
+
+<<instrumentation-point-types,Various ways>> were developed to
+instrument a piece of software for LTTng tracing. The most
+straightforward one is to manually place static instrumentation points,
+called _tracepoints_, in the source code of the application. The Linux
+kernel <<domain,tracing domain>> also makes it possible to dynamically
+add instrumentation points.
+
+If you're only interested in tracing the Linux kernel, your
+instrumentation needs are probably already covered by the built-in
+<<lttng-modules,Linux kernel instrumentation points>> of LTTng. You may
+also wish to have LTTng trace a user application which is already
+instrumented for LTTng tracing. In such cases, skip this whole section
+and read the topics of the ``<<controlling-tracing,Tracing control>>''
+section.
+
+Many methods are available to instrument a piece of software for LTTng
+tracing:
+
+* <<c-application,Instrument a C/$$C++$$ user application>>.
+* <<prebuilt-ust-helpers,Load a prebuilt user space tracing helper>>.
+* <<java-application,Instrument a Java application>>.
+* <<python-application,Instrument a Python application>>.
+* <<proc-lttng-logger-abi,Use the LTTng logger>>.
+* <<instrumenting-linux-kernel,Instrument a Linux kernel image or module>>.
+
+
+[[c-application]]
+=== [[cxx-application]]Instrument a C/$$C++$$ user application
+
+The high level procedure to instrument a C or $$C++$$ user application
+with the <<lttng-ust,LTTng user space tracing library>>, `liblttng-ust`,
+is:
+
+. <<tracepoint-provider,Create the source files of a tracepoint provider
+ package>>.
+
+. <<probing-the-application-source-code,Add tracepoints to
+ the source code of the application>>.
+
+. <<building-tracepoint-providers-and-user-application,Build and link
+ a tracepoint provider package and the user application>>.
+
+If you need quick, man:printf(3)-like instrumentation, skip those steps
+and use <<tracef,`lttng_ust_tracef()`>> or
+<<tracelog,`lttng_ust_tracelog()`>> instead.
+
+IMPORTANT: You need to <<installing-lttng,install>> LTTng-UST to
+instrument a user application with `liblttng-ust`.
+
+
+[[tracepoint-provider]]
+==== Create the source files of a tracepoint provider package
+
+A _tracepoint provider_ is a set of compiled functions which provide
+**tracepoints** to an application, the type of instrumentation point
+which LTTng-UST provides.
+
+Those functions can make LTTng emit events with user-defined fields and
+serialize those events as event records to one or more LTTng-UST
+<<channel,channel>> sub-buffers. The `lttng_ust_tracepoint()` macro,
+which you <<probing-the-application-source-code,insert in the source
+code of a user application>>, calls those functions.
+
+A _tracepoint provider package_ is an object file (`.o`) or a shared
+library (`.so`) which contains one or more tracepoint providers. Its
+source files are:
+
+* One or more <<tpp-header,tracepoint provider header>> (`.h`).
+* A <<tpp-source,tracepoint provider package source>> (`.c`).
+
+A tracepoint provider package is dynamically linked with `liblttng-ust`,
+the LTTng user space tracer, at run time.
+
+[role="img-100"]
+.User application linked with `liblttng-ust` and containing a tracepoint provider.
+image::ust-app.png[]
+
+NOTE: If you need quick, man:printf(3)-like instrumentation, skip
+creating and using a tracepoint provider and use
+<<tracef,`lttng_ust_tracef()`>> or <<tracelog,`lttng_ust_tracelog()`>>
+instead.
+
+
+[[tpp-header]]
+===== Create a tracepoint provider header file template
+
+A _tracepoint provider header file_ contains the tracepoint definitions
+of a tracepoint provider.
+
+To create a tracepoint provider header file:
+
+. Start from this template:
++
+--
+[source,c]
+.Tracepoint provider header file template (`.h` file extension).
+----
+#undef LTTNG_UST_TRACEPOINT_PROVIDER
+#define LTTNG_UST_TRACEPOINT_PROVIDER provider_name
+
+#undef LTTNG_UST_TRACEPOINT_INCLUDE
+#define LTTNG_UST_TRACEPOINT_INCLUDE "./tp.h"
+
+#if !defined(_TP_H) || defined(LTTNG_UST_TRACEPOINT_HEADER_MULTI_READ)
+#define _TP_H
+
+#include <lttng/tracepoint.h>
+
+/*
+ * Use LTTNG_UST_TRACEPOINT_EVENT(), LTTNG_UST_TRACEPOINT_EVENT_CLASS(),
+ * LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(), and
+ * LTTNG_UST_TRACEPOINT_LOGLEVEL() here.
+ */
+
+#endif /* _TP_H */
+
+#include <lttng/tracepoint-event.h>
+----
+--
++
+Replace:
++
+* +__provider_name__+ with the name of your tracepoint provider.
+* `"tp.h"` with the name of your tracepoint provider header file.
+
+. Below the `#include <lttng/tracepoint.h>` line, put your
+ <<defining-tracepoints,tracepoint definitions>>.
+
+Your tracepoint provider name must be unique amongst all the possible
+tracepoint provider names used on the same target system. We suggest to
+include the name of your project or company in the name, for example,
+`org_lttng_my_project_tpp`.
+
+
+[[defining-tracepoints]]
+===== Create a tracepoint definition
+
+A _tracepoint definition_ defines, for a given tracepoint:
+
+* Its **input arguments**.
++
+They're the macro parameters that the `lttng_ust_tracepoint()` macro
+accepts for this particular tracepoint in the source code of the user
+application.
+
+* Its **output event fields**.
++
+They're the sources of event fields that form the payload of any event
+that the execution of the `lttng_ust_tracepoint()` macro emits for this
+particular tracepoint.
+
+Create a tracepoint definition with the
+`LTTNG_UST_TRACEPOINT_EVENT()` macro below the `#include <lttng/tracepoint.h>`
+line in the
+<<tpp-header,tracepoint provider header file template>>.
+
+The syntax of the `LTTNG_UST_TRACEPOINT_EVENT()` macro is:
+
+[source,c]
+.`LTTNG_UST_TRACEPOINT_EVENT()` macro syntax.
+----
+LTTNG_UST_TRACEPOINT_EVENT(
+ /* Tracepoint provider name */
+ provider_name,
+
+ /* Tracepoint name */
+ tracepoint_name,
+
+ /* Input arguments */
+ LTTNG_UST_TP_ARGS(
+ arguments
+ ),
+
+ /* Output event fields */
+ LTTNG_UST_TP_FIELDS(
+ fields
+ )
+)
+----
+
+Replace:
+
+* +__provider_name__+ with your tracepoint provider name.
+* +__tracepoint_name__+ with your tracepoint name.
+* +__arguments__+ with the <<tpp-def-input-args,input arguments>>.
+* +__fields__+ with the <<tpp-def-output-fields,output event field>>
+ definitions.
+
+The full name of this tracepoint is `provider_name:tracepoint_name`.
+
+[IMPORTANT]
+.Event name length limitation
+====
+The concatenation of the tracepoint provider name and the tracepoint
+name must not exceed **254{nbsp}characters**. If it does, the
+instrumented application compiles and runs, but LTTng throws multiple
+warnings and you could experience serious issues.
+====
+
+[[tpp-def-input-args]]The syntax of the `LTTNG_UST_TP_ARGS()` macro is:
+
+[source,c]
+.`LTTNG_UST_TP_ARGS()` macro syntax.
+----
+LTTNG_UST_TP_ARGS(
+ type, arg_name
+)
+----
+
+Replace:
+
+* +__type__+ with the C{nbsp}type of the argument.
+* +__arg_name__+ with the argument name.
+
+You can repeat +__type__+ and +__arg_name__+ up to 10{nbsp}times to have
+more than one argument.
+
+.`LTTNG_UST_TP_ARGS()` usage with three arguments.
+====
+[source,c]
+----
+LTTNG_UST_TP_ARGS(
+ int, count,
+ float, ratio,
+ const char*, query
+)
+----
+====
+
+The `LTTNG_UST_TP_ARGS()` and `LTTNG_UST_TP_ARGS(void)` forms are valid
+to create a tracepoint definition with no input arguments.
+
+[[tpp-def-output-fields]]The `LTTNG_UST_TP_FIELDS()` macro contains a
+list of `lttng_ust_field_*()` macros. Each `lttng_ust_field_*()` macro
+defines one event field. See man:lttng-ust(3) for a complete description
+of the available `lttng_ust_field_*()` macros. A `lttng_ust_field_*()`
+macro specifies the type, size, and byte order of one event field.
+
+Each `lttng_ust_field_*()` macro takes an _argument expression_
+parameter. This is a C{nbsp}expression that the tracer evaluates at the
+`lttng_ust_tracepoint()` macro site in the source code of the
+application. This expression provides the source of data of a field. The
+argument expression can include input argument names listed in the
+`LTTNG_UST_TP_ARGS()` macro.
+
+Each `lttng_ust_field_*()` macro also takes a _field name_ parameter.
+Field names must be unique within a given tracepoint definition.
+
+Here's a complete tracepoint definition example:
+
+.Tracepoint definition.
+====
+The following tracepoint definition defines a tracepoint which takes
+three input arguments and has four output event fields.
+
+[source,c]
+----
+#include "my-custom-structure.h"
+
+LTTNG_UST_TRACEPOINT_EVENT(
+ my_provider,
+ my_tracepoint,
+ LTTNG_UST_TP_ARGS(
+ const struct my_custom_structure *, my_custom_structure,
+ float, ratio,
+ const char *, query
+ ),
+ LTTNG_UST_TP_FIELDS(
+ lttng_ust_field_string(query_field, query)
+ lttng_ust_field_float(double, ratio_field, ratio)
+ lttng_ust_field_integer(int, recv_size,
+ my_custom_structure->recv_size)
+ lttng_ust_field_integer(int, send_size,
+ my_custom_structure->send_size)
+ )
+)
+----
+
+Refer to this tracepoint definition with the `lttng_ust_tracepoint()`
+macro in the source code of your application like this:
+
+[source,c]
+----
+lttng_ust_tracepoint(my_provider, my_tracepoint,
+ my_structure, some_ratio, the_query);
+----
+====
+
+NOTE: The LTTng-UST tracer only evaluates the arguments of a tracepoint
+at run time when such a tracepoint _could_ emit an event. See
+<<event-creation-emission-opti,this note>> to learn more.
+
+
+[[using-tracepoint-classes]]
+===== Use a tracepoint class
+
+A _tracepoint class_ is a class of tracepoints which share the same
+output event field definitions. A _tracepoint instance_ is one
+instance of such a defined tracepoint class, with its own tracepoint
+name.
+
+The <<defining-tracepoints,`LTTNG_UST_TRACEPOINT_EVENT()` macro>> is
+actually a shorthand which defines both a tracepoint class and a
+tracepoint instance at the same time.
+
+When you build a tracepoint provider package, the C or $$C++$$ compiler
+creates one serialization function for each **tracepoint class**. A
+serialization function is responsible for serializing the event fields
+of a tracepoint to a sub-buffer when recording.
+
+For various performance reasons, when your situation requires multiple
+tracepoint definitions with different names, but with the same event
+fields, we recommend that you manually create a tracepoint class and
+instantiate as many tracepoint instances as needed. One positive effect
+of such a design, amongst other advantages, is that all tracepoint
+instances of the same tracepoint class reuse the same serialization
+function, thus reducing
+https://en.wikipedia.org/wiki/Cache_pollution[cache pollution].
+
+.Use a tracepoint class and tracepoint instances.
+====
+Consider the following three tracepoint definitions:
+
+[source,c]
+----
+LTTNG_UST_TRACEPOINT_EVENT(
+ my_app,
+ get_account,
+ LTTNG_UST_TP_ARGS(
+ int, userid,
+ size_t, len
+ ),
+ LTTNG_UST_TP_FIELDS(
+ lttng_ust_field_integer(int, userid, userid)
+ lttng_ust_field_integer(size_t, len, len)
+ )
+)
+
+LTTNG_UST_TRACEPOINT_EVENT(
+ my_app,
+ get_settings,
+ LTTNG_UST_TP_ARGS(
+ int, userid,
+ size_t, len
+ ),
+ LTTNG_UST_TP_FIELDS(
+ lttng_ust_field_integer(int, userid, userid)
+ lttng_ust_field_integer(size_t, len, len)
+ )
+)
+
+LTTNG_UST_TRACEPOINT_EVENT(
+ my_app,
+ get_transaction,
+ LTTNG_UST_TP_ARGS(
+ int, userid,
+ size_t, len
+ ),
+ LTTNG_UST_TP_FIELDS(
+ lttng_ust_field_integer(int, userid, userid)
+ lttng_ust_field_integer(size_t, len, len)
+ )
+)
+----
+
+In this case, we create three tracepoint classes, with one implicit
+tracepoint instance for each of them: `get_account`, `get_settings`, and
+`get_transaction`. However, they all share the same event field names
+and types. Hence three identical, yet independent serialization
+functions are created when you build the tracepoint provider package.
+
+A better design choice is to define a single tracepoint class and three
+tracepoint instances:
+
+[source,c]
+----
+/* The tracepoint class */
+LTTNG_UST_TRACEPOINT_EVENT_CLASS(
+ /* Tracepoint class provider name */
+ my_app,
+
+ /* Tracepoint class name */
+ my_class,
+
+ /* Input arguments */
+ LTTNG_UST_TP_ARGS(
+ int, userid,
+ size_t, len
+ ),
+
+ /* Output event fields */
+ LTTNG_UST_TP_FIELDS(
+ lttng_ust_field_integer(int, userid, userid)
+ lttng_ust_field_integer(size_t, len, len)
+ )
+)
+
+/* The tracepoint instances */
+LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(
+ /* Tracepoint class provider name */
+ my_app,
+
+ /* Tracepoint class name */
+ my_class,
+
+ /* Instance provider name */
+ my_app,
+
+ /* Tracepoint name */
+ get_account,
+
+ /* Input arguments */
+ LTTNG_UST_TP_ARGS(
+ int, userid,
+ size_t, len
+ )
+)
+LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(
+ my_app,
+ my_class,
+ get_settings,
+ LTTNG_UST_TP_ARGS(
+ int, userid,
+ size_t, len
+ )
+)
+LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(
+ my_app,
+ my_class,
+ get_transaction,
+ LTTNG_UST_TP_ARGS(
+ int, userid,
+ size_t, len
+ )
+)
+----
+====
+
+The tracepoint class and instance provider names must be the same if the
+`LTTNG_UST_TRACEPOINT_EVENT_CLASS()` and
+`LTTNG_UST_TRACEPOINT_EVENT_INSTANCE()` expansions are part of the same
+translation unit. See man:lttng-ust(3) to learn more.
+
+
+[[assigning-log-levels]]
+===== Assign a log level to a tracepoint definition
+
+Assign a _log level_ to a <<defining-tracepoints,tracepoint definition>>
+with the `LTTNG_UST_TRACEPOINT_LOGLEVEL()` macro.
+
+Assigning different levels of severity to tracepoint definitions can be
+useful: when you <<enabling-disabling-events,create a recording event
+rule>>, you can target tracepoints having a log level at least as severe
+as a specific value.
+
+The concept of LTTng-UST log levels is similar to the levels found
+in typical logging frameworks:
+
+* In a logging framework, the log level is given by the function
+ or method name you use at the log statement site: `debug()`,
+ `info()`, `warn()`, `error()`, and so on.
+
+* In LTTng-UST, you statically assign the log level to a tracepoint
+ definition; any `lttng_ust_tracepoint()` macro invocation which refers
+ to this definition has this log level.
+
+You must use `LTTNG_UST_TRACEPOINT_LOGLEVEL()` _after_ the
+<<defining-tracepoints,`LTTNG_UST_TRACEPOINT_EVENT()`>> or
+<<using-tracepoint-classes,`LTTNG_UST_TRACEPOINT_INSTANCE()`>> macro for
+a given tracepoint.
+
+The syntax of the `LTTNG_UST_TRACEPOINT_LOGLEVEL()` macro is:
+
+[source,c]
+.`LTTNG_UST_TRACEPOINT_LOGLEVEL()` macro syntax.
+----
+LTTNG_UST_TRACEPOINT_LOGLEVEL(provider_name, tracepoint_name, log_level)
+----
+
+Replace:
+
+* +__provider_name__+ with the tracepoint provider name.
+* +__tracepoint_name__+ with the tracepoint name.
+* +__log_level__+ with the log level to assign to the tracepoint
+ definition named +__tracepoint_name__+ in the +__provider_name__+
+ tracepoint provider.
++
+See man:lttng-ust(3) for a list of available log level names.
+
+.Assign the `LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_UNIT` log level to a
+tracepoint definition.
+====
+[source,c]
+----
+/* Tracepoint definition */
+LTTNG_UST_TRACEPOINT_EVENT(
+ my_app,
+ get_transaction,
+ LTTNG_UST_TP_ARGS(
+ int, userid,
+ size_t, len
+ ),
+ LTTNG_UST_TP_FIELDS(
+ lttng_ust_field_integer(int, userid, userid)
+ lttng_ust_field_integer(size_t, len, len)
+ )
+)
+
+/* Log level assignment */
+LTTNG_UST_TRACEPOINT_LOGLEVEL(my_app, get_transaction,
+ LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_UNIT)
+----
+====
+
+
+[[tpp-source]]
+===== Create a tracepoint provider package source file
+
+A _tracepoint provider package source file_ is a C source file which
+includes a <<tpp-header,tracepoint provider header file>> to expand its
+macros into event serialization and other functions.
+
+Use the following tracepoint provider package source file template:
+
+[source,c]
+.Tracepoint provider package source file template.
+----
+#define LTTNG_UST_TRACEPOINT_CREATE_PROBES
+
+#include "tp.h"
+----
+
+Replace `tp.h` with the name of your <<tpp-header,tracepoint provider
+header file>> name. You may also include more than one tracepoint
+provider header file here to create a tracepoint provider package
+holding more than one tracepoint providers.
+
+
+[[probing-the-application-source-code]]
+==== Add tracepoints to the source code of an application
+
+Once you <<tpp-header,create a tracepoint provider header file>>, use
+the `lttng_ust_tracepoint()` macro in the source code of your
+application to insert the tracepoints that this header
+<<defining-tracepoints,defines>>.
+
+The `lttng_ust_tracepoint()` macro takes at least two parameters: the
+tracepoint provider name and the tracepoint name. The corresponding
+tracepoint definition defines the other parameters.
+
+.`lttng_ust_tracepoint()` usage.
+====
+The following <<defining-tracepoints,tracepoint definition>> defines a
+tracepoint which takes two input arguments and has two output event
+fields.
+
+[source,c]
+.Tracepoint provider header file.
+----
+#include "my-custom-structure.h"
+
+LTTNG_UST_TRACEPOINT_EVENT(
+ my_provider,
+ my_tracepoint,
+ LTTNG_UST_TP_ARGS(
+ int, argc,
+ const char *, cmd_name
+ ),
+ LTTNG_UST_TP_FIELDS(
+ lttng_ust_field_string(cmd_name, cmd_name)
+ lttng_ust_field_integer(int, number_of_args, argc)
+ )
+)
+----
+
+Refer to this tracepoint definition with the `lttng_ust_tracepoint()`
+macro in the source code of your application like this:
+
+[source,c]
+.Application source file.
+----
+#include "tp.h"
+
+int main(int argc, char* argv[])
+{
+ lttng_ust_tracepoint(my_provider, my_tracepoint, argc, argv[0]);
+ return 0;
+}
+----
+
+Note how the source code of the application includes
+the tracepoint provider header file containing the tracepoint
+definitions to use, path:{tp.h}.
+====
+
+.`lttng_ust_tracepoint()` usage with a complex tracepoint definition.
+====
+Consider this complex tracepoint definition, where multiple event
+fields refer to the same input arguments in their argument expression
+parameter:
+
+[source,c]
+.Tracepoint provider header file.
+----
+/* For `struct stat` */
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+LTTNG_UST_TRACEPOINT_EVENT(
+ my_provider,
+ my_tracepoint,
+ LTTNG_UST_TP_ARGS(
+ int, my_int_arg,
+ char *, my_str_arg,
+ struct stat *, st
+ ),
+ LTTNG_UST_TP_FIELDS(
+ lttng_ust_field_integer(int, my_constant_field, 23 + 17)
+ lttng_ust_field_integer(int, my_int_arg_field, my_int_arg)
+ lttng_ust_field_integer(int, my_int_arg_field2,
+ my_int_arg * my_int_arg)
+ lttng_ust_field_integer(int, sum4_field,
+ my_str_arg[0] + my_str_arg[1] +
+ my_str_arg[2] + my_str_arg[3])
+ lttng_ust_field_string(my_str_arg_field, my_str_arg)
+ lttng_ust_field_integer_hex(off_t, size_field, st->st_size)
+ lttng_ust_field_float(double, size_dbl_field, (double) st->st_size)
+ lttng_ust_field_sequence_text(char, half_my_str_arg_field,
+ my_str_arg, size_t,
+ strlen(my_str_arg) / 2)
+ )
+)
+----
+
+Refer to this tracepoint definition with the `lttng_ust_tracepoint()`
+macro in the source code of your application like this:
+
+[source,c]
+.Application source file.
+----
+#define LTTNG_UST_TRACEPOINT_DEFINE
+#include "tp.h"
+
+int main(void)
+{
+ struct stat s;
+
+ stat("/etc/fstab", &s);
+ lttng_ust_tracepoint(my_provider, my_tracepoint, 23,
+ "Hello, World!", &s);
+
+ return 0;
+}
+----
+
+If you look at the event record that LTTng writes when recording this
+program, assuming the file size of path:{/etc/fstab} is 301{nbsp}bytes,
+it should look like this:
+
+.Event record fields
+|====
+|Field name |Field value
+|`my_constant_field` |40
+|`my_int_arg_field` |23
+|`my_int_arg_field2` |529
+|`sum4_field` |389
+|`my_str_arg_field` |`Hello, World!`
+|`size_field` |0x12d
+|`size_dbl_field` |301.0
+|`half_my_str_arg_field` |`Hello,`
+|====
+====
+
+Sometimes, the arguments you pass to `lttng_ust_tracepoint()` are
+expensive to evaluate--they use the call stack, for example. To avoid
+this computation when LTTng wouldn't emit any event anyway, use the
+`lttng_ust_tracepoint_enabled()` and `lttng_ust_do_tracepoint()` macros.
+
+The syntax of the `lttng_ust_tracepoint_enabled()` and
+`lttng_ust_do_tracepoint()` macros is:
+
+[source,c]
+.`lttng_ust_tracepoint_enabled()` and `lttng_ust_do_tracepoint()` macros syntax.
+----
+lttng_ust_tracepoint_enabled(provider_name, tracepoint_name)
+
+lttng_ust_do_tracepoint(provider_name, tracepoint_name, ...)
+----
+
+Replace:
+
+* +__provider_name__+ with the tracepoint provider name.
+* +__tracepoint_name__+ with the tracepoint name.
+
+`lttng_ust_tracepoint_enabled()` returns a non-zero value if executing
+the tracepoint named `tracepoint_name` from the provider named
+`provider_name` _could_ make LTTng emit an event, depending on the
+payload of said event.
+
+`lttng_ust_do_tracepoint()` is like `lttng_ust_tracepoint()`, except
+that it doesn't check what `lttng_ust_tracepoint_enabled()` checks.
+Using `lttng_ust_tracepoint()` with `lttng_ust_tracepoint_enabled()` is
+dangerous because `lttng_ust_tracepoint()` also contains the
+`lttng_ust_tracepoint_enabled()` check; therefore, a race condition is
+possible in this situation:
+
+[source,c]
+.Possible race condition when using `lttng_ust_tracepoint_enabled()` with `lttng_ust_tracepoint()`.
+----
+if (lttng_ust_tracepoint_enabled(my_provider, my_tracepoint)) {
+ stuff = prepare_stuff();
+}
+
+lttng_ust_tracepoint(my_provider, my_tracepoint, stuff);
+----
+
+If `lttng_ust_tracepoint_enabled()` is false, but would be true after
+the conditional block, then `stuff` isn't prepared: the emitted event
+will either contain wrong data, or the whole application could crash
+(with a segmentation fault, for example).
+
+NOTE: Neither `lttng_ust_tracepoint_enabled()` nor
+`lttng_ust_do_tracepoint()` have an `STAP_PROBEV()` call. If you need
+it, you must emit this call yourself.
+
+
+[[building-tracepoint-providers-and-user-application]]
+==== Build and link a tracepoint provider package and an application
+
+Once you have one or more <<tpp-header,tracepoint provider header
+files>> and a <<tpp-source,tracepoint provider package source file>>,
+create the tracepoint provider package by compiling its source
+file. From here, multiple build and run scenarios are possible. The
+following table shows common application and library configurations
+along with the required command lines to achieve them.
+
+In the following diagrams, we use the following file names:
+
+`app`::
+ Executable application.
+
+`app.o`::
+ Application object file.
+
+`tpp.o`::
+ Tracepoint provider package object file.
+
+`tpp.a`::
+ Tracepoint provider package archive file.
+
+`libtpp.so`::
+ Tracepoint provider package shared object file.
+
+`emon.o`::
+ User library object file.
+
+`libemon.so`::
+ User library shared object file.
+
+We use the following symbols in the diagrams of table below:
+
+[role="img-100"]
+.Symbols used in the build scenario diagrams.
+image::ust-sit-symbols.png[]
+
+We assume that path:{.} is part of the env:LD_LIBRARY_PATH environment
+variable in the following instructions.
+
+[role="growable ust-scenarios",cols="asciidoc,asciidoc"]
+.Common tracepoint provider package scenarios.
+|====
+|Scenario |Instructions
+
+|
+The instrumented application is statically linked with
+the tracepoint provider package object.
+
+image::ust-sit+app-linked-with-tp-o+app-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-o.txt[]
+
+To build the instrumented application:
+
+. In path:{app.c}, before including path:{tpp.h}, add the following line:
++
+--
+[source,c]
+----
+#define LTTNG_UST_TRACEPOINT_DEFINE
+----
+--
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o tpp.o -llttng-ust -ldl
+----
+--
+
+To run the instrumented application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The instrumented application is statically linked with the
+tracepoint provider package archive file.
+
+image::ust-sit+app-linked-with-tp-a+app-instrumented.png[]
+
+|
+To create the tracepoint provider package archive file:
+
+. Compile the <<tpp-source,tracepoint provider package source file>>:
++
+--
+[role="term"]
+----
+$ gcc -I. -c tpp.c
+----
+--
+
+. Create the tracepoint provider package archive file:
++
+--
+[role="term"]
+----
+$ ar rcs tpp.a tpp.o
+----
+--
+
+To build the instrumented application:
+
+. In path:{app.c}, before including path:{tpp.h}, add the following line:
++
+--
+[source,c]
+----
+#define LTTNG_UST_TRACEPOINT_DEFINE
+----
+--
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o tpp.a -llttng-ust -ldl
+----
+--
+
+To run the instrumented application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The instrumented application is linked with the tracepoint provider
+package shared object.
+
+image::ust-sit+app-linked-with-tp-so+app-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-so.txt[]
+
+To build the instrumented application:
+
+. In path:{app.c}, before including path:{tpp.h}, add the following line:
++
+--
+[source,c]
+----
+#define LTTNG_UST_TRACEPOINT_DEFINE
+----
+--
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -ldl -L. -ltpp
+----
+--
+
+To run the instrumented application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The tracepoint provider package shared object is preloaded before the
+instrumented application starts.
+
+image::ust-sit+tp-so-preloaded+app-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-so.txt[]
+
+To build the instrumented application:
+
+. In path:{app.c}, before including path:{tpp.h}, add the
+ following lines:
++
+--
+[source,c]
+----
+#define LTTNG_UST_TRACEPOINT_DEFINE
+#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE
+----
+--
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -ldl
+----
+--
+
+To run the instrumented application with tracing support:
+
+* Preload the tracepoint provider package shared object and
+ start the application:
++
+--
+[role="term"]
+----
+$ LD_PRELOAD=./libtpp.so ./app
+----
+--
+
+To run the instrumented application without tracing support:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The instrumented application dynamically loads the tracepoint provider
+package shared object.
+
+image::ust-sit+app-dlopens-tp-so+app-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-so.txt[]
+
+To build the instrumented application:
+
+. In path:{app.c}, before including path:{tpp.h}, add the
+ following lines:
++
+--
+[source,c]
+----
+#define LTTNG_UST_TRACEPOINT_DEFINE
+#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE
+----
+--
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -ldl
+----
+--
+
+To run the instrumented application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The application is linked with the instrumented user library.
+
+The instrumented user library is statically linked with the tracepoint
+provider package object file.
+
+image::ust-sit+app-linked-with-lib+lib-linked-with-tp-o+lib-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-o-fpic.txt[]
+
+To build the instrumented user library:
+
+. In path:{emon.c}, before including path:{tpp.h}, add the
+ following line:
++
+--
+[source,c]
+----
+#define LTTNG_UST_TRACEPOINT_DEFINE
+----
+--
+
+. Compile the user library source file:
++
+--
+[role="term"]
+----
+$ gcc -I. -fpic -c emon.c
+----
+--
+
+. Build the user library shared object:
++
+--
+[role="term"]
+----
+$ gcc -shared -o libemon.so emon.o tpp.o -llttng-ust -ldl
+----
+--
+
+To build the application:
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -L. -lemon
+----
+--
+
+To run the application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The application is linked with the instrumented user library.
+
+The instrumented user library is linked with the tracepoint provider
+package shared object.
+
+image::ust-sit+app-linked-with-lib+lib-linked-with-tp-so+lib-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-so.txt[]
+
+To build the instrumented user library:
+
+. In path:{emon.c}, before including path:{tpp.h}, add the
+ following line:
++
+--
+[source,c]
+----
+#define LTTNG_UST_TRACEPOINT_DEFINE
+----
+--
+
+. Compile the user library source file:
++
+--
+[role="term"]
+----
+$ gcc -I. -fpic -c emon.c
+----
+--
+
+. Build the user library shared object:
++
+--
+[role="term"]
+----
+$ gcc -shared -o libemon.so emon.o -ldl -L. -ltpp
+----
+--
+
+To build the application:
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -L. -lemon
+----
+--
+
+To run the application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The tracepoint provider package shared object is preloaded before the
+application starts.
+
+The application is linked with the instrumented user library.
+
+image::ust-sit+tp-so-preloaded+app-linked-with-lib+lib-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-so.txt[]
+
+To build the instrumented user library:
+
+. In path:{emon.c}, before including path:{tpp.h}, add the
+ following lines:
++
+--
+[source,c]
+----
+#define LTTNG_UST_TRACEPOINT_DEFINE
+#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE
+----
+--
+
+. Compile the user library source file:
++
+--
+[role="term"]
+----
+$ gcc -I. -fpic -c emon.c
+----
+--
+
+. Build the user library shared object:
++
+--
+[role="term"]
+----
+$ gcc -shared -o libemon.so emon.o -ldl
+----
+--
+
+To build the application:
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -L. -lemon
+----
+--
+
+To run the application with tracing support:
+
+* Preload the tracepoint provider package shared object and
+ start the application:
++
+--
+[role="term"]
+----
+$ LD_PRELOAD=./libtpp.so ./app
+----
+--
+
+To run the application without tracing support:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The application is linked with the instrumented user library.
+
+The instrumented user library dynamically loads the tracepoint provider
+package shared object.
+
+image::ust-sit+app-linked-with-lib+lib-dlopens-tp-so+lib-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-so.txt[]
+
+To build the instrumented user library:
+
+. In path:{emon.c}, before including path:{tpp.h}, add the
+ following lines:
++
+--
+[source,c]
+----
+#define LTTNG_UST_TRACEPOINT_DEFINE
+#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE
+----
+--
+
+. Compile the user library source file:
++
+--
+[role="term"]
+----
+$ gcc -I. -fpic -c emon.c
+----
+--
+
+. Build the user library shared object:
++
+--
+[role="term"]
+----
+$ gcc -shared -o libemon.so emon.o -ldl
+----
+--
+
+To build the application:
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -L. -lemon
+----
+--
+
+To run the application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The application dynamically loads the instrumented user library.
+
+The instrumented user library is linked with the tracepoint provider
+package shared object.
+
+image::ust-sit+app-dlopens-lib+lib-linked-with-tp-so+lib-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-so.txt[]
+
+To build the instrumented user library:
+
+. In path:{emon.c}, before including path:{tpp.h}, add the
+ following line:
++
+--
+[source,c]
+----
+#define LTTNG_UST_TRACEPOINT_DEFINE
+----
+--
+
+. Compile the user library source file:
++
+--
+[role="term"]
+----
+$ gcc -I. -fpic -c emon.c
+----
+--
+
+. Build the user library shared object:
++
+--
+[role="term"]
+----
+$ gcc -shared -o libemon.so emon.o -ldl -L. -ltpp
+----
+--
+
+To build the application:
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -ldl -L. -lemon
+----
+--
+
+To run the application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The application dynamically loads the instrumented user library.
+
+The instrumented user library dynamically loads the tracepoint provider
+package shared object.
+
+image::ust-sit+app-dlopens-lib+lib-dlopens-tp-so+lib-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-so.txt[]
+
+To build the instrumented user library:
+
+. In path:{emon.c}, before including path:{tpp.h}, add the
+ following lines:
++
+--
+[source,c]
+----
+#define LTTNG_UST_TRACEPOINT_DEFINE
+#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE
+----
+--
+
+. Compile the user library source file:
++
+--
+[role="term"]
+----
+$ gcc -I. -fpic -c emon.c
+----
+--
+
+. Build the user library shared object:
++
+--
+[role="term"]
+----
+$ gcc -shared -o libemon.so emon.o -ldl
+----
+--
+
+To build the application:
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -ldl -L. -lemon
+----
+--
+
+To run the application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The tracepoint provider package shared object is preloaded before the
+application starts.
+
+The application dynamically loads the instrumented user library.
+
+image::ust-sit+tp-so-preloaded+app-dlopens-lib+lib-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-so.txt[]
+
+To build the instrumented user library:
+
+. In path:{emon.c}, before including path:{tpp.h}, add the
+ following lines:
++
+--
+[source,c]
+----
+#define LTTNG_UST_TRACEPOINT_DEFINE
+#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE
+----
+--
+
+. Compile the user library source file:
++
+--
+[role="term"]
+----
+$ gcc -I. -fpic -c emon.c
+----
+--
+
+. Build the user library shared object:
++
+--
+[role="term"]
+----
+$ gcc -shared -o libemon.so emon.o -ldl
+----
+--
+
+To build the application:
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -L. -lemon
+----
+--
+
+To run the application with tracing support:
+
+* Preload the tracepoint provider package shared object and
+ start the application:
++
+--
+[role="term"]
+----
+$ LD_PRELOAD=./libtpp.so ./app
+----
+--
+
+To run the application without tracing support:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The application is statically linked with the tracepoint provider
+package object file.
+
+The application is linked with the instrumented user library.
+
+image::ust-sit+app-linked-with-tp-o+app-linked-with-lib+lib-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-o.txt[]
+
+To build the instrumented user library:
+
+. In path:{emon.c}, before including path:{tpp.h}, add the
+ following line:
++
+--
+[source,c]
+----
+#define LTTNG_UST_TRACEPOINT_DEFINE
+----
+--
+
+. Compile the user library source file:
++
+--
+[role="term"]
+----
+$ gcc -I. -fpic -c emon.c
+----
+--
+
+. Build the user library shared object:
++
+--
+[role="term"]
+----
+$ gcc -shared -o libemon.so emon.o
+----
+--
+
+To build the application:
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o tpp.o -llttng-ust -ldl -L. -lemon
+----
+--
+
+To run the instrumented application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The application is statically linked with the tracepoint provider
+package object file.
+
+The application dynamically loads the instrumented user library.
+
+image::ust-sit+app-linked-with-tp-o+app-dlopens-lib+lib-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-o.txt[]
+
+To build the application:
+
+. In path:{app.c}, before including path:{tpp.h}, add the following line:
++
+--
+[source,c]
+----
+#define LTTNG_UST_TRACEPOINT_DEFINE
+----
+--
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -Wl,--export-dynamic -o app app.o tpp.o \
+ -llttng-ust -ldl
+----
+--
++
+The `--export-dynamic` option passed to the linker is necessary for the
+dynamically loaded library to ``see'' the tracepoint symbols defined in
+the application.
+
+To build the instrumented user library:
+
+. Compile the user library source file:
++
+--
+[role="term"]
+----
+$ gcc -I. -fpic -c emon.c
+----
+--
+
+. Build the user library shared object:
++
+--
+[role="term"]
+----
+$ gcc -shared -o libemon.so emon.o
+----
+--
+
+To run the application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+|====
+
+
+[[using-lttng-ust-with-daemons]]
+===== Use noch:{LTTng-UST} with daemons
+
+If your instrumented application calls man:fork(2), man:clone(2),
+or BSD's man:rfork(2), without a following man:exec(3)-family
+system call, you must preload the path:{liblttng-ust-fork.so} shared
+object when you start the application.
+
+[role="term"]
+----
+$ LD_PRELOAD=liblttng-ust-fork.so ./my-app
+----
+
+If your tracepoint provider package is
+a shared library which you also preload, you must put both
+shared objects in env:LD_PRELOAD:
+
+[role="term"]
+----
+$ LD_PRELOAD=liblttng-ust-fork.so:/path/to/tp.so ./my-app
+----
+
+
+[role="since-2.9"]
+[[liblttng-ust-fd]]
+===== Use noch:{LTTng-UST} with applications which close file descriptors that don't belong to them
+
+If your instrumented application closes one or more file descriptors
+which it did not open itself, you must preload the
+path:{liblttng-ust-fd.so} shared object when you start the application:
+
+[role="term"]
+----
+$ LD_PRELOAD=liblttng-ust-fd.so ./my-app
+----
+
+Typical use cases include closing all the file descriptors after
+man:fork(2) or man:rfork(2) and buggy applications doing
+``double closes''.
+
+
+[[lttng-ust-pkg-config]]
+===== Use noch:{pkg-config}
+
+On some distributions, LTTng-UST ships with a
+https://www.freedesktop.org/wiki/Software/pkg-config/[pkg-config]
+metadata file. If this is your case, then use cmd:pkg-config to
+build an application on the command line:
+
+[role="term"]
+----
+$ gcc -o my-app my-app.o tp.o $(pkg-config --cflags --libs lttng-ust)
+----
+
+
+[[instrumenting-32-bit-app-on-64-bit-system]]
+===== [[advanced-instrumenting-techniques]]Build a 32-bit instrumented application for a 64-bit target system
+
+In order to trace a 32-bit application running on a 64-bit system,
+LTTng must use a dedicated 32-bit
+<<lttng-consumerd,consumer daemon>>.
+
+The following steps show how to build and install a 32-bit consumer
+daemon, which is _not_ part of the default 64-bit LTTng build, how to
+build and install the 32-bit LTTng-UST libraries, and how to build and
+link an instrumented 32-bit application in that context.
+
+To build a 32-bit instrumented application for a 64-bit target system,
+assuming you have a fresh target system with no installed Userspace RCU
+or LTTng packages:
+
+. Download, build, and install a 32-bit version of Userspace RCU:
++
+--
+[role="term"]
+----
+$ cd $(mktemp -d) &&
+ wget https://lttng.org/files/urcu/userspace-rcu-latest-0.13.tar.bz2 &&
+ tar -xf userspace-rcu-latest-0.13.tar.bz2 &&
+ cd userspace-rcu-0.13.* &&
+ ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 &&
+ make &&
+ sudo make install &&
+ sudo ldconfig
+----
+--
+
+. Using the package manager of your distribution, or from source,
+ install the 32-bit versions of the following dependencies of
+ LTTng-tools and LTTng-UST:
++
+--
+* https://sourceforge.net/projects/libuuid/[libuuid]
+* https://directory.fsf.org/wiki/Popt[popt]
+* https://www.xmlsoft.org/[libxml2]
+* **Optional**: https://github.com/numactl/numactl[numactl]
+--
+
+. Download, build, and install a 32-bit version of the latest
+ LTTng-UST{nbsp}{revision}:
++
+--
+[role="term"]
+----
+$ cd $(mktemp -d) &&
+ wget https://lttng.org/files/lttng-ust/lttng-ust-latest-2.13.tar.bz2 &&
+ tar -xf lttng-ust-latest-2.13.tar.bz2 &&
+ cd lttng-ust-2.13.* &&
+ ./configure --libdir=/usr/local/lib32 \
+ CFLAGS=-m32 CXXFLAGS=-m32 \
+ LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' &&
+ make &&
+ sudo make install &&
+ sudo ldconfig
+----
+--
++
+Add `--disable-numa` to `./configure` if you don't have
+https://github.com/numactl/numactl[numactl].
++
+[NOTE]
+====
+Depending on your distribution, 32-bit libraries could be installed at a
+different location than `/usr/lib32`. For example, Debian is known to
+install some 32-bit libraries in `/usr/lib/i386-linux-gnu`.
+
+In this case, make sure to set `LDFLAGS` to all the
+relevant 32-bit library paths, for example:
+
+[role="term"]
+----
+$ LDFLAGS='-L/usr/lib/i386-linux-gnu -L/usr/lib32'
+----
+====
+
+. Download the latest LTTng-tools{nbsp}{revision}, build, and install
+ the 32-bit consumer daemon:
++
+--
+[role="term"]
+----
+$ cd $(mktemp -d) &&
+ wget https://lttng.org/files/lttng-tools/lttng-tools-latest-2.13.tar.bz2 &&
+ tar -xf lttng-tools-latest-2.13.tar.bz2 &&
+ cd lttng-tools-2.13.* &&
+ ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 CXXFLAGS=-m32 \
+ LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' \
+ --disable-bin-lttng --disable-bin-lttng-crash \
+ --disable-bin-lttng-relayd --disable-bin-lttng-sessiond &&
+ make &&
+ cd src/bin/lttng-consumerd &&
+ sudo make install &&
+ sudo ldconfig
+----
+--
+
+. From your distribution or from source, <<installing-lttng,install>>
+ the 64-bit versions of LTTng-UST and Userspace RCU.
+
+. Download, build, and install the 64-bit version of the
+ latest LTTng-tools{nbsp}{revision}:
++
+--
+[role="term"]
+----
+$ cd $(mktemp -d) &&
+ wget https://lttng.org/files/lttng-tools/lttng-tools-latest-2.13.tar.bz2 &&
+ tar -xf lttng-tools-latest-2.13.tar.bz2 &&
+ cd lttng-tools-2.13.* &&
+ ./configure --with-consumerd32-libdir=/usr/local/lib32 \
+ --with-consumerd32-bin=/usr/local/lib32/lttng/libexec/lttng-consumerd &&
+ make &&
+ sudo make install &&
+ sudo ldconfig
+----
+--
+
+. Pass the following options to man:gcc(1), man:g++(1), or man:clang(1)
+ when linking your 32-bit application:
++
+----
+-m32 -L/usr/lib32 -L/usr/local/lib32 \
+-Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32
+----
++
+For example, let's rebuild the quick start example in
+``<<tracing-your-own-user-application,Record user application events>>''
+as an instrumented 32-bit application:
++
+--
+[role="term"]
+----
+$ gcc -m32 -c -I. hello-tp.c
+$ gcc -m32 -c hello.c
+$ gcc -m32 -o hello hello.o hello-tp.o \
+ -L/usr/lib32 -L/usr/local/lib32 \
+ -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32 \
+ -llttng-ust -ldl
+----
+--
+
+No special action is required to execute the 32-bit application and
+for LTTng to trace it: use the command-line man:lttng(1) tool as usual.
+
+
+[role="since-2.5"]
+[[tracef]]
+==== Use `lttng_ust_tracef()`
+
+man:lttng_ust_tracef(3) is a small LTTng-UST API designed for quick,
+man:printf(3)-like instrumentation without the burden of
+<<tracepoint-provider,creating>> and
+<<building-tracepoint-providers-and-user-application,building>>
+a tracepoint provider package.
+
+To use `lttng_ust_tracef()` in your application:
+
+. In the C or $$C++$$ source files where you need to use
+ `lttng_ust_tracef()`, include `<lttng/tracef.h>`:
++
+--
+[source,c]
+----
+#include <lttng/tracef.h>
+----
+--
+
+. In the source code of the application, use `lttng_ust_tracef()` like
+ you would use man:printf(3):
++
+--
+[source,c]
+----
+ /* ... */
+
+ lttng_ust_tracef("my message: %d (%s)", my_integer, my_string);
+
+ /* ... */
+----
+--
+
+. Link your application with `liblttng-ust`:
++
+--
+[role="term"]
+----
+$ gcc -o app app.c -llttng-ust
+----
+--
+
+To record the events that `lttng_ust_tracef()` calls emit:
+
+* <<enabling-disabling-events,Create a recording event rule>> which
+ matches user space events named `lttng_ust_tracef:*`:
++
+--
+[role="term"]
+----
+$ lttng enable-event --userspace 'lttng_ust_tracef:*'
+----
+--
+
+[IMPORTANT]
+.Limitations of `lttng_ust_tracef()`
+====
+The `lttng_ust_tracef()` utility function was developed to make user
+space tracing super simple, albeit with notable disadvantages compared
+to <<defining-tracepoints,user-defined tracepoints>>:
+
+* All the created events have the same tracepoint provider and
+ tracepoint names, respectively `lttng_ust_tracef` and `event`.
+* There's no static type checking.
+* The only event record field you actually get, named `msg`, is a string
+ potentially containing the values you passed to `lttng_ust_tracef()`
+ using your own format string. This also means that you can't filter
+ events with a custom expression at run time because there are no
+ isolated fields.
+* Since `lttng_ust_tracef()` uses the man:vasprintf(3) function of the
+ C{nbsp}standard library behind the scenes to format the strings at run
+ time, its expected performance is lower than with user-defined
+ tracepoints, which don't require a conversion to a string.
+
+Taking this into consideration, `lttng_ust_tracef()` is useful for some
+quick prototyping and debugging, but you shouldn't consider it for any
+permanent and serious applicative instrumentation.
+====
+
+
+[role="since-2.7"]
+[[tracelog]]
+==== Use `lttng_ust_tracelog()`
+
+The man:tracelog(3) API is very similar to
+<<tracef,`lttng_ust_tracef()`>>, with the difference that it accepts an
+additional log level parameter.
+
+The goal of `lttng_ust_tracelog()` is to ease the migration from logging
+to tracing.
+
+To use `lttng_ust_tracelog()` in your application:
+
+. In the C or $$C++$$ source files where you need to use `tracelog()`,
+ include `<lttng/tracelog.h>`:
++
+--
+[source,c]
+----
+#include <lttng/tracelog.h>
+----
+--
+
+. In the source code of the application, use `lttng_ust_tracelog()` like
+ you would use man:printf(3), except for the first parameter which is
+ the log level:
++
+--
+[source,c]
+----
+ /* ... */
+
+ tracelog(LTTNG_UST_TRACEPOINT_LOGLEVEL_WARNING,
+ "my message: %d (%s)", my_integer, my_string);
+
+ /* ... */
+----
+--
++
+See man:lttng-ust(3) for a list of available log level names.
+
+. Link your application with `liblttng-ust`:
++
+--
+[role="term"]
+----
+$ gcc -o app app.c -llttng-ust
+----
+--
+
+To record the events that `lttng_ust_tracelog()` calls emit with a log
+level _at least as severe as_ a specific log level:
+
+* <<enabling-disabling-events,Create a recording event rule>> which
+ matches user space tracepoint events named `lttng_ust_tracelog:*` and
+ with some minimum level of severity:
++
+--
+[role="term"]
+----
+$ lttng enable-event --userspace 'lttng_ust_tracelog:*' \
+ --loglevel=WARNING
+----
+--
+
+To record the events that `lttng_ust_tracelog()` calls emit with a
+_specific log level_:
+
+* Create a recording event rule which matches tracepoint events named
+ `lttng_ust_tracelog:*` and with a specific log level:
++
+--
+[role="term"]
+----
+$ lttng enable-event --userspace 'lttng_ust_tracelog:*' \
+ --loglevel-only=INFO
+----
+--
+
+
+[[prebuilt-ust-helpers]]
+=== Load a prebuilt user space tracing helper
+
+The LTTng-UST package provides a few helpers in the form of preloadable
+shared objects which automatically instrument system functions and
+calls.
+
+The helper shared objects are normally found in dir:{/usr/lib}. If you
+built LTTng-UST <<building-from-source,from source>>, they're probably
+located in dir:{/usr/local/lib}.
+
+The installed user space tracing helpers in LTTng-UST{nbsp}{revision}
+are:
+
+path:{liblttng-ust-libc-wrapper.so}::
+path:{liblttng-ust-pthread-wrapper.so}::
+ <<liblttng-ust-libc-pthread-wrapper,C{nbsp}standard library
+ memory and POSIX threads function tracing>>.
+
+path:{liblttng-ust-cyg-profile.so}::
+path:{liblttng-ust-cyg-profile-fast.so}::
+ <<liblttng-ust-cyg-profile,Function entry and exit tracing>>.
+
+path:{liblttng-ust-dl.so}::
+ <<liblttng-ust-dl,Dynamic linker tracing>>.
+
+To use a user space tracing helper with any user application:
+
+* Preload the helper shared object when you start the application:
++
+--
+[role="term"]
+----
+$ LD_PRELOAD=liblttng-ust-libc-wrapper.so my-app
+----
+--
++
+You can preload more than one helper:
++
+--
+[role="term"]
+----
+$ LD_PRELOAD=liblttng-ust-libc-wrapper.so:liblttng-ust-dl.so my-app
+----
+--
+
+
+[role="since-2.3"]
+[[liblttng-ust-libc-pthread-wrapper]]
+==== Instrument C standard library memory and POSIX threads functions
+
+The path:{liblttng-ust-libc-wrapper.so} and
+path:{liblttng-ust-pthread-wrapper.so} helpers
+add instrumentation to some C standard library and POSIX
+threads functions.
+
+[role="growable"]
+.Functions instrumented by preloading path:{liblttng-ust-libc-wrapper.so}.
+|====
+|TP provider name |TP name |Instrumented function
+
+.6+|`lttng_ust_libc` |`malloc` |man:malloc(3)
+ |`calloc` |man:calloc(3)
+ |`realloc` |man:realloc(3)
+ |`free` |man:free(3)
+ |`memalign` |man:memalign(3)
+ |`posix_memalign` |man:posix_memalign(3)
+|====
+
+[role="growable"]
+.Functions instrumented by preloading path:{liblttng-ust-pthread-wrapper.so}.
+|====
+|TP provider name |TP name |Instrumented function
+
+.4+|`lttng_ust_pthread` |`pthread_mutex_lock_req` |man:pthread_mutex_lock(3p) (request time)
+ |`pthread_mutex_lock_acq` |man:pthread_mutex_lock(3p) (acquire time)
+ |`pthread_mutex_trylock` |man:pthread_mutex_trylock(3p)
+ |`pthread_mutex_unlock` |man:pthread_mutex_unlock(3p)
+|====
+
+When you preload the shared object, it replaces the functions listed
+in the previous tables by wrappers which contain tracepoints and call
+the replaced functions.
+
+
+[[liblttng-ust-cyg-profile]]
+==== Instrument function entry and exit
+
+The path:{liblttng-ust-cyg-profile*.so} helpers can add instrumentation
+to the entry and exit points of functions.
+
+man:gcc(1) and man:clang(1) have an option named
+https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html[`-finstrument-functions`]
+which generates instrumentation calls for entry and exit to functions.
+The LTTng-UST function tracing helpers,
+path:{liblttng-ust-cyg-profile.so} and
+path:{liblttng-ust-cyg-profile-fast.so}, take advantage of this feature
+to add tracepoints to the two generated functions (which contain
+`cyg_profile` in their names, hence the name of the helper).
+
+To use the LTTng-UST function tracing helper, the source files to
+instrument must be built using the `-finstrument-functions` compiler
+flag.
+
+There are two versions of the LTTng-UST function tracing helper:
+
+* **path:{liblttng-ust-cyg-profile-fast.so}** is a lightweight variant
+ that you should only use when it can be _guaranteed_ that the
+ complete event stream is recorded without any lost event record.
+ Any kind of duplicate information is left out.
++
+Assuming no event record is lost, having only the function addresses on
+entry is enough to create a call graph, since an event record always
+contains the ID of the CPU that generated it.
++
+Use a tool like man:addr2line(1) to convert function addresses back to
+source file names and line numbers.
+
+* **path:{liblttng-ust-cyg-profile.so}** is a more robust variant
+which also works in use cases where event records might get discarded or
+not recorded from application startup.
+In these cases, the trace analyzer needs more information to be
+able to reconstruct the program flow.
+
+See man:lttng-ust-cyg-profile(3) to learn more about the instrumentation
+points of this helper.
+
+All the tracepoints that this helper provides have the log level
+`LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_FUNCTION` (see man:lttng-ust(3)).
+
+TIP: It's sometimes a good idea to limit the number of source files that
+you compile with the `-finstrument-functions` option to prevent LTTng
+from writing an excessive amount of trace data at run time. When using
+man:gcc(1), use the
+`-finstrument-functions-exclude-function-list` option to avoid
+instrument entries and exits of specific function names.
+
+
+[role="since-2.4"]
+[[liblttng-ust-dl]]
+==== Instrument the dynamic linker
+
+The path:{liblttng-ust-dl.so} helper adds instrumentation to the
+man:dlopen(3) and man:dlclose(3) function calls.
+
+See man:lttng-ust-dl(3) to learn more about the instrumentation points
+of this helper.
+
+
+[role="since-2.4"]
+[[java-application]]
+=== Instrument a Java application
+
+You can instrument any Java application which uses one of the following
+logging frameworks:
+
+* The https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[**`java.util.logging`**]
+ (JUL) core logging facilities.
+
+* https://logging.apache.org/log4j/1.2/[**Apache log4j{nbsp}1.2**], since
+ LTTng{nbsp}2.6. Note that Apache Log4j{nbsp}2 isn't supported.
+
+[role="img-100"]
+.LTTng-UST Java agent imported by a Java application.
+image::java-app.png[]
+
+Note that the methods described below are new in LTTng{nbsp}2.8.
+Previous LTTng versions use another technique.
+
+NOTE: We use https://openjdk.java.net/[OpenJDK]{nbsp}8 for development
+and https://ci.lttng.org/[continuous integration], thus this version is
+directly supported. However, the LTTng-UST Java agent is also tested
+with OpenJDK{nbsp}7.
+
+
+[role="since-2.8"]
+[[jul]]
+==== Use the LTTng-UST Java agent for `java.util.logging`
+
+To use the LTTng-UST Java agent in a Java application which uses
+`java.util.logging` (JUL):
+
+. In the source code of the Java application, import the LTTng-UST log
+ handler package for `java.util.logging`:
++
+--
+[source,java]
+----
+import org.lttng.ust.agent.jul.LttngLogHandler;
+----
+--
+
+. Create an LTTng-UST `java.util.logging` log handler:
++
+--
+[source,java]
+----
+Handler lttngUstLogHandler = new LttngLogHandler();
+----
+--
+
+. Add this handler to the `java.util.logging` loggers which should emit
+ LTTng events:
++
+--
+[source,java]
+----
+Logger myLogger = Logger.getLogger("some-logger");
+
+myLogger.addHandler(lttngUstLogHandler);
+----
+--
+
+. Use `java.util.logging` log statements and configuration as usual.
+ The loggers with an attached LTTng-UST log handler can emit
+ LTTng events.
+
+. Before exiting the application, remove the LTTng-UST log handler from
+ the loggers attached to it and call its `close()` method:
++
+--
+[source,java]
+----
+myLogger.removeHandler(lttngUstLogHandler);
+lttngUstLogHandler.close();
+----
+--
++
+This isn't strictly necessary, but it's recommended for a clean
+disposal of the resources of the handler.
+
+. Include the common and JUL-specific JAR files of the LTTng-UST Java agent,
+ path:{lttng-ust-agent-common.jar} and path:{lttng-ust-agent-jul.jar},
+ in the
+ https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class
+ path] when you build the Java application.
++
+The JAR files are typically located in dir:{/usr/share/java}.
++
+IMPORTANT: The LTTng-UST Java agent must be
+<<installing-lttng,installed>> for the logging framework your
+application uses.
+
+.Use the LTTng-UST Java agent for `java.util.logging`.
+====
+[source,java]
+.path:{Test.java}
+----
+import java.io.IOException;
+import java.util.logging.Handler;
+import java.util.logging.Logger;
+import org.lttng.ust.agent.jul.LttngLogHandler;
+
+public class Test
+{
+ private static final int answer = 42;
+
+ public static void main(String[] argv) throws Exception
+ {
+ // Create a logger
+ Logger logger = Logger.getLogger("jello");
+
+ // Create an LTTng-UST log handler
+ Handler lttngUstLogHandler = new LttngLogHandler();
+
+ // Add the LTTng-UST log handler to our logger
+ logger.addHandler(lttngUstLogHandler);
+
+ // Log at will!
+ logger.info("some info");
+ logger.warning("some warning");
+ Thread.sleep(500);
+ logger.finer("finer information; the answer is " + answer);
+ Thread.sleep(123);
+ logger.severe("error!");
+
+ // Not mandatory, but cleaner
+ logger.removeHandler(lttngUstLogHandler);
+ lttngUstLogHandler.close();
+ }
+}
+----
+
+Build this example:
+
+[role="term"]
+----
+$ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java
+----
+
+<<creating-destroying-tracing-sessions,Create a recording session>>,
+<<enabling-disabling-events,create a recording event rule>> matching JUL
+events named `jello`, and <<basic-tracing-session-control,start
+recording>>:
+
+[role="term"]
+----
+$ lttng create
+$ lttng enable-event --jul jello
+$ lttng start
+----
+
+Run the compiled class:
+
+[role="term"]
+----
+$ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test
+----
+
+<<basic-tracing-session-control,Stop recording>> and inspect the
+recorded events:
+
+[role="term"]
+----
+$ lttng stop
+$ lttng view
+----
+====
+
+In the resulting trace, an <<event,event record>> which a Java
+application using `java.util.logging` generated is named
+`lttng_jul:event` and has the following fields:
+
+`msg`::
+ Log record message.
+
+`logger_name`::
+ Logger name.
+
+`class_name`::
+ Name of the class in which the log statement was executed.
+
+`method_name`::
+ Name of the method in which the log statement was executed.
+
+`long_millis`::
+ Logging time (timestamp in milliseconds).
+
+`int_loglevel`::
+ Log level integer value.
+
+`int_threadid`::
+ ID of the thread in which the log statement was executed.
+
+Use the opt:lttng-enable-event(1):--loglevel or
+opt:lttng-enable-event(1):--loglevel-only option of the
+man:lttng-enable-event(1) command to target a range of
+`java.util.logging` log levels or a specific `java.util.logging` log
+level.
+
+
+[role="since-2.8"]
+[[log4j]]
+==== Use the LTTng-UST Java agent for Apache log4j
+
+To use the LTTng-UST Java agent in a Java application which uses
+Apache log4j{nbsp}1.2:
+
+. In the source code of the Java application, import the LTTng-UST log
+ appender package for Apache log4j:
++
+--
+[source,java]
+----
+import org.lttng.ust.agent.log4j.LttngLogAppender;
+----
+--
+
+. Create an LTTng-UST log4j log appender:
++
+--
+[source,java]
+----
+Appender lttngUstLogAppender = new LttngLogAppender();
+----
+--
+
+. Add this appender to the log4j loggers which should emit LTTng events:
++
+--
+[source,java]
+----
+Logger myLogger = Logger.getLogger("some-logger");
+
+myLogger.addAppender(lttngUstLogAppender);
+----
+--
+
+. Use Apache log4j log statements and configuration as usual. The
+ loggers with an attached LTTng-UST log appender can emit LTTng events.
+
+. Before exiting the application, remove the LTTng-UST log appender from
+ the loggers attached to it and call its `close()` method:
++
+--
+[source,java]
+----
+myLogger.removeAppender(lttngUstLogAppender);
+lttngUstLogAppender.close();
+----
+--
++
+This isn't strictly necessary, but it's recommended for a clean
+disposal of the resources of the appender.
+
+. Include the common and log4j-specific JAR
+ files of the LTTng-UST Java agent, path:{lttng-ust-agent-common.jar} and
+ path:{lttng-ust-agent-log4j.jar}, in the
+ https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class
+ path] when you build the Java application.
++
+The JAR files are typically located in dir:{/usr/share/java}.
++
+IMPORTANT: The LTTng-UST Java agent must be
+<<installing-lttng,installed>> for the logging framework your
+application uses.
+
+.Use the LTTng-UST Java agent for Apache log4j.
+====
+[source,java]
+.path:{Test.java}
+----
+import org.apache.log4j.Appender;
+import org.apache.log4j.Logger;
+import org.lttng.ust.agent.log4j.LttngLogAppender;
+
+public class Test
+{
+ private static final int answer = 42;
+
+ public static void main(String[] argv) throws Exception
+ {
+ // Create a logger
+ Logger logger = Logger.getLogger("jello");
+
+ // Create an LTTng-UST log appender
+ Appender lttngUstLogAppender = new LttngLogAppender();
+
+ // Add the LTTng-UST log appender to our logger
+ logger.addAppender(lttngUstLogAppender);
+
+ // Log at will!
+ logger.info("some info");
+ logger.warn("some warning");
+ Thread.sleep(500);
+ logger.debug("debug information; the answer is " + answer);
+ Thread.sleep(123);
+ logger.fatal("error!");
+
+ // Not mandatory, but cleaner
+ logger.removeAppender(lttngUstLogAppender);
+ lttngUstLogAppender.close();
+ }
+}
+
+----
+
+Build this example (`$LOG4JPATH` is the path to the Apache log4j JAR
+file):
+
+[role="term"]
+----
+$ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH Test.java
+----
+
+<<creating-destroying-tracing-sessions,Create a recording session>>,
+<<enabling-disabling-events,create a recording event rule>> matching
+log4j events named `jello`, and <<basic-tracing-session-control,start
+recording>>:
+
+[role="term"]
+----
+$ lttng create
+$ lttng enable-event --log4j jello
+$ lttng start
+----
+
+Run the compiled class:
+
+[role="term"]
+----
+$ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH:. Test
+----
+
+<<basic-tracing-session-control,Stop recording>> and inspect the
+recorded events:
+
+[role="term"]
+----
+$ lttng stop
+$ lttng view
+----
+====
+
+In the resulting trace, an <<event,event record>> which a Java
+application using log4j generated is named `lttng_log4j:event` and
+has the following fields:
+
+`msg`::
+ Log record message.
+
+`logger_name`::
+ Logger name.
+
+`class_name`::
+ Name of the class in which the log statement was executed.
+
+`method_name`::
+ Name of the method in which the log statement was executed.
+
+`filename`::
+ Name of the file in which the executed log statement is located.
+
+`line_number`::
+ Line number at which the log statement was executed.
+
+`timestamp`::
+ Logging timestamp.
+
+`int_loglevel`::
+ Log level integer value.
+
+`thread_name`::
+ Name of the Java thread in which the log statement was executed.
+
+Use the opt:lttng-enable-event(1):--loglevel or
+opt:lttng-enable-event(1):--loglevel-only option of the
+man:lttng-enable-event(1) command to target a range of Apache log4j
+log levels or a specific log4j log level.
+
+
+[role="since-2.8"]
+[[java-application-context]]
+==== Provide application-specific context fields in a Java application
+
+A Java application-specific context field is a piece of state which
+the Java application provides. You can <<adding-context,add>> such
+a context field to be recorded, using the
+man:lttng-add-context(1) command, to each <<event,event record>>
+which the log statements of this application produce.
+
+For example, a given object might have a current request ID variable.
+You can create a context information retriever for this object and
+assign a name to this current request ID. You can then, using the
+man:lttng-add-context(1) command, add this context field by name so that
+LTTng writes it to the event records of a given `java.util.logging` or
+log4j <<channel,channel>>.
+
+To provide application-specific context fields in a Java application:
+
+. In the source code of the Java application, import the LTTng-UST
+ Java agent context classes and interfaces:
++
+--
+[source,java]
+----
+import org.lttng.ust.agent.context.ContextInfoManager;
+import org.lttng.ust.agent.context.IContextInfoRetriever;
+----
+--
+
+. Create a context information retriever class, that is, a class which
+ implements the `IContextInfoRetriever` interface:
++
+--
+[source,java]
+----
+class MyContextInfoRetriever implements IContextInfoRetriever
+{
+ @Override
+ public Object retrieveContextInfo(String key)
+ {
+ if (key.equals("intCtx")) {
+ return (short) 17;
+ } else if (key.equals("strContext")) {
+ return "context value!";
+ } else {
+ return null;
+ }
+ }
+}
+----
+--
++
+This `retrieveContextInfo()` method is the only member of the
+`IContextInfoRetriever` interface. Its role is to return the current
+value of a state by name to create a context field. The names of the
+context fields and which state variables they return depends on your
+specific scenario.
++
+All primitive types and objects are supported as context fields.
+When `retrieveContextInfo()` returns an object, the context field
+serializer calls its `toString()` method to add a string field to
+event records. The method can also return `null`, which means that
+no context field is available for the required name.
+
+. Register an instance of your context information retriever class to
+ the context information manager singleton:
++
+--
+[source,java]
+----
+IContextInfoRetriever cir = new MyContextInfoRetriever();
+ContextInfoManager cim = ContextInfoManager.getInstance();
+cim.registerContextInfoRetriever("retrieverName", cir);
+----
+--
+
+. Before exiting the application, remove your context information
+ retriever from the context information manager singleton:
++
+--
+[source,java]
+----
+ContextInfoManager cim = ContextInfoManager.getInstance();
+cim.unregisterContextInfoRetriever("retrieverName");
+----
+--
++
+This isn't strictly necessary, but it's recommended for a clean
+disposal of some resources of the manager.
+
+. Build your Java application with LTTng-UST Java agent support as
+ usual, following the procedure for either the
+ <<jul,`java.util.logging`>> or <<log4j,Apache log4j>> framework.
+
+.Provide application-specific context fields in a Java application.
+====
+[source,java]
+.path:{Test.java}
+----
+import java.util.logging.Handler;
+import java.util.logging.Logger;
+import org.lttng.ust.agent.jul.LttngLogHandler;
+import org.lttng.ust.agent.context.ContextInfoManager;
+import org.lttng.ust.agent.context.IContextInfoRetriever;
+
+public class Test
+{
+ // Our context information retriever class
+ private static class MyContextInfoRetriever
+ implements IContextInfoRetriever
+ {
+ @Override
+ public Object retrieveContextInfo(String key) {
+ if (key.equals("intCtx")) {
+ return (short) 17;
+ } else if (key.equals("strContext")) {
+ return "context value!";
+ } else {
+ return null;
+ }
+ }
+ }
+
+ private static final int answer = 42;
+
+ public static void main(String args[]) throws Exception
+ {
+ // Get the context information manager instance
+ ContextInfoManager cim = ContextInfoManager.getInstance();
+
+ // Create and register our context information retriever
+ IContextInfoRetriever cir = new MyContextInfoRetriever();
+ cim.registerContextInfoRetriever("myRetriever", cir);
+
+ // Create a logger
+ Logger logger = Logger.getLogger("jello");
+
+ // Create an LTTng-UST log handler
+ Handler lttngUstLogHandler = new LttngLogHandler();
+
+ // Add the LTTng-UST log handler to our logger
+ logger.addHandler(lttngUstLogHandler);
+
+ // Log at will!
+ logger.info("some info");
+ logger.warning("some warning");
+ Thread.sleep(500);
+ logger.finer("finer information; the answer is " + answer);
+ Thread.sleep(123);
+ logger.severe("error!");
+
+ // Not mandatory, but cleaner
+ logger.removeHandler(lttngUstLogHandler);
+ lttngUstLogHandler.close();
+ cim.unregisterContextInfoRetriever("myRetriever");
+ }
+}
+----
+
+Build this example:
+
+[role="term"]
+----
+$ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java
+----
+
+<<creating-destroying-tracing-sessions,Create a recording session>> and
+<<enabling-disabling-events,create a recording event rule>> matching
+`java.util.logging` events named `jello`:
+
+[role="term"]
+----
+$ lttng create
+$ lttng enable-event --jul jello
+----
+
+<<adding-context,Add the application-specific context fields>> to be
+recorded to the event records of the `java.util.logging` channel:
+
+[role="term"]
+----
+$ lttng add-context --jul --type='$app.myRetriever:intCtx'
+$ lttng add-context --jul --type='$app.myRetriever:strContext'
+----
+
+<<basic-tracing-session-control,Start recording>>:
+
+[role="term"]
+----
+$ lttng start
+----
+
+Run the compiled class:
+
+[role="term"]
+----
+$ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test
+----
+
+<<basic-tracing-session-control,Stop recording>> and inspect the
+recorded events:
+
+[role="term"]
+----
+$ lttng stop
+$ lttng view
+----
+====
+
+
+[role="since-2.7"]
+[[python-application]]
+=== Instrument a Python application
+
+You can instrument a Python{nbsp}2 or Python{nbsp}3 application which
+uses the standard
+https://docs.python.org/3/library/logging.html[`logging`] package.
+
+Each log statement creates an LTTng event once the application module
+imports the <<lttng-ust-agents,LTTng-UST Python agent>> package.
+
+[role="img-100"]
+.A Python application importing the LTTng-UST Python agent.
+image::python-app.png[]
+
+To use the LTTng-UST Python agent:
+
+. In the source code of the Python application, import the LTTng-UST
+ Python agent:
++
+--
+[source,python]
+----
+import lttngust
+----
+--
++
+The LTTng-UST Python agent automatically adds its logging handler to the
+root logger at import time.
++
+A log statement that the application executes before this import doesn't
+create an LTTng event.
++
+IMPORTANT: The LTTng-UST Python agent must be
+<<installing-lttng,installed>>.
+
+. Use log statements and logging configuration as usual.
+ Since the LTTng-UST Python agent adds a handler to the _root_
+ logger, any log statement from any logger can emit an LTTng event.
+
+.Use the LTTng-UST Python agent.
+====
+[source,python]
+.path:{test.py}
+----
+import lttngust
+import logging
+import time
+
+
+def example():
+ logging.basicConfig()
+ logger = logging.getLogger('my-logger')
+
+ while True:
+ logger.debug('debug message')
+ logger.info('info message')
+ logger.warn('warn message')
+ logger.error('error message')
+ logger.critical('critical message')
+ time.sleep(1)
+
+
+if __name__ == '__main__':
+ example()
+----
+
+NOTE: `logging.basicConfig()`, which adds to the root logger a basic
+logging handler which prints to the standard error stream, isn't
+strictly required for LTTng-UST tracing to work, but in versions of
+Python preceding{nbsp}3.2, you could see a warning message which
+indicates that no handler exists for the logger `my-logger`.
+
+<<creating-destroying-tracing-sessions,Create a recording session>>,
+<<enabling-disabling-events,create a recording event rule>> matching
+Python logging events named `my-logger`, and
+<<basic-tracing-session-control,start recording>>:
+
+[role="term"]
+----
+$ lttng create
+$ lttng enable-event --python my-logger
+$ lttng start
+----
+
+Run the Python script:
+
+[role="term"]
+----
+$ python test.py
+----
+
+<<basic-tracing-session-control,Stop recording>> and inspect the
+recorded events:
+
+[role="term"]
+----
+$ lttng stop
+$ lttng view
+----
+====
+
+In the resulting trace, an <<event,event record>> which a Python
+application generated is named `lttng_python:event` and has the
+following fields:
+
+`asctime`::
+ Logging time (string).
+
+`msg`::
+ Log record message.
+
+`logger_name`::
+ Logger name.
+
+`funcName`::
+ Name of the function in which the log statement was executed.
+
+`lineno`::
+ Line number at which the log statement was executed.
+
+`int_loglevel`::
+ Log level integer value.
+
+`thread`::
+ ID of the Python thread in which the log statement was executed.
+
+`threadName`::
+ Name of the Python thread in which the log statement was executed.
+
+Use the opt:lttng-enable-event(1):--loglevel or
+opt:lttng-enable-event(1):--loglevel-only option of the
+man:lttng-enable-event(1) command to target a range of Python log levels
+or a specific Python log level.
+
+When an application imports the LTTng-UST Python agent, the agent tries
+to register to a <<lttng-sessiond,session daemon>>. Note that you must
+<<start-sessiond,start the session daemon>> _before_ you run the Python
+application. If a session daemon is found, the agent tries to register
+to it during five seconds, after which the application continues
+without LTTng tracing support. Override this timeout value with
+the env:LTTNG_UST_PYTHON_REGISTER_TIMEOUT environment variable
+(milliseconds).
+
+If the session daemon stops while a Python application with an imported
+LTTng-UST Python agent runs, the agent retries to connect and to
+register to a session daemon every three seconds. Override this
+delay with the env:LTTNG_UST_PYTHON_REGISTER_RETRY_DELAY environment
+variable.
+
+
+[role="since-2.5"]
+[[proc-lttng-logger-abi]]
+=== Use the LTTng logger
+
+The `lttng-tracer` Linux kernel module, part of
+<<lttng-modules,LTTng-modules>>, creates the special LTTng logger files
+path:{/proc/lttng-logger} and path:{/dev/lttng-logger} (since
+LTTng{nbsp}2.11) when it's loaded. Any application can write text data
+to any of those files to create one or more LTTng events.
+
+[role="img-100"]
+.An application writes to the LTTng logger file to create one or more LTTng events.
+image::lttng-logger.png[]
+
+The LTTng logger is the quickest method--not the most efficient,
+however--to add instrumentation to an application. It's designed
+mostly to instrument shell scripts:
+
+[role="term"]
+----
+$ echo "Some message, some $variable" > /dev/lttng-logger
+----
+
+Any event that the LTTng logger creates is named `lttng_logger` and
+belongs to the Linux kernel <<domain,tracing domain>>. However, unlike
+other instrumentation points in the kernel tracing domain, **any Unix
+user** can <<enabling-disabling-events,create a recording event rule>>
+which matches events named `lttng_logger`, not only the root user or
+users in the <<tracing-group,tracing group>>.
+
+To use the LTTng logger:
+
+* From any application, write text data to the path:{/dev/lttng-logger}
+ file.
+
+The `msg` field of `lttng_logger` event records contains the
+recorded message.
+
+NOTE: The maximum message length of an LTTng logger event is
+1024{nbsp}bytes. Writing more than this makes the LTTng logger emit more
+than one event to contain the remaining data.
+
+You shouldn't use the LTTng logger to trace a user application which you
+can instrument in a more efficient way, namely:
+
+* <<c-application,C and $$C++$$ applications>>.
+* <<java-application,Java applications>>.
+* <<python-application,Python applications>>.
+
+.Use the LTTng logger.
+====
+[source,bash]
+.path:{test.bash}
+----
+echo 'Hello, World!' > /dev/lttng-logger
+sleep 2
+df --human-readable --print-type / > /dev/lttng-logger
+----
+
+<<creating-destroying-tracing-sessions,Create a recording session>>,
+<<enabling-disabling-events,create a recording event rule>> matching
+Linux kernel tracepoint events named `lttng_logger`, and
+<<basic-tracing-session-control,start recording>>:
+
+[role="term"]
+----
+$ lttng create
+$ lttng enable-event --kernel lttng_logger
+$ lttng start
+----
+
+Run the Bash script:
+
+[role="term"]
+----
+$ bash test.bash
+----
+
+<<basic-tracing-session-control,Stop recording>> and inspect the recorded
+events:
+
+[role="term"]
+----
+$ lttng stop
+$ lttng view
+----
+====
+
+
+[[instrumenting-linux-kernel]]
+=== Instrument a Linux kernel image or module
+
+NOTE: This section shows how to _add_ instrumentation points to the
+Linux kernel. The subsystems of the kernel are already thoroughly
+instrumented at strategic points for LTTng when you
+<<installing-lttng,install>> the <<lttng-modules,LTTng-modules>>
+package.
+
+
+[[linux-add-lttng-layer]]
+==== [[instrumenting-linux-kernel-itself]][[mainline-trace-event]][[lttng-adaptation-layer]]Add an LTTng layer to an existing ftrace tracepoint
+
+This section shows how to add an LTTng layer to existing ftrace
+instrumentation using the `TRACE_EVENT()` API.
+
+This section doesn't document the `TRACE_EVENT()` macro. Read the
+following articles to learn more about this API:
+
+* https://lwn.net/Articles/379903/[Using the TRACE_EVENT() macro (Part{nbsp}1)]
+* https://lwn.net/Articles/381064/[Using the TRACE_EVENT() macro (Part{nbsp}2)]
+* https://lwn.net/Articles/383362/[Using the TRACE_EVENT() macro (Part{nbsp}3)]
+
+The following procedure assumes that your ftrace tracepoints are
+correctly defined in their own header and that they're created in
+one source file using the `CREATE_TRACE_POINTS` definition.
+
+To add an LTTng layer over an existing ftrace tracepoint:
+
+. Make sure the following kernel configuration options are
+ enabled:
++
+--
+* `CONFIG_MODULES`
+* `CONFIG_KALLSYMS`
+* `CONFIG_HIGH_RES_TIMERS`
+* `CONFIG_TRACEPOINTS`
+--
+
+. Build the Linux source tree with your custom ftrace tracepoints.
+. Boot the resulting Linux image on your target system.
++
+Confirm that the tracepoints exist by looking for their names in the
+dir:{/sys/kernel/debug/tracing/events/subsys} directory, where `subsys`
+is your subsystem name.
+
+. Get a copy of the latest LTTng-modules{nbsp}{revision}:
++
+--
+[role="term"]
+----
+$ cd $(mktemp -d) &&
+ wget https://lttng.org/files/lttng-modules/lttng-modules-latest-2.13.tar.bz2 &&
+ tar -xf lttng-modules-latest-2.13.tar.bz2 &&
+ cd lttng-modules-2.13.*
+----
+--
+
+. In dir:{instrumentation/events/lttng-module}, relative to the root
+ of the LTTng-modules source tree, create a header file named
+ +__subsys__.h+ for your custom subsystem +__subsys__+ and write your
+ LTTng-modules tracepoint definitions using the LTTng-modules
+ macros in it.
++
+Start with this template:
++
+--
+[source,c]
+.path:{instrumentation/events/lttng-module/my_subsys.h}
+----
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM my_subsys
+
+#if !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _LTTNG_MY_SUBSYS_H
+
+#include "../../../probes/lttng-tracepoint-event.h"
+#include <linux/tracepoint.h>
+
+LTTNG_TRACEPOINT_EVENT(
+ /*
+ * Format is identical to the TRACE_EVENT() version for the three
+ * following macro parameters:
+ */
+ my_subsys_my_event,
+ TP_PROTO(int my_int, const char *my_string),
+ TP_ARGS(my_int, my_string),
+
+ /* LTTng-modules specific macros */
+ TP_FIELDS(
+ ctf_integer(int, my_int_field, my_int)
+ ctf_string(my_bar_field, my_bar)
+ )
+)
+
+#endif /* !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ) */
+
+#include "../../../probes/define_trace.h"
+----
+--
++
+The entries in the `TP_FIELDS()` section are the list of fields for the
+LTTng tracepoint. This is similar to the `TP_STRUCT__entry()` part of
+the `TRACE_EVENT()` ftrace macro.
++
+See ``<<lttng-modules-tp-fields,Tracepoint fields macros>>'' for a
+complete description of the available `ctf_*()` macros.
+
+. Create the kernel module C{nbsp}source file of the LTTng-modules
+ probe, +probes/lttng-probe-__subsys__.c+, where +__subsys__+ is your
+ subsystem name:
++
+--
+[source,c]
+.path:{probes/lttng-probe-my-subsys.c}
+----
+#include <linux/module.h>
+#include "../lttng-tracer.h"
+
+/*
+ * Build-time verification of mismatch between mainline
+ * TRACE_EVENT() arguments and the LTTng-modules adaptation
+ * layer LTTNG_TRACEPOINT_EVENT() arguments.
+ */
+#include <trace/events/my_subsys.h>
+
+/* Create LTTng tracepoint probes */
+#define LTTNG_PACKAGE_BUILD
+#define CREATE_TRACE_POINTS
+#define TRACE_INCLUDE_PATH ../instrumentation/events/lttng-module
+
+#include "../instrumentation/events/lttng-module/my_subsys.h"
+
+MODULE_LICENSE("GPL and additional rights");
+MODULE_AUTHOR("Your name <your-email>");
+MODULE_DESCRIPTION("LTTng my_subsys probes");
+MODULE_VERSION(__stringify(LTTNG_MODULES_MAJOR_VERSION) "."
+ __stringify(LTTNG_MODULES_MINOR_VERSION) "."
+ __stringify(LTTNG_MODULES_PATCHLEVEL_VERSION)
+ LTTNG_MODULES_EXTRAVERSION);
+----
+--
+
+. Edit path:{probes/KBuild} and add your new kernel module object
+ next to the existing ones:
++
+--
+[source,make]
+.path:{probes/KBuild}
+----
+# ...
+
+obj-m += lttng-probe-module.o
+obj-m += lttng-probe-power.o
+
+obj-m += lttng-probe-my-subsys.o
+
+# ...
+----
+--
+
+. Build and install the LTTng kernel modules:
++
+--
+[role="term"]
+----
+$ make KERNELDIR=/path/to/linux
+# make modules_install && depmod -a
+----
+--
++
+Replace `/path/to/linux` with the path to the Linux source tree where
+you defined and used tracepoints with the `TRACE_EVENT()` ftrace macro.
+
+Note that you can also use the
+<<lttng-tracepoint-event-code,`LTTNG_TRACEPOINT_EVENT_CODE()` macro>>
+instead of `LTTNG_TRACEPOINT_EVENT()` to use custom local variables and
+C{nbsp}code that need to be executed before LTTng records the event
+fields.
+
+The best way to learn how to use the previous LTTng-modules macros is to
+inspect the existing LTTng-modules tracepoint definitions in the
+dir:{instrumentation/events/lttng-module} header files. Compare them
+with the Linux kernel mainline versions in the
+dir:{include/trace/events} directory of the Linux source tree.
+
+
+[role="since-2.7"]
+[[lttng-tracepoint-event-code]]
+===== Use custom C code to access the data for tracepoint fields
+
+Although we recommended to always use the
+<<lttng-adaptation-layer,`LTTNG_TRACEPOINT_EVENT()`>> macro to describe
+the arguments and fields of an LTTng-modules tracepoint when possible,
+sometimes you need a more complex process to access the data that the
+tracer records as event record fields. In other words, you need local
+variables and multiple C{nbsp}statements instead of simple
+argument-based expressions that you pass to the
+<<lttng-modules-tp-fields,`ctf_*()` macros of `TP_FIELDS()`>>.
+
+Use the `LTTNG_TRACEPOINT_EVENT_CODE()` macro instead of
+`LTTNG_TRACEPOINT_EVENT()` to declare custom local variables and define
+a block of C{nbsp}code to be executed before LTTng records the fields.
+The structure of this macro is:
+
+[source,c]
+.`LTTNG_TRACEPOINT_EVENT_CODE()` macro syntax.
+----
+LTTNG_TRACEPOINT_EVENT_CODE(
+ /*
+ * Format identical to the LTTNG_TRACEPOINT_EVENT()
+ * version for the following three macro parameters:
+ */
+ my_subsys_my_event,
+ TP_PROTO(int my_int, const char *my_string),
+ TP_ARGS(my_int, my_string),
+
+ /* Declarations of custom local variables */
+ TP_locvar(
+ int a = 0;
+ unsigned long b = 0;
+ const char *name = "(undefined)";
+ struct my_struct *my_struct;
+ ),
+
+ /*
+ * Custom code which uses both tracepoint arguments
+ * (in TP_ARGS()) and local variables (in TP_locvar()).
+ *
+ * Local variables are actually members of a structure pointed
+ * to by the special variable tp_locvar.
+ */
+ TP_code(
+ if (my_int) {
+ tp_locvar->a = my_int + 17;
+ tp_locvar->my_struct = get_my_struct_at(tp_locvar->a);
+ tp_locvar->b = my_struct_compute_b(tp_locvar->my_struct);
+ tp_locvar->name = my_struct_get_name(tp_locvar->my_struct);
+ put_my_struct(tp_locvar->my_struct);
+
+ if (tp_locvar->b) {
+ tp_locvar->a = 1;
+ }
+ }
+ ),
+
+ /*
+ * Format identical to the LTTNG_TRACEPOINT_EVENT()
+ * version for this, except that tp_locvar members can be
+ * used in the argument expression parameters of
+ * the ctf_*() macros.
+ */
+ TP_FIELDS(
+ ctf_integer(unsigned long, my_struct_b, tp_locvar->b)
+ ctf_integer(int, my_struct_a, tp_locvar->a)
+ ctf_string(my_string_field, my_string)
+ ctf_string(my_struct_name, tp_locvar->name)
+ )
+)
+----
+
+IMPORTANT: The C code defined in `TP_code()` must not have any side
+effects when executed. In particular, the code must not allocate
+memory or get resources without deallocating this memory or putting
+those resources afterwards.
+
+
+[[instrumenting-linux-kernel-tracing]]
+==== Load and unload a custom probe kernel module
+
+You must load a <<lttng-adaptation-layer,created LTTng-modules probe
+kernel module>> in the kernel before it can emit LTTng events.
+
+To load the default probe kernel modules and a custom probe kernel
+module:
+
+* Use the opt:lttng-sessiond(8):--extra-kmod-probes option to give extra
+ probe modules to load when starting a root <<lttng-sessiond,session
+ daemon>>:
++
+--
+.Load the `my_subsys`, `usb`, and the default probe modules.
+====
+[role="term"]
+----
+# lttng-sessiond --extra-kmod-probes=my_subsys,usb
+----
+====
+--
++
+You only need to pass the subsystem name, not the whole kernel module
+name.
+
+To load _only_ a given custom probe kernel module:
+
+* Use the opt:lttng-sessiond(8):--kmod-probes option to give the probe
+ modules to load when starting a root session daemon:
++
+--
+.Load only the `my_subsys` and `usb` probe modules.
+====
+[role="term"]
+----
+# lttng-sessiond --kmod-probes=my_subsys,usb
+----
+====
+--
+
+To confirm that a probe module is loaded:
+
+* Use man:lsmod(8):
++
+--
+[role="term"]
+----
+$ lsmod | grep lttng_probe_usb
+----
+--
+
+To unload the loaded probe modules:
+
+* Kill the session daemon with `SIGTERM`:
++
+--
+[role="term"]
+----
+# pkill lttng-sessiond
+----
+--
++
+You can also use the `--remove` option of man:modprobe(8) if the session
+daemon terminates abnormally.
+
+
+[[controlling-tracing]]
+== Tracing control
+
+Once an application or a Linux kernel is <<instrumenting,instrumented>>
+for LTTng tracing, you can _trace_ it.
+
+In the LTTng context, _tracing_ means making sure that LTTng attempts to
+execute some action(s) when a CPU executes an instrumentation point.
+
+This section is divided in topics on how to use the various
+<<plumbing,components of LTTng>>, in particular the
+<<lttng-cli,cmd:lttng command-line tool>>, to _control_ the LTTng
+daemons and tracers.
+
+NOTE: In the following subsections, we refer to an man:lttng(1) command
+using its man page name. For example, instead of ``Run the `create`
+command to'', we write ``Run the man:lttng-create(1) command to''.
+
+
+[[start-sessiond]]
+=== Start a session daemon
+
+In some situations, you need to run a <<lttng-sessiond,session daemon>>
+(man:lttng-sessiond(8)) _before_ you can use the man:lttng(1)
+command-line tool.
+
+You will see the following error when you run a command while no session
+daemon is running:
+
+----
+Error: No session daemon is available
+----
+
+The only command that automatically runs a session daemon is
+man:lttng-create(1), which you use to
+<<creating-destroying-tracing-sessions,create a recording session>>. While
+this could be your most used first operation, sometimes it's not. Some
+examples are:
+
+* <<list-instrumentation-points,List the available instrumentation points>>.
+* <<saving-loading-tracing-session,Load a recording session configuration>>.
+* <<add-event-rule-matches-trigger,Add a trigger>>.
+
+All the examples above don't require a recording session to operate on.
+
+[[tracing-group]] Each Unix user can have its own running session daemon
+to use the user space LTTng tracer. The session daemon that the `root`
+user starts is the only one allowed to control the LTTng kernel tracer.
+Members of the Unix _tracing group_ may connect to and control the root
+session daemon, even for user space tracing. See the ``Session daemon
+connection'' section of man:lttng(1) to learn more about the Unix
+tracing group.
+
+To start a user session daemon:
+
+* Run man:lttng-sessiond(8):
++
+--
+[role="term"]
+----
+$ lttng-sessiond --daemonize
+----
+--
+
+To start the root session daemon:
+
+* Run man:lttng-sessiond(8) as the `root` user:
++
+--
+[role="term"]
+----
+# lttng-sessiond --daemonize
+----
+--
+
+In both cases, remove the opt:lttng-sessiond(8):--daemonize option to
+start the session daemon in foreground.
+
+To stop a session daemon, kill its process (see man:kill(1)) with the
+standard `TERM` signal.
+
+Note that some Linux distributions could manage the LTTng session daemon
+as a service. In this case, we suggest that you use the service manager
+to start, restart, and stop session daemons.
+
+
+[[creating-destroying-tracing-sessions]]
+=== Create and destroy a recording session
+
+Many LTTng control operations happen in the scope of a
+<<tracing-session,recording session>>, which is the dialogue between the
+<<lttng-sessiond,session daemon>> and you for everything related to
+<<event,event recording>>.
+
+To create a recording session with a generated name:
+
+* Use the man:lttng-create(1) command:
++
+--
+[role="term"]
+----
+$ lttng create
+----
+--
+
+The name of the created recording session is `auto` followed by the
+creation date.
+
+To create a recording session with a specific name:
+
+* Use the optional argument of the man:lttng-create(1) command:
++
+--
+[role="term"]
+----
+$ lttng create SESSION
+----
+--
++
+Replace +__SESSION__+ with your specific recording session name.
+
+In <<local-mode,local mode>>, LTTng writes the traces of a recording
+session to the +$LTTNG_HOME/lttng-traces/__NAME__-__DATE__-__TIME__+
+directory by default, where +__NAME__+ is the name of the recording
+session. Note that the env:LTTNG_HOME environment variable defaults to
+`$HOME` if not set.
+
+To output LTTng traces to a non-default location:
+
+* Use the opt:lttng-create(1):--output option of the man:lttng-create(1)
+ command:
++
+--
+[role="term"]
+----
+$ lttng create my-session --output=/tmp/some-directory
+----
+--
+
+You may create as many recording sessions as you wish.
+
+To list all the existing recording sessions for your Unix user, or for
+all users if your Unix user is `root`:
+
+* Use the man:lttng-list(1) command:
++
+--
+[role="term"]
+----
+$ lttng list
+----
+--
+
+[[cur-tracing-session]]When you create a recording session, the
+man:lttng-create(1) command sets it as the _current recording session_.
+The following man:lttng(1) commands operate on the current recording
+session when you don't specify one:
+
+[role="list-3-cols"]
+* man:lttng-add-context(1)
+* man:lttng-clear(1)
+* man:lttng-destroy(1)
+* man:lttng-disable-channel(1)
+* man:lttng-disable-event(1)
+* man:lttng-disable-rotation(1)
+* man:lttng-enable-channel(1)
+* man:lttng-enable-event(1)
+* man:lttng-enable-rotation(1)
+* man:lttng-load(1)
+* man:lttng-regenerate(1)
+* man:lttng-rotate(1)
+* man:lttng-save(1)
+* man:lttng-snapshot(1)
+* man:lttng-start(1)
+* man:lttng-status(1)
+* man:lttng-stop(1)
+* man:lttng-track(1)
+* man:lttng-untrack(1)
+* man:lttng-view(1)
+
+To change the current recording session:
+
+* Use the man:lttng-set-session(1) command:
++
+--
+[role="term"]
+----
+$ lttng set-session SESSION
+----
+--
++
+Replace +__SESSION__+ with the name of the new current recording session.
+
+When you're done recording in a given recording session, destroy it.
+This operation frees the resources taken by the recording session to
+destroy; it doesn't destroy the trace data that LTTng wrote for this
+recording session (see ``<<clear,Clear a recording session>>'' for one
+way to do this).
+
+To destroy the current recording session:
+
+* Use the man:lttng-destroy(1) command:
++
+--
+[role="term"]
+----
+$ lttng destroy
+----
+--
+
+The man:lttng-destroy(1) command also runs the man:lttng-stop(1) command
+implicitly (see ``<<basic-tracing-session-control,Start and stop a
+recording session>>''). You need to stop recording to make LTTng flush the
+remaining trace data and make the trace readable.
+
+
+[[list-instrumentation-points]]
+=== List the available instrumentation points
+
+The <<lttng-sessiond,session daemon>> can query the running instrumented
+user applications and the Linux kernel to get a list of available
+instrumentation points:
+
+* LTTng tracepoints and system calls for the Linux kernel
+ <<domain,tracing domain>>.
+
+* LTTng tracepoints for the user space tracing domain.
+
+To list the available instrumentation points:
+
+. <<start-sessiond,Make sure>> there's a running
+ <<lttng-sessiond,session daemon>> to which your Unix user can
+ connect.
+
+. Use the man:lttng-list(1) command with the option of the requested
+ tracing domain amongst:
++
+--
+opt:lttng-list(1):--kernel::
+ Linux kernel tracepoints.
++
+Your Unix user must be `root`, or it must be a member of the Unix
+<<tracing-group,tracing group>>.
+
+opt:lttng-list(1):--kernel with opt:lttng-list(1):--syscall::
+ Linux kernel system calls.
++
+Your Unix user must be `root`, or it must be a member of the Unix
+<<tracing-group,tracing group>>.
+
+opt:lttng-list(1):--userspace::
+ User space tracepoints.
+
+opt:lttng-list(1):--jul::
+ `java.util.logging` loggers.
+
+opt:lttng-list(1):--log4j::
+ Apache log4j loggers.
+
+opt:lttng-list(1):--python::
+ Python loggers.
+--
+
+.List the available user space tracepoints.
+====
+[role="term"]
+----
+$ lttng list --userspace
+----
+====
+
+.List the available Linux kernel system calls.
+====
+[role="term"]
+----
+$ lttng list --kernel --syscall
+----
+====
+
+
+[[enabling-disabling-events]]
+=== Create and enable a recording event rule
+
+Once you <<creating-destroying-tracing-sessions,create a recording
+session>>, you can create <<event,recording event rules>> with the
+man:lttng-enable-event(1) command.
+
+The man:lttng-enable-event(1) command always attaches an event rule to a
+<<channel,channel>> on creation. The command can create a _default
+channel_, named `channel0`, for you. The man:lttng-enable-event(1)
+command reuses the default channel each time you run it for the same
+tracing domain and session.
+
+A recording event rule is always enabled at creation time.
+
+The following examples show how to combine the command-line arguments of
+the man:lttng-enable-event(1) command to create simple to more complex
+recording event rules within the <<cur-tracing-session,current recording
+session>>.
+
+.Create a recording event rule matching specific Linux kernel tracepoint events (default channel).
+====
+[role="term"]
+----
+# lttng enable-event --kernel sched_switch
+----
+====
+
+.Create a recording event rule matching Linux kernel system call events with four specific names (default channel).
+====
+[role="term"]
+----
+# lttng enable-event --kernel --syscall open,write,read,close
+----
+====
+
+.Create recording event rules matching tracepoint events which satisfy a filter expressions (default channel).
+====
+[role="term"]
+----
+# lttng enable-event --kernel sched_switch --filter='prev_comm == "bash"'
+----
+
+[role="term"]
+----
+# lttng enable-event --kernel --all \
+ --filter='$ctx.tid == 1988 || $ctx.tid == 1534'
+----
+
+[role="term"]
+----
+$ lttng enable-event --jul my_logger \
+ --filter='$app.retriever:cur_msg_id > 3'
+----
+
+IMPORTANT: Make sure to always single-quote the filter string when you
+run man:lttng(1) from a shell.
+
+See also ``<<pid-tracking,Allow specific processes to record events>>''
+which offers another, more efficient filtering mechanism for process ID,
+user ID, and group ID attributes.
+====
+
+.Create a recording event rule matching any user space event from the `my_app` tracepoint provider and with a log level range (default channel).
+====
+[role="term"]
+----
+$ lttng enable-event --userspace my_app:'*' --loglevel=INFO
+----
+
+IMPORTANT: Make sure to always single-quote the wildcard character when
+you run man:lttng(1) from a shell.
+====
+
+.Create a recording event rule matching user space events named specifically, but with name exclusions (default channel).
+====
+[role="term"]
+----
+$ lttng enable-event --userspace my_app:'*' \
+ --exclude=my_app:set_user,my_app:handle_sig
+----
+====
+
+.Create a recording event rule matching any Apache log4j event with a specific log level (default channel).
+====
+[role="term"]
+----
+$ lttng enable-event --log4j --all --loglevel-only=WARN
+----
+====
+
+.Create a recording event rule, attached to a specific channel, and matching user space tracepoint events named `my_app:my_tracepoint`.
+====
+[role="term"]
+----
+$ lttng enable-event --userspace my_app:my_tracepoint \
+ --channel=my-channel
+----
+====
+
+.Create a recording event rule matching user space probe events for the `malloc` function entry in path:{/usr/lib/libc.so.6}:
+====
+[role="term"]
+----
+# lttng enable-event --kernel \
+ --userspace-probe=/usr/lib/libc.so.6:malloc \
+ libc_malloc
+----
+====
+
+.Create a recording event rule matching user space probe events for the `server`/`accept_request` https://www.sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps[USDT probe] in path:{/usr/bin/serv}:
+====
+[role="term"]
+----
+# lttng enable-event --kernel \
+ --userspace-probe=sdt:serv:server:accept_request \
+ server_accept_request
+----
+====
+
+The recording event rules of a given channel form a whitelist: as soon
+as an event rule matches an event, LTTng emits it _once_ and therefore
+<<channel-overwrite-mode-vs-discard-mode,can>> record it. For example,
+the following rules both match user space tracepoint events named
+`my_app:my_tracepoint` with an `INFO` log level:
+
+[role="term"]
+----
+$ lttng enable-event --userspace my_app:my_tracepoint
+$ lttng enable-event --userspace my_app:my_tracepoint \
+ --loglevel=INFO
+----
+
+The second recording event rule is redundant: the first one includes the
+second one.
+
+
+[[disable-event-rule]]
+=== Disable a recording event rule
+
+To disable a <<event,recording event rule>> that you
+<<enabling-disabling-events,created>> previously, use the
+man:lttng-disable-event(1) command.
+
+man:lttng-disable-event(1) can only find recording event rules to
+disable by their <<instrumentation-point-types,instrumentation point
+type>> and event name conditions. Therefore, you cannot disable
+recording event rules having a specific instrumentation point log level
+condition, for example.
+
+LTTng doesn't emit (and, therefore, won't record) an event which only
+_disabled_ recording event rules match.
+
+.Disable event rules matching Python logging events from the `my-logger` logger (default <<channel,channel>>, <<cur-tracing-session,current recording session>>).
+====
+[role="term"]
+----
+$ lttng disable-event --python my-logger
+----
+====
+
+.Disable event rules matching all `java.util.logging` events (default channel, recording session `my-session`).
+====
+[role="term"]
+----
+$ lttng disable-event --jul --session=my-session '*'
+----
+====
+
+.Disable _all_ the Linux kernel recording event rules (channel `my-chan`, current recording session).
+====
+The opt:lttng-disable-event(1):--all-events option isn't, like the
+opt:lttng-enable-event(1):--all option of the man:lttng-enable-event(1)
+command, an alias for the event name globbing pattern `*`: it disables
+_all_ the recording event rules of a given channel.
+
+[role="term"]
+----
+# lttng disable-event --kernel --channel=my-chan --all-events
+----
+====
+
+NOTE: You can't _remove_ a recording event rule once you create it.
+
+
+[[status]]
+=== Get the status of a recording session
+
+To get the status of the <<cur-tracing-session,current recording
+session>>, that is, its parameters, its channels, recording event rules,
+and their attributes:
+
+* Use the man:lttng-status(1) command:
++
+--
+[role="term"]
+----
+$ lttng status
+----
+--
+
+To get the status of any recording session:
+
+* Use the man:lttng-list(1) command with the name of the recording
+ session:
++
+--
+[role="term"]
+----
+$ lttng list SESSION
+----
+--
++
+Replace +__SESSION__+ with the recording session name.
+
+
+[[basic-tracing-session-control]]
+=== Start and stop a recording session
+
+Once you <<creating-destroying-tracing-sessions,create a recording
+session>> and <<enabling-disabling-events,create one or more recording
+event rules>>, you can start and stop the tracers for this recording
+session.
+
+To start the <<cur-tracing-session,current recording session>>:
+
+* Use the man:lttng-start(1) command:
++
+--
+[role="term"]
+----
+$ lttng start
+----
+--
+
+LTTng is flexible: you can launch user applications before or after you
+start the tracers. An LTTng tracer only <<event,records an event>> if a
+recording event rule matches it, which means the tracer is active.
+
+The `start-session` <<trigger,trigger>> action can also start a recording
+session.
+
+To stop the current recording session:
+
+* Use the man:lttng-stop(1) command:
++
+--
+[role="term"]
+----
+$ lttng stop
+----
+--
++
+If there were <<channel-overwrite-mode-vs-discard-mode,lost event
+records>> or lost sub-buffers since the last time you ran
+man:lttng-start(1), the man:lttng-stop(1) command prints corresponding
+warnings.
+
+IMPORTANT: You need to stop recording to make LTTng flush the remaining
+trace data and make the trace readable. Note that the
+man:lttng-destroy(1) command (see
+``<<creating-destroying-tracing-sessions,Create and destroy a recording
+session>>'') also runs the man:lttng-stop(1) command implicitly.
+
+The `stop-session` <<trigger,trigger>> action can also stop a recording
+session.
+
+[role="since-2.12"]
+[[clear]]
+=== Clear a recording session
+
+You might need to remove all the current tracing data of one or more
+<<tracing-session,recording sessions>> between multiple attempts to
+reproduce a problem without interrupting the LTTng recording activity.
+
+To clear the tracing data of the
+<<cur-tracing-session,current recording session>>:
+
+* Use the man:lttng-clear(1) command:
++
+--
+[role="term"]
+----
+$ lttng clear
+----
+--
+
+To clear the tracing data of all the recording sessions:
+
+* Use the `lttng clear` command with its opt:lttng-clear(1):--all
+ option:
++
+--
+[role="term"]
+----
+$ lttng clear --all
+----
+--
+
+
+[[enabling-disabling-channels]]
+=== Create a channel
+
+Once you <<creating-destroying-tracing-sessions,create a recording
+session>>, you can create a <<channel,channel>> with the
+man:lttng-enable-channel(1) command.
+
+Note that LTTng can automatically create a default channel when you
+<<enabling-disabling-events,create a recording event rule>>.
+Therefore, you only need to create a channel when you need non-default
+attributes.
+
+Specify each non-default channel attribute with a command-line
+option when you run the man:lttng-enable-channel(1) command.
+
+You can only create a custom channel in the Linux kernel and user space
+<<domain,tracing domains>>: the Java/Python logging tracing domains have
+their own default channel which LTTng automatically creates when you
+<<enabling-disabling-events,create a recording event rule>>.
+
+[IMPORTANT]
+====
+As of LTTng{nbsp}{revision}, you may _not_ perform the
+following operations with the man:lttng-enable-channel(1) command:
+
+* Change an attribute of an existing channel.
+
+* Enable a disabled channel once its recording session has been
+ <<basic-tracing-session-control,active>> at least once.
+
+* Create a channel once its recording session has been active at
+ least once.
+
+* Create a user space channel with a given
+ <<channel-buffering-schemes,buffering scheme>> and create a second
+ user space channel with a different buffering scheme in the same
+ recording session.
+====
+
+The following examples show how to combine the command-line options of
+the man:lttng-enable-channel(1) command to create simple to more complex
+channels within the <<cur-tracing-session,current recording session>>.
+
+.Create a Linux kernel channel with default attributes.
+====
+[role="term"]
+----
+# lttng enable-channel --kernel my-channel
+----
+====
+
+.Create a user space channel with four sub-buffers or 1{nbsp}MiB each, per CPU, per instrumented process.
+====
+[role="term"]
+----
+$ lttng enable-channel --userspace --num-subbuf=4 --subbuf-size=1M \
+ --buffers-pid my-channel
+----
+====
+
+.[[blocking-timeout-example]]Create a default user space channel with an infinite blocking timeout.
+====
+<<creating-destroying-tracing-sessions,Create a recording session>>,
+create the channel, <<enabling-disabling-events,create a recording event
+rule>>, and <<basic-tracing-session-control,start recording>>:
+
+[role="term"]
+----
+$ lttng create
+$ lttng enable-channel --userspace --blocking-timeout=inf blocking-chan
+$ lttng enable-event --userspace --channel=blocking-chan --all
+$ lttng start
+----
+
+Run an application instrumented with LTTng-UST tracepoints and allow it
+to block:
+
+[role="term"]
+----
+$ LTTNG_UST_ALLOW_BLOCKING=1 my-app
+----
+====
+
+.Create a Linux kernel channel which rotates eight trace files of 4{nbsp}MiB each for each stream.
+====
+[role="term"]
+----
+# lttng enable-channel --kernel --tracefile-count=8 \
+ --tracefile-size=4194304 my-channel
+----
+====
+
+.Create a user space channel in <<overwrite-mode,overwrite>> (or ``flight recorder'') mode.
+====
+[role="term"]
+----
+$ lttng enable-channel --userspace --overwrite my-channel
+----
+====
+
+.<<enabling-disabling-events,Create>> the same <<event,recording event rule>> attached to two different channels.
+====
+[role="term"]
+----
+$ lttng enable-event --userspace --channel=my-channel app:tp
+$ lttng enable-event --userspace --channel=other-channel app:tp
+----
+
+When a CPU executes the `app:tp` <<c-application,user space
+tracepoint>>, the two recording event rules above match the created
+event, making LTTng emit the event. Because the recording event rules
+are not attached to the same channel, LTTng records the event twice.
+====
+
+
+[[disable-channel]]
+=== Disable a channel
+
+To disable a specific channel that you
+<<enabling-disabling-channels,created>> previously, use the
+man:lttng-disable-channel(1) command.
+
+.Disable a specific Linux kernel channel (<<cur-tracing-session,current recording session>>).
+====
+[role="term"]
+----
+# lttng disable-channel --kernel my-channel
+----
+====
+
+An enabled channel is an implicit <<event,recording event rule>>
+condition.
+
+NOTE: As of LTTng{nbsp}{revision}, you may _not_ enable a disabled
+channel once its recording session has been
+<<basic-tracing-session-control,started>> at least once.
+
+
+[[adding-context]]
+=== Add context fields to be recorded to the event records of a channel
+
+<<event,Event record>> fields in trace files provide important
+information about previously emitted events, but sometimes some external
+context may help you solve a problem faster.
+
+Examples of context fields are:
+
+* The **process ID**, **thread ID**, **process name**, and
+ **process priority** of the thread from which LTTng emits the event.
+
+* The **hostname** of the system on which LTTng emits the event.
+
+* The Linux kernel and user call stacks (since LTTng{nbsp}2.11).
+
+* The current values of many possible **performance counters** using
+ perf, for example:
+
+** CPU cycles, stalled cycles, idle cycles, and the other cycle types.
+** Cache misses.
+** Branch instructions, misses, and loads.
+** CPU faults.
+
+* Any state defined at the application level (supported for the
+ `java.util.logging` and Apache log4j <<domain,tracing domains>>).
+
+To get the full list of available context fields:
+
+* Use the opt:lttng-add-context(1):--list option of the
+ man:lttng-add-context(1) command:
++
+[role="term"]
+----
+$ lttng add-context --list
+----
+
+.Add context fields to be recorded to the event records of all the <<channel,channels>> of the <<cur-tracing-session,current recording session>>.
+====
+The following command line adds the virtual process identifier and the
+per-thread CPU cycles count fields to all the user space channels of the
+current recording session.
+
+[role="term"]
+----
+$ lttng add-context --userspace --type=vpid --type=perf:thread:cpu-cycles
+----
+====
+
+.Add performance counter context fields by raw ID
+====
+See man:lttng-add-context(1) for the exact format of the context field
+type, which is partly compatible with the format used in
+man:perf-record(1).
+
+[role="term"]
+----
+# lttng add-context --userspace --type=perf:thread:raw:r0110:test
+# lttng add-context --kernel --type=perf:cpu:raw:r0013c:x86unhalted
+----
+====
+
+.Add context fields to be recorded to the event records of a specific channel.
+====
+The following command line adds the thread identifier and user call
+stack context fields to the Linux kernel channel named `my-channel` of
+the <<cur-tracing-session,current recording session>>.
+
+[role="term"]
+----
+# lttng add-context --kernel --channel=my-channel \
+ --type=tid --type=callstack-user
+----
+====
+
+.Add an <<java-application-context,application-specific context field>> to be recorded to the event records of a specific channel.
+====
+The following command line makes sure LTTng writes the `cur_msg_id`
+context field of the `retriever` context retriever to all the Java
+logging <<event,event records>> of the channel named `my-channel`:
+
+[role="term"]
+----
+# lttng add-context --kernel --channel=my-channel \
+ --type='$app:retriever:cur_msg_id'
+----
+
+IMPORTANT: Make sure to always single-quote the `$` character when you
+run man:lttng-add-context(1) from a shell.
+====
+
+NOTE: You can't undo what the man:lttng-add-context(1) command does.
+
+
+[role="since-2.7"]
+[[pid-tracking]]
+=== Allow specific processes to record events
+
+It's often useful to only allow processes with specific attributes to
+record events. For example, you may wish to record all the system calls
+which a given process makes (à la man:strace(1)).
+
+The man:lttng-track(1) and man:lttng-untrack(1) commands serve this
+purpose. Both commands operate on _inclusion sets_ of process
+attributes. The available process attribute types are:
+
+Linux kernel <<domain,tracing domain>>::
++
+* Process ID (PID).
+
+* Virtual process ID (VPID).
++
+This is the PID as seen by the application.
+
+* Unix user ID (UID).
+
+* Virtual Unix user ID (VUID).
++
+This is the UID as seen by the application.
+
+* Unix group ID (GID).
+
+* Virtual Unix group ID (VGID).
++
+This is the GID as seen by the application.
+
+User space tracing domain::
++
+* VPID
+* VUID
+* VGID
+
+A <<tracing-session,recording session>> has nine process
+attribute inclusion sets: six for the Linux kernel <<domain,tracing domain>>
+and three for the user space tracing domain.
+
+For a given recording session, a process{nbsp}__P__ is allowed to record
+LTTng events for a given <<domain,tracing domain>>{nbsp}__D__ if _all_
+the attributes of{nbsp}__P__ are part of the inclusion sets
+of{nbsp}__D__.
+
+Whether a process is allowed or not to record LTTng events is an
+implicit condition of all <<event,recording event rules>>. Therefore, if
+LTTng creates an event{nbsp}__E__ for a given process, but this process
+may not record events, then no recording event rule matches{nbsp}__E__,
+which means LTTng won't emit and record{nbsp}__E__.
+
+When you <<creating-destroying-tracing-sessions,create a recording
+session>>, all its process attribute inclusion sets contain all the
+possible values. In other words, all processes are allowed to record
+events.
+
+Add values to an inclusion set with the man:lttng-track(1) command and
+remove values with the man:lttng-untrack(1) command.
+
+[NOTE]
+====
+The process attribute values are _numeric_.
+
+Should a process with a given ID (part of an inclusion set), for
+example, exit, and then a new process be given this same ID, then the
+latter would also be allowed to record events.
+
+With the man:lttng-track(1) command, you can add Unix user and group
+_names_ to the user and group inclusion sets: the
+<<lttng-sessiond,session daemon>> finds the corresponding UID, VUID,
+GID, or VGID once on _addition_ to the inclusion set. This means that if
+you rename the user or group after you run the man:lttng-track(1)
+command, its user/group ID remains part of the inclusion sets.
+====
+
+.Allow processes to record events based on their virtual process ID (VPID).
+====
+For the sake of the following example, assume the target system has
+16{nbsp}possible VPIDs.
+
+When you
+<<creating-destroying-tracing-sessions,create a recording session>>,
+the user space VPID inclusion set contains _all_ the possible VPIDs:
+
+[role="img-100"]
+.The VPID inclusion set is full.
+image::track-all.png[]
+
+When the inclusion set is full and you run the man:lttng-track(1)
+command to specify some VPIDs, LTTng:
+
+. Clears the inclusion set.
+. Adds the specific VPIDs to the inclusion set.
+
+After:
+
+[role="term"]
+----
+$ lttng track --userspace --vpid=3,4,7,10,13
+----
+
+the VPID inclusion set is:
+
+[role="img-100"]
+.The VPID inclusion set contains the VPIDs 3, 4, 7, 10, and 13.
+image::track-3-4-7-10-13.png[]
+
+Add more VPIDs to the inclusion set afterwards:
+
+[role="term"]
+----
+$ lttng track --userspace --vpid=1,15,16
+----
+
+The result is:
+
+[role="img-100"]
+.VPIDs 1, 15, and 16 are added to the inclusion set.
+image::track-1-3-4-7-10-13-15-16.png[]
+
+The man:lttng-untrack(1) command removes entries from process attribute
+inclusion sets. Given the previous example, the following command:
+
+[role="term"]
+----
+$ lttng untrack --userspace --vpid=3,7,10,13
+----
+
+leads to this VPID inclusion set:
+
+[role="img-100"]
+.VPIDs 3, 7, 10, and 13 are removed from the inclusion set.
+image::track-1-4-15-16.png[]
+
+You can make the VPID inclusion set full again with the
+opt:lttng-track(1):--all option:
+
+[role="term"]
+----
+$ lttng track --userspace --vpid --all
+----
+
+The result is, again:
+
+[role="img-100"]
+.The VPID inclusion set is full.
+image::track-all.png[]
+====
+
+.Allow specific processes to record events based on their user ID (UID).
+====
+A typical use case with process attribute inclusion sets is to start
+with an empty inclusion set, then <<basic-tracing-session-control,start
+the tracers>>, and finally add values manually while the tracers are
+active.
+
+Use the opt:lttng-untrack(1):--all option of the
+man:lttng-untrack(1) command to clear the inclusion set after you
+<<creating-destroying-tracing-sessions,create a recording session>>, for
+example (with UIDs):
+
+[role="term"]
+----
+# lttng untrack --kernel --uid --all
+----
+
+gives:
+
+[role="img-100"]
+.The UID inclusion set is empty.
+image::untrack-all.png[]
+
+If the LTTng tracer runs with this inclusion set configuration, it
+records no events within the <<cur-tracing-session,current recording
+session>> because no processes is allowed to do so. Use the
+man:lttng-track(1) command as usual to add specific values to the UID
+inclusion set when you need to, for example:
+
+[role="term"]
+----
+# lttng track --kernel --uid=http,11
+----
+
+Result:
+
+[role="img-100"]
+.UIDs 6 (`http`) and 11 are part of the UID inclusion set.
+image::track-6-11.png[]
+====
+
+
+[role="since-2.5"]
+[[saving-loading-tracing-session]]
+=== Save and load recording session configurations
+
+Configuring a <<tracing-session,recording session>> can be long. Some of
+the tasks involved are:
+
+* <<enabling-disabling-channels,Create channels>> with
+ specific attributes.
+
+* <<adding-context,Add context fields>> to be recorded to the
+ <<event,event records>> of specific channels.
+
+* <<enabling-disabling-events,Create recording event rules>> with
+ specific log level, filter, and other conditions.
+
+If you use LTTng to solve real world problems, chances are you have to
+record events using the same recording session setup over and over,
+modifying a few variables each time in your instrumented program or
+environment.
+
+To avoid constant recording session reconfiguration, the man:lttng(1)
+command-line tool can save and load recording session configurations
+to/from XML files.
+
+To save a given recording session configuration:
+
+* Use the man:lttng-save(1) command:
++
+--
+[role="term"]
+----
+$ lttng save SESSION
+----
+--
++
+Replace +__SESSION__+ with the name of the recording session to save.
+
+LTTng saves recording session configurations to
+dir:{$LTTNG_HOME/.lttng/sessions} by default. Note that the
+env:LTTNG_HOME environment variable defaults to `$HOME` if not set. See
+man:lttng-save(1) to learn more about the recording session configuration
+output path.
+
+LTTng saves all configuration parameters, for example:
+
+* The recording session name.
+* The trace data output path.
+* The <<channel,channels>>, with their state and all their attributes.
+* The context fields you added to channels.
+* The <<event,recording event rules>> with their state and conditions.
+
+To load a recording session:
+
+* Use the man:lttng-load(1) command:
++
+--
+[role="term"]
+----
+$ lttng load SESSION
+----
+--
++
+Replace +__SESSION__+ with the name of the recording session to load.
+
+When LTTng loads a configuration, it restores your saved recording session
+as if you just configured it manually.
+
+You can also save and load many sessions at a time; see
+man:lttng-save(1) and man:lttng-load(1) to learn more.
+
+
+[[sending-trace-data-over-the-network]]
+=== Send trace data over the network
+
+LTTng can send the recorded trace data of a <<tracing-session,recording
+session>> to a remote system over the network instead of writing it to
+the local file system.
+
+To send the trace data over the network:
+
+. On the _remote_ system (which can also be the target system),
+ start an LTTng <<lttng-relayd,relay daemon>> (man:lttng-relayd(8)):
++
+--
+[role="term"]
+----
+$ lttng-relayd
+----
+--
+
+. On the _target_ system, create a recording session
+ <<net-streaming-mode,configured>> to send trace data over the network:
++
+--
+[role="term"]
+----
+$ lttng create my-session --set-url=net://remote-system
+----
+--
++
+Replace +__remote-system__+ with the host name or IP address of the
+remote system. See man:lttng-create(1) for the exact URL format.
+
+. On the target system, use the man:lttng(1) command-line tool as usual.
++
+When recording is <<basic-tracing-session-control,active>>, the
+<<lttng-consumerd,consumer daemon>> of the target sends the contents of
+<<channel,sub-buffers>> to the remote relay daemon instead of flushing
+them to the local file system. The relay daemon writes the received
+packets to its local file system.
+
+See the ``Output directory'' section of man:lttng-relayd(8) to learn
+where a relay daemon writes its received trace data.
+
+
+[role="since-2.4"]
+[[lttng-live]]
+=== View events as LTTng records them (noch:{LTTng} live)
+
+_LTTng live_ is a network protocol implemented by the
+<<lttng-relayd,relay daemon>> (man:lttng-relayd(8)) to allow compatible
+trace readers to display or analyze <<event,event records>> as LTTng
+records events on the target system while recording is
+<<basic-tracing-session-control,active>>.
+
+The relay daemon creates a _tee_: it forwards the trace data to both the
+local file system and to connected live readers:
+
+[role="img-90"]
+.The relay daemon creates a _tee_, forwarding the trace data to both trace files and a connected live reader.
+image::live.png[]
+
+To use LTTng live:
+
+. On the _target system_, create a <<tracing-session,recording session>>
+ in _live mode_:
++
+--
+[role="term"]
+----
+$ lttng create my-session --live
+----
+--
++
+This operation spawns a local relay daemon.
+
+. Start the live reader and configure it to connect to the relay daemon.
++
+For example, with man:babeltrace2(1):
++
+--
+[role="term"]
+----
+$ babeltrace2 net://localhost/host/HOSTNAME/my-session
+----
+--
++
+Replace +__HOSTNAME__+ with the host name of the target system.
+
+. Configure the recording session as usual with the man:lttng(1)
+ command-line tool, and <<basic-tracing-session-control,start recording>>.
+
+List the available live recording sessions with man:babeltrace2(1):
+
+[role="term"]
+----
+$ babeltrace2 net://localhost
+----
+
+You can start the relay daemon on another system. In this case, you need
+to specify the URL of the relay daemon when you
+<<creating-destroying-tracing-sessions,create the recording session>> with
+the opt:lttng-create(1):--set-url option of the man:lttng-create(1)
+command. You also need to replace +__localhost__+ in the procedure above
+with the host name of the system on which the relay daemon runs.
+
+
+[role="since-2.3"]
+[[taking-a-snapshot]]
+=== Take a snapshot of the current sub-buffers of a recording session
+
+The normal behavior of LTTng is to append full sub-buffers to growing
+trace data files. This is ideal to keep a full history of the events
+which the target system emitted, but it can represent too much data in
+some situations.
+
+For example, you may wish to have LTTng record your application
+continuously until some critical situation happens, in which case you
+only need the latest few recorded events to perform the desired
+analysis, not multi-gigabyte trace files.
+
+With the man:lttng-snapshot(1) command, you can take a _snapshot_ of the
+current <<channel,sub-buffers>> of a given <<tracing-session,recording
+session>>. LTTng can write the snapshot to the local file system or send
+it over the network.
+
+[role="img-100"]
+.A snapshot is a copy of the current sub-buffers, which LTTng does _not_ clear after the operation.
+image::snapshot.png[]
+
+The snapshot feature of LTTng is similar to how a
+https://en.wikipedia.org/wiki/Flight_recorder[flight recorder] or the
+``roll'' mode of an oscilloscope work.
+
+TIP: If you wish to create unmanaged, self-contained, non-overlapping
+trace chunk archives instead of a simple copy of the current
+sub-buffers, see the <<session-rotation,recording session rotation>>
+feature (available since LTTng{nbsp}2.11).
+
+To take a snapshot of the <<cur-tracing-session,current recording
+session>>:
+
+. Create a recording session in <<snapshot-mode,snapshot mode>>:
++
+--
+[role="term"]
+----
+$ lttng create my-session --snapshot
+----
+--
++
+The <<channel-overwrite-mode-vs-discard-mode,event record loss mode>> of
+<<channel,channels>> created in this mode is automatically set to
+<<overwrite-mode,_overwrite_>>.
+
+. Configure the recording session as usual with the man:lttng(1)
+ command-line tool, and <<basic-tracing-session-control,start
+ recording>>.
+
+. **Optional**: When you need to take a snapshot,
+ <<basic-tracing-session-control,stop recording>>.
++
+You can take a snapshot when the tracers are active, but if you stop
+them first, you're guaranteed that the trace data in the sub-buffers
+doesn't change before you actually take the snapshot.
+
+. Take a snapshot:
++
+--
+[role="term"]
+----
+$ lttng snapshot record --name=my-first-snapshot
+----
+--
++
+LTTng writes the current sub-buffers of all the channels of the
+<<cur-tracing-session,current recording session>> to
+trace files on the local file system. Those trace files have
+`my-first-snapshot` in their name.
+
+There's no difference between the format of a normal trace file and the
+format of a snapshot: LTTng trace readers also support LTTng snapshots.
+
+By default, LTTng writes snapshot files to the path shown by
+
+[role="term"]
+----
+$ lttng snapshot list-output
+----
+
+You can change this path or decide to send snapshots over the network
+using either:
+
+. An output path or URL that you specify when you
+ <<creating-destroying-tracing-sessions,create the recording session>>.
+
+. A snapshot output path or URL that you add using the
+ `add-output` action of the man:lttng-snapshot(1) command.
+
+. An output path or URL that you provide directly to the
+ `record` action of the man:lttng-snapshot(1) command.
+
+Method{nbsp}3 overrides method{nbsp}2, which overrides method 1. When
+you specify a URL, a <<lttng-relayd,relay daemon>> must listen on a
+remote system (see ``<<sending-trace-data-over-the-network,Send trace
+data over the network>>'').
+
+The `snapshot-session` <<trigger,trigger>> action can also take
+a recording session snapshot.
+
+
+[role="since-2.11"]
+[[session-rotation]]
+=== Archive the current trace chunk (rotate a recording session)
+
+The <<taking-a-snapshot,snapshot user guide>> shows how to dump the
+current sub-buffers of a recording session to the file system or send them
+over the network. When you take a snapshot, LTTng doesn't clear the ring
+buffers of the recording session: if you take another snapshot immediately
+after, both snapshots could contain overlapping trace data.
+
+Inspired by https://en.wikipedia.org/wiki/Log_rotation[log rotation],
+_recording session rotation_ is a feature which appends the content of the
+ring buffers to what's already on the file system or sent over the
+network since the creation of the recording session or since the last
+rotation, and then clears those ring buffers to avoid trace data
+overlaps.
+
+What LTTng is about to write when performing a recording session rotation
+is called the _current trace chunk_. When LTTng writes or sends over the
+network this current trace chunk, it becomes a _trace chunk archive_.
+Therefore, a recording session rotation operation _archives_ the current
+trace chunk.
+
+[role="img-100"]
+.A recording session rotation operation _archives_ the current trace chunk.
+image::rotation.png[]
+
+A trace chunk archive is a self-contained LTTng trace which LTTng
+doesn't manage anymore: you can read it, modify it, move it, or remove
+it.
+
+As of LTTng{nbsp}{revision}, there are three methods to perform a
+recording session rotation:
+
+* <<immediate-rotation,Immediately>>.
+
+* With a <<rotation-schedule,rotation schedule>>.
+
+* Through the execution of a `rotate-session` <<trigger,trigger>>
+ action.
+
+[[immediate-rotation]]To perform an immediate rotation of the
+<<cur-tracing-session,current recording session>>:
+
+. <<creating-destroying-tracing-sessions,Create a recording session>> in
+ <<local-mode,local mode>> or <<net-streaming-mode,network streaming
+ mode>> (only those two recording session modes support recording session
+ rotation):
++
+--
+[role="term"]
+----
+# lttng create my-session
+----
+--
+
+. <<enabling-disabling-events,Create one or more recording event rules>>
+ and <<basic-tracing-session-control,start recording>>:
++
+--
+[role="term"]
+----
+# lttng enable-event --kernel sched_'*'
+# lttng start
+----
+--
+
+. When needed, immediately rotate the current recording session:
++
+--
+[role="term"]
+----
+# lttng rotate
+----
+--
++
+The man:lttng-rotate(1) command prints the path to the created trace
+chunk archive. See its manual page to learn about the format of trace
+chunk archive directory names.
++
+Perform other immediate rotations while the recording session is active.
+It's guaranteed that all the trace chunk archives don't contain
+overlapping trace data. You can also perform an immediate rotation once
+you have <<basic-tracing-session-control,stopped>> the recording session.
+
+. When you're done recording,
+ <<creating-destroying-tracing-sessions,destroy the current recording
+ session>>:
++
+--
+[role="term"]
+----
+# lttng destroy
+----
+--
++
+The recording session destruction operation creates one last trace chunk
+archive from the current trace chunk.
+
+[[rotation-schedule]]A recording session rotation schedule is a planned
+rotation which LTTng performs automatically based on one of the
+following conditions:
+
+* A timer with a configured period expires.
+
+* The total size of the _flushed_ part of the current trace chunk
+ becomes greater than or equal to a configured value.
+
+To schedule a rotation of the <<cur-tracing-session,current recording
+session>>, set a _rotation schedule_:
+
+. <<creating-destroying-tracing-sessions,Create a recording session>> in
+ <<local-mode,local mode>> or <<net-streaming-mode,network streaming
+ mode>> (only those two creation modes support recording session
+ rotation):
++
+--
+[role="term"]
+----
+# lttng create my-session
+----
+--
+
+. <<enabling-disabling-events,Create one or more recording event rules>>:
++
+--
+[role="term"]
+----
+# lttng enable-event --kernel sched_'*'
+----
+--
+
+. Set a recording session rotation schedule:
++
+--
+[role="term"]
+----
+# lttng enable-rotation --timer=10s
+----
+--
++
+In this example, we set a rotation schedule so that LTTng performs a
+recording session rotation every ten seconds.
++
+See man:lttng-enable-rotation(1) to learn more about other ways to set a
+rotation schedule.
+
+. <<basic-tracing-session-control,Start recording>>:
++
+--
+[role="term"]
+----
+# lttng start
+----
+--
++
+LTTng performs recording session rotations automatically while the
+recording session is active thanks to the rotation schedule.
+
+. When you're done recording,
+ <<creating-destroying-tracing-sessions,destroy the current recording
+ session>>:
++
+--
+[role="term"]
+----
+# lttng destroy
+----
+--
++
+The recording session destruction operation creates one last trace chunk
+archive from the current trace chunk.
+
+Unset a recording session rotation schedule with the
+man:lttng-disable-rotation(1) command.
+
+
+[role="since-2.13"]
+[[add-event-rule-matches-trigger]]
+=== Add an ``event rule matches'' trigger to a session daemon
+
+With the man:lttng-add-trigger(1) command, you can add a
+<<trigger,trigger>> to a <<lttng-sessiond,session daemon>>.
+
+A trigger associates an LTTng tracing condition to one or more actions:
+when the condition is satisfied, LTTng attempts to execute the actions.
+
+A trigger doesn't need any <<tracing-session,recording session>> to exist:
+it belongs to a session daemon.
+
+As of LTTng{nbsp}{revision}, many condition types are available through
+the <<liblttng-ctl-lttng,`liblttng-ctl`>> C{nbsp}API, but the
+man:lttng-add-trigger(1) command only accepts the ``event rule matches''
+condition.
+
+An ``event rule matches'' condition is satisfied when its event rule
+matches an event.
+
+Unlike a <<event,recording event rule>>, the event rule of an
+``event rule matches'' trigger condition has no implicit conditions,
+that is:
+
+* It has no enabled/disabled state.
+* It has no attached <<channel,channel>>.
+* It doesn't belong to a <<tracing-session,recording session>>.
+
+Both the man:lttng-add-trigger(1) and man:lttng-enable-event(1) commands
+accept command-line arguments to specify an <<event-rule,event rule>>.
+That being said, the former is a more recent command and therefore
+follows the common event rule specification format (see
+man:lttng-event-rule(7)).
+
+.Start a <<tracing-session,recording session>> when an event rule matches.
+====
+This example shows how to add the following trigger to the root
+<<lttng-sessiond,session daemon>>:
+
+Condition::
+ An event rule matches a Linux kernel system call event of which the
+ name starts with `exec` and `*/ls` matches the `filename` payload
+ field.
++
+With such an event rule, LTTng emits an event when the cmd:ls program
+starts.
+
+Action::
+ <<basic-tracing-session-control,Start the recording session>>
+ named `pitou`.
+
+To add such a trigger to the root session daemon:
+
+. **If there's no currently running LTTng root session daemon**, start
+ one:
++
+[role="term"]
+----
+# lttng-sessiond --daemonize
+----
+
+. <<creating-destroying-tracing-sessions,Create a recording session>>
+ named `pitou` and
+ <<enabling-disabling-events,create a recording event rule>> matching
+ all the system call events:
++
+[role="term"]
+----
+# lttng create pitou
+# lttng enable-event --kernel --syscall --all
+----
+
+. Add the trigger to the root session daemon:
++
+[role="term"]
+----
+# lttng add-trigger --condition=event-rule-matches \
+ --type=syscall --name='exec*' \
+ --filter='filename == "*/ls"' \
+ --action=start-session pitou
+----
++
+Confirm that the trigger exists with the man:lttng-list-triggers(1)
+command:
++
+[role="term"]
+----
+# lttng list-triggers
+----
+
+. Make sure the `pitou` recording session is still inactive (stopped):
++
+[role="term"]
+----
+# lttng list pitou
+----
++
+The first line should be something like:
++
+----
+Recording session pitou: [inactive]
+----
+
+Run the cmd:ls program to fire the LTTng trigger above:
+
+[role="term"]
+----
+$ ls ~
+----
+
+At this point, the `pitou` recording session should be active
+(started). Confirm this with the man:lttng-list(1) command again:
+
+[role="term"]
+----
+# lttng list pitou
+----
+
+The first line should now look like:
+
+----
+Recording session pitou: [active]
+----
+
+This line confirms that the LTTng trigger you added fired, therefore
+starting the `pitou` recording session.
+====
+
+.[[trigger-event-notif]]Send a notification to a user application when an event rule matches.
+====
+This example shows how to add the following trigger to the root
+<<lttng-sessiond,session daemon>>:
+
+Condition::
+ An event rule matches a Linux kernel tracepoint event named
+ `sched_switch` and of which the value of the `next_comm` payload
+ field is `bash`.
++
+With such an event rule, LTTng emits an event when Linux gives access to
+the processor to a process named `bash`.
+
+Action::
+ Send an LTTng notification to a user application.
+
+Moreover, we'll specify a _capture descriptor_ with the
+`event-rule-matches` trigger condition so that the user application can
+get the value of a specific `sched_switch` event payload field.
+
+First, write and build the user application:
+
+. Create the C{nbsp}source file of the application:
++
+--
+[source,c]
+.path:{notif-app.c}
+----
+#include <stdlib.h>
+#include <stdio.h>
+#include <stdbool.h>
+#include <assert.h>
+#include <string.h>
+#include <lttng/lttng.h>
+
+/*
+ * Subscribes to notifications, through the notification channel
+ * `notification_channel`, which match the condition of the trigger
+ * named `trigger_name`.
+ *
+ * Returns `true` on success.
+ */
+static bool subscribe(struct lttng_notification_channel *notification_channel,
+ const char *trigger_name)
+{
+ const struct lttng_condition *condition = NULL;
+ struct lttng_triggers *triggers = NULL;
+ unsigned int trigger_count;
+ unsigned int i;
+ enum lttng_error_code error_code;
+ enum lttng_trigger_status trigger_status;
+ bool ret = false;
+
+ /* Get all LTTng triggers */
+ error_code = lttng_list_triggers(&triggers);
+ assert(error_code == LTTNG_OK);
+
+ /* Get the number of triggers */
+ trigger_status = lttng_triggers_get_count(triggers, &trigger_count);
+ assert(trigger_status == LTTNG_TRIGGER_STATUS_OK);
+
+ /* Find the trigger named `trigger_name` */
+ for (i = 0; i < trigger_count; i++) {
+ const struct lttng_trigger *trigger;
+ const char *this_trigger_name;
+
+ trigger = lttng_triggers_get_at_index(triggers, i);
+ trigger_status = lttng_trigger_get_name(trigger, &this_trigger_name);
+ assert(trigger_status == LTTNG_TRIGGER_STATUS_OK);
+
+ if (strcmp(this_trigger_name, trigger_name) == 0) {
+ /* Trigger found: subscribe with its condition */
+ enum lttng_notification_channel_status notification_channel_status;
+
+ notification_channel_status = lttng_notification_channel_subscribe(
+ notification_channel,
+ lttng_trigger_get_const_condition(trigger));
+ assert(notification_channel_status ==
+ LTTNG_NOTIFICATION_CHANNEL_STATUS_OK);
+ ret = true;
+ break;
+ }
+ }
+
+ lttng_triggers_destroy(triggers);
+ return ret;
+}
+
+/*
+ * Handles the evaluation `evaluation` of a single notification.
+ */
+static void handle_evaluation(const struct lttng_evaluation *evaluation)
+{
+ enum lttng_evaluation_status evaluation_status;
+ const struct lttng_event_field_value *array_field_value;
+ const struct lttng_event_field_value *string_field_value;
+ enum lttng_event_field_value_status event_field_value_status;
+ const char *string_field_string_value;
+
+ /* Get the value of the first captured (string) field */
+ evaluation_status = lttng_evaluation_event_rule_matches_get_captured_values(
+ evaluation, &array_field_value);
+ assert(evaluation_status == LTTNG_EVALUATION_STATUS_OK);
+ event_field_value_status =
+ lttng_event_field_value_array_get_element_at_index(
+ array_field_value, 0, &string_field_value);
+ assert(event_field_value_status == LTTNG_EVENT_FIELD_VALUE_STATUS_OK);
+ assert(lttng_event_field_value_get_type(string_field_value) ==
+ LTTNG_EVENT_FIELD_VALUE_TYPE_STRING);
+ event_field_value_status = lttng_event_field_value_string_get_value(
+ string_field_value, &string_field_string_value);
+ assert(event_field_value_status == LTTNG_EVENT_FIELD_VALUE_STATUS_OK);
+
+ /* Print the string value of the field */
+ puts(string_field_string_value);
+}
+
+int main(int argc, char *argv[])
+{
+ int exit_status = EXIT_SUCCESS;
+ struct lttng_notification_channel *notification_channel;
+ enum lttng_notification_channel_status notification_channel_status;
+ const struct lttng_condition *condition;
+ const char *trigger_name;
+ bool subscribe_res;
+
+ assert(argc >= 2);
+ trigger_name = argv[1];
+
+ /*
+ * Create a notification channel.
+ *
+ * A notification channel connects the user application to the LTTng
+ * session daemon.
+ *
+ * You can use this notification channel to listen to various types
+ * of notifications.
+ */
+ notification_channel = lttng_notification_channel_create(
+ lttng_session_daemon_notification_endpoint);
+ assert(notification_channel);
+
+ /*
+ * Subscribe to notifications which match the condition of the
+ * trigger named `trigger_name`.
+ */
+ if (!subscribe(notification_channel, trigger_name)) {
+ fprintf(stderr,
+ "Error: Failed to subscribe to notifications (trigger `%s`).\n",
+ trigger_name);
+ exit_status = EXIT_FAILURE;
+ goto end;
+ }
+
+ /*
+ * Notification loop.
+ *
+ * Put this in a dedicated thread to avoid blocking the main thread.
+ */
+ while (true) {
+ struct lttng_notification *notification;
+ enum lttng_notification_channel_status status;
+ const struct lttng_evaluation *notification_evaluation;
+
+ /* Receive the next notification */
+ status = lttng_notification_channel_get_next_notification(
+ notification_channel, ¬ification);
+
+ switch (status) {
+ case LTTNG_NOTIFICATION_CHANNEL_STATUS_OK:
+ break;
+ case LTTNG_NOTIFICATION_CHANNEL_STATUS_NOTIFICATIONS_DROPPED:
+ /*
+ * The session daemon can drop notifications if a receiving
+ * application doesn't consume the notifications fast
+ * enough.
+ */
+ continue;
+ case LTTNG_NOTIFICATION_CHANNEL_STATUS_CLOSED:
+ /*
+ * The session daemon closed the notification channel.
+ *
+ * This is typically caused by a session daemon shutting
+ * down.
+ */
+ goto end;
+ default:
+ /* Unhandled conditions or errors */
+ exit_status = EXIT_FAILURE;
+ goto end;
+ }
+
+ /*
+ * Handle the condition evaluation.
+ *
+ * A notification provides, amongst other things:
+ *
+ * * The condition that caused LTTng to send this notification.
+ *
+ * * The condition evaluation, which provides more specific
+ * information on the evaluation of the condition.
+ */
+ handle_evaluation(lttng_notification_get_evaluation(notification));
+
+ /* Destroy the notification object */
+ lttng_notification_destroy(notification);
+ }
+
+end:
+ lttng_notification_channel_destroy(notification_channel);
+ return exit_status;
+}
+----
+--
++
+This application prints the first captured string field value of the
+condition evaluation of each LTTng notification it receives.
+
+. Build the `notif-app` application,
+ using https://www.freedesktop.org/wiki/Software/pkg-config/[pkg-config]
+ to provide the right compiler and linker flags:
++
+--
+[role="term"]
+----
+$ gcc -o notif-app notif-app.c $(pkg-config --cflags --libs lttng-ctl)
+----
+--
+
+Now, to add the trigger to the root session daemon:
+
+[start=3]
+. **If there's no currently running LTTng root session daemon**, start
+ one:
++
+[role="term"]
+----
+# lttng-sessiond --daemonize
+----
+
+. Add the trigger, naming it `sched-switch-notif`, to the root
+ session daemon:
++
+[role="term"]
+----
+# lttng add-trigger --name=sched-switch-notif \
+ --condition=event-rule-matches \
+ --type=kernel --name=sched_switch \
+ --filter='next_comm == "bash"' --capture=prev_comm \
+ --action=notify
+----
++
+Confirm that the `sched-switch-notif` trigger exists with the
+man:lttng-list-triggers(1) command:
++
+[role="term"]
+----
+# lttng list-triggers
+----
+
+Run the cmd:notif-app application, passing the name of the trigger
+of which to watch the notifications:
+
+[role="term"]
+----
+# ./notif-app sched-switch-notif
+----
+
+Now, in an interactive Bash, type a few keys to fire the
+`sched-switch-notif` trigger. Watch the `notif-app` application print
+the previous process names.
+====
+
+[role="since-2.6"]
+[[mi]]
+=== Use the machine interface
+
+With any command of the man:lttng(1) command-line tool, set the
+opt:lttng(1):--mi option to `xml` (before the command name) to get an
+XML machine interface output, for example:
+
+[role="term"]
+----
+$ lttng --mi=xml list my-session
+----
+
+A schema definition (XSD) is
+https://github.com/lttng/lttng-tools/blob/stable-{revision}/src/common/mi-lttng-4.0.xsd[available]
+to ease the integration with external tools as much as possible.
+
+
+[role="since-2.8"]
+[[metadata-regenerate]]
+=== Regenerate the metadata of an LTTng trace
+
+An LTTng trace, which is a https://diamon.org/ctf[CTF] trace, has both
+data stream files and a metadata stream file. This metadata file
+contains, amongst other things, information about the offset of the
+clock sources which LTTng uses to assign timestamps to <<event,event
+records>> when recording.
+
+If, once a <<tracing-session,recording session>> is
+<<basic-tracing-session-control,started>>, a major
+https://en.wikipedia.org/wiki/Network_Time_Protocol[NTP] correction
+happens, the clock offset of the trace also needs to be updated. Use
+the `metadata` item of the man:lttng-regenerate(1) command to do so.
+
+The main use case of this command is to allow a system to boot with
+an incorrect wall time and have LTTng trace it before its wall time
+is corrected. Once the system is known to be in a state where its
+wall time is correct, you can run `lttng regenerate metadata`.
+
+To regenerate the metadata stream files of the
+<<cur-tracing-session,current recording session>>:
+
+* Use the `metadata` item of the man:lttng-regenerate(1) command:
++
+--
+[role="term"]
+----
+$ lttng regenerate metadata
+----
+--
+
+
+[role="since-2.9"]
+[[regenerate-statedump]]
+=== Regenerate the state dump event records of a recording session
+
+The LTTng kernel and user space tracers generate state dump
+<<event,event records>> when the application starts or when you
+<<basic-tracing-session-control,start a recording session>>.
+
+An analysis can use the state dump event records to set an initial state
+before it builds the rest of the state from the subsequent event
+records. http://tracecompass.org/[Trace Compass] and
+https://github.com/lttng/lttng-analyses[LTTng analyses] are notable
+examples of applications which use the state dump of an LTTng trace.
+
+When you <<taking-a-snapshot,take a snapshot>>, it's possible that the
+state dump event records aren't included in the snapshot trace files
+because they were recorded to a <<channel,sub-buffer>> that has been
+consumed or <<overwrite-mode,overwritten>> already.
+
+Use the `statedump` item of the man:lttng-regenerate(1) command to emit
+and record the state dump events again.
+
+To regenerate the state dump of the <<cur-tracing-session,current
+recording session>>, provided you created it in <<snapshot-mode,snapshot
+mode>>, before you take a snapshot:
+
+. Use the `statedump` item of the man:lttng-regenerate(1) command:
++
+--
+[role="term"]
+----
+$ lttng regenerate statedump
+----
+--
+
+. <<basic-tracing-session-control,Stop the recording session>>:
++
+--
+[role="term"]
+----
+$ lttng stop
+----
+--
+
+. <<taking-a-snapshot,Take a snapshot>>:
++
+--
+[role="term"]
+----
+$ lttng snapshot record --name=my-snapshot
+----
+--
+
+Depending on the event throughput, you should run steps{nbsp}1
+and{nbsp}2 as closely as possible.
+
+[NOTE]
+====
+To record the state dump events, you need to
+<<enabling-disabling-events,create recording event rules>> which enable
+them:
+
+* The names of LTTng-UST state dump tracepoints start with
+ `lttng_ust_statedump:`.
+
+* The names of LTTng-modules state dump tracepoints start with
+ `lttng_statedump_`.
+====
+
+
+[role="since-2.7"]
+[[persistent-memory-file-systems]]
+=== Record trace data on persistent memory file systems
+
+https://en.wikipedia.org/wiki/Non-volatile_random-access_memory[Non-volatile
+random-access memory] (NVRAM) is random-access memory that retains its
+information when power is turned off (non-volatile). Systems with such
+memory can store data structures in RAM and retrieve them after a
+reboot, without flushing to typical _storage_.
+
+Linux supports NVRAM file systems thanks to either
+http://pramfs.sourceforge.net/[PRAMFS] or
+https://www.kernel.org/doc/Documentation/filesystems/dax.txt[DAX]{nbsp}+{nbsp}http://lkml.iu.edu/hypermail/linux/kernel/1504.1/03463.html[pmem]
+(requires Linux{nbsp}4.1+).
+
+This section doesn't describe how to operate such file systems; we
+assume that you have a working persistent memory file system.
+
+When you <<creating-destroying-tracing-sessions,create a recording
+session>>, you can specify the path of the shared memory holding the
+sub-buffers. If you specify a location on an NVRAM file system, then you
+can retrieve the latest recorded trace data when the system reboots
+after a crash.
+
+To record trace data on a persistent memory file system and retrieve the
+trace data after a system crash:
+
+. Create a recording session with a <<channel,sub-buffer>> shared memory
+ path located on an NVRAM file system:
++
+--
+[role="term"]
+----
+$ lttng create my-session --shm-path=/path/to/shm/on/nvram
+----
+--
+
+. Configure the recording session as usual with the man:lttng(1)
+ command-line tool, and <<basic-tracing-session-control,start
+ recording>>.
+
+. After a system crash, use the man:lttng-crash(1) command-line tool to
+ read the trace data recorded on the NVRAM file system:
++
+--
+[role="term"]
+----
+$ lttng-crash /path/to/shm/on/nvram
+----
+--
+
+The binary layout of the ring buffer files isn't exactly the same as the
+trace files layout. This is why you need to use man:lttng-crash(1)
+instead of some standard LTTng trace reader.
+
+To convert the ring buffer files to LTTng trace files:
+
+* Use the opt:lttng-crash(1):--extract option of man:lttng-crash(1):
++
+--
+[role="term"]
+----
+$ lttng-crash --extract=/path/to/trace /path/to/shm/on/nvram
+----
+--
+
+
+[role="since-2.10"]
+[[notif-trigger-api]]
+=== Get notified when the buffer usage of a channel is too high or too low
+
+With the notification and <<trigger,trigger>> C{nbsp}API of
+<<liblttng-ctl-lttng,`liblttng-ctl`>>, LTTng can notify your user
+application when the buffer usage of one or more <<channel,channels>>
+becomes too low or too high.
+
+Use this API and enable or disable <<event,recording event rules>> while
+a recording session <<basic-tracing-session-control,is active>> to avoid
+<<channel-overwrite-mode-vs-discard-mode,discarded event records>>, for
+example.
+
+.Send a notification to a user application when the buffer usage of an LTTng channel is too high.
+====
+In this example, we create and build an application which gets notified
+when the buffer usage of a specific LTTng channel is higher than
+75{nbsp}%.
+
+We only print that it's the case in this example, but we could as well
+use the `liblttng-ctl` C{nbsp}API to <<enabling-disabling-events,disable
+recording event rules>> when this happens, for example.
+
+. Create the C{nbsp}source file of the application:
++
+--
+[source,c]
+.path:{notif-app.c}
+----
+#include <stdlib.h>
+#include <stdio.h>
+#include <assert.h>
+#include <lttng/lttng.h>
+
+int main(int argc, char *argv[])
+{
+ int exit_status = EXIT_SUCCESS;
+ struct lttng_notification_channel *notification_channel;
+ struct lttng_condition *condition;
+ struct lttng_action *action;
+ struct lttng_trigger *trigger;
+ const char *recording_session_name;
+ const char *channel_name;
+
+ assert(argc >= 3);
+ recording_session_name = argv[1];
+ channel_name = argv[2];
+
+ /*
+ * Create a notification channel.
+ *
+ * A notification channel connects the user application to the LTTng
+ * session daemon.
+ *
+ * You can use this notification channel to listen to various types
+ * of notifications.
+ */
+ notification_channel = lttng_notification_channel_create(
+ lttng_session_daemon_notification_endpoint);
+
+ /*
+ * Create a "buffer usage becomes greater than" condition.
+ *
+ * In this case, the condition is satisfied when the buffer usage
+ * becomes greater than or equal to 75 %.
+ *
+ * We create the condition for a specific recording session name,
+ * channel name, and for the user space tracing domain.
+ *
+ * The following condition types also exist:
+ *
+ * * The buffer usage of a channel becomes less than a given value.
+ *
+ * * The consumed data size of a recording session becomes greater
+ * than a given value.
+ *
+ * * A recording session rotation becomes ongoing.
+ *
+ * * A recording session rotation becomes completed.
+ *
+ * * A given event rule matches an event.
+ */
+ condition = lttng_condition_buffer_usage_high_create();
+ lttng_condition_buffer_usage_set_threshold_ratio(condition, .75);
+ lttng_condition_buffer_usage_set_session_name(condition,
+ recording_session_name);
+ lttng_condition_buffer_usage_set_channel_name(condition,
+ channel_name);
+ lttng_condition_buffer_usage_set_domain_type(condition,
+ LTTNG_DOMAIN_UST);
+
+ /*
+ * Create an action (receive a notification) to execute when the
+ * condition created above is satisfied.
+ */
+ action = lttng_action_notify_create();
+
+ /*
+ * Create a trigger.
+ *
+ * A trigger associates a condition to an action: LTTng executes
+ * the action when the condition is satisfied.
+ */
+ trigger = lttng_trigger_create(condition, action);
+
+ /* Register the trigger to the LTTng session daemon. */
+ lttng_register_trigger(trigger);
+
+ /*
+ * Now that we have registered a trigger, LTTng will send a
+ * notification every time its condition is met through a
+ * notification channel.
+ *
+ * To receive this notification, we must subscribe to notifications
+ * which match the same condition.
+ */
+ lttng_notification_channel_subscribe(notification_channel,
+ condition);
+
+ /*
+ * Notification loop.
+ *
+ * Put this in a dedicated thread to avoid blocking the main thread.
+ */
+ for (;;) {
+ struct lttng_notification *notification;
+ enum lttng_notification_channel_status status;
+ const struct lttng_evaluation *notification_evaluation;
+ const struct lttng_condition *notification_condition;
+ double buffer_usage;
+
+ /* Receive the next notification. */
+ status = lttng_notification_channel_get_next_notification(
+ notification_channel, ¬ification);
+
+ switch (status) {
+ case LTTNG_NOTIFICATION_CHANNEL_STATUS_OK:
+ break;
+ case LTTNG_NOTIFICATION_CHANNEL_STATUS_NOTIFICATIONS_DROPPED:
+ /*
+ * The session daemon can drop notifications if a monitoring
+ * application isn't consuming the notifications fast
+ * enough.
+ */
+ continue;
+ case LTTNG_NOTIFICATION_CHANNEL_STATUS_CLOSED:
+ /*
+ * The session daemon closed the notification channel.
+ *
+ * This is typically caused by a session daemon shutting
+ * down.
+ */
+ goto end;
+ default:
+ /* Unhandled conditions or errors. */
+ exit_status = EXIT_FAILURE;
+ goto end;
+ }
+
+ /*
+ * A notification provides, amongst other things:
+ *
+ * * The condition that caused LTTng to send this notification.
+ *
+ * * The condition evaluation, which provides more specific
+ * information on the evaluation of the condition.
+ *
+ * The condition evaluation provides the buffer usage
+ * value at the moment the condition was satisfied.
+ */
+ notification_condition = lttng_notification_get_condition(
+ notification);
+ notification_evaluation = lttng_notification_get_evaluation(
+ notification);
+
+ /* We're subscribed to only one condition. */
+ assert(lttng_condition_get_type(notification_condition) ==
+ LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH);
+
+ /*
+ * Get the exact sampled buffer usage from the condition
+ * evaluation.
+ */
+ lttng_evaluation_buffer_usage_get_usage_ratio(
+ notification_evaluation, &buffer_usage);
+
+ /*
+ * At this point, instead of printing a message, we could do
+ * something to reduce the buffer usage of the channel, like
+ * disable specific events, for example.
+ */
+ printf("Buffer usage is %f %% in recording session \"%s\", "
+ "user space channel \"%s\".\n", buffer_usage * 100,
+ recording_session_name, channel_name);
+
+ /* Destroy the notification object. */
+ lttng_notification_destroy(notification);
+ }
+
+end:
+ lttng_action_destroy(action);
+ lttng_condition_destroy(condition);
+ lttng_trigger_destroy(trigger);
+ lttng_notification_channel_destroy(notification_channel);
+ return exit_status;
+}
+----
+--
+
+. Build the `notif-app` application, linking it with `liblttng-ctl`:
++
+--
+[role="term"]
+----
+$ gcc -o notif-app notif-app.c $(pkg-config --cflags --libs lttng-ctl)
+----
+--
+
+. <<creating-destroying-tracing-sessions,Create a recording session>>,
+ <<enabling-disabling-events,create a recording event rule>> matching
+ all the user space tracepoint events, and
+ <<basic-tracing-session-control,start recording>>:
++
+--
+[role="term"]
+----
+$ lttng create my-session
+$ lttng enable-event --userspace --all
+$ lttng start
+----
+--
++
+If you create the channel manually with the man:lttng-enable-channel(1)
+command, you can set its <<channel-monitor-timer,monitor timer>> to
+control how frequently LTTng samples the current values of the channel
+properties to evaluate user conditions.
+
+. Run the `notif-app` application.
++
+This program accepts the <<tracing-session,recording session>> and
+user space channel names as its two first arguments. The channel
+which LTTng automatically creates with the man:lttng-enable-event(1)
+command above is named `channel0`:
++
+--
+[role="term"]
+----
+$ ./notif-app my-session channel0
+----
+--
+
+. In another terminal, run an application with a very high event
+ throughput so that the 75{nbsp}% buffer usage condition is reached.
++
+In the first terminal, the application should print lines like this:
++
+----
+Buffer usage is 81.45197 % in recording session "my-session", user space
+channel "channel0".
+----
++
+If you don't see anything, try to make the threshold of the condition in
+path:{notif-app.c} lower (0.1{nbsp}%, for example), and then rebuild the
+`notif-app` application (step{nbsp}2) and run it again (step{nbsp}4).
+====
+
+
+[[reference]]
+== Reference
+
+[[lttng-modules-ref]]
+=== noch:{LTTng-modules}
+
+
+[role="since-2.9"]
+[[lttng-tracepoint-enum]]
+==== `LTTNG_TRACEPOINT_ENUM()` usage
+
+Use the `LTTNG_TRACEPOINT_ENUM()` macro to define an enumeration:
+
+[source,c]
+----
+LTTNG_TRACEPOINT_ENUM(name, TP_ENUM_VALUES(entries))
+----
+
+Replace:
+
+* `name` with the name of the enumeration (C identifier, unique
+ amongst all the defined enumerations).
+* `entries` with a list of enumeration entries.
+
+The available enumeration entry macros are:
+
++ctf_enum_value(__name__, __value__)+::
+ Entry named +__name__+ mapped to the integral value +__value__+.
+
++ctf_enum_range(__name__, __begin__, __end__)+::
+ Entry named +__name__+ mapped to the range of integral values between
+ +__begin__+ (included) and +__end__+ (included).
+
++ctf_enum_auto(__name__)+::
+ Entry named +__name__+ mapped to the integral value following the
+ last mapping value.
++
+The last value of a `ctf_enum_value()` entry is its +__value__+
+parameter.
++
+The last value of a `ctf_enum_range()` entry is its +__end__+ parameter.
++
+If `ctf_enum_auto()` is the first entry in the list, its integral
+value is 0.
+
+Use the `ctf_enum()` <<lttng-modules-tp-fields,field definition macro>>
+to use a defined enumeration as a tracepoint field.
+
+.Define an enumeration with `LTTNG_TRACEPOINT_ENUM()`.
+====
+[source,c]
+----
+LTTNG_TRACEPOINT_ENUM(
+ my_enum,
+ TP_ENUM_VALUES(
+ ctf_enum_auto("AUTO: EXPECT 0")
+ ctf_enum_value("VALUE: 23", 23)
+ ctf_enum_value("VALUE: 27", 27)
+ ctf_enum_auto("AUTO: EXPECT 28")
+ ctf_enum_range("RANGE: 101 TO 303", 101, 303)
+ ctf_enum_auto("AUTO: EXPECT 304")
+ )
+)
+----
+====
+
+
+[role="since-2.7"]
+[[lttng-modules-tp-fields]]
+==== Tracepoint fields macros (for `TP_FIELDS()`)
+
+[[tp-fast-assign]][[tp-struct-entry]]The available macros to define
+tracepoint fields, which must be listed within `TP_FIELDS()` in
+`LTTNG_TRACEPOINT_EVENT()`, are:
+
+[role="func-desc growable",cols="asciidoc,asciidoc"]
+.Available macros to define LTTng-modules tracepoint fields
+|====
+|Macro |Description and parameters
+
+|
++ctf_integer(__t__, __n__, __e__)+
+
++ctf_integer_nowrite(__t__, __n__, __e__)+
+
++ctf_user_integer(__t__, __n__, __e__)+
+
++ctf_user_integer_nowrite(__t__, __n__, __e__)+
+|
+Standard integer, displayed in base{nbsp}10.
+
++__t__+::
+ Integer C type (`int`, `long`, `size_t`, ...).
+
++__n__+::
+ Field name.
+
++__e__+::
+ Argument expression.
+
+|
++ctf_integer_hex(__t__, __n__, __e__)+
+
++ctf_user_integer_hex(__t__, __n__, __e__)+
+|
+Standard integer, displayed in base{nbsp}16.
+
++__t__+::
+ Integer C type.
+
++__n__+::
+ Field name.
+
++__e__+::
+ Argument expression.
+
+|+ctf_integer_oct(__t__, __n__, __e__)+
+|
+Standard integer, displayed in base{nbsp}8.
+
++__t__+::
+ Integer C type.
+
++__n__+::
+ Field name.
+
++__e__+::
+ Argument expression.
+
+|
++ctf_integer_network(__t__, __n__, __e__)+
+
++ctf_user_integer_network(__t__, __n__, __e__)+
+|
+Integer in network byte order (big-endian), displayed in base{nbsp}10.
+
++__t__+::
+ Integer C type.
+
++__n__+::
+ Field name.
+
++__e__+::
+ Argument expression.
+
+|
++ctf_integer_network_hex(__t__, __n__, __e__)+
+
++ctf_user_integer_network_hex(__t__, __n__, __e__)+
+|
+Integer in network byte order, displayed in base{nbsp}16.
+
++__t__+::
+ Integer C type.
+
++__n__+::
+ Field name.
+
++__e__+::
+ Argument expression.
+
+|
++ctf_enum(__N__, __t__, __n__, __e__)+
+
++ctf_enum_nowrite(__N__, __t__, __n__, __e__)+
+
++ctf_user_enum(__N__, __t__, __n__, __e__)+
+
++ctf_user_enum_nowrite(__N__, __t__, __n__, __e__)+
+|
+Enumeration.
+
++__N__+::
+ Name of a <<lttng-tracepoint-enum,previously defined enumeration>>.
+
++__t__+::
+ Integer C type (`int`, `long`, `size_t`, ...).
+
++__n__+::
+ Field name.
+
++__e__+::
+ Argument expression.
+
+|
++ctf_string(__n__, __e__)+
+
++ctf_string_nowrite(__n__, __e__)+
+
++ctf_user_string(__n__, __e__)+
+
++ctf_user_string_nowrite(__n__, __e__)+
+|
+Null-terminated string; undefined behavior if +__e__+ is `NULL`.
+
++__n__+::
+ Field name.
+
++__e__+::
+ Argument expression.
+
+|
++ctf_array(__t__, __n__, __e__, __s__)+
+
++ctf_array_nowrite(__t__, __n__, __e__, __s__)+
+
++ctf_user_array(__t__, __n__, __e__, __s__)+
+
++ctf_user_array_nowrite(__t__, __n__, __e__, __s__)+
+|
+Statically-sized array of integers.
+
++__t__+::
+ Array element C type.
+
++__n__+::
+ Field name.
+
++__e__+::
+ Argument expression.
+
++__s__+::
+ Number of elements.
+
+|
++ctf_array_bitfield(__t__, __n__, __e__, __s__)+
+
++ctf_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+
+
++ctf_user_array_bitfield(__t__, __n__, __e__, __s__)+
+
++ctf_user_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+
+|
+Statically-sized array of bits.
+
+The type of +__e__+ must be an integer type. +__s__+ is the number
+of elements of such type in +__e__+, not the number of bits.
+
++__t__+::
+ Array element C type.
+
++__n__+::
+ Field name.
+
++__e__+::
+ Argument expression.
+
++__s__+::
+ Number of elements.
+
+|
++ctf_array_text(__t__, __n__, __e__, __s__)+
+
++ctf_array_text_nowrite(__t__, __n__, __e__, __s__)+
+
++ctf_user_array_text(__t__, __n__, __e__, __s__)+
+
++ctf_user_array_text_nowrite(__t__, __n__, __e__, __s__)+
+|
+Statically-sized array, printed as text.
+
+The string doesn't need to be null-terminated.
+
++__t__+::
+ Array element C type (always `char`).
+
++__n__+::
+ Field name.
+
++__e__+::
+ Argument expression.
+
++__s__+::
+ Number of elements.
+
+|
++ctf_sequence(__t__, __n__, __e__, __T__, __E__)+
+
++ctf_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+
+
++ctf_user_sequence(__t__, __n__, __e__, __T__, __E__)+
+
++ctf_user_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+
+|
+Dynamically-sized array of integers.
+
+The type of +__E__+ must be unsigned.
+
++__t__+::
+ Array element C type.
+
++__n__+::
+ Field name.
+
++__e__+::
+ Argument expression.
+
++__T__+::
+ Length expression C type.
+
++__E__+::
+ Length expression.
+
+|
++ctf_sequence_hex(__t__, __n__, __e__, __T__, __E__)+
+
++ctf_user_sequence_hex(__t__, __n__, __e__, __T__, __E__)+
+|
+Dynamically-sized array of integers, displayed in base{nbsp}16.
+
+The type of +__E__+ must be unsigned.
+
++__t__+::
+ Array element C type.
+
++__n__+::
+ Field name.
+
++__e__+::
+ Argument expression.
+
++__T__+::
+ Length expression C type.
+
++__E__+::
+ Length expression.
+
+|+ctf_sequence_network(__t__, __n__, __e__, __T__, __E__)+
+|
+Dynamically-sized array of integers in network byte order (big-endian),
+displayed in base{nbsp}10.
+
+The type of +__E__+ must be unsigned.
+
++__t__+::
+ Array element C type.
+
++__n__+::
+ Field name.
+
++__e__+::
+ Argument expression.
+
++__T__+::
+ Length expression C type.
+
++__E__+::
+ Length expression.
+
+|
++ctf_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+
+
++ctf_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+
+
++ctf_user_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+
+
++ctf_user_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+
+|
+Dynamically-sized array of bits.
+
+The type of +__e__+ must be an integer type. +__s__+ is the number
+of elements of such type in +__e__+, not the number of bits.
+
+The type of +__E__+ must be unsigned.
+
++__t__+::
+ Array element C type.
+
++__n__+::
+ Field name.
+
++__e__+::
+ Argument expression.
+
++__T__+::
+ Length expression C type.
+
++__E__+::
+ Length expression.
+
+|
++ctf_sequence_text(__t__, __n__, __e__, __T__, __E__)+
+
++ctf_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+
+
++ctf_user_sequence_text(__t__, __n__, __e__, __T__, __E__)+
+
++ctf_user_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+
+|
+Dynamically-sized array, displayed as text.
+
+The string doesn't need to be null-terminated.
+
+The type of +__E__+ must be unsigned.
+
+The behaviour is undefined if +__e__+ is `NULL`.
+
++__t__+::
+ Sequence element C type (always `char`).
+
++__n__+::
+ Field name.
+
++__e__+::
+ Argument expression.
+
++__T__+::
+ Length expression C type.
+
++__E__+::
+ Length expression.
+|====
+
+Use the `_user` versions when the argument expression, `e`, is
+a user space address. In the cases of `ctf_user_integer*()` and
+`ctf_user_float*()`, `&e` must be a user space address, thus `e` must
+be addressable.
+
+The `_nowrite` versions omit themselves from the trace data, but are
+otherwise identical. This means LTTng won't write the `_nowrite` fields
+to the recorded trace. Their primary purpose is to make some of the
+event context available to the <<enabling-disabling-events,recording
+event rule filters>> without having to commit the data to
+<<channel,sub-buffers>>.
+
+
+[[glossary]]
+== Glossary
+
+Terms related to LTTng and to tracing in general:
+
+[[def-action]]action::
+ The part of a <<def-trigger,trigger>> which LTTng executes when the
+ trigger <<def-condition,condition>> is satisfied.
+
+Babeltrace::
+ The https://diamon.org/babeltrace[Babeltrace] project, which includes:
++
+* The
+ https://babeltrace.org/docs/v2.0/man1/babeltrace2.1/[cmd:babeltrace2]
+ command-line interface.
+* The libbabeltrace2 library which offers a
+ https://babeltrace.org/docs/v2.0/libbabeltrace2/[C API].
+* https://babeltrace.org/docs/v2.0/python/bt2/[Python{nbsp}3 bindings].
+* Plugins.
+
+[[def-buffering-scheme]]<<channel-buffering-schemes,buffering scheme>>::
+ A layout of <<def-sub-buffer,sub-buffers>> applied to a given channel.
+
+[[def-channel]]<<channel,channel>>::
+ An entity which is responsible for a set of
+ <<def-ring-buffer,ring buffers>>.
++
+<<def-recording-event-rule,Recording event rules>> are always attached
+to a specific channel.
+
+clock::
+ A source of time for a <<def-tracer,tracer>>.
+
+[[def-condition]]condition::
+ The part of a <<def-trigger,trigger>> which must be satisfied for
+ LTTng to attempt to execute the trigger <<def-action,actions>>.
+
+[[def-consumer-daemon]]<<lttng-consumerd,consumer daemon>>::
+ A program which is responsible for consuming the full
+ <<def-sub-buffer,sub-buffers>> and write them to a file system or
+ send them over the network.
+
+[[def-current-trace-chunk]]current trace chunk::
+ A <<def-trace-chunk,trace chunk>> which includes the current content
+ of all the <<def-sub-buffer,sub-buffers>> of the
+ <<def-tracing-session,recording session>> and the stream files
+ produced since the latest event amongst:
++
+* The creation of the recording session.
+* The last <<def-tracing-session-rotation,recording session rotation>>, if
+ any.
+
+<<channel-overwrite-mode-vs-discard-mode,discard mode>>::
+ The <<def-event-record-loss-mode,event record loss mode>> in which
+ the <<def-tracer,tracer>> _discards_ new <<def-event-record,event
+ records>> when there's no <<def-sub-buffer,sub-buffer>> space left to
+ store them.
+
+[[def-event]]event::
+ The execution of an <<def-instrumentation-point,instrumentation
+ point>>, like a <<def-tracepoint,tracepoint>> that you manually place
+ in some source code, or a Linux kprobe.
++
+When an instrumentation point is executed, LTTng creates an event.
++
+When an <<def-event-rule,event rule>> matches the event,
+<<def-lttng,LTTng>> executes some action, for example:
++
+* Record its payload to a <<def-sub-buffer,sub-buffer>> as an
+ <<def-event-record,event record>>.
+* Attempt to execute the user-defined actions of a
+ <<def-trigger,trigger>> with an
+ <<add-event-rule-matches-trigger,``event rule matches''>> condition.
+
+[[def-event-name]]event name::
+ The name of an <<def-event,event>>, which is also the name of the
+ <<def-event-record,event record>>.
++
+This is also called the _instrumentation point name_.
+
+[[def-event-record]]event record::
+ A record (binary serialization), in a <<def-trace,trace>>, of the
+ payload of an <<def-event,event>>.
++
+The payload of an event record has zero or more _fields_.
+
+[[def-event-record-loss-mode]]<<channel-overwrite-mode-vs-discard-mode,event record loss mode>>::
+ The mechanism by which event records of a given
+ <<def-channel,channel>> are lost (not recorded) when there's no
+ <<def-sub-buffer,sub-buffer>> space left to store them.
+
+[[def-event-rule]]<<event-rule,event rule>>::
+ Set of conditions which an <<def-event,event>> must satisfy
+ for LTTng to execute some action.
++
+An event rule is said to _match_ events, like a
+https://en.wikipedia.org/wiki/Regular_expression[regular expression]
+matches strings.
++
+A <<def-recording-event-rule,recording event rule>> is a specific type
+of event rule of which the action is to <<def-record,record>> the event
+to a <<def-sub-buffer,sub-buffer>>.
+
+[[def-incl-set]]inclusion set::
+ In the <<pid-tracking,process attribute inclusion set>> context: a
+ set of <<def-proc-attr,process attributes>> of a given type.
+
+<<instrumenting,instrumentation>>::
+ The use of <<def-lttng,LTTng>> probes to make a kernel or
+ <<def-user-application,user application>> traceable.
+
+[[def-instrumentation-point]]instrumentation point::
+ A point in the execution path of a kernel or
+ <<def-user-application,user application>> which, when executed,
+ create an <<def-event,event>>.
+
+instrumentation point name::
+ See _<<def-event-name,event name>>_.
+
+`java.util.logging`::
+ The
+ https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[core logging facilities]
+ of the Java platform.
+
+log4j::
+ A https://logging.apache.org/log4j/1.2/[logging library] for Java
+ developed by the Apache Software Foundation.
+
+log level::
+ Level of severity of a log statement or user space
+ <<def-instrumentation-point,instrumentation point>>.
+
+[[def-lttng]]LTTng::
+ The _Linux Trace Toolkit: next generation_ project.
+
+<<lttng-cli,cmd:lttng>>::
+ A command-line tool provided by the <<def-lttng-tools,LTTng-tools>>
+ project which you can use to send and receive control messages to and
+ from a <<def-session-daemon,session daemon>>.
+
+LTTng analyses::
+ The https://github.com/lttng/lttng-analyses[LTTng analyses] project,
+ which is a set of analyzing programs that you can use to obtain a
+ higher level view of an <<def-lttng,LTTng>> <<def-trace,trace>>.
+
+cmd:lttng-consumerd::
+ The name of the <<def-consumer-daemon,consumer daemon>> program.
+
+cmd:lttng-crash::
+ A utility provided by the <<def-lttng-tools,LTTng-tools>> project
+ which can convert <<def-ring-buffer,ring buffer>> files (usually
+ <<persistent-memory-file-systems,saved on a persistent memory file
+ system>>) to <<def-trace,trace>> files.
++
+See man:lttng-crash(1).
+
+LTTng Documentation::
+ This document.
+
+<<lttng-live,LTTng live>>::
+ A communication protocol between the <<lttng-relayd,relay daemon>> and
+ live readers which makes it possible to show or analyze
+ <<def-event-record,event records>> ``live'', as they're received by
+ the <<def-relay-daemon,relay daemon>>.
+
+<<lttng-modules,LTTng-modules>>::
+ The https://github.com/lttng/lttng-modules[LTTng-modules] project,
+ which contains the Linux kernel modules to make the Linux kernel
+ <<def-instrumentation-point,instrumentation points>> available for
+ <<def-lttng,LTTng>> tracing.
+
+cmd:lttng-relayd::
+ The name of the <<def-relay-daemon,relay daemon>> program.
+
+cmd:lttng-sessiond::
+ The name of the <<def-session-daemon,session daemon>> program.
+
+[[def-lttng-tools]]LTTng-tools::
+ The https://github.com/lttng/lttng-tools[LTTng-tools] project, which
+ contains the various programs and libraries used to
+ <<controlling-tracing,control tracing>>.
+
+[[def-lttng-ust]]<<lttng-ust,LTTng-UST>>::
+ The https://github.com/lttng/lttng-ust[LTTng-UST] project, which
+ contains libraries to instrument
+ <<def-user-application,user applications>>.
+
+<<lttng-ust-agents,LTTng-UST Java agent>>::
+ A Java package provided by the <<def-lttng-ust,LTTng-UST>> project to
+ allow the LTTng instrumentation of `java.util.logging` and Apache
+ log4j{nbsp}1.2 logging statements.
+
+<<lttng-ust-agents,LTTng-UST Python agent>>::
+ A Python package provided by the <<def-lttng-ust,LTTng-UST>> project
+ to allow the <<def-lttng,LTTng>> instrumentation of Python logging
+ statements.
+
+<<channel-overwrite-mode-vs-discard-mode,overwrite mode>>::
+ The <<def-event-record-loss-mode,event record loss mode>> in which new
+ <<def-event-record,event records>> _overwrite_ older event records
+ when there's no <<def-sub-buffer,sub-buffer>> space left to store
+ them.
+
+<<channel-buffering-schemes,per-process buffering>>::
+ A <<def-buffering-scheme,buffering scheme>> in which each instrumented
+ process has its own <<def-sub-buffer,sub-buffers>> for a given user
+ space <<def-channel,channel>>.
+
+<<channel-buffering-schemes,per-user buffering>>::
+ A <<def-buffering-scheme,buffering scheme>> in which all the processes
+ of a Unix user share the same <<def-sub-buffer,sub-buffers>> for a
+ given user space <<def-channel,channel>>.
+
+[[def-proc-attr]]process attribute::
+ In the <<pid-tracking,process attribute inclusion set>> context:
++
+* A process ID.
+* A virtual process ID.
+* A Unix user ID.
+* A virtual Unix user ID.
+* A Unix group ID.
+* A virtual Unix group ID.
+
+record (_noun_)::
+ See <<def-event-record,_event record_>>.
+
+[[def-record]]record (_verb_)::
+ Serialize the binary payload of an <<def-event,event>> to a
+ <<def-sub-buffer,sub-buffer>>.
+
+[[def-recording-event-rule]]<<event,recording event rule>>::
+ Specific type of <<def-event-rule,event rule>> of which the action is
+ to <<def-record,record>> the matched event to a
+ <<def-sub-buffer,sub-buffer>>.
+
+[[def-tracing-session]][[def-recording-session]]<<tracing-session,recording session>>::
+ A stateful dialogue between you and a <<lttng-sessiond,session daemon>>.
+
+[[def-tracing-session-rotation]]<<session-rotation,recording session rotation>>::
+ The action of archiving the
+ <<def-current-trace-chunk,current trace chunk>> of a
+ <<def-tracing-session,recording session>>.
+
+[[def-relay-daemon]]<<lttng-relayd,relay daemon>>::
+ A process which is responsible for receiving the <<def-trace,trace>>
+ data which a distant <<def-consumer-daemon,consumer daemon>> sends.
+
+[[def-ring-buffer]]ring buffer::
+ A set of <<def-sub-buffer,sub-buffers>>.
+
+rotation::
+ See _<<def-tracing-session-rotation,recording session rotation>>_.
+
+[[def-session-daemon]]<<lttng-sessiond,session daemon>>::
+ A process which receives control commands from you and orchestrates
+ the <<def-tracer,tracers>> and various <<def-lttng,LTTng>> daemons.
+
+<<taking-a-snapshot,snapshot>>::
+ A copy of the current data of all the <<def-sub-buffer,sub-buffers>>
+ of a given <<def-tracing-session,recording session>>, saved as
+ <<def-trace,trace>> files.
+
+[[def-sub-buffer]]sub-buffer::
+ One part of an <<def-lttng,LTTng>> <<def-ring-buffer,ring buffer>>
+ which contains <<def-event-record,event records>>.
+
+timestamp::
+ The time information attached to an <<def-event,event>> when LTTng
+ creates it.
+
+[[def-trace]]trace (_noun_)::
+ A set of:
++
+* One https://diamon.org/ctf/[CTF] metadata stream file.
+* One or more CTF data stream files which are the concatenations of one
+ or more flushed <<def-sub-buffer,sub-buffers>>.
+
+[[def-trace-verb]]trace (_verb_)::
+ From the perspective of a <<def-tracer,tracer>>: attempt to execute
+ one or more actions when emitting an <<def-event,event>> in an
+ application or in a system.
+
+[[def-trace-chunk]]trace chunk::
+ A self-contained <<def-trace,trace>> which is part of a
+ <<def-tracing-session,recording session>>. Each
+ <<def-tracing-session-rotation, recording session rotation>> produces a
+ <<def-trace-chunk-archive,trace chunk archive>>.
+
+[[def-trace-chunk-archive]]trace chunk archive::
+ The result of a <<def-tracing-session-rotation, recording session
+ rotation>>.
++
+<<def-lttng,LTTng>> doesn't manage any trace chunk archive, even if its
+containing <<def-tracing-session,recording session>> is still active: you
+are free to read it, modify it, move it, or remove it.
+
+Trace Compass::
+ The http://tracecompass.org[Trace Compass] project and application.
+
+[[def-tracepoint]]tracepoint::
+ An instrumentation point using the tracepoint mechanism of the Linux
+ kernel or of <<def-lttng-ust,LTTng-UST>>.
+
+tracepoint definition::
+ The definition of a single <<def-tracepoint,tracepoint>>.
+
+tracepoint name::
+ The name of a <<def-tracepoint,tracepoint>>.
+
+[[def-tracepoint-provider]]tracepoint provider::
+ A set of functions providing <<def-tracepoint,tracepoints>> to an
+ instrumented <<def-user-application,user application>>.
++
+Not to be confused with a <<def-tracepoint-provider-package,tracepoint
+provider package>>: many tracepoint providers can exist within a
+tracepoint provider package.
+
+[[def-tracepoint-provider-package]]tracepoint provider package::
+ One or more <<def-tracepoint-provider,tracepoint providers>> compiled
+ as an https://en.wikipedia.org/wiki/Object_file[object file] or as a
+ link:https://en.wikipedia.org/wiki/Library_(computing)#Shared_libraries[shared
+ library].
+
+[[def-tracer]]tracer::
+ A piece of software which executes some action when it emits
+ an <<def-event,event>>, like <<def-record,record>> it to some
+ buffer.
+
+<<domain,tracing domain>>::
+ A type of LTTng <<def-tracer,tracer>>.
+
+<<tracing-group,tracing group>>::
+ The Unix group which a Unix user can be part of to be allowed to
+ control the Linux kernel LTTng <<def-tracer,tracer>>.
+
+[[def-trigger]]<<trigger,trigger>>::
+ A <<def-condition,condition>>-<<def-action,actions>> pair; when the
+ condition of a trigger is satisfied, LTTng attempts to execute its
+ actions.
+
+[[def-user-application]]user application::
+ An application (program or library) running in user space, as opposed
+ to a Linux kernel module, for example.
projects / lttng-docs.git / commitdiff
