--- /dev/null
+The LTTng Documentation
+=======================
+Philippe Proulx <pproulx@efficios.com>
+v2.11, 12 November 2018
+
+
+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.
++
+You can skip this section if you’re familiar with software tracing and
+with the LTTng project.
+
+* **<<installing-lttng,Installation>>** describes the steps to
+ install the LTTng packages on common Linux distributions and from
+ their sources.
++
+You can skip this section if you already properly installed LTTng on
+your target system.
+
+* **<<getting-started,Quick start>>** is a concise guide to
+ getting started quickly with LTTng kernel and user space tracing.
++
+We recommend this section if you're new to LTTng or to software tracing
+in general.
++
+You can skip this section if you're not new to LTTng.
+
+* **<<core-concepts,Core concepts>>** explains the concepts at
+ the heart of LTTng.
++
+It's a good idea to become familiar with the core concepts
+before attempting to use the toolkit.
+
+* **<<plumbing,Components of LTTng>>** describes the various components
+ of the LTTng machinery, like the daemons, the libraries, and the
+ command-line interface.
+* **<<instrumenting,Instrumentation>>** shows different ways to
+ instrument user applications and the Linux kernel.
++
+Instrumenting source code is essential to provide a meaningful
+source of events.
++
+You can skip this section if you do not have a programming background.
+
+* **<<controlling-tracing,Tracing control>>** is divided into topics
+ which demonstrate how to use the vast array of features that
+ LTTng{nbsp}{revision} offers.
+* **<<reference,Reference>>** contains reference tables.
+* **<<glossary,Glossary>>** is a specialized dictionary of terms related
+ to LTTng or to the field of software tracing.
+
+
+include::../common/convention.txt[]
+
+
+include::../common/acknowledgements.txt[]
+
+
+[[whats-new]]
+== What's new in LTTng{nbsp}{revision}?
+
+LTTng{nbsp}{revision} bears the name _Lafontaine_. This modern
+https://en.wikipedia.org/wiki/Saison[saison] from the
+https://oshlag.com/[Oshlag] microbrewery is a refreshing--zesty--rice
+beer with hints of fruit and spices. Some even say it makes for a great
+https://en.wikipedia.org/wiki/Somaek[Somaek] when mixed with
+Chamisul Soju, not that we've tried!
+
+New features and changes in LTTng{nbsp}{revision}:
+
+* Just like you can typically perform
+ https://en.wikipedia.org/wiki/Log_rotation[log rotation], you can
+ now <<session-rotation,_rotate_ a tracing session>>, that
+ is, according to man:lttng-rotate(1), archive the current trace
+ chunk (all the tracing session's trace data since the last rotation
+ or since its inception) so that LTTng does not manage it anymore.
++
+Once LTTng archives a trace chunk, you are free to read it, modify it,
+move it, or remove it.
++
+You can rotate a tracing session immediately or set a rotation schedule
+to automate rotations.
+
+* When you <<enabling-disabling-events,create an event rule>>, the
+ filter expression syntax now supports the following new operators:
++
+--
+** `~` (bitwise not)
+** `<<` (bitwise left shift)
+** `>>` (bitwise right shift)
+** `&` (bitwise AND)
+** `^` (bitwise XOR)
+** `|` (bitwise OR)
+--
++
+The syntax also supports array indexing with the usual square brackets:
++
+----
+regs[3][1] & 0xff7 == 0x240
+----
++
+There are peculiarities for both the new operators and the array
+indexing brackets, like a custom precedence table and implicit casting.
+See man:lttng-enable-event(1) to get all the details about the filter
+expression syntax.
+
+* You can now dynamically instrument any application's or library's
+ function entry by symbol name thanks to the new
+ opt:lttng-enable-event(1):--userspace-probe option of
+ the `lttng enable-event` command:
++
+[role="term"]
+----
+$ lttng enable-event --kernel \
+ --userspace-probe=/usr/lib/libc.so.6:malloc libc_malloc
+----
++
+The option also supports tracing existing
+https://www.sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps[SystemTap
+Statically Defined Tracing] (SDT) probe (DTrace-style marker). For example,
+given the following probe:
++
+[source,c]
+----
+DTRACE_PROBE2("server", "accept-request", request_id, ip_addr);
+----
++
+You can trace this probe with:
++
+----
+$ lttng enable-event --kernel \
+ --userspace-probe=sdt:/path/to/server:server:accept-request \
+ server_accept_request
+----
++
+This feature makes use of Linux's
+https://www.kernel.org/doc/Documentation/trace/uprobetracer.txt[uprobe]
+mechanism, therefore you must use the `--userspace-probe`
+instrumentation option with the opt:lttng-enable-event(1):--kernel
+domain option.
++
+NOTE: As of LTTng{nbsp}{revision}, LTTng does not record function
+parameters with the opt:lttng-enable-event(1):--userspace-probe option.
+
+* Two new <<adding-context,context>> fields are available for Linux
+ kernel <<channel,channels>>:
++
+--
+** `callstack-kernel`
+** `callstack-user`
+--
++
+Thanks to those, you can record the Linux kernel and user call stacks
+when a kernel event occurs. For example:
++
+[role="term"]
+----
+$ lttng enable-event --kernel --syscall read
+$ lttng add-context --kernel --type=callstack-kernel --type=callstack-user
+----
++
+When an man:open(2) system call occurs, LTTng attaches the kernel and
+user call stacks to the recorded event.
++
+NOTE: LTTng cannot always sample the user space call stack reliably.
+For instance, LTTng cannot sample the call stack of user applications
+and libraries compiled with the
+https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html[`-fomit-frame pointer`]
+option. In such a case, the tracing is not affected, but the sampled
+user space call stack may only contain the user call stack's topmost
+address.
+
+* The <<lttng-relayd,relay daemon>> is more efficient and presents fewer
+ connectivity issues, especially when a large number of targets send
+ trace data to a given relay daemon.
+
+* User applications and libraries instrumented with
+ <<lttng-ust,LTTng-UST>> can now safely unload (man:dlclose(3)) a
+ dynamically loaded
+ <<building-tracepoint-providers-and-user-application,tracepoint
+ provider package>>.
+
+
+[[nuts-and-bolts]]
+== Nuts and bolts
+
+What is LTTng? As its name suggests, the _Linux Trace Toolkit: next
+generation_ is a modern toolkit for tracing Linux systems and
+applications. So your first question might be:
+**what is tracing?**
+
+
+[[what-is-tracing]]
+=== What is tracing?
+
+As the history of software engineering progressed and led to what
+we now take for granted--complex, numerous and
+interdependent software applications running in parallel on
+sophisticated operating systems like Linux--the authors of such
+components, software developers, began feeling a natural
+urge to have tools that would ensure the robustness and good performance
+of their masterpieces.
+
+One major achievement in this field is, inarguably, the
+https://www.gnu.org/software/gdb/[GNU debugger (GDB)],
+an essential tool for developers to find and fix bugs. But even the best
+debugger won't help make your software run faster, and nowadays, faster
+software means either more work done by the same hardware, or cheaper
+hardware for the same work.
+
+A _profiler_ is often the tool of choice to identify performance
+bottlenecks. Profiling is suitable to identify _where_ performance is
+lost in a given software. The profiler outputs a profile, a statistical
+summary of observed events, which you may use to discover which
+functions took the most time to execute. However, a profiler won't
+report _why_ some identified functions are the bottleneck. Bottlenecks
+might only occur when specific conditions are met, conditions that are
+sometimes impossible to capture by a statistical profiler, or impossible
+to reproduce with an application altered by the overhead of an
+event-based profiler. For a thorough investigation of software
+performance issues, a history of execution is essential, with the
+recorded values of variables and context fields you choose, and
+with as little influence as possible on the instrumented software. This
+is where tracing comes in handy.
+
+_Tracing_ is a technique used to understand what goes on in a running
+software system. The software used for tracing is called a _tracer_,
+which is conceptually similar to a tape recorder. When recording,
+specific instrumentation points placed in the software source code
+generate events that are saved on a giant tape: a _trace_ file. You
+can trace user applications and the operating system at the same time,
+opening the possibility of resolving a wide range of problems that would
+otherwise be extremely challenging.
+
+Tracing is often compared to _logging_. However, tracers and loggers are
+two different tools, serving two different purposes. Tracers are
+designed to record much lower-level events that occur much more
+frequently than log messages, often in the range of thousands per
+second, with very little execution overhead. Logging is more appropriate
+for a very high-level analysis of less frequent events: user accesses,
+exceptional conditions (errors and warnings, for example), database
+transactions, instant messaging communications, and such. Simply put,
+logging is one of the many use cases that can be satisfied with tracing.
+
+The list of recorded events inside a trace file can be read manually
+like a log file for the maximum level of detail, but it is generally
+much more interesting to perform application-specific analyses to
+produce reduced statistics and graphs that are useful to resolve a
+given problem. Trace viewers and analyzers are specialized tools
+designed to do this.
+
+In the end, this is what LTTng is: a powerful, open source set of
+tools to trace the Linux kernel and user applications at the same time.
+LTTng is composed of several components actively maintained and
+developed by its link:/community/#where[community].
+
+
+[[lttng-alternatives]]
+=== Alternatives to noch:{LTTng}
+
+Excluding proprietary solutions, a few competing software tracers
+exist for Linux:
+
+* https://github.com/dtrace4linux/linux[dtrace4linux] is a port of
+ Sun Microsystems's DTrace to Linux. The cmd:dtrace tool interprets
+ user scripts and is responsible for loading code into the
+ Linux kernel for further execution and collecting the outputted data.
+* https://en.wikipedia.org/wiki/Berkeley_Packet_Filter[eBPF] is a
+ subsystem in the Linux kernel in which a virtual machine can execute
+ programs passed from the user space to the kernel. You can attach
+ such programs to tracepoints and kprobes thanks to a system call, and
+ they can output data to the user space when executed thanks to
+ different mechanisms (pipe, VM register values, and eBPF maps, to name
+ a few).
+* https://www.kernel.org/doc/Documentation/trace/ftrace.txt[ftrace]
+ is the de facto function tracer of the Linux kernel. Its user
+ interface is a set of special files in sysfs.
+* https://perf.wiki.kernel.org/[perf] is
+ a performance analyzing tool for Linux which supports hardware
+ performance counters, tracepoints, as well as other counters and
+ types of probes. perf's controlling utility is the cmd:perf command
+ line/curses tool.
+* http://linux.die.net/man/1/strace[strace]
+ is a command-line utility which records system calls made by a
+ user process, as well as signal deliveries and changes of process
+ state. strace makes use of https://en.wikipedia.org/wiki/Ptrace[ptrace]
+ to fulfill its function.
+* http://www.sysdig.org/[sysdig], like SystemTap, uses scripts to
+ analyze Linux kernel events. You write scripts, or _chisels_ in
+ sysdig's jargon, in Lua and sysdig executes them while it traces the
+ system or afterwards. sysdig's interface is the cmd:sysdig
+ command-line tool as well as the curses-based cmd:csysdig tool.
+* https://sourceware.org/systemtap/[SystemTap] is a Linux kernel and
+ user space tracer which uses custom user scripts to produce plain text
+ traces. SystemTap converts the scripts to the C language, and then
+ compiles them as Linux kernel modules which are loaded to produce
+ trace data. SystemTap's primary user interface is the cmd:stap
+ command-line tool.
+
+The main distinctive features of LTTng is that it produces correlated
+kernel and user space traces, as well as doing so with the lowest
+overhead amongst other solutions. It produces trace files in the
+http://diamon.org/ctf[CTF] format, a file format optimized
+for the production and analyses of multi-gigabyte data.
+
+LTTng is the result of more than 10{nbsp}years of active open source
+development by a community of passionate developers.
+LTTng{nbsp}{revision} is currently available on major desktop and server
+Linux distributions.
+
+The main interface for tracing control is a single command-line tool
+named cmd:lttng. The latter can create several tracing sessions, enable
+and disable events on the fly, filter events efficiently with custom
+user expressions, start and stop tracing, and much more. LTTng can
+record the traces on the file system or send them over the network, and
+keep them totally or partially. You can view the traces once tracing
+becomes inactive or in real-time.
+
+<<installing-lttng,Install LTTng now>> and
+<<getting-started,start tracing>>!
+
+
+[[installing-lttng]]
+== Installation
+
+**LTTng** is a set of software <<plumbing,components>> which interact to
+<<instrumenting,instrument>> the Linux kernel and user applications, and
+to <<controlling-tracing,control tracing>> (start and stop
+tracing, enable and disable event rules, and the rest). Those
+components are bundled into the following packages:
+
+* **LTTng-tools**: Libraries and command-line interface to
+ control tracing.
+* **LTTng-modules**: Linux kernel modules to instrument and
+ trace the kernel.
+* **LTTng-UST**: Libraries and Java/Python packages to instrument and
+ trace user applications.
+
+Most distributions mark the LTTng-modules and LTTng-UST packages as
+optional when installing LTTng-tools (which is always required). 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 trace the
+ Linux kernel.
+* You only need to install LTTng-UST if you intend to trace user
+ applications.
+
+[role="growable"]
+.Availability of LTTng{nbsp}{revision} for major Linux distributions as of 12 November 2018.
+|====
+|Distribution |Available in releases |Alternatives
+
+|https://www.ubuntu.com/[Ubuntu]
+|Ubuntu{nbsp}14.04 _Trusty Tahr_, Ubuntu{nbsp}16.04 _Xenial Xerus_,
+and Ubuntu{nbsp}18.04 _Bionic Beaver_:
+<<ubuntu-ppa,use the LTTng Stable{nbsp}{revision} PPA>>.
+|<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
+
+|https://getfedora.org/[Fedora]
+|_Not available_
+|
+link:/docs/v2.10#doc-fedora[LTTng{nbsp}2.10 for Fedora{nbsp}27,
+Fedora{nbsp}28, and Fedora{nbsp}29].
+
+<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
+
+|https://www.debian.org/[Debian]
+|_Not available_
+|link:/docs/v2.10#doc-debian[LTTng{nbsp}2.10 for Debian "buster" (testing)].
+
+<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
+
+|https://www.archlinux.org/[Arch Linux]
+|_Not available_
+|<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
+
+|https://alpinelinux.org/[Alpine Linux]
+|_Not available_
+|
+link:/docs/v2.10#doc-alpine-linux[LTTng{nbsp}2.10 for
+Alpine Linux{nbsp}3.7 and Alpine Linux{nbsp}3.8].
+
+<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
+
+|https://www.opensuse.org/[openSUSE]
+|_Not available_
+|link:/docs/v2.10#doc-opensuse[LTTng{nbsp}2.10 for openSUSE Leap{nbsp}15.0].
+
+<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
+
+|https://www.redhat.com/[RHEL] and https://www.suse.com/[SLES]
+|See http://packages.efficios.com/[EfficiOS Enterprise Packages].
+|
+
+|https://buildroot.org/[Buildroot]
+|_Not available_
+|
+link:/docs/v2.10#doc-buildroot[LTTng{nbsp}2.10 for Buildroot{nbsp}2018.02,
+Buildroot{nbsp}2018.05, Buildroot{nbsp}2018.08, and Buildroot{nbsp}2018.11].
+
+<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
+
+|http://www.openembedded.org/wiki/Main_Page[OpenEmbedded] and
+https://www.yoctoproject.org/[Yocto]
+|_Not available_
+|<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
+|====
+
+
+[[ubuntu-ppa]]
+=== Ubuntu: noch:{LTTng} Stable {revision} PPA
+
+The https://launchpad.net/~lttng/+archive/ubuntu/stable-{revision}[LTTng
+Stable{nbsp}{revision} PPA] offers the latest stable
+LTTng{nbsp}{revision} packages for:
+
+* Ubuntu{nbsp}14.04 _Trusty Tahr_
+* Ubuntu{nbsp}16.04 _Xenial Xerus_
+* Ubuntu{nbsp}18.04 _Bionic Beaver_
+
+To install LTTng{nbsp}{revision} from the LTTng Stable{nbsp}{revision} PPA:
+
+. Add the LTTng Stable{nbsp}{revision} PPA repository and update the
+ list of packages:
++
+--
+[role="term"]
+----
+# apt-add-repository ppa:lttng/stable-2.11
+# apt-get update
+----
+--
+
+. Install the main LTTng{nbsp}{revision} packages:
++
+--
+[role="term"]
+----
+# apt-get install lttng-tools
+# apt-get install lttng-modules-dkms
+# apt-get install liblttng-ust-dev
+----
+--
+
+. **If you need to instrument and trace
+ <<java-application,Java applications>>**, install the LTTng-UST
+ Java agent:
++
+--
+[role="term"]
+----
+# apt-get install liblttng-ust-agent-java
+----
+--
+
+. **If you need to instrument and trace
+ <<python-application,Python{nbsp}3 applications>>**, install the
+ LTTng-UST Python agent:
++
+--
+[role="term"]
+----
+# apt-get install python3-lttngust
+----
+--
+
+
+[[enterprise-distributions]]
+=== RHEL, SUSE, and other enterprise distributions
+
+To install LTTng on enterprise Linux distributions, such as Red Hat
+Enterprise Linux (RHEL) and SUSE Linux Enterprise Server (SUSE), please
+see http://packages.efficios.com/[EfficiOS Enterprise Packages].
+
+
+[[building-from-source]]
+=== Build from source
+
+To build and install LTTng{nbsp}{revision} from source:
+
+. Using your distribution's package manager, or from source, install
+ the following dependencies of LTTng-tools and LTTng-UST:
++
+--
+* https://sourceforge.net/projects/libuuid/[libuuid]
+* http://directory.fsf.org/wiki/Popt[popt]
+* http://liburcu.org/[Userspace RCU]
+* http://www.xmlsoft.org/[libxml2]
+* **Optional**: https://github.com/numactl/numactl[numactl]
+--
+
+. Download, build, and install the latest LTTng-modules{nbsp}{revision}:
++
+--
+[role="term"]
+----
+$ cd $(mktemp -d) &&
+wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.11.tar.bz2 &&
+tar -xf lttng-modules-latest-2.11.tar.bz2 &&
+cd lttng-modules-2.11.* &&
+make &&
+sudo make modules_install &&
+sudo depmod -a
+----
+--
+
+. Download, build, and install the latest LTTng-UST{nbsp}{revision}:
++
+--
+[role="term"]
+----
+$ cd $(mktemp -d) &&
+wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.11.tar.bz2 &&
+tar -xf lttng-ust-latest-2.11.tar.bz2 &&
+cd lttng-ust-2.11.* &&
+./configure &&
+make &&
+sudo make install &&
+sudo ldconfig
+----
+--
++
+Add `--disable-numa` to `./configure` if you don't have
+https://github.com/numactl/numactl[numactl].
++
+--
+[IMPORTANT]
+.Java and Python application tracing
+====
+If you need to instrument and trace <<java-application,Java
+applications>>, pass the `--enable-java-agent-jul`,
+`--enable-java-agent-log4j`, or `--enable-java-agent-all` options to the
+`configure` script, depending on which Java logging framework you use.
+
+If you need to instrument and trace <<python-application,Python
+applications>>, pass the `--enable-python-agent` option to the
+`configure` script. You can set the `PYTHON` environment variable to the
+path to the Python interpreter for which to install the LTTng-UST Python
+agent package.
+====
+--
++
+--
+[NOTE]
+====
+By default, LTTng-UST libraries are installed to
+dir:{/usr/local/lib}, which is the de facto directory in which to
+keep self-compiled and third-party libraries.
+
+When <<building-tracepoint-providers-and-user-application,linking an
+instrumented user application with `liblttng-ust`>>:
+
+* Append `/usr/local/lib` to the env:LD_LIBRARY_PATH environment
+ variable.
+* Pass the `-L/usr/local/lib` and `-Wl,-rpath,/usr/local/lib` options to
+ man:gcc(1), man:g++(1), or man:clang(1).
+====
+--
+
+. Download, build, and install the latest LTTng-tools{nbsp}{revision}:
++
+--
+[role="term"]
+----
+$ cd $(mktemp -d) &&
+wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.11.tar.bz2 &&
+tar -xf lttng-tools-latest-2.11.tar.bz2 &&
+cd lttng-tools-2.11.* &&
+./configure &&
+make &&
+sudo make install &&
+sudo ldconfig
+----
+--
+
+TIP: The https://github.com/eepp/vlttng[vlttng tool] can do all the
+previous steps automatically for a given version of LTTng and confine
+the installed files in a specific directory. This can be useful to test
+LTTng without installing it on your system.
+
+
+[[getting-started]]
+== Quick start
+
+This is a short guide to get started quickly with LTTng kernel and user
+space tracing.
+
+Before you follow this guide, make sure to <<installing-lttng,install>>
+LTTng.
+
+This tutorial walks you through the steps to:
+
+. <<tracing-the-linux-kernel,Trace the Linux kernel>>.
+. <<tracing-your-own-user-application,Trace a user application>> written
+ in C.
+. <<viewing-and-analyzing-your-traces,View and analyze the
+ recorded events>>.
+
+
+[[tracing-the-linux-kernel]]
+=== Trace the Linux kernel
+
+The following command lines start with the `#` prompt because you need
+root privileges to trace the Linux kernel. You can also trace the kernel
+as a regular user if your Unix user is a member of the
+<<tracing-group,tracing group>>.
+
+. Create a <<tracing-session,tracing session>> which writes its traces
+ to dir:{/tmp/my-kernel-trace}:
++
+--
+[role="term"]
+----
+# lttng create my-kernel-session --output=/tmp/my-kernel-trace
+----
+--
+
+. List the available kernel tracepoints and system calls:
++
+--
+[role="term"]
+----
+# lttng list --kernel
+# lttng list --kernel --syscall
+----
+--
+
+. Create <<event,event rules>> which match the desired instrumentation
+ point names, for example the `sched_switch` and `sched_process_fork`
+ tracepoints, and the man:open(2) and man:close(2) system calls:
++
+--
+[role="term"]
+----
+# lttng enable-event --kernel sched_switch,sched_process_fork
+# lttng enable-event --kernel --syscall open,close
+----
+--
++
+You can also create an event rule which matches _all_ the Linux kernel
+tracepoints (this will generate a lot of data when tracing):
++
+--
+[role="term"]
+----
+# lttng enable-event --kernel --all
+----
+--
+
+. <<basic-tracing-session-control,Start tracing>>:
++
+--
+[role="term"]
+----
+# lttng start
+----
+--
+
+. Do some operation on your system for a few seconds. For example,
+ load a website, or list the files of a directory.
+. <<basic-tracing-session-control,Stop tracing>> and destroy the
+ tracing session:
++
+--
+[role="term"]
+----
+# lttng stop
+# lttng destroy
+----
+--
++
+The man:lttng-destroy(1) command does not destroy the trace data; it
+only destroys the state of the tracing session.
+
+. For the sake of this example, make the recorded trace accessible to
+ the non-root users:
++
+--
+[role="term"]
+----
+# chown -R $(whoami) /tmp/my-kernel-trace
+----
+--
+
+See <<viewing-and-analyzing-your-traces,View and analyze the
+recorded events>> to view the recorded events.
+
+
+[[tracing-your-own-user-application]]
+=== Trace a user application
+
+This section steps you through a simple example to trace a
+_Hello world_ program written in C.
+
+To create the traceable user application:
+
+. Create the tracepoint provider header file, which defines the
+ tracepoints and the events they can generate:
++
+--
+[source,c]
+.path:{hello-tp.h}
+----
+#undef TRACEPOINT_PROVIDER
+#define TRACEPOINT_PROVIDER hello_world
+
+#undef TRACEPOINT_INCLUDE
+#define TRACEPOINT_INCLUDE "./hello-tp.h"
+
+#if !defined(_HELLO_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
+#define _HELLO_TP_H
+
+#include <lttng/tracepoint.h>
+
+TRACEPOINT_EVENT(
+ hello_world,
+ my_first_tracepoint,
+ TP_ARGS(
+ int, my_integer_arg,
+ char*, my_string_arg
+ ),
+ TP_FIELDS(
+ ctf_string(my_string_field, my_string_arg)
+ ctf_integer(int, my_integer_field, my_integer_arg)
+ )
+)
+
+#endif /* _HELLO_TP_H */
+
+#include <lttng/tracepoint-event.h>
+----
+--
+
+. Create the tracepoint provider package source file:
++
+--
+[source,c]
+.path:{hello-tp.c}
+----
+#define TRACEPOINT_CREATE_PROBES
+#define TRACEPOINT_DEFINE
+
+#include "hello-tp.h"
+----
+--
+
+. Build the tracepoint provider package:
++
+--
+[role="term"]
+----
+$ gcc -c -I. hello-tp.c
+----
+--
+
+. Create the _Hello World_ application source file:
++
+--
+[source,c]
+.path:{hello.c}
+----
+#include <stdio.h>
+#include "hello-tp.h"
+
+int main(int argc, char *argv[])
+{
+ int x;
+
+ puts("Hello, World!\nPress Enter to continue...");
+
+ /*
+ * The following getchar() call is only placed here for the purpose
+ * of this demonstration, to pause the application in order for
+ * you to have time to list its tracepoints. It is not
+ * needed otherwise.
+ */
+ getchar();
+
+ /*
+ * A tracepoint() call.
+ *
+ * Arguments, as defined in hello-tp.h:
+ *
+ * 1. Tracepoint provider name (required)
+ * 2. Tracepoint name (required)
+ * 3. my_integer_arg (first user-defined argument)
+ * 4. my_string_arg (second user-defined argument)
+ *
+ * Notice the tracepoint provider and tracepoint names are
+ * NOT strings: they are in fact parts of variables that the
+ * macros in hello-tp.h create.
+ */
+ tracepoint(hello_world, my_first_tracepoint, 23, "hi there!");
+
+ for (x = 0; x < argc; ++x) {
+ tracepoint(hello_world, my_first_tracepoint, x, argv[x]);
+ }
+
+ puts("Quitting now!");
+ tracepoint(hello_world, my_first_tracepoint, x * x, "x^2");
+
+ return 0;
+}
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -c hello.c
+----
+--
+
+. Link the application with the tracepoint provider package,
+ `liblttng-ust`, and `libdl`:
++
+--
+[role="term"]
+----
+$ gcc -o hello hello.o hello-tp.o -llttng-ust -ldl
+----
+--
+
+Here's the whole build process:
+
+[role="img-100"]
+.User space tracing tutorial's build steps.
+image::ust-flow.png[]
+
+To trace the user application:
+
+. Run the application with a few arguments:
++
+--
+[role="term"]
+----
+$ ./hello world and beyond
+----
+--
++
+You see:
++
+--
+----
+Hello, World!
+Press Enter to continue...
+----
+--
+
+. Start an LTTng <<lttng-sessiond,session daemon>>:
++
+--
+[role="term"]
+----
+$ lttng-sessiond --daemonize
+----
+--
++
+Note that a session daemon might already be running, for example as
+a service that the distribution's service manager started.
+
+. List the available user space tracepoints:
++
+--
+[role="term"]
+----
+$ lttng list --userspace
+----
+--
++
+You see the `hello_world:my_first_tracepoint` tracepoint listed
+under the `./hello` process.
+
+. Create a <<tracing-session,tracing session>>:
++
+--
+[role="term"]
+----
+$ lttng create my-user-space-session
+----
+--
+
+. Create an <<event,event rule>> which matches the
+ `hello_world:my_first_tracepoint` event name:
++
+--
+[role="term"]
+----
+$ lttng enable-event --userspace hello_world:my_first_tracepoint
+----
+--
+
+. <<basic-tracing-session-control,Start tracing>>:
++
+--
+[role="term"]
+----
+$ lttng start
+----
+--
+
+. Go back to the running `hello` application and press Enter. The
+ program executes all `tracepoint()` instrumentation points and exits.
+. <<basic-tracing-session-control,Stop tracing>> and destroy the
+ tracing session:
++
+--
+[role="term"]
+----
+$ lttng stop
+$ lttng destroy
+----
+--
++
+The man:lttng-destroy(1) command does not destroy the trace data; it
+only destroys the state of the tracing session.
+
+By default, LTTng saves the traces in
++$LTTNG_HOME/lttng-traces/__name__-__date__-__time__+,
+where +__name__+ is the tracing session name. The
+env:LTTNG_HOME environment variable defaults to `$HOME` if not set.
+
+See <<viewing-and-analyzing-your-traces,View and analyze the
+recorded events>> to view the recorded events.
+
+
+[[viewing-and-analyzing-your-traces]]
+=== View and analyze the recorded events
+
+Once you have completed the <<tracing-the-linux-kernel,Trace the Linux
+kernel>> and <<tracing-your-own-user-application,Trace a user
+application>> tutorials, you can inspect the recorded events.
+
+Many tools are available to read LTTng traces:
+
+* **cmd:babeltrace** is a command-line utility which converts trace
+ formats; it supports the format that LTTng produces, CTF, as well as a
+ basic text output which can be ++grep++ed. The cmd:babeltrace command
+ is part of the http://diamon.org/babeltrace[Babeltrace] project.
+* Babeltrace also includes
+ **https://www.python.org/[Python] bindings** so
+ that you can easily open and read an LTTng trace with your own script,
+ benefiting from the power of Python.
+* http://tracecompass.org/[**Trace Compass**]
+ is a graphical user interface for viewing and analyzing any type of
+ logs or traces, including LTTng's.
+* https://github.com/lttng/lttng-analyses[**LTTng analyses**] is a
+ project which includes many high-level analyses of LTTng kernel
+ traces, like scheduling statistics, interrupt frequency distribution,
+ top CPU usage, and more.
+
+NOTE: This section assumes that LTTng saved 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:babeltrace command-line tool
+
+The simplest way to list all the recorded events of a trace is to pass
+its path to cmd:babeltrace with no options:
+
+[role="term"]
+----
+$ babeltrace ~/lttng-traces/my-user-space-session*
+----
+
+cmd:babeltrace finds all traces recursively within the given path and
+prints all their events, merging them in chronological order.
+
+You can pipe the output of cmd:babeltrace into a tool like man:grep(1) for
+further filtering:
+
+[role="term"]
+----
+$ babeltrace /tmp/my-kernel-trace | grep _switch
+----
+
+You can pipe the output of cmd:babeltrace into a tool like man:wc(1) to
+count the recorded events:
+
+[role="term"]
+----
+$ babeltrace /tmp/my-kernel-trace | grep _open | wc --lines
+----
+
+
+[[viewing-and-analyzing-your-traces-bt-python]]
+==== Use the Babeltrace Python bindings
+
+The <<viewing-and-analyzing-your-traces-bt,text output of cmd:babeltrace>>
+is useful to isolate events by simple matching using man:grep(1) and
+similar utilities. However, more elaborate filters, such as keeping only
+event records with a field value falling within a specific range, are
+not trivial to write using a shell. Moreover, reductions and even the
+most basic computations involving multiple event records are virtually
+impossible to implement.
+
+Fortunately, Babeltrace ships with Python{nbsp}3 bindings which makes it
+easy to read the event records of an LTTng trace sequentially and
+compute the desired information.
+
+The following script accepts an LTTng Linux kernel trace path as its
+first argument and prints the short names of the top five running
+processes on CPU{nbsp}0 during the whole trace:
+
+[source,python]
+.path:{top5proc.py}
+----
+from collections import Counter
+import babeltrace
+import sys
+
+
+def top5proc():
+ if len(sys.argv) != 2:
+ msg = 'Usage: python3 {} TRACEPATH'.format(sys.argv[0])
+ print(msg, file=sys.stderr)
+ return False
+
+ # A trace collection contains one or more traces
+ col = babeltrace.TraceCollection()
+
+ # Add the trace provided by the user (LTTng traces always have
+ # the 'ctf' format)
+ if col.add_trace(sys.argv[1], 'ctf') is None:
+ raise RuntimeError('Cannot add trace')
+
+ # This counter dict contains execution times:
+ #
+ # task command name -> total execution time (ns)
+ exec_times = Counter()
+
+ # This contains the last `sched_switch` timestamp
+ last_ts = None
+
+ # Iterate on events
+ for event in col.events:
+ # Keep only `sched_switch` events
+ if event.name != 'sched_switch':
+ continue
+
+ # Keep only events which happened on CPU 0
+ if event['cpu_id'] != 0:
+ continue
+
+ # Event timestamp
+ cur_ts = event.timestamp
+
+ if last_ts is None:
+ # We start here
+ last_ts = cur_ts
+
+ # Previous task command (short) name
+ prev_comm = event['prev_comm']
+
+ # Initialize entry in our dict if not yet done
+ if prev_comm not in exec_times:
+ exec_times[prev_comm] = 0
+
+ # Compute previous command execution time
+ diff = cur_ts - last_ts
+
+ # Update execution time of this command
+ exec_times[prev_comm] += diff
+
+ # Update last timestamp
+ last_ts = cur_ts
+
+ # Display top 5
+ for name, ns in exec_times.most_common(5):
+ s = ns / 1000000000
+ print('{:20}{} s'.format(name, s))
+
+ return True
+
+
+if __name__ == '__main__':
+ sys.exit(0 if top5proc() else 1)
+----
+
+Run this script:
+
+[role="term"]
+----
+$ python3 top5proc.py /tmp/my-kernel-trace/kernel
+----
+
+Output example:
+
+----
+swapper/0 48.607245889 s
+chromium 7.192738188 s
+pavucontrol 0.709894415 s
+Compositor 0.660867933 s
+Xorg.bin 0.616753786 s
+----
+
+Note that `swapper/0` is the "idle" process of CPU{nbsp}0 on Linux;
+since we weren't using the CPU that much when tracing, its first
+position in the list makes sense.
+
+
+[[core-concepts]]
+== [[understanding-lttng]]Core concepts
+
+From a user's perspective, the LTTng system is built on a few concepts,
+or objects, on which the <<lttng-cli,cmd:lttng command-line tool>>
+operates by sending commands to the <<lttng-sessiond,session daemon>>.
+Understanding how those objects relate to eachother is key in mastering
+the toolkit.
+
+The core concepts are:
+
+* <<tracing-session,Tracing session>>
+* <<domain,Tracing domain>>
+* <<channel,Channel and ring buffer>>
+* <<"event","Instrumentation point, event rule, event, and event record">>
+
+
+[[tracing-session]]
+=== Tracing session
+
+A _tracing session_ is a stateful dialogue between you and
+a <<lttng-sessiond,session daemon>>. You can
+<<creating-destroying-tracing-sessions,create a new tracing
+session>> with the `lttng create` command.
+
+Anything that you do when you control LTTng tracers happens within a
+tracing session. In particular, a tracing session:
+
+* Has its own name.
+* Has its own set of trace files.
+* Has its own state of activity (started or stopped).
+* Has its own <<tracing-session-mode,mode>> (local, network streaming,
+ snapshot, or live).
+* Has its own <<channel,channels>> to which are associated their own
+ <<event,event rules>>.
+
+[role="img-100"]
+.A _tracing session_ contains <<channel,channels>> that are members of <<domain,tracing domains>> and contain <<event,event rules>>.
+image::concepts.png[]
+
+Those attributes and objects are completely isolated between different
+tracing sessions.
+
+A tracing session is analogous to a cash machine session:
+the operations you do on the banking system through the cash machine do
+not alter the data of other users of the same system. In the case of
+the cash machine, a session lasts as long as your bank card is inside.
+In the case of LTTng, a tracing session lasts from the `lttng create`
+command to the `lttng destroy` command.
+
+[role="img-100"]
+.Each Unix user has its own set of tracing sessions.
+image::many-sessions.png[]
+
+
+[[tracing-session-mode]]
+==== Tracing session mode
+
+LTTng can send the generated trace data to different locations. The
+_tracing session mode_ dictates where to send it. The following modes
+are available in LTTng{nbsp}{revision}:
+
+Local mode::
+ LTTng writes the traces to the file system of the machine it traces
+ (target system).
+
+Network streaming mode::
+ LTTng sends the traces over the network to a
+ <<lttng-relayd,relay daemon>> running on a remote system.
+
+Snapshot mode::
+ LTTng does not write the traces by default. Instead, you can request
+ LTTng to <<taking-a-snapshot,take a snapshot>>, that is, a copy of the
+ tracing session's current sub-buffers, and to write it to the
+ target's file system or to send it over the network to a
+ <<lttng-relayd,relay daemon>> running on a remote system.
+
+Live mode::
+ This mode is similar to the network streaming mode, but a live
+ trace viewer can connect to the distant relay daemon to
+ <<lttng-live,view event records as LTTng generates them>>.
+
+
+[[domain]]
+=== Tracing domain
+
+A _tracing domain_ is a namespace for event sources. A tracing domain
+has its own properties and features.
+
+There are currently five available tracing domains:
+
+* Linux kernel
+* User space
+* `java.util.logging` (JUL)
+* log4j
+* Python
+
+You must specify a tracing domain when using some commands to avoid
+ambiguity. For example, since all the domains support named tracepoints
+as event sources (instrumentation points that you manually insert in the
+source code), you need to specify a tracing domain when
+<<enabling-disabling-events,creating an event rule>> because all the
+tracing domains could have tracepoints with the same names.
+
+Some features are reserved to specific tracing domains. Dynamic function
+entry and return instrumentation points, for example, are currently only
+supported in the Linux kernel tracing domain, but support for other
+tracing domains could be added in the future.
+
+You can create <<channel,channels>> in the Linux kernel and user space
+tracing domains. The other tracing domains have a single default
+channel.
+
+
+[[channel]]
+=== Channel and ring buffer
+
+A _channel_ is an object which is responsible for a set of ring buffers.
+Each ring buffer is divided into multiple sub-buffers. When an LTTng
+tracer emits an event, it can record it to one or more
+sub-buffers. The attributes of a channel determine what to do when
+there's no space left for a new event record because all sub-buffers
+are full, where to send a full sub-buffer, and other behaviours.
+
+A channel is always associated to a <<domain,tracing domain>>. The
+`java.util.logging` (JUL), log4j, and Python tracing domains each have
+a default channel which you cannot configure.
+
+A channel also owns <<event,event rules>>. When an LTTng tracer emits
+an event, it records it to the sub-buffers of all
+the enabled channels with a satisfied event rule, as long as those
+channels are part of active <<tracing-session,tracing sessions>>.
+
+
+[[channel-buffering-schemes]]
+==== Per-user vs. per-process buffering schemes
+
+A channel has at least one ring buffer _per CPU_. LTTng always
+records an event to the ring buffer associated to the CPU on which it
+occurs.
+
+Two _buffering schemes_ are available when you
+<<enabling-disabling-channels,create a channel>> in the
+user space <<domain,tracing domain>>:
+
+Per-user buffering::
+ Allocate one set of ring buffers--one per CPU--shared by all the
+ instrumented processes of each Unix user.
++
+--
+[role="img-100"]
+.Per-user buffering scheme.
+image::per-user-buffering.png[]
+--
+
+Per-process buffering::
+ Allocate one set of ring buffers--one per CPU--for each
+ instrumented process.
++
+--
+[role="img-100"]
+.Per-process buffering scheme.
+image::per-process-buffering.png[]
+--
++
+The per-process buffering scheme tends to consume more memory than the
+per-user option because systems generally have more instrumented
+processes than Unix users running instrumented processes. However, the
+per-process buffering scheme ensures that one process having a high
+event throughput won't fill all the shared sub-buffers of the same
+user, only its own.
+
+The Linux kernel tracing domain has only one available buffering scheme
+which is to allocate a single set of ring buffers for the whole system.
+This scheme is similar to the per-user option, but with a single, global
+user "running" the kernel.
+
+
+[[channel-overwrite-mode-vs-discard-mode]]
+==== Overwrite vs. discard event record loss modes
+
+When an event occurs, LTTng records it to a specific sub-buffer (yellow
+arc in the following animations) of a specific channel's ring buffer.
+When there's no space left in a sub-buffer, the tracer marks it as
+consumable (red) and another, empty sub-buffer starts receiving the
+following event records. A <<lttng-consumerd,consumer daemon>>
+eventually consumes the marked sub-buffer (returns to white).
+
+[NOTE]
+[role="docsvg-channel-subbuf-anim"]
+====
+{note-no-anim}
+====
+
+In an ideal world, sub-buffers are consumed faster than they are filled,
+as it is the case in the previous animation. In the real world,
+however, all sub-buffers can be full at some point, leaving no space to
+record the following events.
+
+By default, LTTng-modules and LTTng-UST are _non-blocking_ tracers: when
+no empty sub-buffer is available, it is acceptable to lose event records
+when the alternative would be to cause substantial delays in the
+instrumented application's execution. LTTng privileges performance over
+integrity; it aims at perturbing the target system as little as possible
+in order to make tracing 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 no empty sub-buffer is
+available, or because the <<opt-blocking-timeout,blocking timeout>> is
+reached, the channel's _event record loss mode_ determines what to do.
+The available event record loss modes are:
+
+Discard mode::
+ Drop the newest event records until a the tracer releases a
+ sub-buffer.
++
+This is the only available mode when you specify a
+<<opt-blocking-timeout,blocking timeout>>.
+
+Overwrite mode::
+ Clear the sub-buffer containing the oldest event records and start
+ writing the newest event records there.
++
+This mode is sometimes called _flight recorder mode_ because it's
+similar to a
+https://en.wikipedia.org/wiki/Flight_recorder[flight recorder]:
+always keep a fixed amount of the latest data.
+
+Which mechanism you should choose depends on your context: prioritize
+the newest or the oldest event records in the ring buffer?
+
+Beware that, in overwrite mode, the tracer abandons a _whole sub-buffer_
+as soon as a there's no space left for a new event record, whereas in
+discard mode, the tracer only discards the event record that doesn't
+fit.
+
+In discard mode, LTTng increments a count of lost event records when an
+event record is lost and saves this count to the trace. In overwrite
+mode, since LTTng{nbsp}2.8, LTTng increments a count of lost sub-buffers
+when a sub-buffer is lost and saves this count to the trace. In this
+mode, LTTng does not write to the trace the exact number of lost event
+records in those lost sub-buffers. Trace analyses can use the trace's
+saved discarded event record and sub-buffer counts to decide whether or
+not to perform the analyses even if trace data is known to be missing.
+
+There are a few ways to decrease your probability of losing event
+records.
+<<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>> shows
+how you can fine-tune the sub-buffer count and size of a channel to
+virtually stop losing event records, though at the cost of greater
+memory usage.
+
+
+[[channel-subbuf-size-vs-subbuf-count]]
+==== Sub-buffer count and size
+
+When you <<enabling-disabling-channels,create a channel>>, you can
+set its number of sub-buffers and their size.
+
+Note that there is noticeable CPU overhead introduced when
+switching sub-buffers (marking a full one as consumable and switching
+to an empty one for the following events to be recorded). Knowing this,
+the following list presents a few practical situations along with how
+to configure the sub-buffer count and size for them:
+
+* **High event throughput**: In general, prefer bigger sub-buffers to
+ lower the risk of losing event records.
++
+Having bigger sub-buffers also ensures a lower
+<<channel-switch-timer,sub-buffer switching frequency>>.
++
+The number of sub-buffers is only meaningful if you create the channel
+in overwrite mode: in this case, if a sub-buffer overwrite happens, the
+other sub-buffers are left unaltered.
+
+* **Low event throughput**: In general, prefer smaller sub-buffers
+ since the risk of losing event records is low.
++
+Because events occur less frequently, the sub-buffer switching frequency
+should remain low and thus the tracer's overhead should not be a
+problem.
+
+* **Low memory system**: If your target system has a low memory
+ limit, prefer fewer first, then smaller sub-buffers.
++
+Even if the system is limited in memory, you want to keep the
+sub-buffers as big as possible to avoid a high sub-buffer switching
+frequency.
+
+Note that LTTng uses http://diamon.org/ctf/[CTF] as its trace format,
+which means event data is very compact. For example, the average
+LTTng kernel event record weights about 32{nbsp}bytes. Thus, a
+sub-buffer size of 1{nbsp}MiB is considered big.
+
+The previous situations highlight the major trade-off between a few big
+sub-buffers and more, smaller sub-buffers: sub-buffer switching
+frequency vs. how much data is lost in overwrite mode. Assuming a
+constant event throughput and using the overwrite mode, the two
+following configurations have the same ring buffer total size:
+
+[NOTE]
+[role="docsvg-channel-subbuf-size-vs-count-anim"]
+====
+{note-no-anim}
+====
+
+* **Two sub-buffers of 4{nbsp}MiB each**: Expect a very low sub-buffer
+ switching frequency, but if a sub-buffer overwrite happens, half of
+ the event records so far (4{nbsp}MiB) are definitely lost.
+* **Eight sub-buffers of 1{nbsp}MiB each**: Expect four times the tracer's
+ overhead as the previous configuration, but if a sub-buffer
+ overwrite happens, only the eighth of event records so far are
+ definitely lost.
+
+In discard mode, the sub-buffers count parameter is pointless: use two
+sub-buffers and set their size according to the requirements of your
+situation.
+
+
+[[channel-switch-timer]]
+==== Switch timer period
+
+The _switch timer period_ is an important configurable attribute of
+a channel to ensure periodic sub-buffer flushing.
+
+When the _switch timer_ expires, a sub-buffer switch happens. You can
+set the switch timer period attribute when you
+<<enabling-disabling-channels,create a channel>> to ensure that LTTng
+consumes and commits trace data to trace files or to a distant relay
+daemon periodically in case of a low event throughput.
+
+[NOTE]
+[role="docsvg-channel-switch-timer"]
+====
+{note-no-anim}
+====
+
+This attribute is also convenient when you use big sub-buffers to cope
+with a sporadic high event throughput, even if the throughput is
+normally low.
+
+
+[[channel-read-timer]]
+==== Read timer period
+
+By default, the LTTng tracers use a notification mechanism to signal a
+full sub-buffer so that a consumer daemon can consume it. When such
+notifications must be avoided, for example in real-time applications,
+you can use the channel's _read timer_ instead. When the read timer
+fires, the <<lttng-consumerd,consumer daemon>> checks for full,
+consumable sub-buffers.
+
+
+[[tracefile-rotation]]
+==== Trace file count and size
+
+By default, trace files can grow as large as needed. You can set the
+maximum size of each trace file that a channel writes when you
+<<enabling-disabling-channels,create a channel>>. When the size of
+a trace file reaches the channel's fixed maximum size, LTTng creates
+another file to contain the next event records. LTTng appends a file
+count to each trace file name in this case.
+
+If you set the trace file size attribute when you create a channel, the
+maximum number of trace files that LTTng creates is _unlimited_ by
+default. To limit them, you can also set a maximum number of trace
+files. When the number of trace files reaches the channel's fixed
+maximum count, the oldest trace file is overwritten. This mechanism is
+called _trace file rotation_.
+
+Even if you don't limit the trace file count, you cannot assume that
+LTTng doesn't manage any trace file. In other words, there is 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 tracing session is
+with the <<session-rotation,tracing session rotation>> feature
+(available since LTTng{nbsp}2.11).
+
+
+[[event]]
+=== Instrumentation point, event rule, event, and event record
+
+An _event rule_ is a set of conditions which must be **all** satisfied
+for LTTng to record an occuring event.
+
+You set the conditions when you <<enabling-disabling-events,create
+an event rule>>.
+
+You always attach an event rule to <<channel,channel>> when you create
+it.
+
+When an event passes the conditions of an event rule, LTTng records it
+in one of the attached channel's sub-buffers.
+
+The available conditions, as of LTTng{nbsp}{revision}, are:
+
+* The event rule _is enabled_.
+* The instrumentation point's type _is{nbsp}T_.
+* The instrumentation point's name (sometimes called _event name_)
+ _matches{nbsp}N_, but _is not{nbsp}E_.
+* The instrumentation point's log level _is as severe as{nbsp}L_, or
+ _is exactly{nbsp}L_.
+* The fields of the event's payload _satisfy_ a filter
+ expression{nbsp}__F__.
+
+As you can see, all the conditions but the dynamic filter are related to
+the event rule's status or to the instrumentation point, not to the
+occurring events. This is why, without a filter, checking if an event
+passes an event rule is not a dynamic task: when you create or modify an
+event rule, all the tracers of its tracing domain enable or disable the
+instrumentation points themselves once. This is possible because the
+attributes of an instrumentation point (type, name, and log level) are
+defined statically. In other words, without a dynamic filter, the tracer
+_does not evaluate_ the arguments of an instrumentation point unless it
+matches an enabled event rule.
+
+Note that, for LTTng to record an event, the <<channel,channel>> to
+which a matching event rule is attached must also be enabled, and the
+<<tracing-session,tracing session>> owning this channel must be active
+(started).
+
+[role="img-100"]
+.Logical path from an instrumentation point to an event record.
+image::event-rule.png[]
+
+.Event, event record, or event rule?
+****
+With so many similar terms, it's easy to get confused.
+
+An **event** is the consequence of the execution of an _instrumentation
+point_, like a tracepoint that you manually place in some source code,
+or a Linux kernel kprobe. An event is said to _occur_ at a specific
+time. Different actions can be taken upon the occurrence of an event,
+like record the event's payload to a buffer.
+
+An **event record** is the representation of an event in a sub-buffer. A
+tracer is responsible for capturing the payload of an event, current
+context variables, the event's ID, and the event's timestamp. LTTng
+can append this sub-buffer to a trace file.
+
+An **event rule** is a set of conditions which must _all_ be satisfied
+for LTTng to record an occuring event. Events still occur without
+satisfying event rules, but LTTng does not record them.
+****
+
+
+[[plumbing]]
+== Components of noch:{LTTng}
+
+The second _T_ in _LTTng_ stands for _toolkit_: it would be wrong
+to call LTTng a simple _tool_ since it is composed of multiple
+interacting components. This section describes those components,
+explains their respective roles, and shows how they connect together to
+form the LTTng ecosystem.
+
+The following diagram shows how the most important components of LTTng
+interact with user applications, the Linux kernel, and you:
+
+[role="img-100"]
+.Control and trace data paths between LTTng components.
+image::plumbing.png[]
+
+The LTTng project incorporates:
+
+* **LTTng-tools**: Libraries and command-line interface to
+ control tracing sessions.
+** <<lttng-sessiond,Session daemon>> (man:lttng-sessiond(8)).
+** <<lttng-consumerd,Consumer daemon>> (cmd:lttng-consumerd).
+** <<lttng-relayd,Relay daemon>> (man:lttng-relayd(8)).
+** <<liblttng-ctl-lttng,Tracing control library>> (`liblttng-ctl`).
+** <<lttng-cli,Tracing control command-line tool>> (man:lttng(1)).
+* **LTTng-UST**: Libraries and Java/Python packages to trace user
+ applications.
+** <<lttng-ust,User space tracing library>> (`liblttng-ust`) and its
+ headers to instrument and trace any native user application.
+** <<prebuilt-ust-helpers,Preloadable user space tracing helpers>>:
+*** `liblttng-ust-libc-wrapper`
+*** `liblttng-ust-pthread-wrapper`
+*** `liblttng-ust-cyg-profile`
+*** `liblttng-ust-cyg-profile-fast`
+*** `liblttng-ust-dl`
+** User space tracepoint provider source files generator command-line
+ tool (man:lttng-gen-tp(1)).
+** <<lttng-ust-agents,LTTng-UST Java agent>> to instrument and trace
+ Java applications using `java.util.logging` or
+ Apache log4j{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 trace
+ the kernel.
+** LTTng kernel tracer module.
+** Tracing ring buffer kernel modules.
+** Probe kernel modules.
+** LTTng logger kernel module.
+
+
+[[lttng-cli]]
+=== Tracing control command-line interface
+
+[role="img-100"]
+.The tracing control command-line interface.
+image::plumbing-lttng-cli.png[]
+
+The _man:lttng(1) command-line tool_ is the standard user interface to
+control LTTng <<tracing-session,tracing sessions>>. The cmd:lttng tool
+is part of LTTng-tools.
+
+The cmd:lttng tool is linked with
+<<liblttng-ctl-lttng,`liblttng-ctl`>> to communicate with
+one or more <<lttng-sessiond,session daemons>> behind the scenes.
+
+The cmd:lttng tool has a Git-like interface:
+
+[role="term"]
+----
+$ lttng <GENERAL OPTIONS> <COMMAND> <COMMAND OPTIONS>
+----
+
+The <<controlling-tracing,Tracing control>> section explores the
+available features of LTTng using the cmd:lttng tool.
+
+
+[[liblttng-ctl-lttng]]
+=== Tracing control library
+
+[role="img-100"]
+.The tracing control library.
+image::plumbing-liblttng-ctl.png[]
+
+The _LTTng control library_, `liblttng-ctl`, is used to communicate
+with a <<lttng-sessiond,session daemon>> using a C API that hides the
+underlying protocol's details. `liblttng-ctl` is part of LTTng-tools.
+
+The <<lttng-cli,cmd:lttng command-line tool>>
+is linked with `liblttng-ctl`.
+
+You can use `liblttng-ctl` in C or $$C++$$ source code by including its
+"master" header:
+
+[source,c]
+----
+#include <lttng/lttng.h>
+----
+
+Some objects are referenced by name (C string), such as tracing
+sessions, but most of them require to create a handle first using
+`lttng_create_handle()`.
+
+The best available developer documentation for `liblttng-ctl` is, as of
+LTTng{nbsp}{revision}, its installed header files. Every function and
+structure is thoroughly documented.
+
+
+[[lttng-ust]]
+=== User space tracing library
+
+[role="img-100"]
+.The user space tracing library.
+image::plumbing-liblttng-ust.png[]
+
+The _user space tracing library_, `liblttng-ust` (see man:lttng-ust(3)),
+is the LTTng user space tracer. It receives commands from a
+<<lttng-sessiond,session daemon>>, for example to
+enable and disable specific instrumentation points, and writes event
+records to ring buffers shared with a
+<<lttng-consumerd,consumer daemon>>.
+`liblttng-ust` is part of LTTng-UST.
+
+Public C header files are installed beside `liblttng-ust` to
+instrument any <<c-application,C or $$C++$$ application>>.
+
+<<lttng-ust-agents,LTTng-UST agents>>, which are regular Java and Python
+packages, use their own library providing tracepoints which is
+linked with `liblttng-ust`.
+
+An application or library does not have to initialize `liblttng-ust`
+manually: its constructor does the necessary tasks to properly register
+to a session daemon. The initialization phase also enables the
+instrumentation points matching the <<event,event rules>> that you
+already created.
+
+
+[[lttng-ust-agents]]
+=== User space tracing agents
+
+[role="img-100"]
+.The user space tracing agents.
+image::plumbing-lttng-ust-agents.png[]
+
+The _LTTng-UST Java and Python agents_ are regular Java and Python
+packages which add LTTng tracing capabilities to the
+native logging frameworks. The LTTng-UST agents are part of LTTng-UST.
+
+In the case of Java, the
+https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[`java.util.logging`
+core logging facilities] and
+https://logging.apache.org/log4j/1.2/[Apache log4j{nbsp}1.2] are supported.
+Note that Apache Log4{nbsp}2 is not supported.
+
+In the case of Python, the standard
+https://docs.python.org/3/library/logging.html[`logging`] package
+is supported. Both Python{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 trace the log statements. 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 application executes a log statement, the root logger passes it
+to the agent's log handler. The agent's log handler calls a native
+function in a tracepoint provider package shared library linked with
+<<lttng-ust,`liblttng-ust`>>, passing the formatted log message and
+other fields, like its logger name and its log level. This native
+function contains a user space instrumentation point, hence tracing the
+log statement.
+
+The log level condition of an
+<<event,event rule>> is considered when tracing
+a Java or a Python application, and it's compatible with the standard
+JUL, log4j, and Python log levels.
+
+
+[[lttng-modules]]
+=== LTTng kernel modules
+
+[role="img-100"]
+.The LTTng kernel modules.
+image::plumbing-lttng-modules.png[]
+
+The _LTTng kernel modules_ are a set of Linux kernel modules
+which implement the kernel tracer of the LTTng project. The LTTng
+kernel modules are part of LTTng-modules.
+
+The LTTng kernel modules include:
+
+* A set of _probe_ modules.
++
+Each module attaches to a specific subsystem
+of the Linux kernel using its tracepoint instrument points. There are
+also modules to attach to the entry and return points of the Linux
+system call functions.
+
+* _Ring buffer_ modules.
++
+A ring buffer implementation is provided as kernel modules. The LTTng
+kernel tracer writes to the ring buffer; a
+<<lttng-consumerd,consumer daemon>> reads from the ring buffer.
+
+* The _LTTng kernel tracer_ module.
+* The _LTTng logger_ module.
++
+The LTTng logger module implements the special path:{/proc/lttng-logger}
+file so that any executable can generate LTTng events by opening and
+writing to this file.
++
+See <<proc-lttng-logger-abi,LTTng logger>>.
+
+Generally, you do not have to load the LTTng kernel modules manually
+(using man:modprobe(8), for example): a root <<lttng-sessiond,session
+daemon>> loads the necessary modules when starting. If you have extra
+probe modules, you can specify to load them to the session daemon on
+the command line.
+
+The LTTng kernel modules are installed in
++/usr/lib/modules/__release__/extra+ by default, where +__release__+ is
+the kernel release (see `uname --kernel-release`).
+
+
+[[lttng-sessiond]]
+=== Session daemon
+
+[role="img-100"]
+.The session daemon.
+image::plumbing-sessiond.png[]
+
+The _session daemon_, man:lttng-sessiond(8), is a daemon responsible for
+managing tracing sessions and for controlling the various components of
+LTTng. The session daemon is part of LTTng-tools.
+
+The session daemon sends control requests to and receives control
+responses from:
+
+* The <<lttng-ust,user space tracing library>>.
++
+Any instance of the user space tracing library first registers to
+a session daemon. Then, the session daemon can send requests to
+this instance, such as:
++
+--
+** Get the list of tracepoints.
+** Share an <<event,event rule>> so that the user space tracing library
+ can enable or disable tracepoints. Amongst the possible conditions
+ of an event rule is a filter expression which `liblttng-ust` evalutes
+ when an event occurs.
+** Share <<channel,channel>> attributes and ring buffer locations.
+--
++
+The session daemon and the user space tracing library use a Unix
+domain socket for their communication.
+
+* The <<lttng-ust-agents,user space tracing agents>>.
++
+Any instance of a user space tracing agent first registers to
+a session daemon. Then, the session daemon can send requests to
+this instance, such as:
++
+--
+** Get the list of loggers.
+** Enable or disable a specific logger.
+--
++
+The session daemon and the user space tracing agent use a TCP connection
+for their communication.
+
+* The <<lttng-modules,LTTng kernel tracer>>.
+* The <<lttng-consumerd,consumer daemon>>.
++
+The session daemon sends requests to the consumer daemon to instruct
+it where to send the trace data streams, amongst other information.
+
+* The <<lttng-relayd,relay daemon>>.
+
+The session daemon receives commands from the
+<<liblttng-ctl-lttng,tracing control library>>.
+
+The root session daemon loads the appropriate
+<<lttng-modules,LTTng kernel modules>> on startup. It also spawns
+a <<lttng-consumerd,consumer daemon>> as soon as you create
+an <<event,event rule>>.
+
+The session daemon does not send and receive trace data: this is the
+role of the <<lttng-consumerd,consumer daemon>> and
+<<lttng-relayd,relay daemon>>. It does, however, generate the
+http://diamon.org/ctf/[CTF] metadata stream.
+
+Each Unix user can have its own session daemon instance. The
+tracing sessions 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 thus to trace the Linux
+kernel.
+
+The <<lttng-cli,cmd:lttng command-line tool>> automatically starts a
+session daemon when using its `create` command if none is currently
+running. You can also start the session daemon manually.
+
+
+[[lttng-consumerd]]
+=== Consumer daemon
+
+[role="img-100"]
+.The consumer daemon.
+image::plumbing-consumerd.png[]
+
+The _consumer daemon_, cmd:lttng-consumerd, is a daemon which shares
+ring buffers with user applications or with the LTTng kernel modules to
+collect trace data and send it to some location (on disk or to a
+<<lttng-relayd,relay daemon>> over the network). The consumer daemon
+is part of LTTng-tools.
+
+You do not start a consumer daemon manually: a consumer daemon is always
+spawned by a <<lttng-sessiond,session daemon>> as soon as you create an
+<<event,event rule>>, that is, before you start tracing. When you kill
+its owner session daemon, the consumer daemon also exits because it is
+the session daemon's child process. Command-line options of
+man:lttng-sessiond(8) target the consumer daemon process.
+
+There are up to two running consumer daemons per Unix user, whereas only
+one session daemon can run per user. This is because each process can be
+either 32-bit or 64-bit: if the target system runs a mixture of 32-bit
+and 64-bit processes, it is more efficient to have separate
+corresponding 32-bit and 64-bit consumer daemons. The root user is an
+exception: it can have up to _three_ running consumer daemons: 32-bit
+and 64-bit instances for its user applications, and one more
+reserved for collecting kernel trace data.
+
+
+[[lttng-relayd]]
+=== Relay daemon
+
+[role="img-100"]
+.The relay daemon.
+image::plumbing-relayd.png[]
+
+The _relay daemon_, man:lttng-relayd(8), is a daemon acting as a bridge
+between remote session and consumer daemons, local trace files, and a
+remote live trace viewer. The relay daemon is part of LTTng-tools.
+
+The main purpose of the relay daemon is to implement a receiver of
+<<sending-trace-data-over-the-network,trace data over the network>>.
+This is useful when the target system does not have much file system
+space to record trace files locally.
+
+The relay daemon is also a server to which a
+<<lttng-live,live trace viewer>> can
+connect. The live trace viewer sends requests to the relay daemon to
+receive trace data as the target system emits events. The
+communication protocol is named _LTTng live_; it is used over TCP
+connections.
+
+Note that you can start the relay daemon on the target system directly.
+This is the setup of choice when the use case is to view events as
+the target system emits them without the need of a remote system.
+
+
+[[instrumenting]]
+== [[using-lttng]]Instrumentation
+
+There are many examples of tracing and monitoring in our everyday life:
+
+* You have access to real-time and historical weather reports and
+ forecasts thanks to weather stations installed around the country.
+* You know your heart is safe thanks to an electrocardiogram.
+* You make sure not to drive your car too fast and to have enough fuel
+ to reach your destination thanks to gauges visible on your dashboard.
+
+All the previous examples have something in common: they rely on
+**instruments**. Without the electrodes attached to the surface of your
+body's skin, cardiac monitoring is futile.
+
+LTTng, as a tracer, is no different from those real life examples. If
+you're about to trace a software system or, in other words, record its
+history of execution, you better have **instrumentation points** in the
+subject you're tracing, that is, the actual software.
+
+Various ways were developed to instrument a piece of software for LTTng
+tracing. The most straightforward one is to manually place
+instrumentation points, called _tracepoints_, in the software's source
+code. It is also possible to add instrumentation points dynamically in
+the Linux kernel <<domain,tracing domain>>.
+
+If you're only interested in tracing the Linux kernel, your
+instrumentation needs are probably already covered by LTTng's built-in
+<<lttng-modules,Linux kernel tracepoints>>. You may also wish to trace a
+user application which is already instrumented for LTTng tracing.
+In such cases, you can skip this whole section and read the topics of
+the <<controlling-tracing,Tracing control>> section.
+
+Many methods are available to instrument a piece of software for LTTng
+tracing. They are:
+
+* <<c-application,User space instrumentation for C and $$C++$$
+ applications>>.
+* <<prebuilt-ust-helpers,Prebuilt user space tracing helpers>>.
+* <<java-application,User space Java agent>>.
+* <<python-application,User space Python agent>>.
+* <<proc-lttng-logger-abi,LTTng logger>>.
+* <<instrumenting-linux-kernel,LTTng kernel tracepoints>>.
+
+
+[[c-application]]
+=== [[cxx-application]]User space instrumentation for C and $$C++$$ applications
+
+The procedure to instrument a C or $$C++$$ user application with
+the <<lttng-ust,LTTng user space tracing library>>, `liblttng-ust`, is:
+
+. <<tracepoint-provider,Create the source files of a tracepoint provider
+ package>>.
+. <<probing-the-application-source-code,Add tracepoints to
+ the application's source code>>.
+. <<building-tracepoint-providers-and-user-application,Build and link
+ a tracepoint provider package and the user application>>.
+
+If you need quick, man:printf(3)-like instrumentation, you can skip
+those steps and use <<tracef,`tracef()`>> or <<tracelog,`tracelog()`>>
+instead.
+
+IMPORTANT: You need to <<installing-lttng,install>> LTTng-UST to
+instrument a user application with `liblttng-ust`.
+
+
+[[tracepoint-provider]]
+==== Create the source files of a tracepoint provider package
+
+A _tracepoint provider_ is a set of compiled functions which provide
+**tracepoints** to an application, the type of instrumentation point
+supported by LTTng-UST. Those functions can emit events with
+user-defined fields and serialize those events as event records to one
+or more LTTng-UST <<channel,channel>> sub-buffers. The `tracepoint()`
+macro, which you <<probing-the-application-source-code,insert in a user
+application's source code>>, calls those functions.
+
+A _tracepoint provider package_ is an object file (`.o`) or a shared
+library (`.so`) which contains one or more tracepoint providers.
+Its source files are:
+
+* One or more <<tpp-header,tracepoint provider header>> (`.h`).
+* A <<tpp-source,tracepoint provider package source>> (`.c`).
+
+A tracepoint provider package is dynamically linked with `liblttng-ust`,
+the LTTng user space tracer, at run time.
+
+[role="img-100"]
+.User application linked with `liblttng-ust` and containing a tracepoint provider.
+image::ust-app.png[]
+
+NOTE: If you need quick, man:printf(3)-like instrumentation, you can
+skip creating and using a tracepoint provider and use
+<<tracef,`tracef()`>> or <<tracelog,`tracelog()`>> instead.
+
+
+[[tpp-header]]
+===== Create a tracepoint provider header file template
+
+A _tracepoint provider header file_ contains the tracepoint
+definitions of a tracepoint provider.
+
+To create a tracepoint provider header file:
+
+. Start from this template:
++
+--
+[source,c]
+.Tracepoint provider header file template (`.h` file extension).
+----
+#undef TRACEPOINT_PROVIDER
+#define TRACEPOINT_PROVIDER provider_name
+
+#undef TRACEPOINT_INCLUDE
+#define TRACEPOINT_INCLUDE "./tp.h"
+
+#if !defined(_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
+#define _TP_H
+
+#include <lttng/tracepoint.h>
+
+/*
+ * Use TRACEPOINT_EVENT(), TRACEPOINT_EVENT_CLASS(),
+ * TRACEPOINT_EVENT_INSTANCE(), and TRACEPOINT_LOGLEVEL() here.
+ */
+
+#endif /* _TP_H */
+
+#include <lttng/tracepoint-event.h>
+----
+--
+
+. Replace:
++
+* `provider_name` with the name of your tracepoint provider.
+* `"tp.h"` with the name of your tracepoint provider header file.
+
+. Below the `#include <lttng/tracepoint.h>` line, put your
+ <<defining-tracepoints,tracepoint definitions>>.
+
+Your tracepoint provider name must be unique amongst all the possible
+tracepoint provider names used on the same target system. We
+suggest to include the name of your project or company in the name,
+for example, `org_lttng_my_project_tpp`.
+
+TIP: [[lttng-gen-tp]]You can use the man:lttng-gen-tp(1) tool to create
+this boilerplate for you. When using cmd:lttng-gen-tp, all you need to
+write are the <<defining-tracepoints,tracepoint definitions>>.
+
+
+[[defining-tracepoints]]
+===== Create a tracepoint definition
+
+A _tracepoint definition_ defines, for a given tracepoint:
+
+* Its **input arguments**. They are the macro parameters that the
+ `tracepoint()` macro accepts for this particular tracepoint
+ in the user application's source code.
+* Its **output event fields**. They are the sources of event fields
+ that form the payload of any event that the execution of the
+ `tracepoint()` macro emits for this particular tracepoint.
+
+You can create a tracepoint definition by using the
+`TRACEPOINT_EVENT()` macro below the `#include <lttng/tracepoint.h>`
+line in the
+<<tpp-header,tracepoint provider header file template>>.
+
+The syntax of the `TRACEPOINT_EVENT()` macro is:
+
+[source,c]
+.`TRACEPOINT_EVENT()` macro syntax.
+----
+TRACEPOINT_EVENT(
+ /* Tracepoint provider name */
+ provider_name,
+
+ /* Tracepoint name */
+ tracepoint_name,
+
+ /* Input arguments */
+ TP_ARGS(
+ arguments
+ ),
+
+ /* Output event fields */
+ TP_FIELDS(
+ fields
+ )
+)
+----
+
+Replace:
+
+* `provider_name` with your tracepoint provider name.
+* `tracepoint_name` with your tracepoint name.
+* `arguments` with the <<tpp-def-input-args,input arguments>>.
+* `fields` with the <<tpp-def-output-fields,output event field>>
+ definitions.
+
+This tracepoint emits events named `provider_name:tracepoint_name`.
+
+[IMPORTANT]
+.Event name's length limitation
+====
+The concatenation of the tracepoint provider name and the
+tracepoint name must not exceed **254{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 `TP_ARGS()` macro is:
+
+[source,c]
+.`TP_ARGS()` macro syntax.
+----
+TP_ARGS(
+ type, arg_name
+)
+----
+
+Replace:
+
+* `type` with the C type of the argument.
+* `arg_name` with the argument name.
+
+You can repeat `type` and `arg_name` up to 10{nbsp}times to have
+more than one argument.
+
+.`TP_ARGS()` usage with three arguments.
+====
+[source,c]
+----
+TP_ARGS(
+ int, count,
+ float, ratio,
+ const char*, query
+)
+----
+====
+
+The `TP_ARGS()` and `TP_ARGS(void)` forms are valid to create a
+tracepoint definition with no input arguments.
+
+[[tpp-def-output-fields]]The `TP_FIELDS()` macro contains a list of
+`ctf_*()` macros. Each `ctf_*()` macro defines one event field. See
+man:lttng-ust(3) for a complete description of the available `ctf_*()`
+macros. A `ctf_*()` macro specifies the type, size, and byte order of
+one event field.
+
+Each `ctf_*()` macro takes an _argument expression_ parameter. This is a
+C expression that the tracer evalutes at the `tracepoint()` macro site
+in the application's source code. This expression provides a field's
+source of data. The argument expression can include input argument names
+listed in the `TP_ARGS()` macro.
+
+Each `ctf_*()` macro also takes a _field name_ parameter. Field names
+must be unique within a given tracepoint definition.
+
+Here's a complete tracepoint definition example:
+
+.Tracepoint definition.
+====
+The following tracepoint definition defines a tracepoint which takes
+three input arguments and has four output event fields.
+
+[source,c]
+----
+#include "my-custom-structure.h"
+
+TRACEPOINT_EVENT(
+ my_provider,
+ my_tracepoint,
+ TP_ARGS(
+ const struct my_custom_structure*, my_custom_structure,
+ float, ratio,
+ const char*, query
+ ),
+ TP_FIELDS(
+ ctf_string(query_field, query)
+ ctf_float(double, ratio_field, ratio)
+ ctf_integer(int, recv_size, my_custom_structure->recv_size)
+ ctf_integer(int, send_size, my_custom_structure->send_size)
+ )
+)
+----
+
+You can refer to this tracepoint definition with the `tracepoint()`
+macro in your application's source code like this:
+
+[source,c]
+----
+tracepoint(my_provider, my_tracepoint,
+ my_structure, some_ratio, the_query);
+----
+====
+
+NOTE: The LTTng tracer only evaluates tracepoint arguments at run time
+if they satisfy an enabled <<event,event rule>>.
+
+
+[[using-tracepoint-classes]]
+===== Use a tracepoint class
+
+A _tracepoint class_ is a class of tracepoints which share the same
+output event field definitions. A _tracepoint instance_ is one
+instance of such a defined tracepoint class, with its own tracepoint
+name.
+
+The <<defining-tracepoints,`TRACEPOINT_EVENT()` macro>> is actually a
+shorthand which defines both a tracepoint class and a tracepoint
+instance at the same time.
+
+When you build a tracepoint provider package, the C or $$C++$$ compiler
+creates one serialization function for each **tracepoint class**. A
+serialization function is responsible for serializing the event fields
+of a tracepoint to a sub-buffer when tracing.
+
+For various performance reasons, when your situation requires multiple
+tracepoint definitions with different names, but with the same event
+fields, we recommend that you manually create a tracepoint class
+and instantiate as many tracepoint instances as needed. One positive
+effect of such a design, amongst other advantages, is that all
+tracepoint instances of the same tracepoint class reuse the same
+serialization function, thus reducing
+https://en.wikipedia.org/wiki/Cache_pollution[cache pollution].
+
+.Use a tracepoint class and tracepoint instances.
+====
+Consider the following three tracepoint definitions:
+
+[source,c]
+----
+TRACEPOINT_EVENT(
+ my_app,
+ get_account,
+ TP_ARGS(
+ int, userid,
+ size_t, len
+ ),
+ TP_FIELDS(
+ ctf_integer(int, userid, userid)
+ ctf_integer(size_t, len, len)
+ )
+)
+
+TRACEPOINT_EVENT(
+ my_app,
+ get_settings,
+ TP_ARGS(
+ int, userid,
+ size_t, len
+ ),
+ TP_FIELDS(
+ ctf_integer(int, userid, userid)
+ ctf_integer(size_t, len, len)
+ )
+)
+
+TRACEPOINT_EVENT(
+ my_app,
+ get_transaction,
+ TP_ARGS(
+ int, userid,
+ size_t, len
+ ),
+ TP_FIELDS(
+ ctf_integer(int, userid, userid)
+ ctf_integer(size_t, len, len)
+ )
+)
+----
+
+In this case, we create three tracepoint classes, with one implicit
+tracepoint instance for each of them: `get_account`, `get_settings`, and
+`get_transaction`. However, they all share the same event field names
+and types. Hence three identical, yet independent serialization
+functions are created when you build the tracepoint provider package.
+
+A better design choice is to define a single tracepoint class and three
+tracepoint instances:
+
+[source,c]
+----
+/* The tracepoint class */
+TRACEPOINT_EVENT_CLASS(
+ /* Tracepoint provider name */
+ my_app,
+
+ /* Tracepoint class name */
+ my_class,
+
+ /* Input arguments */
+ TP_ARGS(
+ int, userid,
+ size_t, len
+ ),
+
+ /* Output event fields */
+ TP_FIELDS(
+ ctf_integer(int, userid, userid)
+ ctf_integer(size_t, len, len)
+ )
+)
+
+/* The tracepoint instances */
+TRACEPOINT_EVENT_INSTANCE(
+ /* Tracepoint provider name */
+ my_app,
+
+ /* Tracepoint class name */
+ my_class,
+
+ /* Tracepoint name */
+ get_account,
+
+ /* Input arguments */
+ TP_ARGS(
+ int, userid,
+ size_t, len
+ )
+)
+TRACEPOINT_EVENT_INSTANCE(
+ my_app,
+ my_class,
+ get_settings,
+ TP_ARGS(
+ int, userid,
+ size_t, len
+ )
+)
+TRACEPOINT_EVENT_INSTANCE(
+ my_app,
+ my_class,
+ get_transaction,
+ TP_ARGS(
+ int, userid,
+ size_t, len
+ )
+)
+----
+====
+
+
+[[assigning-log-levels]]
+===== Assign a log level to a tracepoint definition
+
+You can assign an optional _log level_ to a
+<<defining-tracepoints,tracepoint definition>>.
+
+Assigning different levels of severity to tracepoint definitions can
+be useful: when you <<enabling-disabling-events,create an event rule>>,
+you can target tracepoints having a log level as severe as a specific
+value.
+
+The concept of LTTng-UST log levels is similar to the levels found
+in typical logging frameworks:
+
+* In a logging framework, the log level is given by the function
+ or method name you use at the log statement site: `debug()`,
+ `info()`, `warn()`, `error()`, and so on.
+* In LTTng-UST, you statically assign the log level to a tracepoint
+ definition; any `tracepoint()` macro invocation which refers to
+ this definition has this log level.
+
+You can assign a log level to a tracepoint definition with the
+`TRACEPOINT_LOGLEVEL()` macro. You must use this macro _after_ the
+<<defining-tracepoints,`TRACEPOINT_EVENT()`>> or
+<<using-tracepoint-classes,`TRACEPOINT_INSTANCE()`>> macro for a given
+tracepoint.
+
+The syntax of the `TRACEPOINT_LOGLEVEL()` macro is:
+
+[source,c]
+.`TRACEPOINT_LOGLEVEL()` macro syntax.
+----
+TRACEPOINT_LOGLEVEL(provider_name, tracepoint_name, log_level)
+----
+
+Replace:
+
+* `provider_name` with the tracepoint provider name.
+* `tracepoint_name` with the tracepoint name.
+* `log_level` with the log level to assign to the tracepoint
+ definition named `tracepoint_name` in the `provider_name`
+ tracepoint provider.
++
+See man:lttng-ust(3) for a list of available log level names.
+
+.Assign the `TRACE_DEBUG_UNIT` log level to a tracepoint definition.
+====
+[source,c]
+----
+/* Tracepoint definition */
+TRACEPOINT_EVENT(
+ my_app,
+ get_transaction,
+ TP_ARGS(
+ int, userid,
+ size_t, len
+ ),
+ TP_FIELDS(
+ ctf_integer(int, userid, userid)
+ ctf_integer(size_t, len, len)
+ )
+)
+
+/* Log level assignment */
+TRACEPOINT_LOGLEVEL(my_app, get_transaction, TRACE_DEBUG_UNIT)
+----
+====
+
+
+[[tpp-source]]
+===== Create a tracepoint provider package source file
+
+A _tracepoint provider package source file_ is a C source file which
+includes a <<tpp-header,tracepoint provider header file>> to expand its
+macros into event serialization and other functions.
+
+You can always use the following tracepoint provider package source
+file template:
+
+[source,c]
+.Tracepoint provider package source file template.
+----
+#define TRACEPOINT_CREATE_PROBES
+
+#include "tp.h"
+----
+
+Replace `tp.h` with the name of your <<tpp-header,tracepoint provider
+header file>> name. You may also include more than one tracepoint
+provider header file here to create a tracepoint provider package
+holding more than one tracepoint providers.
+
+
+[[probing-the-application-source-code]]
+==== Add tracepoints to an application's source code
+
+Once you <<tpp-header,create a tracepoint provider header file>>, you
+can use the `tracepoint()` macro in your application's
+source code to insert the tracepoints that this header
+<<defining-tracepoints,defines>>.
+
+The `tracepoint()` macro takes at least two parameters: the tracepoint
+provider name and the tracepoint name. The corresponding tracepoint
+definition defines the other parameters.
+
+.`tracepoint()` usage.
+====
+The following <<defining-tracepoints,tracepoint definition>> defines a
+tracepoint which takes two input arguments and has two output event
+fields.
+
+[source,c]
+.Tracepoint provider header file.
+----
+#include "my-custom-structure.h"
+
+TRACEPOINT_EVENT(
+ my_provider,
+ my_tracepoint,
+ TP_ARGS(
+ int, argc,
+ const char*, cmd_name
+ ),
+ TP_FIELDS(
+ ctf_string(cmd_name, cmd_name)
+ ctf_integer(int, number_of_args, argc)
+ )
+)
+----
+
+You can refer to this tracepoint definition with the `tracepoint()`
+macro in your application's source code like this:
+
+[source,c]
+.Application's source file.
+----
+#include "tp.h"
+
+int main(int argc, char* argv[])
+{
+ tracepoint(my_provider, my_tracepoint, argc, argv[0]);
+
+ return 0;
+}
+----
+
+Note how the application's source code includes
+the tracepoint provider header file containing the tracepoint
+definitions to use, path:{tp.h}.
+====
+
+.`tracepoint()` usage with a complex tracepoint definition.
+====
+Consider this complex tracepoint definition, where multiple event
+fields refer to the same input arguments in their argument expression
+parameter:
+
+[source,c]
+.Tracepoint provider header file.
+----
+/* For `struct stat` */
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+TRACEPOINT_EVENT(
+ my_provider,
+ my_tracepoint,
+ TP_ARGS(
+ int, my_int_arg,
+ char*, my_str_arg,
+ struct stat*, st
+ ),
+ TP_FIELDS(
+ ctf_integer(int, my_constant_field, 23 + 17)
+ ctf_integer(int, my_int_arg_field, my_int_arg)
+ ctf_integer(int, my_int_arg_field2, my_int_arg * my_int_arg)
+ ctf_integer(int, sum4_field, my_str_arg[0] + my_str_arg[1] +
+ my_str_arg[2] + my_str_arg[3])
+ ctf_string(my_str_arg_field, my_str_arg)
+ ctf_integer_hex(off_t, size_field, st->st_size)
+ ctf_float(double, size_dbl_field, (double) st->st_size)
+ ctf_sequence_text(char, half_my_str_arg_field, my_str_arg,
+ size_t, strlen(my_str_arg) / 2)
+ )
+)
+----
+
+You can refer to this tracepoint definition with the `tracepoint()`
+macro in your application's source code like this:
+
+[source,c]
+.Application's source file.
+----
+#define TRACEPOINT_DEFINE
+#include "tp.h"
+
+int main(void)
+{
+ struct stat s;
+
+ stat("/etc/fstab", &s);
+ tracepoint(my_provider, my_tracepoint, 23, "Hello, World!", &s);
+
+ return 0;
+}
+----
+
+If you look at the event record that LTTng writes when tracing this
+program, assuming the file size of path:{/etc/fstab} is 301{nbsp}bytes,
+it should look like this:
+
+.Event record fields
+|====
+|Field's name |Field's value
+|`my_constant_field` |40
+|`my_int_arg_field` |23
+|`my_int_arg_field2` |529
+|`sum4_field` |389
+|`my_str_arg_field` |`Hello, World!`
+|`size_field` |0x12d
+|`size_dbl_field` |301.0
+|`half_my_str_arg_field` |`Hello,`
+|====
+====
+
+Sometimes, the arguments you pass to `tracepoint()` are expensive to
+compute--they use the call stack, for example. To avoid this
+computation when the tracepoint is disabled, you can use the
+`tracepoint_enabled()` and `do_tracepoint()` macros.
+
+The syntax of the `tracepoint_enabled()` and `do_tracepoint()` macros
+is:
+
+[source,c]
+.`tracepoint_enabled()` and `do_tracepoint()` macros syntax.
+----
+tracepoint_enabled(provider_name, tracepoint_name)
+do_tracepoint(provider_name, tracepoint_name, ...)
+----
+
+Replace:
+
+* `provider_name` with the tracepoint provider name.
+* `tracepoint_name` with the tracepoint name.
+
+`tracepoint_enabled()` returns a non-zero value if the tracepoint named
+`tracepoint_name` from the provider named `provider_name` is enabled
+**at run time**.
+
+`do_tracepoint()` is like `tracepoint()`, except that it doesn't check
+if the tracepoint is enabled. Using `tracepoint()` with
+`tracepoint_enabled()` is dangerous since `tracepoint()` also contains
+the `tracepoint_enabled()` check, thus a race condition is
+possible in this situation:
+
+[source,c]
+.Possible race condition when using `tracepoint_enabled()` with `tracepoint()`.
+----
+if (tracepoint_enabled(my_provider, my_tracepoint)) {
+ stuff = prepare_stuff();
+}
+
+tracepoint(my_provider, my_tracepoint, stuff);
+----
+
+If the tracepoint is enabled after the condition, then `stuff` is not
+prepared: the emitted event will either contain wrong data, or the whole
+application could crash (segmentation fault, for example).
+
+NOTE: Neither `tracepoint_enabled()` nor `do_tracepoint()` have an
+`STAP_PROBEV()` call. If you need it, you must emit
+this call yourself.
+
+
+[[building-tracepoint-providers-and-user-application]]
+==== Build and link a tracepoint provider package and an application
+
+Once you have one or more <<tpp-header,tracepoint provider header
+files>> and a <<tpp-source,tracepoint provider package source file>>,
+you can create the tracepoint provider package by compiling its source
+file. From here, multiple build and run scenarios are possible. The
+following table shows common application and library configurations
+along with the required command lines to achieve them.
+
+In the following diagrams, we use the following file names:
+
+`app`::
+ Executable application.
+
+`app.o`::
+ Application's object file.
+
+`tpp.o`::
+ Tracepoint provider package object file.
+
+`tpp.a`::
+ Tracepoint provider package archive file.
+
+`libtpp.so`::
+ Tracepoint provider package shared object file.
+
+`emon.o`::
+ User library object file.
+
+`libemon.so`::
+ User library shared object file.
+
+We use the following symbols in the diagrams of table below:
+
+[role="img-100"]
+.Symbols used in the build scenario diagrams.
+image::ust-sit-symbols.png[]
+
+We assume that path:{.} is part of the env:LD_LIBRARY_PATH environment
+variable in the following instructions.
+
+[role="growable ust-scenarios",cols="asciidoc,asciidoc"]
+.Common tracepoint provider package scenarios.
+|====
+|Scenario |Instructions
+
+|
+The instrumented application is statically linked with
+the tracepoint provider package object.
+
+image::ust-sit+app-linked-with-tp-o+app-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-o.txt[]
+
+To build the instrumented application:
+
+. In path:{app.c}, before including path:{tpp.h}, add the following line:
++
+--
+[source,c]
+----
+#define TRACEPOINT_DEFINE
+----
+--
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o tpp.o -llttng-ust -ldl
+----
+--
+
+To run the instrumented application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The instrumented application is statically linked with the
+tracepoint provider package archive file.
+
+image::ust-sit+app-linked-with-tp-a+app-instrumented.png[]
+
+|
+To create the tracepoint provider package archive file:
+
+. Compile the <<tpp-source,tracepoint provider package source file>>:
++
+--
+[role="term"]
+----
+$ gcc -I. -c tpp.c
+----
+--
+
+. Create the tracepoint provider package archive file:
++
+--
+[role="term"]
+----
+$ ar rcs tpp.a tpp.o
+----
+--
+
+To build the instrumented application:
+
+. In path:{app.c}, before including path:{tpp.h}, add the following line:
++
+--
+[source,c]
+----
+#define TRACEPOINT_DEFINE
+----
+--
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o tpp.a -llttng-ust -ldl
+----
+--
+
+To run the instrumented application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The instrumented application is linked with the tracepoint provider
+package shared object.
+
+image::ust-sit+app-linked-with-tp-so+app-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-so.txt[]
+
+To build the instrumented application:
+
+. In path:{app.c}, before including path:{tpp.h}, add the following line:
++
+--
+[source,c]
+----
+#define TRACEPOINT_DEFINE
+----
+--
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -ldl -L. -ltpp
+----
+--
+
+To run the instrumented application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The tracepoint provider package shared object is preloaded before the
+instrumented application starts.
+
+image::ust-sit+tp-so-preloaded+app-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-so.txt[]
+
+To build the instrumented application:
+
+. In path:{app.c}, before including path:{tpp.h}, add the
+ following lines:
++
+--
+[source,c]
+----
+#define TRACEPOINT_DEFINE
+#define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
+----
+--
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -ldl
+----
+--
+
+To run the instrumented application with tracing support:
+
+* Preload the tracepoint provider package shared object and
+ start the application:
++
+--
+[role="term"]
+----
+$ LD_PRELOAD=./libtpp.so ./app
+----
+--
+
+To run the instrumented application without tracing support:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The instrumented application dynamically loads the tracepoint provider
+package shared object.
+
+image::ust-sit+app-dlopens-tp-so+app-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-so.txt[]
+
+To build the instrumented application:
+
+. In path:{app.c}, before including path:{tpp.h}, add the
+ following lines:
++
+--
+[source,c]
+----
+#define TRACEPOINT_DEFINE
+#define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
+----
+--
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -ldl
+----
+--
+
+To run the instrumented application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The application is linked with the instrumented user library.
+
+The instrumented user library is statically linked with the tracepoint
+provider package object file.
+
+image::ust-sit+app-linked-with-lib+lib-linked-with-tp-o+lib-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-o-fpic.txt[]
+
+To build the instrumented user library:
+
+. In path:{emon.c}, before including path:{tpp.h}, add the
+ following line:
++
+--
+[source,c]
+----
+#define TRACEPOINT_DEFINE
+----
+--
+
+. Compile the user library source file:
++
+--
+[role="term"]
+----
+$ gcc -I. -fpic -c emon.c
+----
+--
+
+. Build the user library shared object:
++
+--
+[role="term"]
+----
+$ gcc -shared -o libemon.so emon.o tpp.o -llttng-ust -ldl
+----
+--
+
+To build the application:
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -L. -lemon
+----
+--
+
+To run the application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The application is linked with the instrumented user library.
+
+The instrumented user library is linked with the tracepoint provider
+package shared object.
+
+image::ust-sit+app-linked-with-lib+lib-linked-with-tp-so+lib-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-so.txt[]
+
+To build the instrumented user library:
+
+. In path:{emon.c}, before including path:{tpp.h}, add the
+ following line:
++
+--
+[source,c]
+----
+#define TRACEPOINT_DEFINE
+----
+--
+
+. Compile the user library source file:
++
+--
+[role="term"]
+----
+$ gcc -I. -fpic -c emon.c
+----
+--
+
+. Build the user library shared object:
++
+--
+[role="term"]
+----
+$ gcc -shared -o libemon.so emon.o -ldl -L. -ltpp
+----
+--
+
+To build the application:
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -L. -lemon
+----
+--
+
+To run the application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The tracepoint provider package shared object is preloaded before the
+application starts.
+
+The application is linked with the instrumented user library.
+
+image::ust-sit+tp-so-preloaded+app-linked-with-lib+lib-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-so.txt[]
+
+To build the instrumented user library:
+
+. In path:{emon.c}, before including path:{tpp.h}, add the
+ following lines:
++
+--
+[source,c]
+----
+#define TRACEPOINT_DEFINE
+#define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
+----
+--
+
+. Compile the user library source file:
++
+--
+[role="term"]
+----
+$ gcc -I. -fpic -c emon.c
+----
+--
+
+. Build the user library shared object:
++
+--
+[role="term"]
+----
+$ gcc -shared -o libemon.so emon.o -ldl
+----
+--
+
+To build the application:
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -L. -lemon
+----
+--
+
+To run the application with tracing support:
+
+* Preload the tracepoint provider package shared object and
+ start the application:
++
+--
+[role="term"]
+----
+$ LD_PRELOAD=./libtpp.so ./app
+----
+--
+
+To run the application without tracing support:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The application is linked with the instrumented user library.
+
+The instrumented user library dynamically loads the tracepoint provider
+package shared object.
+
+image::ust-sit+app-linked-with-lib+lib-dlopens-tp-so+lib-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-so.txt[]
+
+To build the instrumented user library:
+
+. In path:{emon.c}, before including path:{tpp.h}, add the
+ following lines:
++
+--
+[source,c]
+----
+#define TRACEPOINT_DEFINE
+#define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
+----
+--
+
+. Compile the user library source file:
++
+--
+[role="term"]
+----
+$ gcc -I. -fpic -c emon.c
+----
+--
+
+. Build the user library shared object:
++
+--
+[role="term"]
+----
+$ gcc -shared -o libemon.so emon.o -ldl
+----
+--
+
+To build the application:
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -L. -lemon
+----
+--
+
+To run the application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The application dynamically loads the instrumented user library.
+
+The instrumented user library is linked with the tracepoint provider
+package shared object.
+
+image::ust-sit+app-dlopens-lib+lib-linked-with-tp-so+lib-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-so.txt[]
+
+To build the instrumented user library:
+
+. In path:{emon.c}, before including path:{tpp.h}, add the
+ following line:
++
+--
+[source,c]
+----
+#define TRACEPOINT_DEFINE
+----
+--
+
+. Compile the user library source file:
++
+--
+[role="term"]
+----
+$ gcc -I. -fpic -c emon.c
+----
+--
+
+. Build the user library shared object:
++
+--
+[role="term"]
+----
+$ gcc -shared -o libemon.so emon.o -ldl -L. -ltpp
+----
+--
+
+To build the application:
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -ldl -L. -lemon
+----
+--
+
+To run the application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The application dynamically loads the instrumented user library.
+
+The instrumented user library dynamically loads the tracepoint provider
+package shared object.
+
+image::ust-sit+app-dlopens-lib+lib-dlopens-tp-so+lib-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-so.txt[]
+
+To build the instrumented user library:
+
+. In path:{emon.c}, before including path:{tpp.h}, add the
+ following lines:
++
+--
+[source,c]
+----
+#define TRACEPOINT_DEFINE
+#define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
+----
+--
+
+. Compile the user library source file:
++
+--
+[role="term"]
+----
+$ gcc -I. -fpic -c emon.c
+----
+--
+
+. Build the user library shared object:
++
+--
+[role="term"]
+----
+$ gcc -shared -o libemon.so emon.o -ldl
+----
+--
+
+To build the application:
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -ldl -L. -lemon
+----
+--
+
+To run the application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The tracepoint provider package shared object is preloaded before the
+application starts.
+
+The application dynamically loads the instrumented user library.
+
+image::ust-sit+tp-so-preloaded+app-dlopens-lib+lib-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-so.txt[]
+
+To build the instrumented user library:
+
+. In path:{emon.c}, before including path:{tpp.h}, add the
+ following lines:
++
+--
+[source,c]
+----
+#define TRACEPOINT_DEFINE
+#define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
+----
+--
+
+. Compile the user library source file:
++
+--
+[role="term"]
+----
+$ gcc -I. -fpic -c emon.c
+----
+--
+
+. Build the user library shared object:
++
+--
+[role="term"]
+----
+$ gcc -shared -o libemon.so emon.o -ldl
+----
+--
+
+To build the application:
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o -L. -lemon
+----
+--
+
+To run the application with tracing support:
+
+* Preload the tracepoint provider package shared object and
+ start the application:
++
+--
+[role="term"]
+----
+$ LD_PRELOAD=./libtpp.so ./app
+----
+--
+
+To run the application without tracing support:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The application is statically linked with the tracepoint provider
+package object file.
+
+The application is linked with the instrumented user library.
+
+image::ust-sit+app-linked-with-tp-o+app-linked-with-lib+lib-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-o.txt[]
+
+To build the instrumented user library:
+
+. In path:{emon.c}, before including path:{tpp.h}, add the
+ following line:
++
+--
+[source,c]
+----
+#define TRACEPOINT_DEFINE
+----
+--
+
+. Compile the user library source file:
++
+--
+[role="term"]
+----
+$ gcc -I. -fpic -c emon.c
+----
+--
+
+. Build the user library shared object:
++
+--
+[role="term"]
+----
+$ gcc -shared -o libemon.so emon.o
+----
+--
+
+To build the application:
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -o app app.o tpp.o -llttng-ust -ldl -L. -lemon
+----
+--
+
+To run the instrumented application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+
+|
+The application is statically linked with the tracepoint provider
+package object file.
+
+The application dynamically loads the instrumented user library.
+
+image::ust-sit+app-linked-with-tp-o+app-dlopens-lib+lib-instrumented.png[]
+
+|
+include::../common/ust-sit-step-tp-o.txt[]
+
+To build the application:
+
+. In path:{app.c}, before including path:{tpp.h}, add the following line:
++
+--
+[source,c]
+----
+#define TRACEPOINT_DEFINE
+----
+--
+
+. Compile the application source file:
++
+--
+[role="term"]
+----
+$ gcc -c app.c
+----
+--
+
+. Build the application:
++
+--
+[role="term"]
+----
+$ gcc -Wl,--export-dynamic -o app app.o tpp.o \
+ -llttng-ust -ldl
+----
+--
++
+The `--export-dynamic` option passed to the linker is necessary for the
+dynamically loaded library to ``see'' the tracepoint symbols defined in
+the application.
+
+To build the instrumented user library:
+
+. Compile the user library source file:
++
+--
+[role="term"]
+----
+$ gcc -I. -fpic -c emon.c
+----
+--
+
+. Build the user library shared object:
++
+--
+[role="term"]
+----
+$ gcc -shared -o libemon.so emon.o
+----
+--
+
+To run the application:
+
+* Start the application:
++
+--
+[role="term"]
+----
+$ ./app
+----
+--
+|====
+
+
+[[using-lttng-ust-with-daemons]]
+===== Use noch:{LTTng-UST} with daemons
+
+If your instrumented application calls man:fork(2), man:clone(2),
+or BSD's man:rfork(2), without a following man:exec(3)-family
+system call, you must preload the path:{liblttng-ust-fork.so} shared
+object when you start the application.
+
+[role="term"]
+----
+$ LD_PRELOAD=liblttng-ust-fork.so ./my-app
+----
+
+If your tracepoint provider package is
+a shared library which you also preload, you must put both
+shared objects in env:LD_PRELOAD:
+
+[role="term"]
+----
+$ LD_PRELOAD=liblttng-ust-fork.so:/path/to/tp.so ./my-app
+----
+
+
+[role="since-2.9"]
+[[liblttng-ust-fd]]
+===== Use noch:{LTTng-UST} with applications which close file descriptors that don't belong to them
+
+If your instrumented application closes one or more file descriptors
+which it did not open itself, you must preload the
+path:{liblttng-ust-fd.so} shared object when you start the application:
+
+[role="term"]
+----
+$ LD_PRELOAD=liblttng-ust-fd.so ./my-app
+----
+
+Typical use cases include closing all the file descriptors after
+man:fork(2) or man:rfork(2) and buggy applications doing
+``double closes''.
+
+
+[[lttng-ust-pkg-config]]
+===== Use noch:{pkg-config}
+
+On some distributions, LTTng-UST ships with a
+https://www.freedesktop.org/wiki/Software/pkg-config/[pkg-config]
+metadata file. If this is your case, then you can use cmd:pkg-config to
+build an application on the command line:
+
+[role="term"]
+----
+$ gcc -o my-app my-app.o tp.o $(pkg-config --cflags --libs lttng-ust)
+----
+
+
+[[instrumenting-32-bit-app-on-64-bit-system]]
+===== [[advanced-instrumenting-techniques]]Build a 32-bit instrumented application for a 64-bit target system
+
+In order to trace a 32-bit application running on a 64-bit system,
+LTTng must use a dedicated 32-bit
+<<lttng-consumerd,consumer daemon>>.
+
+The following steps show how to build and install a 32-bit consumer
+daemon, which is _not_ part of the default 64-bit LTTng build, how to
+build and install the 32-bit LTTng-UST libraries, and how to build and
+link an instrumented 32-bit application in that context.
+
+To build a 32-bit instrumented application for a 64-bit target system,
+assuming you have a fresh target system with no installed Userspace RCU
+or LTTng packages:
+
+. Download, build, and install a 32-bit version of Userspace RCU:
++
+--
+[role="term"]
+----
+$ cd $(mktemp -d) &&
+wget http://lttng.org/files/urcu/userspace-rcu-latest-0.9.tar.bz2 &&
+tar -xf userspace-rcu-latest-0.9.tar.bz2 &&
+cd userspace-rcu-0.9.* &&
+./configure --libdir=/usr/local/lib32 CFLAGS=-m32 &&
+make &&
+sudo make install &&
+sudo ldconfig
+----
+--
+
+. Using your distribution's package manager, or from source, install
+ the following 32-bit versions of the following dependencies of
+ LTTng-tools and LTTng-UST:
++
+--
+* https://sourceforge.net/projects/libuuid/[libuuid]
+* http://directory.fsf.org/wiki/Popt[popt]
+* http://www.xmlsoft.org/[libxml2]
+--
+
+. Download, build, and install a 32-bit version of the latest
+ LTTng-UST{nbsp}{revision}:
++
+--
+[role="term"]
+----
+$ cd $(mktemp -d) &&
+wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.11.tar.bz2 &&
+tar -xf lttng-ust-latest-2.11.tar.bz2 &&
+cd lttng-ust-2.11.* &&
+./configure --libdir=/usr/local/lib32 \
+ CFLAGS=-m32 CXXFLAGS=-m32 \
+ LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' &&
+make &&
+sudo make install &&
+sudo ldconfig
+----
+--
++
+[NOTE]
+====
+Depending on your distribution,
+32-bit libraries could be installed at a different location than
+`/usr/lib32`. For example, Debian is known to install
+some 32-bit libraries in `/usr/lib/i386-linux-gnu`.
+
+In this case, make sure to set `LDFLAGS` to all the
+relevant 32-bit library paths, for example:
+
+[role="term"]
+----
+$ LDFLAGS='-L/usr/lib/i386-linux-gnu -L/usr/lib32'
+----
+====
+
+. Download the latest LTTng-tools{nbsp}{revision}, build, and install
+ the 32-bit consumer daemon:
++
+--
+[role="term"]
+----
+$ cd $(mktemp -d) &&
+wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.11.tar.bz2 &&
+tar -xf lttng-tools-latest-2.11.tar.bz2 &&
+cd lttng-tools-2.11.* &&
+./configure --libdir=/usr/local/lib32 CFLAGS=-m32 CXXFLAGS=-m32 \
+ LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' \
+ --disable-bin-lttng --disable-bin-lttng-crash \
+ --disable-bin-lttng-relayd --disable-bin-lttng-sessiond &&
+make &&
+cd src/bin/lttng-consumerd &&
+sudo make install &&
+sudo ldconfig
+----
+--
+
+. From your distribution or from source,
+ <<installing-lttng,install>> the 64-bit versions of
+ LTTng-UST and Userspace RCU.
+. Download, build, and install the 64-bit version of the
+ latest LTTng-tools{nbsp}{revision}:
++
+--
+[role="term"]
+----
+$ cd $(mktemp -d) &&
+wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.11.tar.bz2 &&
+tar -xf lttng-tools-latest-2.11.tar.bz2 &&
+cd lttng-tools-2.11.* &&
+./configure --with-consumerd32-libdir=/usr/local/lib32 \
+ --with-consumerd32-bin=/usr/local/lib32/lttng/libexec/lttng-consumerd &&
+make &&
+sudo make install &&
+sudo ldconfig
+----
+--
+
+. Pass the following options to man:gcc(1), man:g++(1), or man:clang(1)
+ when linking your 32-bit application:
++
+----
+-m32 -L/usr/lib32 -L/usr/local/lib32 \
+-Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32
+----
++
+For example, let's rebuild the quick start example in
+<<tracing-your-own-user-application,Trace a user application>> as an
+instrumented 32-bit application:
++
+--
+[role="term"]
+----
+$ gcc -m32 -c -I. hello-tp.c
+$ gcc -m32 -c hello.c
+$ gcc -m32 -o hello hello.o hello-tp.o \
+ -L/usr/lib32 -L/usr/local/lib32 \
+ -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32 \
+ -llttng-ust -ldl
+----
+--
+
+No special action is required to execute the 32-bit application and
+to trace it: use the command-line man:lttng(1) tool as usual.
+
+
+[role="since-2.5"]
+[[tracef]]
+==== Use `tracef()`
+
+man:tracef(3) is a small LTTng-UST API designed for quick,
+man:printf(3)-like instrumentation without the burden of
+<<tracepoint-provider,creating>> and
+<<building-tracepoint-providers-and-user-application,building>>
+a tracepoint provider package.
+
+To use `tracef()` in your application:
+
+. In the C or C++ source files where you need to use `tracef()`,
+ include `<lttng/tracef.h>`:
++
+--
+[source,c]
+----
+#include <lttng/tracef.h>
+----
+--
+
+. In the application's source code, use `tracef()` like you would use
+ man:printf(3):
++
+--
+[source,c]
+----
+ /* ... */
+
+ tracef("my message: %d (%s)", my_integer, my_string);
+
+ /* ... */
+----
+--
+
+. Link your application with `liblttng-ust`:
++
+--
+[role="term"]
+----
+$ gcc -o app app.c -llttng-ust
+----
+--
+
+To trace the events that `tracef()` calls emit:
+
+* <<enabling-disabling-events,Create an event rule>> which matches the
+ `lttng_ust_tracef:*` event name:
++
+--
+[role="term"]
+----
+$ lttng enable-event --userspace 'lttng_ust_tracef:*'
+----
+--
+
+[IMPORTANT]
+.Limitations of `tracef()`
+====
+The `tracef()` utility function was developed to make user space tracing
+super simple, albeit with notable disadvantages compared to
+<<defining-tracepoints,user-defined tracepoints>>:
+
+* All the emitted events have the same tracepoint provider and
+ tracepoint names, respectively `lttng_ust_tracef` and `event`.
+* There is no static type checking.
+* The only event record field you actually get, named `msg`, is a string
+ potentially containing the values you passed to `tracef()`
+ using your own format string. This also means that you cannot filter
+ events with a custom expression at run time because there are no
+ isolated fields.
+* Since `tracef()` uses the C standard library's man:vasprintf(3)
+ function behind the scenes to format the strings at run time, its
+ expected performance is lower than with user-defined tracepoints,
+ which do not require a conversion to a string.
+
+Taking this into consideration, `tracef()` is useful for some quick
+prototyping and debugging, but you should not consider it for any
+permanent and serious applicative instrumentation.
+====
+
+
+[role="since-2.7"]
+[[tracelog]]
+==== Use `tracelog()`
+
+The man:tracelog(3) API is very similar to <<tracef,`tracef()`>>, with
+the difference that it accepts an additional log level parameter.
+
+The goal of `tracelog()` is to ease the migration from logging to
+tracing.
+
+To use `tracelog()` in your application:
+
+. In the C or C++ source files where you need to use `tracelog()`,
+ include `<lttng/tracelog.h>`:
++
+--
+[source,c]
+----
+#include <lttng/tracelog.h>
+----
+--
+
+. In the application's source code, use `tracelog()` like you would use
+ man:printf(3), except for the first parameter which is the log
+ level:
++
+--
+[source,c]
+----
+ /* ... */
+
+ tracelog(TRACE_WARNING, "my message: %d (%s)",
+ my_integer, my_string);
+
+ /* ... */
+----
+--
++
+See man:lttng-ust(3) for a list of available log level names.
+
+. Link your application with `liblttng-ust`:
++
+--
+[role="term"]
+----
+$ gcc -o app app.c -llttng-ust
+----
+--
+
+To trace the events that `tracelog()` calls emit with a log level
+_as severe as_ a specific log level:
+
+* <<enabling-disabling-events,Create an event rule>> which matches the
+ `lttng_ust_tracelog:*` event name and a minimum level
+ of severity:
++
+--
+[role="term"]
+----
+$ lttng enable-event --userspace 'lttng_ust_tracelog:*'
+ --loglevel=TRACE_WARNING
+----
+--
+
+To trace the events that `tracelog()` calls emit with a
+_specific log level_:
+
+* Create an event rule which matches the `lttng_ust_tracelog:*`
+ event name and a specific log level:
++
+--
+[role="term"]
+----
+$ lttng enable-event --userspace 'lttng_ust_tracelog:*'
+ --loglevel-only=TRACE_INFO
+----
+--
+
+
+[[prebuilt-ust-helpers]]
+=== Prebuilt user space tracing helpers
+
+The LTTng-UST package provides a few helpers in the form or preloadable
+shared objects which automatically instrument system functions and
+calls.
+
+The helper shared objects are normally found in dir:{/usr/lib}. If you
+built LTTng-UST <<building-from-source,from source>>, they are probably
+located in dir:{/usr/local/lib}.
+
+The installed user space tracing helpers in LTTng-UST{nbsp}{revision}
+are:
+
+path:{liblttng-ust-libc-wrapper.so}::
+path:{liblttng-ust-pthread-wrapper.so}::
+ <<liblttng-ust-libc-pthread-wrapper,C{nbsp}standard library
+ memory and POSIX threads function tracing>>.
+
+path:{liblttng-ust-cyg-profile.so}::
+path:{liblttng-ust-cyg-profile-fast.so}::
+ <<liblttng-ust-cyg-profile,Function entry and exit tracing>>.
+
+path:{liblttng-ust-dl.so}::
+ <<liblttng-ust-dl,Dynamic linker tracing>>.
+
+To use a user space tracing helper with any user application:
+
+* Preload the helper shared object when you start the application:
++
+--
+[role="term"]
+----
+$ LD_PRELOAD=liblttng-ust-libc-wrapper.so my-app
+----
+--
++
+You can preload more than one helper:
++
+--
+[role="term"]
+----
+$ LD_PRELOAD=liblttng-ust-libc-wrapper.so:liblttng-ust-dl.so my-app
+----
+--
+
+
+[role="since-2.3"]
+[[liblttng-ust-libc-pthread-wrapper]]
+==== Instrument C standard library memory and POSIX threads functions
+
+The path:{liblttng-ust-libc-wrapper.so} and
+path:{liblttng-ust-pthread-wrapper.so} helpers
+add instrumentation to some C standard library and POSIX
+threads functions.
+
+[role="growable"]
+.Functions instrumented by preloading path:{liblttng-ust-libc-wrapper.so}.
+|====
+|TP provider name |TP name |Instrumented function
+
+.6+|`lttng_ust_libc` |`malloc` |man:malloc(3)
+ |`calloc` |man:calloc(3)
+ |`realloc` |man:realloc(3)
+ |`free` |man:free(3)
+ |`memalign` |man:memalign(3)
+ |`posix_memalign` |man:posix_memalign(3)
+|====
+
+[role="growable"]
+.Functions instrumented by preloading path:{liblttng-ust-pthread-wrapper.so}.
+|====
+|TP provider name |TP name |Instrumented function
+
+.4+|`lttng_ust_pthread` |`pthread_mutex_lock_req` |man:pthread_mutex_lock(3p) (request time)
+ |`pthread_mutex_lock_acq` |man:pthread_mutex_lock(3p) (acquire time)
+ |`pthread_mutex_trylock` |man:pthread_mutex_trylock(3p)
+ |`pthread_mutex_unlock` |man:pthread_mutex_unlock(3p)
+|====
+
+When you preload the shared object, it replaces the functions listed
+in the previous tables by wrappers which contain tracepoints and call
+the replaced functions.
+
+
+[[liblttng-ust-cyg-profile]]
+==== Instrument function entry and exit
+
+The path:{liblttng-ust-cyg-profile*.so} helpers can add instrumentation
+to the entry and exit points of functions.
+
+man:gcc(1) and man:clang(1) have an option named
+https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html[`-finstrument-functions`]
+which generates instrumentation calls for entry and exit to functions.
+The LTTng-UST function tracing helpers,
+path:{liblttng-ust-cyg-profile.so} and
+path:{liblttng-ust-cyg-profile-fast.so}, take advantage of this feature
+to add tracepoints to the two generated functions (which contain
+`cyg_profile` in their names, hence the helper's name).
+
+To use the LTTng-UST function tracing helper, the source files to
+instrument must be built using the `-finstrument-functions` compiler
+flag.
+
+There are two versions of the LTTng-UST function tracing helper:
+
+* **path:{liblttng-ust-cyg-profile-fast.so}** is a lightweight variant
+ that you should only use when it can be _guaranteed_ that the
+ complete event stream is recorded without any lost event record.
+ Any kind of duplicate information is left out.
++
+Assuming no event record is lost, having only the function addresses on
+entry is enough to create a call graph, since an event record always
+contains the ID of the CPU that generated it.
++
+You can use a tool like man:addr2line(1) to convert function addresses
+back to source file names and line numbers.
+
+* **path:{liblttng-ust-cyg-profile.so}** is a more robust variant
+which also works in use cases where event records might get discarded or
+not recorded from application startup.
+In these cases, the trace analyzer needs more information to be
+able to reconstruct the program flow.
+
+See man:lttng-ust-cyg-profile(3) to learn more about the instrumentation
+points of this helper.
+
+All the tracepoints that this helper provides have the
+log level `TRACE_DEBUG_FUNCTION` (see man:lttng-ust(3)).
+
+TIP: It's sometimes a good idea to limit the number of source files that
+you compile with the `-finstrument-functions` option to prevent LTTng
+from writing an excessive amount of trace data at run time. When using
+man:gcc(1), you can use the
+`-finstrument-functions-exclude-function-list` option to avoid
+instrument entries and exits of specific function names.
+
+
+[role="since-2.4"]
+[[liblttng-ust-dl]]
+==== Instrument the dynamic linker
+
+The path:{liblttng-ust-dl.so} helper adds instrumentation to the
+man:dlopen(3) and man:dlclose(3) function calls.
+
+See man:lttng-ust-dl(3) to learn more about the instrumentation points
+of this helper.
+
+
+[role="since-2.4"]
+[[java-application]]
+=== User space Java agent
+
+You can instrument any Java application which uses one of the following
+logging frameworks:
+
+* The https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[**`java.util.logging`**]
+ (JUL) core logging facilities.
+* http://logging.apache.org/log4j/1.2/[**Apache log4j{nbsp}1.2**], since
+ LTTng{nbsp}2.6. Note that Apache Log4j{nbsp}2 is not supported.
+
+[role="img-100"]
+.LTTng-UST Java agent imported by a Java application.
+image::java-app.png[]
+
+Note that the methods described below are new in LTTng{nbsp}{revision}.
+Previous LTTng versions use another technique.
+
+NOTE: We use http://openjdk.java.net/[OpenJDK]{nbsp}8 for development
+and https://ci.lttng.org/[continuous integration], thus this version is
+directly supported. However, the LTTng-UST Java agent is also tested
+with OpenJDK{nbsp}7.
+
+
+[role="since-2.8"]
+[[jul]]
+==== Use the LTTng-UST Java agent for `java.util.logging`
+
+To use the LTTng-UST Java agent in a Java application which uses
+`java.util.logging` (JUL):
+
+. In the Java application's source code, import the LTTng-UST
+ log handler package for `java.util.logging`:
++
+--
+[source,java]
+----
+import org.lttng.ust.agent.jul.LttngLogHandler;
+----
+--
+
+. Create an LTTng-UST JUL log handler:
++
+--
+[source,java]
+----
+Handler lttngUstLogHandler = new LttngLogHandler();
+----
+--
+
+. Add this handler to the JUL loggers which should emit LTTng events:
++
+--
+[source,java]
+----
+Logger myLogger = Logger.getLogger("some-logger");
+
+myLogger.addHandler(lttngUstLogHandler);
+----
+--
+
+. Use `java.util.logging` log statements and configuration as usual.
+ The loggers with an attached LTTng-UST log handler can emit
+ LTTng events.
+
+. Before exiting the application, remove the LTTng-UST log handler from
+ the loggers attached to it and call its `close()` method:
++
+--
+[source,java]
+----
+myLogger.removeHandler(lttngUstLogHandler);
+lttngUstLogHandler.close();
+----
+--
++
+This is not strictly necessary, but it is recommended for a clean
+disposal of the handler's resources.
+
+. Include the LTTng-UST Java agent's common and JUL-specific JAR files,
+ path:{lttng-ust-agent-common.jar} and path:{lttng-ust-agent-jul.jar},
+ in the
+ https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class
+ path] when you build the Java application.
++
+The JAR files are typically located in dir:{/usr/share/java}.
++
+IMPORTANT: The LTTng-UST Java agent must be
+<<installing-lttng,installed>> for the logging framework your
+application uses.
+
+.Use the LTTng-UST Java agent for `java.util.logging`.
+====
+[source,java]
+.path:{Test.java}
+----
+import java.io.IOException;
+import java.util.logging.Handler;
+import java.util.logging.Logger;
+import org.lttng.ust.agent.jul.LttngLogHandler;
+
+public class Test
+{
+ private static final int answer = 42;
+
+ public static void main(String[] argv) throws Exception
+ {
+ // Create a logger
+ Logger logger = Logger.getLogger("jello");
+
+ // Create an LTTng-UST log handler
+ Handler lttngUstLogHandler = new LttngLogHandler();
+
+ // Add the LTTng-UST log handler to our logger
+ logger.addHandler(lttngUstLogHandler);
+
+ // Log at will!
+ logger.info("some info");
+ logger.warning("some warning");
+ Thread.sleep(500);
+ logger.finer("finer information; the answer is " + answer);
+ Thread.sleep(123);
+ logger.severe("error!");
+
+ // Not mandatory, but cleaner
+ logger.removeHandler(lttngUstLogHandler);
+ lttngUstLogHandler.close();
+ }
+}
+----
+
+Build this example:
+
+[role="term"]
+----
+$ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java
+----
+
+<<creating-destroying-tracing-sessions,Create a tracing session>>,
+<<enabling-disabling-events,create an event rule>> matching the
+`jello` JUL logger, and <<basic-tracing-session-control,start tracing>>:
+
+[role="term"]
+----
+$ lttng create
+$ lttng enable-event --jul jello
+$ lttng start
+----
+
+Run the compiled class:
+
+[role="term"]
+----
+$ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test
+----
+
+<<basic-tracing-session-control,Stop tracing>> and inspect the
+recorded events:
+
+[role="term"]
+----
+$ lttng stop
+$ lttng view
+----
+====
+
+In the resulting trace, an <<event,event record>> generated by a Java
+application using `java.util.logging` is named `lttng_jul:event` and
+has the following fields:
+
+`msg`::
+ Log record's message.
+
+`logger_name`::
+ Logger name.
+
+`class_name`::
+ Name of the class in which the log statement was executed.
+
+`method_name`::
+ Name of the method in which the log statement was executed.
+
+`long_millis`::
+ Logging time (timestamp in milliseconds).
+
+`int_loglevel`::
+ Log level integer value.
+
+`int_threadid`::
+ ID of the thread in which the log statement was executed.
+
+You can use the opt:lttng-enable-event(1):--loglevel or
+opt:lttng-enable-event(1):--loglevel-only option of the
+man:lttng-enable-event(1) command to target a range of JUL log levels
+or a specific JUL log level.
+
+
+[role="since-2.8"]
+[[log4j]]
+==== Use the LTTng-UST Java agent for Apache log4j
+
+To use the LTTng-UST Java agent in a Java application which uses
+Apache log4j{nbsp}1.2:
+
+. In the Java application's source code, import the LTTng-UST
+ log appender package for Apache log4j:
++
+--
+[source,java]
+----
+import org.lttng.ust.agent.log4j.LttngLogAppender;
+----
+--
+
+. Create an LTTng-UST log4j log appender:
++
+--
+[source,java]
+----
+Appender lttngUstLogAppender = new LttngLogAppender();
+----
+--
+
+. Add this appender to the log4j loggers which should emit LTTng events:
++
+--
+[source,java]
+----
+Logger myLogger = Logger.getLogger("some-logger");
+
+myLogger.addAppender(lttngUstLogAppender);
+----
+--
+
+. Use Apache log4j log statements and configuration as usual. The
+ loggers with an attached LTTng-UST log appender can emit LTTng events.
+
+. Before exiting the application, remove the LTTng-UST log appender from
+ the loggers attached to it and call its `close()` method:
++
+--
+[source,java]
+----
+myLogger.removeAppender(lttngUstLogAppender);
+lttngUstLogAppender.close();
+----
+--
++
+This is not strictly necessary, but it is recommended for a clean
+disposal of the appender's resources.
+
+. Include the LTTng-UST Java agent's common and log4j-specific JAR
+ files, path:{lttng-ust-agent-common.jar} and
+ path:{lttng-ust-agent-log4j.jar}, in the
+ https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class
+ path] when you build the Java application.
++
+The JAR files are typically located in dir:{/usr/share/java}.
++
+IMPORTANT: The LTTng-UST Java agent must be
+<<installing-lttng,installed>> for the logging framework your
+application uses.
+
+.Use the LTTng-UST Java agent for Apache log4j.
+====
+[source,java]
+.path:{Test.java}
+----
+import org.apache.log4j.Appender;
+import org.apache.log4j.Logger;
+import org.lttng.ust.agent.log4j.LttngLogAppender;
+
+public class Test
+{
+ private static final int answer = 42;
+
+ public static void main(String[] argv) throws Exception
+ {
+ // Create a logger
+ Logger logger = Logger.getLogger("jello");
+
+ // Create an LTTng-UST log appender
+ Appender lttngUstLogAppender = new LttngLogAppender();
+
+ // Add the LTTng-UST log appender to our logger
+ logger.addAppender(lttngUstLogAppender);
+
+ // Log at will!
+ logger.info("some info");
+ logger.warn("some warning");
+ Thread.sleep(500);
+ logger.debug("debug information; the answer is " + answer);
+ Thread.sleep(123);
+ logger.fatal("error!");
+
+ // Not mandatory, but cleaner
+ logger.removeAppender(lttngUstLogAppender);
+ lttngUstLogAppender.close();
+ }
+}
+
+----
+
+Build this example (`$LOG4JPATH` is the path to the Apache log4j JAR
+file):
+
+[role="term"]
+----
+$ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH Test.java
+----
+
+<<creating-destroying-tracing-sessions,Create a tracing session>>,
+<<enabling-disabling-events,create an event rule>> matching the
+`jello` log4j logger, and <<basic-tracing-session-control,start tracing>>:
+
+[role="term"]
+----
+$ lttng create
+$ lttng enable-event --log4j jello
+$ lttng start
+----
+
+Run the compiled class:
+
+[role="term"]
+----
+$ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH:. Test
+----
+
+<<basic-tracing-session-control,Stop tracing>> and inspect the
+recorded events:
+
+[role="term"]
+----
+$ lttng stop
+$ lttng view
+----
+====
+
+In the resulting trace, an <<event,event record>> generated by a Java
+application using log4j is named `lttng_log4j:event` and
+has the following fields:
+
+`msg`::
+ Log record's message.
+
+`logger_name`::
+ Logger name.
+
+`class_name`::
+ Name of the class in which the log statement was executed.
+
+`method_name`::
+ Name of the method in which the log statement was executed.
+
+`filename`::
+ Name of the file in which the executed log statement is located.
+
+`line_number`::
+ Line number at which the log statement was executed.
+
+`timestamp`::
+ Logging timestamp.
+
+`int_loglevel`::
+ Log level integer value.
+
+`thread_name`::
+ Name of the Java thread in which the log statement was executed.
+
+You can use the opt:lttng-enable-event(1):--loglevel or
+opt:lttng-enable-event(1):--loglevel-only option of the
+man:lttng-enable-event(1) command to target a range of Apache log4j log levels
+or a specific log4j log level.
+
+
+[role="since-2.8"]
+[[java-application-context]]
+==== Provide application-specific context fields in a Java application
+
+A Java application-specific context field is a piece of state provided
+by the application which <<adding-context,you can add>>, using the
+man:lttng-add-context(1) command, to each <<event,event record>>
+produced by the log statements of this application.
+
+For example, a given object might have a current request ID variable.
+You can create a context information retriever for this object and
+assign a name to this current request ID. You can then, using the
+man:lttng-add-context(1) command, add this context field by name to
+the JUL or log4j <<channel,channel>>.
+
+To provide application-specific context fields in a Java application:
+
+. In the Java application's source code, import the LTTng-UST
+ Java agent context classes and interfaces:
++
+--
+[source,java]
+----
+import org.lttng.ust.agent.context.ContextInfoManager;
+import org.lttng.ust.agent.context.IContextInfoRetriever;
+----
+--
+
+. Create a context information retriever class, that is, a class which
+ implements the `IContextInfoRetriever` interface:
++
+--
+[source,java]
+----
+class MyContextInfoRetriever implements IContextInfoRetriever
+{
+ @Override
+ public Object retrieveContextInfo(String key)
+ {
+ if (key.equals("intCtx")) {
+ return (short) 17;
+ } else if (key.equals("strContext")) {
+ return "context value!";
+ } else {
+ return null;
+ }
+ }
+}
+----
+--
++
+This `retrieveContextInfo()` method is the only member of the
+`IContextInfoRetriever` interface. Its role is to return the current
+value of a state by name to create a context field. The names of the
+context fields and which state variables they return depends on your
+specific scenario.
++
+All primitive types and objects are supported as context fields.
+When `retrieveContextInfo()` returns an object, the context field
+serializer calls its `toString()` method to add a string field to
+event records. The method can also return `null`, which means that
+no context field is available for the required name.
+
+. Register an instance of your context information retriever class to
+ the context information manager singleton:
++
+--
+[source,java]
+----
+IContextInfoRetriever cir = new MyContextInfoRetriever();
+ContextInfoManager cim = ContextInfoManager.getInstance();
+cim.registerContextInfoRetriever("retrieverName", cir);
+----
+--
+
+. Before exiting the application, remove your context information
+ retriever from the context information manager singleton:
++
+--
+[source,java]
+----
+ContextInfoManager cim = ContextInfoManager.getInstance();
+cim.unregisterContextInfoRetriever("retrieverName");
+----
+--
++
+This is not strictly necessary, but it is recommended for a clean
+disposal of some manager's resources.
+
+. Build your Java application with LTTng-UST Java agent support as
+ usual, following the procedure for either the <<jul,JUL>> or
+ <<log4j,Apache log4j>> framework.
+
+
+.Provide application-specific context fields in a Java application.
+====
+[source,java]
+.path:{Test.java}
+----
+import java.util.logging.Handler;
+import java.util.logging.Logger;
+import org.lttng.ust.agent.jul.LttngLogHandler;
+import org.lttng.ust.agent.context.ContextInfoManager;
+import org.lttng.ust.agent.context.IContextInfoRetriever;
+
+public class Test
+{
+ // Our context information retriever class
+ private static class MyContextInfoRetriever
+ implements IContextInfoRetriever
+ {
+ @Override
+ public Object retrieveContextInfo(String key) {
+ if (key.equals("intCtx")) {
+ return (short) 17;
+ } else if (key.equals("strContext")) {
+ return "context value!";
+ } else {
+ return null;
+ }
+ }
+ }
+
+ private static final int answer = 42;
+
+ public static void main(String args[]) throws Exception
+ {
+ // Get the context information manager instance
+ ContextInfoManager cim = ContextInfoManager.getInstance();
+
+ // Create and register our context information retriever
+ IContextInfoRetriever cir = new MyContextInfoRetriever();
+ cim.registerContextInfoRetriever("myRetriever", cir);
+
+ // Create a logger
+ Logger logger = Logger.getLogger("jello");
+
+ // Create an LTTng-UST log handler
+ Handler lttngUstLogHandler = new LttngLogHandler();
+
+ // Add the LTTng-UST log handler to our logger
+ logger.addHandler(lttngUstLogHandler);
+
+ // Log at will!
+ logger.info("some info");
+ logger.warning("some warning");
+ Thread.sleep(500);
+ logger.finer("finer information; the answer is " + answer);
+ Thread.sleep(123);
+ logger.severe("error!");
+
+ // Not mandatory, but cleaner
+ logger.removeHandler(lttngUstLogHandler);
+ lttngUstLogHandler.close();
+ cim.unregisterContextInfoRetriever("myRetriever");
+ }
+}
+----
+
+Build this example:
+
+[role="term"]
+----
+$ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java
+----
+
+<<creating-destroying-tracing-sessions,Create a tracing session>>
+and <<enabling-disabling-events,create an event rule>> matching the
+`jello` JUL logger:
+
+[role="term"]
+----
+$ lttng create
+$ lttng enable-event --jul jello
+----
+
+<<adding-context,Add the application-specific context fields>> to the
+JUL channel:
+
+[role="term"]
+----
+$ lttng add-context --jul --type='$app.myRetriever:intCtx'
+$ lttng add-context --jul --type='$app.myRetriever:strContext'
+----
+
+<<basic-tracing-session-control,Start tracing>>:
+
+[role="term"]
+----
+$ lttng start
+----
+
+Run the compiled class:
+
+[role="term"]
+----
+$ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test
+----
+
+<<basic-tracing-session-control,Stop tracing>> and inspect the
+recorded events:
+
+[role="term"]
+----
+$ lttng stop
+$ lttng view
+----
+====
+
+
+[role="since-2.7"]
+[[python-application]]
+=== User space Python agent
+
+You can instrument a Python{nbsp}2 or Python{nbsp}3 application which
+uses the standard
+https://docs.python.org/3/library/logging.html[`logging`] package.
+
+Each log statement emits an LTTng event once the
+application module imports the
+<<lttng-ust-agents,LTTng-UST Python agent>> package.
+
+[role="img-100"]
+.A Python application importing the LTTng-UST Python agent.
+image::python-app.png[]
+
+To use the LTTng-UST Python agent:
+
+. In the Python application's source code, import the LTTng-UST Python
+ agent:
++
+--
+[source,python]
+----
+import lttngust
+----
+--
++
+The LTTng-UST Python agent automatically adds its logging handler to the
+root logger at import time.
++
+Any log statement that the application executes before this import does
+not emit an LTTng event.
++
+IMPORTANT: The LTTng-UST Python agent must be
+<<installing-lttng,installed>>.
+
+. Use log statements and logging configuration as usual.
+ Since the LTTng-UST Python agent adds a handler to the _root_
+ logger, you can trace any log statement from any logger.
+
+.Use the LTTng-UST Python agent.
+====
+[source,python]
+.path:{test.py}
+----
+import lttngust
+import logging
+import time
+
+
+def example():
+ logging.basicConfig()
+ logger = logging.getLogger('my-logger')
+
+ while True:
+ logger.debug('debug message')
+ logger.info('info message')
+ logger.warn('warn message')
+ logger.error('error message')
+ logger.critical('critical message')
+ time.sleep(1)
+
+
+if __name__ == '__main__':
+ example()
+----
+
+NOTE: `logging.basicConfig()`, which adds to the root logger a basic
+logging handler which prints to the standard error stream, is not
+strictly required for LTTng-UST tracing to work, but in versions of
+Python preceding{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 tracing session>>,
+<<enabling-disabling-events,create an event rule>> matching the
+`my-logger` Python logger, and <<basic-tracing-session-control,start
+tracing>>:
+
+[role="term"]
+----
+$ lttng create
+$ lttng enable-event --python my-logger
+$ lttng start
+----
+
+Run the Python script:
+
+[role="term"]
+----
+$ python test.py
+----
+
+<<basic-tracing-session-control,Stop tracing>> and inspect the recorded
+events:
+
+[role="term"]
+----
+$ lttng stop
+$ lttng view
+----
+====
+
+In the resulting trace, an <<event,event record>> generated by a Python
+application is named `lttng_python:event` and has the following fields:
+
+`asctime`::
+ Logging time (string).
+
+`msg`::
+ Log record's message.
+
+`logger_name`::
+ Logger name.
+
+`funcName`::
+ Name of the function in which the log statement was executed.
+
+`lineno`::
+ Line number at which the log statement was executed.
+
+`int_loglevel`::
+ Log level integer value.
+
+`thread`::
+ ID of the Python thread in which the log statement was executed.
+
+`threadName`::
+ Name of the Python thread in which the log statement was executed.
+
+You can use the opt:lttng-enable-event(1):--loglevel or
+opt:lttng-enable-event(1):--loglevel-only option of the
+man:lttng-enable-event(1) command to target a range of Python log levels
+or a specific Python log level.
+
+When an application imports the LTTng-UST Python agent, the agent tries
+to register to a <<lttng-sessiond,session daemon>>. Note that you must
+<<start-sessiond,start the session daemon>> _before_ you run the Python
+application. If a session daemon is found, the agent tries to register
+to it during five seconds, after which the application continues
+without LTTng tracing support. You can override this timeout value with
+the env:LTTNG_UST_PYTHON_REGISTER_TIMEOUT environment variable
+(milliseconds).
+
+If the session daemon stops while a Python application with an imported
+LTTng-UST Python agent runs, the agent retries to connect and to
+register to a session daemon every three seconds. You can override this
+delay with the env:LTTNG_UST_PYTHON_REGISTER_RETRY_DELAY environment
+variable.
+
+
+[role="since-2.5"]
+[[proc-lttng-logger-abi]]
+=== LTTng logger
+
+The `lttng-tracer` Linux kernel module, part of
+<<lttng-modules,LTTng-modules>>, creates the special LTTng logger file
+path:{/proc/lttng-logger} when it's loaded. Any application can write
+text data to this file to emit an LTTng event.
+
+[role="img-100"]
+.An application writes to the LTTng logger file to emit an LTTng event.
+image::lttng-logger.png[]
+
+The LTTng logger is the quickest method--not the most efficient,
+however--to add instrumentation to an application. It is designed
+mostly to instrument shell scripts:
+
+[role="term"]
+----
+$ echo "Some message, some $variable" > /proc/lttng-logger
+----
+
+Any event that the LTTng logger emits is named `lttng_logger` and
+belongs to the Linux kernel <<domain,tracing domain>>. However, unlike
+other instrumentation points in the kernel tracing domain, **any Unix
+user** can <<enabling-disabling-events,create an event rule>> which
+matches its event name, not only the root user or users in the
+<<tracing-group,tracing group>>.
+
+To use the LTTng logger:
+
+* From any application, write text data to the path:{/proc/lttng-logger}
+ file.
+
+The `msg` field of `lttng_logger` event records contains the
+recorded message.
+
+NOTE: The maximum message length of an LTTng logger event is
+1024{nbsp}bytes. Writing more than this makes the LTTng logger emit more
+than one event to contain the remaining data.
+
+You should not use the LTTng logger to trace a user application which
+can be instrumented in a more efficient way, namely:
+
+* <<c-application,C and $$C++$$ applications>>.
+* <<java-application,Java applications>>.
+* <<python-application,Python applications>>.
+
+.Use the LTTng logger.
+====
+[source,bash]
+.path:{test.bash}
+----
+echo 'Hello, World!' > /proc/lttng-logger
+sleep 2
+df --human-readable --print-type / > /proc/lttng-logger
+----
+
+<<creating-destroying-tracing-sessions,Create a tracing session>>,
+<<enabling-disabling-events,create an event rule>> matching the
+`lttng_logger` Linux kernel tracepoint, and
+<<basic-tracing-session-control,start tracing>>:
+
+[role="term"]
+----
+$ lttng create
+$ lttng enable-event --kernel lttng_logger
+$ lttng start
+----
+
+Run the Bash script:
+
+[role="term"]
+----
+$ bash test.bash
+----
+
+<<basic-tracing-session-control,Stop tracing>> and inspect the recorded
+events:
+
+[role="term"]
+----
+$ lttng stop
+$ lttng view
+----
+====
+
+
+[[instrumenting-linux-kernel]]
+=== LTTng kernel tracepoints
+
+NOTE: This section shows how to _add_ instrumentation points to the
+Linux kernel. The kernel's subsystems are already thoroughly
+instrumented at strategic places for LTTng when you
+<<installing-lttng,install>> the <<lttng-modules,LTTng-modules>>
+package.
+
+////
+There are two methods to instrument the Linux kernel:
+
+. <<linux-add-lttng-layer,Add an LTTng layer>> over an existing ftrace
+ tracepoint which uses the `TRACE_EVENT()` API.
++
+Choose this if you want to instrumentation a Linux kernel tree with an
+instrumentation point compatible with ftrace, perf, and SystemTap.
+
+. Use an <<linux-lttng-tracepoint-event,LTTng-only approach>> to
+ instrument an out-of-tree kernel module.
++
+Choose this if you don't need ftrace, perf, or SystemTap support.
+////
+
+
+[[linux-add-lttng-layer]]
+==== [[instrumenting-linux-kernel-itself]][[mainline-trace-event]][[lttng-adaptation-layer]]Add an LTTng layer to an existing ftrace tracepoint
+
+This section shows how to add an LTTng layer to existing ftrace
+instrumentation using the `TRACE_EVENT()` API.
+
+This section does not document the `TRACE_EVENT()` macro. You can
+read the following articles to learn more about this API:
+
+* http://lwn.net/Articles/379903/[Using the TRACE_EVENT() macro (Part{nbsp}1)]
+* http://lwn.net/Articles/381064/[Using the TRACE_EVENT() macro (Part{nbsp}2)]
+* http://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 are created in
+one source file using the `CREATE_TRACE_POINTS` definition.
+
+To add an LTTng layer over an existing ftrace tracepoint:
+
+. Make sure the following kernel configuration options are
+ enabled:
++
+--
+* `CONFIG_MODULES`
+* `CONFIG_KALLSYMS`
+* `CONFIG_HIGH_RES_TIMERS`
+* `CONFIG_TRACEPOINTS`
+--
+
+. Build the Linux source tree with your custom ftrace tracepoints.
+. Boot the resulting Linux image on your target system.
++
+Confirm that the tracepoints exist by looking for their names in the
+dir:{/sys/kernel/debug/tracing/events/subsys} directory, where `subsys`
+is your subsystem's name.
+
+. Get a copy of the latest LTTng-modules{nbsp}{revision}:
++
+--
+[role="term"]
+----
+$ cd $(mktemp -d) &&
+wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.11.tar.bz2 &&
+tar -xf lttng-modules-latest-2.11.tar.bz2 &&
+cd lttng-modules-2.11.*
+----
+--
+
+. In dir:{instrumentation/events/lttng-module}, relative to the root
+ of the LTTng-modules source tree, create a header file named
+ +__subsys__.h+ for your custom subsystem +__subsys__+ and write your
+ LTTng-modules tracepoint definitions using the LTTng-modules
+ macros in it.
++
+Start with this template:
++
+--
+[source,c]
+.path:{instrumentation/events/lttng-module/my_subsys.h}
+----
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM my_subsys
+
+#if !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _LTTNG_MY_SUBSYS_H
+
+#include "../../../probes/lttng-tracepoint-event.h"
+#include <linux/tracepoint.h>
+
+LTTNG_TRACEPOINT_EVENT(
+ /*
+ * Format is identical to TRACE_EVENT()'s version for the three
+ * following macro parameters:
+ */
+ my_subsys_my_event,
+ TP_PROTO(int my_int, const char *my_string),
+ TP_ARGS(my_int, my_string),
+
+ /* LTTng-modules specific macros */
+ TP_FIELDS(
+ ctf_integer(int, my_int_field, my_int)
+ ctf_string(my_bar_field, my_bar)
+ )
+)
+
+#endif /* !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ) */
+
+#include "../../../probes/define_trace.h"
+----
+--
++
+The entries in the `TP_FIELDS()` section are the list of fields for the
+LTTng tracepoint. This is similar to the `TP_STRUCT__entry()` part of
+ftrace's `TRACE_EVENT()` macro.
++
+See <<lttng-modules-tp-fields,Tracepoint fields macros>> for a
+complete description of the available `ctf_*()` macros.
+
+. Create the LTTng-modules probe's kernel module C source file,
+ +probes/lttng-probe-__subsys__.c+, where +__subsys__+ is your
+ subsystem name:
++
+--
+[source,c]
+.path:{probes/lttng-probe-my-subsys.c}
+----
+#include <linux/module.h>
+#include "../lttng-tracer.h"
+
+/*
+ * Build-time verification of mismatch between mainline
+ * TRACE_EVENT() arguments and the LTTng-modules adaptation
+ * layer LTTNG_TRACEPOINT_EVENT() arguments.
+ */
+#include <trace/events/my_subsys.h>
+
+/* Create LTTng tracepoint probes */
+#define LTTNG_PACKAGE_BUILD
+#define CREATE_TRACE_POINTS
+#define TRACE_INCLUDE_PATH ../instrumentation/events/lttng-module
+
+#include "../instrumentation/events/lttng-module/my_subsys.h"
+
+MODULE_LICENSE("GPL and additional rights");
+MODULE_AUTHOR("Your name <your-email>");
+MODULE_DESCRIPTION("LTTng my_subsys probes");
+MODULE_VERSION(__stringify(LTTNG_MODULES_MAJOR_VERSION) "."
+ __stringify(LTTNG_MODULES_MINOR_VERSION) "."
+ __stringify(LTTNG_MODULES_PATCHLEVEL_VERSION)
+ LTTNG_MODULES_EXTRAVERSION);
+----
+--
+
+. Edit path:{probes/KBuild} and add your new kernel module object
+ next to the existing ones:
++
+--
+[source,make]
+.path:{probes/KBuild}
+----
+# ...
+
+obj-m += lttng-probe-module.o
+obj-m += lttng-probe-power.o
+
+obj-m += lttng-probe-my-subsys.o
+
+# ...
+----
+--
+
+. Build and install the LTTng kernel modules:
++
+--
+[role="term"]
+----
+$ make KERNELDIR=/path/to/linux
+# make modules_install && depmod -a
+----
+--
++
+Replace `/path/to/linux` with the path to the Linux source tree where
+you defined and used tracepoints with ftrace's `TRACE_EVENT()` macro.
+
+Note that you can also use the
+<<lttng-tracepoint-event-code,`LTTNG_TRACEPOINT_EVENT_CODE()` macro>>
+instead of `LTTNG_TRACEPOINT_EVENT()` to use custom local variables and
+C code that need to be executed before the event fields are recorded.
+
+The best way to learn how to use the previous LTTng-modules macros is to
+inspect the existing LTTng-modules tracepoint definitions in the
+dir:{instrumentation/events/lttng-module} header files. Compare them
+with the Linux kernel mainline versions in the
+dir:{include/trace/events} directory of the Linux source tree.
+
+
+[role="since-2.7"]
+[[lttng-tracepoint-event-code]]
+===== Use custom C code to access the data for tracepoint fields
+
+Although we recommended to always use the
+<<lttng-adaptation-layer,`LTTNG_TRACEPOINT_EVENT()`>> macro to describe
+the arguments and fields of an LTTng-modules tracepoint when possible,
+sometimes you need a more complex process to access the data that the
+tracer records as event record fields. In other words, you need local
+variables and multiple C{nbsp}statements instead of simple
+argument-based expressions that you pass to the
+<<lttng-modules-tp-fields,`ctf_*()` macros of `TP_FIELDS()`>>.
+
+You can use the `LTTNG_TRACEPOINT_EVENT_CODE()` macro instead of
+`LTTNG_TRACEPOINT_EVENT()` to declare custom local variables and define
+a block of C{nbsp}code to be executed before LTTng records the fields.
+The structure of this macro is:
+
+[source,c]
+.`LTTNG_TRACEPOINT_EVENT_CODE()` macro syntax.
+----
+LTTNG_TRACEPOINT_EVENT_CODE(
+ /*
+ * Format identical to the LTTNG_TRACEPOINT_EVENT()
+ * version for the following three macro parameters:
+ */
+ my_subsys_my_event,
+ TP_PROTO(int my_int, const char *my_string),
+ TP_ARGS(my_int, my_string),
+
+ /* Declarations of custom local variables */
+ TP_locvar(
+ int a = 0;
+ unsigned long b = 0;
+ const char *name = "(undefined)";
+ struct my_struct *my_struct;
+ ),
+
+ /*
+ * Custom code which uses both tracepoint arguments
+ * (in TP_ARGS()) and local variables (in TP_locvar()).
+ *
+ * Local variables are actually members of a structure pointed
+ * to by the special variable tp_locvar.
+ */
+ TP_code(
+ if (my_int) {
+ tp_locvar->a = my_int + 17;
+ tp_locvar->my_struct = get_my_struct_at(tp_locvar->a);
+ tp_locvar->b = my_struct_compute_b(tp_locvar->my_struct);
+ tp_locvar->name = my_struct_get_name(tp_locvar->my_struct);
+ put_my_struct(tp_locvar->my_struct);
+
+ if (tp_locvar->b) {
+ tp_locvar->a = 1;
+ }
+ }
+ ),
+
+ /*
+ * Format identical to the LTTNG_TRACEPOINT_EVENT()
+ * version for this, except that tp_locvar members can be
+ * used in the argument expression parameters of
+ * the ctf_*() macros.
+ */
+ TP_FIELDS(
+ ctf_integer(unsigned long, my_struct_b, tp_locvar->b)
+ ctf_integer(int, my_struct_a, tp_locvar->a)
+ ctf_string(my_string_field, my_string)
+ ctf_string(my_struct_name, tp_locvar->name)
+ )
+)
+----
+
+IMPORTANT: The C code defined in `TP_code()` must not have any side
+effects when executed. In particular, the code must not allocate
+memory or get resources without deallocating this memory or putting
+those resources afterwards.
+
+
+[[instrumenting-linux-kernel-tracing]]
+==== Load and unload a custom probe kernel module
+
+You must load a <<lttng-adaptation-layer,created LTTng-modules probe
+kernel module>> in the kernel before it can emit LTTng events.
+
+To load the default probe kernel modules and a custom probe kernel
+module:
+
+* Use the opt:lttng-sessiond(8):--extra-kmod-probes option to give extra
+ probe modules to load when starting a root <<lttng-sessiond,session
+ daemon>>:
++
+--
+.Load the `my_subsys`, `usb`, and the default probe modules.
+====
+[role="term"]
+----
+# lttng-sessiond --extra-kmod-probes=my_subsys,usb
+----
+====
+--
++
+You only need to pass the subsystem name, not the whole kernel module
+name.
+
+To load _only_ a given custom probe kernel module:
+
+* Use the opt:lttng-sessiond(8):--kmod-probes option to give the probe
+ modules to load when starting a root session daemon:
++
+--
+.Load only the `my_subsys` and `usb` probe modules.
+====
+[role="term"]
+----
+# lttng-sessiond --kmod-probes=my_subsys,usb
+----
+====
+--
+
+To confirm that a probe module is loaded:
+
+* Use man:lsmod(8):
++
+--
+[role="term"]
+----
+$ lsmod | grep lttng_probe_usb
+----
+--
+
+To unload the loaded probe modules:
+
+* Kill the session daemon with `SIGTERM`:
++
+--
+[role="term"]
+----
+# pkill lttng-sessiond
+----
+--
++
+You can also use man:modprobe(8)'s `--remove` option if the session
+daemon terminates abnormally.
+
+
+[[controlling-tracing]]
+== Tracing control
+
+Once an application or a Linux kernel is
+<<instrumenting,instrumented>> for LTTng tracing,
+you can _trace_ it.
+
+This section is divided in topics on how to use the various
+<<plumbing,components of LTTng>>, in particular the <<lttng-cli,cmd:lttng
+command-line tool>>, to _control_ the LTTng daemons and tracers.
+
+NOTE: In the following subsections, we refer to an man:lttng(1) command
+using its man page name. For example, instead of _Run the `create`
+command to..._, we use _Run the man:lttng-create(1) command to..._.
+
+
+[[start-sessiond]]
+=== Start a session daemon
+
+In some situations, you need to run a <<lttng-sessiond,session daemon>>
+(man:lttng-sessiond(8)) _before_ you can use the man:lttng(1)
+command-line tool.
+
+You will see the following error when you run a command while no session
+daemon is running:
+
+----
+Error: No session daemon is available
+----
+
+The only command that automatically runs a session daemon is
+man:lttng-create(1), which you use to
+<<creating-destroying-tracing-sessions,create a tracing session>>. While
+this is most of the time the first operation that you do, sometimes it's
+not. Some examples are:
+
+* <<list-instrumentation-points,List the available instrumentation points>>.
+* <<saving-loading-tracing-session,Load a tracing session configuration>>.
+
+[[tracing-group]] Each Unix user must have its own running session
+daemon to trace user applications. The session daemon that the root user
+starts is the only one allowed to control the LTTng kernel tracer. Users
+that are part of the _tracing group_ can control the root session
+daemon. The default tracing group name is `tracing`; you can set it to
+something else with the opt:lttng-sessiond(8):--group option when you
+start the root session daemon.
+
+To start a user session daemon:
+
+* Run man:lttng-sessiond(8):
++
+--
+[role="term"]
+----
+$ lttng-sessiond --daemonize
+----
+--
+
+To start the root session daemon:
+
+* Run man:lttng-sessiond(8) as the root user:
++
+--
+[role="term"]
+----
+# lttng-sessiond --daemonize
+----
+--
+
+In both cases, remove the opt:lttng-sessiond(8):--daemonize option to
+start the session daemon in foreground.
+
+To stop a session daemon, use man:kill(1) on its process ID (standard
+`TERM` signal).
+
+Note that some Linux distributions could manage the LTTng session daemon
+as a service. In this case, you should use the service manager to
+start, restart, and stop session daemons.
+
+
+[[creating-destroying-tracing-sessions]]
+=== Create and destroy a tracing session
+
+Almost all the LTTng control operations happen in the scope of
+a <<tracing-session,tracing session>>, which is the dialogue between the
+<<lttng-sessiond,session daemon>> and you.
+
+To create a tracing session with a generated name:
+
+* Use the man:lttng-create(1) command:
++
+--
+[role="term"]
+----
+$ lttng create
+----
+--
+
+The created tracing session's name is `auto` followed by the
+creation date.
+
+To create a tracing session with a specific name:
+
+* Use the optional argument of the man:lttng-create(1) command:
++
+--
+[role="term"]
+----
+$ lttng create my-session
+----
+--
++
+Replace `my-session` with the specific tracing session name.
+
+LTTng appends the creation date to the created tracing session's name.
+
+LTTng writes the traces of a tracing session in
++$LTTNG_HOME/lttng-trace/__name__+ by default, where +__name__+ is the
+name of the tracing session. Note that the env:LTTNG_HOME environment
+variable defaults to `$HOME` if not set.
+
+To output LTTng traces to a non-default location:
+
+* Use the opt:lttng-create(1):--output option of the man:lttng-create(1) command:
++
+--
+[role="term"]
+----
+$ lttng create my-session --output=/tmp/some-directory
+----
+--
+
+You may create as many tracing sessions as you wish.
+
+To list all the existing tracing sessions for your Unix user:
+
+* Use the man:lttng-list(1) command:
++
+--
+[role="term"]
+----
+$ lttng list
+----
+--
+
+When you create a tracing session, it is set as the _current tracing
+session_. The following man:lttng(1) commands operate on the current
+tracing session when you don't specify one:
+
+[role="list-3-cols"]
+* man:lttng-add-context(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 tracing session:
+
+* Use the man:lttng-set-session(1) command:
++
+--
+[role="term"]
+----
+$ lttng set-session new-session
+----
+--
++
+Replace `new-session` by the name of the new current tracing session.
+
+When you are done tracing in a given tracing session, you can destroy
+it. This operation frees the resources taken by the tracing session
+to destroy; it does not destroy the trace data that LTTng wrote for
+this tracing session.
+
+To destroy the current tracing session:
+
+* Use the man:lttng-destroy(1) command:
++
+--
+[role="term"]
+----
+$ lttng destroy
+----
+--
+
+
+[[list-instrumentation-points]]
+=== List the available instrumentation points
+
+The <<lttng-sessiond,session daemon>> can query the running instrumented
+user applications and the Linux kernel to get a list of available
+instrumentation points. For the Linux kernel <<domain,tracing domain>>,
+they are tracepoints and system calls. For the user space tracing
+domain, they are tracepoints. For the other tracing domains, they are
+logger names.
+
+To list the available instrumentation points:
+
+* Use the man:lttng-list(1) command with the requested tracing domain's
+ option amongst:
++
+--
+* opt:lttng-list(1):--kernel: Linux kernel tracepoints (your Unix user
+ must be a root user, or it must be a member of the
+ <<tracing-group,tracing group>>).
+* opt:lttng-list(1):--kernel with opt:lttng-list(1):--syscall: Linux
+ kernel system calls (your Unix user must be a root user, or it must be
+ a member of the tracing group).
+* opt:lttng-list(1):--userspace: user space tracepoints.
+* opt:lttng-list(1):--jul: `java.util.logging` loggers.
+* opt:lttng-list(1):--log4j: Apache log4j loggers.
+* opt:lttng-list(1):--python: Python loggers.
+--
+
+.List the available user space tracepoints.
+====
+[role="term"]
+----
+$ lttng list --userspace
+----
+====
+
+.List the available Linux kernel system call tracepoints.
+====
+[role="term"]
+----
+$ lttng list --kernel --syscall
+----
+====
+
+
+[[enabling-disabling-events]]
+=== Create and enable an event rule
+
+Once you <<creating-destroying-tracing-sessions,create a tracing
+session>>, you can create <<event,event rules>> with the
+man:lttng-enable-event(1) command.
+
+You specify each condition with a command-line option. The available
+condition arguments are shown in the following table.
+
+[role="growable",cols="asciidoc,asciidoc,default"]
+.Condition command-line arguments for the man:lttng-enable-event(1) command.
+|====
+|Argument |Description |Applicable tracing domains
+
+|
+One of:
+
+. `--syscall`
+. +--probe=__ADDR__+
+. +--function=__ADDR__+
+. +--userspace-probe=__PATH__:__SYMBOL__+
+. +--userspace-probe=sdt:__PATH__:__PROVIDER__:__NAME__+
+
+|
+Instead of using the default _tracepoint_ instrumentation type, use:
+
+. A Linux system call (entry and exit).
+. A Linux https://lwn.net/Articles/132196/[kprobe] (symbol or address).
+. The entry and return points of a Linux function (symbol or address).
+. The entry point of a user application or library function (path to
+ application/library and symbol).
+. A https://www.sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps[SystemTap
+ Statically Defined Tracing] (SDT) probe (path to application/library,
+ provider and probe names).
+
+|Linux kernel.
+
+|First positional argument.
+
+|
+Tracepoint or system call name.
+
+With the opt:lttng-enable-event(1):--probe,
+opt:lttng-enable-event(1):--function, and
+opt:lttng-enable-event(1):--userspace-probe options, this is a custom
+name given to the event rule. With the JUL, log4j, and Python domains,
+this is a logger name.
+
+With a tracepoint, logger, or system call name, you can use the special
+`*` globbing character to match anything (for example, `sched_*`,
+`my_comp*:*msg_*`).
+
+|All.
+
+|
+One of:
+
+. +--loglevel=__LEVEL__+
+. +--loglevel-only=__LEVEL__+
+
+|
+. Match only tracepoints or log statements with a logging level at
+ least as severe as +__LEVEL__+.
+. Match only tracepoints or log statements with a logging level
+ equal to +__LEVEL__+.
+
+See man:lttng-enable-event(1) for the list of available logging level
+names.
+
+|User space, JUL, log4j, and Python.
+
+|+--exclude=__EXCLUSIONS__+
+
+|
+When you use a `*` character at the end of the tracepoint or logger
+name (first positional argument), exclude the specific names in the
+comma-delimited list +__EXCLUSIONS__+.
+
+|
+User space, JUL, log4j, and Python.
+
+|+--filter=__EXPR__+
+
+|
+Match only events which satisfy the expression +__EXPR__+.
+
+See man:lttng-enable-event(1) to learn more about the syntax of a
+filter expression.
+
+|All.
+
+|====
+
+You attach an event rule to a <<channel,channel>> on creation. If you do
+not specify the channel with the opt:lttng-enable-event(1):--channel
+option, and if the event rule to create is the first in its
+<<domain,tracing domain>> for a given tracing session, then LTTng
+creates a _default channel_ for you. This default channel is reused in
+subsequent invocations of the man:lttng-enable-event(1) command for the
+same tracing domain.
+
+An event rule is always enabled at creation time.
+
+The following examples show how you can combine the previous
+command-line options to create simple to more complex event rules.
+
+.Create an event rule targetting a Linux kernel tracepoint (default channel).
+====
+[role="term"]
+----
+$ lttng enable-event --kernel sched_switch
+----
+====
+
+.Create an event rule matching four Linux kernel system calls (default channel).
+====
+[role="term"]
+----
+$ lttng enable-event --kernel --syscall open,write,read,close
+----
+====
+
+.Create event rules matching tracepoints with filter expressions (default channel).
+====
+[role="term"]
+----
+$ lttng enable-event --kernel sched_switch --filter='prev_comm == "bash"'
+----
+
+[role="term"]
+----
+$ lttng enable-event --kernel --all \
+ --filter='$ctx.tid == 1988 || $ctx.tid == 1534'
+----
+
+[role="term"]
+----
+$ lttng enable-event --jul my_logger \
+ --filter='$app.retriever:cur_msg_id > 3'
+----
+
+IMPORTANT: Make sure to always quote the filter string when you
+use man:lttng(1) from a shell.
+====
+
+.Create an event rule matching any user space tracepoint of a given tracepoint provider with a log level range (default channel).
+====
+[role="term"]
+----
+$ lttng enable-event --userspace my_app:'*' --loglevel=TRACE_INFO
+----
+
+IMPORTANT: Make sure to always quote the wildcard character when you
+use man:lttng(1) from a shell.
+====
+
+.Create an event rule matching multiple Python loggers with a wildcard and with exclusions (default channel).
+====
+[role="term"]
+----
+$ lttng enable-event --python my-app.'*' \
+ --exclude='my-app.module,my-app.hello'
+----
+====
+
+.Create an event rule matching any Apache log4j logger with a specific log level (default channel).
+====
+[role="term"]
+----
+$ lttng enable-event --log4j --all --loglevel-only=LOG4J_WARN
+----
+====
+
+.Create an event rule attached to a specific channel matching a specific user space tracepoint provider and tracepoint.
+====
+[role="term"]
+----
+$ lttng enable-event --userspace my_app:my_tracepoint --channel=my-channel
+----
+====
+
+.Create an event rule matching 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 an event rule matching the `server`/`accept_request` https://www.sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps[SDT probe] in path:{/usr/bin/serv}:
+====
+[role="term"]
+----
+$ lttng enable-event --kernel --userspace-probe=sdt:serv:server:accept_request \
+ server_accept_request
+----
+====
+
+The event rules of a given channel form a whitelist: as soon as an
+emitted event passes one of them, LTTng can record the event. For
+example, an event named `my_app:my_tracepoint` emitted from a user space
+tracepoint with a `TRACE_ERROR` log level passes both of the following
+rules:
+
+[role="term"]
+----
+$ lttng enable-event --userspace my_app:my_tracepoint
+$ lttng enable-event --userspace my_app:my_tracepoint \
+ --loglevel=TRACE_INFO
+----
+
+The second event rule is redundant: the first one includes
+the second one.
+
+
+[[disable-event-rule]]
+=== Disable an event rule
+
+To disable an event rule that you <<enabling-disabling-events,created>>
+previously, use the man:lttng-disable-event(1) command. This command
+disables _all_ the event rules (of a given tracing domain and channel)
+which match an instrumentation point. The other conditions are not
+supported as of LTTng{nbsp}{revision}.
+
+The LTTng tracer does not record an emitted event which passes
+a _disabled_ event rule.
+
+.Disable an event rule matching a Python logger (default channel).
+====
+[role="term"]
+----
+$ lttng disable-event --python my-logger
+----
+====
+
+.Disable an event rule matching all `java.util.logging` loggers (default channel).
+====
+[role="term"]
+----
+$ lttng disable-event --jul '*'
+----
+====
+
+.Disable _all_ the event rules of the default channel.
+====
+The opt:lttng-disable-event(1):--all-events option is not, like the
+opt:lttng-enable-event(1):--all option of man:lttng-enable-event(1), the
+equivalent of the event name `*` (wildcard): it disables _all_ the event
+rules of a given channel.
+
+[role="term"]
+----
+$ lttng disable-event --jul --all-events
+----
+====
+
+NOTE: You cannot delete an event rule once you create it.
+
+
+[[status]]
+=== Get the status of a tracing session
+
+To get the status of the current tracing session, that is, its
+parameters, its channels, event rules, and their attributes:
+
+* Use the man:lttng-status(1) command:
++
+--
+[role="term"]
+----
+$ lttng status
+----
+--
++
+
+To get the status of any tracing session:
+
+* Use the man:lttng-list(1) command with the tracing session's name:
++
+--
+[role="term"]
+----
+$ lttng list my-session
+----
+--
++
+Replace `my-session` with the desired tracing session's name.
+
+
+[[basic-tracing-session-control]]
+=== Start and stop a tracing session
+
+Once you <<creating-destroying-tracing-sessions,create a tracing
+session>> and
+<<enabling-disabling-events,create one or more event rules>>,
+you can start and stop the tracers for this tracing session.
+
+To start tracing in the current tracing session:
+
+* Use the man:lttng-start(1) command:
++
+--
+[role="term"]
+----
+$ lttng start
+----
+--
+
+LTTng is very flexible: you can launch user applications before
+or after the you start the tracers. The tracers only record the events
+if they pass enabled event rules and if they occur while the tracers are
+started.
+
+To stop tracing in the current tracing session:
+
+* Use the man:lttng-stop(1) command:
++
+--
+[role="term"]
+----
+$ lttng stop
+----
+--
++
+If there were <<channel-overwrite-mode-vs-discard-mode,lost event
+records>> or lost sub-buffers since the last time you ran
+man:lttng-start(1), warnings are printed when you run the
+man:lttng-stop(1) command.
+
+
+[[enabling-disabling-channels]]
+=== Create a channel
+
+Once you create a tracing session, you can create a <<channel,channel>>
+with the man:lttng-enable-channel(1) command.
+
+Note that LTTng automatically creates a default channel when, for a
+given <<domain,tracing domain>>, no channels exist and you
+<<enabling-disabling-events,create>> the first event rule. This default
+channel is named `channel0` and its attributes are set to reasonable
+values. Therefore, you only need to create a channel when you need
+non-default attributes.
+
+You specify each non-default channel attribute with a command-line
+option when you use the man:lttng-enable-channel(1) command. The
+available command-line options are:
+
+[role="growable",cols="asciidoc,asciidoc"]
+.Command-line options for the man:lttng-enable-channel(1) command.
+|====
+|Option |Description
+
+|`--overwrite`
+
+|
+Use the _overwrite_
+<<channel-overwrite-mode-vs-discard-mode,event record loss mode>> instead
+of the default _discard_ mode.
+
+|`--buffers-pid` (user space tracing domain only)
+
+|
+Use the per-process <<channel-buffering-schemes,buffering scheme>>
+instead of the default per-user buffering scheme.
+
+|+--subbuf-size=__SIZE__+
+
+|
+Allocate sub-buffers of +__SIZE__+ bytes (power of two), for each CPU,
+either for each Unix user (default), or for each instrumented process.
+
+See <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>>.
+
+|+--num-subbuf=__COUNT__+
+
+|
+Allocate +__COUNT__+ sub-buffers (power of two), for each CPU, either
+for each Unix user (default), or for each instrumented process.
+
+See <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>>.
+
+|+--tracefile-size=__SIZE__+
+
+|
+Set the maximum size of each trace file that this channel writes within
+a stream to +__SIZE__+ bytes instead of no maximum.
+
+See <<tracefile-rotation,Trace file count and size>>.
+
+|+--tracefile-count=__COUNT__+
+
+|
+Limit the number of trace files that this channel creates to
++__COUNT__+ channels instead of no limit.
+
+See <<tracefile-rotation,Trace file count and size>>.
+
+|+--switch-timer=__PERIODUS__+
+
+|
+Set the <<channel-switch-timer,switch timer period>>
+to +__PERIODUS__+{nbsp}µs.
+
+|+--read-timer=__PERIODUS__+
+
+|
+Set the <<channel-read-timer,read timer period>>
+to +__PERIODUS__+{nbsp}µs.
+
+|[[opt-blocking-timeout]]+--blocking-timeout=__TIMEOUTUS__+
+
+|
+Set the timeout of user space applications which load LTTng-UST
+in blocking mode to +__TIMEOUTUS__+:
+
+0 (default)::
+ Never block (non-blocking mode).
+
+`inf`::
+ Block forever until space is available in a sub-buffer to record
+ the event.
+
+__n__, a positive value::
+ Wait for at most __n__ µs when trying to write into a sub-buffer.
+
+Note that, for this option to have any effect on an instrumented
+user space application, you need to run the application with a set
+env:LTTNG_UST_ALLOW_BLOCKING environment variable.
+
+|+--output=__TYPE__+ (Linux kernel tracing domain only)
+
+|
+Set the channel's output type to +__TYPE__+, either `mmap` or `splice`.
+
+|====
+
+You can only create a channel in the Linux kernel and user space
+<<domain,tracing domains>>: other tracing domains have their own channel
+created on the fly when <<enabling-disabling-events,creating event
+rules>>.
+
+[IMPORTANT]
+====
+Because of a current LTTng limitation, you must create all channels
+_before_ you <<basic-tracing-session-control,start tracing>> in a given
+tracing session, that is, before the first time you run
+man:lttng-start(1).
+
+Since LTTng automatically creates a default channel when you use the
+man:lttng-enable-event(1) command with a specific tracing domain, you
+cannot, for example, create a Linux kernel event rule, start tracing,
+and then create a user space event rule, because no user space channel
+exists yet and it's too late to create one.
+
+For this reason, make sure to configure your channels properly
+before starting the tracers for the first time!
+====
+
+The following examples show how you can combine the previous
+command-line options to create simple to more complex channels.
+
+.Create a Linux kernel channel with default attributes.
+====
+[role="term"]
+----
+$ lttng enable-channel --kernel my-channel
+----
+====
+
+.Create a user space channel with 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 tracing-session>>,
+create the channel, <<enabling-disabling-events,create an event rule>>,
+and <<basic-tracing-session-control,start tracing>>:
+
+[role="term"]
+----
+$ lttng create
+$ lttng enable-channel --userspace --blocking-timeout=inf blocking-channel
+$ lttng enable-event --userspace --channel=blocking-channel --all
+$ lttng start
+----
+
+Run an application instrumented with LTTng-UST and allow it to block:
+
+[role="term"]
+----
+$ LTTNG_UST_ALLOW_BLOCKING=1 my-app
+----
+====
+
+.Create a Linux kernel channel which rotates 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 (or _flight recorder_) mode.
+====
+[role="term"]
+----
+$ lttng enable-channel --userspace --overwrite my-channel
+----
+====
+
+You can <<enabling-disabling-events,create>> the same event rule in
+two different channels:
+
+[role="term"]
+----
+$ lttng enable-event --userspace --channel=my-channel app:tp
+$ lttng enable-event --userspace --channel=other-channel app:tp
+----
+
+If both channels are enabled, when a tracepoint named `app:tp` is
+reached, LTTng records two events, one for each channel.
+
+
+[[disable-channel]]
+=== Disable a channel
+
+To disable a specific channel that you <<enabling-disabling-channels,created>>
+previously, use the man:lttng-disable-channel(1) command.
+
+.Disable a specific Linux kernel channel.
+====
+[role="term"]
+----
+$ lttng disable-channel --kernel my-channel
+----
+====
+
+The state of a channel precedes the individual states of event rules
+attached to it: event rules which belong to a disabled channel, even if
+they are enabled, are also considered disabled.
+
+
+[[adding-context]]
+=== Add context fields to a channel
+
+Event record fields in trace files provide important information about
+events that occured previously, but sometimes some external context may
+help you solve a problem faster. Examples of context fields are:
+
+* The **process ID**, **thread ID**, **process name**, and
+ **process priority** of the thread in which the event occurs.
+* The **hostname** of the system on which the event occurs.
+* The Linux kernel and user call stacks (since
+ LTTng{nbsp}{revision}).
+* The current values of many possible **performance counters** using
+ perf, for example:
+** CPU cycles, stalled cycles, idle cycles, and the other cycle types.
+** Cache misses.
+** Branch instructions, misses, and loads.
+** CPU faults.
+* Any context defined at the application level (supported for the
+ JUL and log4j <<domain,tracing domains>>).
+
+To get the full list of available context fields, see
+`lttng add-context --list`. Some context fields are reserved for a
+specific <<domain,tracing domain>> (Linux kernel or user space).
+
+You add context fields to <<channel,channels>>. All the events
+that a channel with added context fields records contain those fields.
+
+To add context fields to one or all the channels of a given tracing
+session:
+
+* Use the man:lttng-add-context(1) command.
+
+.Add context fields to all the channels of the current tracing session.
+====
+The following command line adds the virtual process identifier and
+the per-thread CPU cycles count fields to all the user space channels
+of the current tracing session.
+
+[role="term"]
+----
+$ lttng add-context --userspace --type=vpid --type=perf:thread:cpu-cycles
+----
+====
+
+.Add performance counter context fields by raw ID
+====
+See man:lttng-add-context(1) for the exact format of the context field
+type, which is partly compatible with the format used in
+man:perf-record(1).
+
+[role="term"]
+----
+$ lttng add-context --userspace --type=perf:thread:raw:r0110:test
+$ lttng add-context --kernel --type=perf:cpu:raw:r0013c:x86unhalted
+----
+====
+
+.Add context fields to 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` in
+the current tracing session.
+
+[role="term"]
+----
+$ lttng add-context --kernel --channel=my-channel \
+ --type=tid --type=callstack-user
+----
+====
+
+.Add an application-specific context field to a specific channel.
+====
+The following command line adds the `cur_msg_id` context field of the
+`retriever` context retriever for all the instrumented
+<<java-application,Java applications>> recording <<event,event records>>
+in the channel named `my-channel`:
+
+[role="term"]
+----
+$ lttng add-context --kernel --channel=my-channel \
+ --type='$app:retriever:cur_msg_id'
+----
+
+IMPORTANT: Make sure to always quote the `$` character when you
+use man:lttng-add-context(1) from a shell.
+====
+
+NOTE: You cannot remove context fields from a channel once you add it.
+
+
+[role="since-2.7"]
+[[pid-tracking]]
+=== Track process IDs
+
+It's often useful to allow only specific process IDs (PIDs) to emit
+events. For example, you may wish to record all the system calls made by
+a given process (à la http://linux.die.net/man/1/strace[strace]).
+
+The man:lttng-track(1) and man:lttng-untrack(1) commands serve this
+purpose. Both commands operate on a whitelist of process IDs. You _add_
+entries to this whitelist with the man:lttng-track(1) command and remove
+entries with the man:lttng-untrack(1) command. Any process which has one
+of the PIDs in the whitelist is allowed to emit LTTng events which pass
+an enabled <<event,event rule>>.
+
+NOTE: The PID tracker tracks the _numeric process IDs_. Should a
+process with a given tracked ID exit and another process be given this
+ID, then the latter would also be allowed to emit events.
+
+.Track and untrack process IDs.
+====
+For the sake of the following example, assume the target system has
+16{nbsp}possible PIDs.
+
+When you
+<<creating-destroying-tracing-sessions,create a tracing session>>,
+the whitelist contains all the possible PIDs:
+
+[role="img-100"]
+.All PIDs are tracked.
+image::track-all.png[]
+
+When the whitelist is full and you use the man:lttng-track(1) command to
+specify some PIDs to track, LTTng first clears the whitelist, then it
+tracks the specific PIDs. After:
+
+[role="term"]
+----
+$ lttng track --pid=3,4,7,10,13
+----
+
+the whitelist is:
+
+[role="img-100"]
+.PIDs 3, 4, 7, 10, and 13 are tracked.
+image::track-3-4-7-10-13.png[]
+
+You can add more PIDs to the whitelist afterwards:
+
+[role="term"]
+----
+$ lttng track --pid=1,15,16
+----
+
+The result is:
+
+[role="img-100"]
+.PIDs 1, 15, and 16 are added to the whitelist.
+image::track-1-3-4-7-10-13-15-16.png[]
+
+The man:lttng-untrack(1) command removes entries from the PID tracker's
+whitelist. Given the previous example, the following command:
+
+[role="term"]
+----
+$ lttng untrack --pid=3,7,10,13
+----
+
+leads to this whitelist:
+
+[role="img-100"]
+.PIDs 3, 7, 10, and 13 are removed from the whitelist.
+image::track-1-4-15-16.png[]
+
+LTTng can track all possible PIDs again using the
+opt:lttng-track(1):--all option:
+
+[role="term"]
+----
+$ lttng track --pid --all
+----
+
+The result is, again:
+
+[role="img-100"]
+.All PIDs are tracked.
+image::track-all.png[]
+====
+
+.Track only specific PIDs
+====
+A very typical use case with PID tracking is to start with an empty
+whitelist, then <<basic-tracing-session-control,start the tracers>>, and
+then add PIDs manually while tracers are active. You can accomplish this
+by using the opt:lttng-untrack(1):--all option of the
+man:lttng-untrack(1) command to clear the whitelist after you
+<<creating-destroying-tracing-sessions,create a tracing session>>:
+
+[role="term"]
+----
+$ lttng untrack --pid --all
+----
+
+gives:
+
+[role="img-100"]
+.No PIDs are tracked.
+image::untrack-all.png[]
+
+If you trace with this whitelist configuration, the tracer records no
+events for this <<domain,tracing domain>> because no processes are
+tracked. You can use the man:lttng-track(1) command as usual to track
+specific PIDs, for example:
+
+[role="term"]
+----
+$ lttng track --pid=6,11
+----
+
+Result:
+
+[role="img-100"]
+.PIDs 6 and 11 are tracked.
+image::track-6-11.png[]
+====
+
+
+[role="since-2.5"]
+[[saving-loading-tracing-session]]
+=== Save and load tracing session configurations
+
+Configuring a <<tracing-session,tracing session>> can be long. Some of
+the tasks involved are:
+
+* <<enabling-disabling-channels,Create channels>> with
+ specific attributes.
+* <<adding-context,Add context fields>> to specific channels.
+* <<enabling-disabling-events,Create event rules>> with specific log
+ level and filter conditions.
+
+If you use LTTng to solve real world problems, chances are you have to
+record events using the same tracing session setup over and over,
+modifying a few variables each time in your instrumented program
+or environment. To avoid constant tracing session reconfiguration,
+the man:lttng(1) command-line tool can save and load tracing session
+configurations to/from XML files.
+
+To save a given tracing session configuration:
+
+* Use the man:lttng-save(1) command:
++
+--
+[role="term"]
+----
+$ lttng save my-session
+----
+--
++
+Replace `my-session` with the name of the tracing session to save.
+
+LTTng saves tracing session configurations to
+dir:{$LTTNG_HOME/.lttng/sessions} by default. Note that the
+env:LTTNG_HOME environment variable defaults to `$HOME` if not set. Use
+the opt:lttng-save(1):--output-path option to change this destination
+directory.
+
+LTTng saves all configuration parameters, for example:
+
+* The tracing session name.
+* The trace data output path.
+* The channels with their state and all their attributes.
+* The context fields you added to channels.
+* The event rules with their state, log level and filter conditions.
+
+To load a tracing session:
+
+* Use the man:lttng-load(1) command:
++
+--
+[role="term"]
+----
+$ lttng load my-session
+----
+--
++
+Replace `my-session` with the name of the tracing session to load.
+
+When LTTng loads a configuration, it restores your saved tracing session
+as if you just configured it manually.
+
+See man:lttng(1) for the complete list of command-line options. You
+can also save and load all many sessions at a time, and decide in which
+directory to output the XML files.
+
+
+[[sending-trace-data-over-the-network]]
+=== Send trace data over the network
+
+LTTng can send the recorded trace data to a remote system over the
+network instead of writing it to the local file system.
+
+To send the trace data over the network:
+
+. On the _remote_ system (which can also be the target system),
+ start an LTTng <<lttng-relayd,relay daemon>> (man:lttng-relayd(8)):
++
+--
+[role="term"]
+----
+$ lttng-relayd
+----
+--
+
+. On the _target_ system, create a tracing session configured to
+ send trace data over the network:
++
+--
+[role="term"]
+----
+$ lttng create my-session --set-url=net://remote-system
+----
+--
++
+Replace `remote-system` by the host name or IP address of the
+remote system. See man:lttng-create(1) for the exact URL format.
+
+. On the target system, use the man:lttng(1) command-line tool as usual.
+ When tracing is active, the target's consumer daemon sends sub-buffers
+ to the relay daemon running on the remote system instead of flushing
+ them to the local file system. The relay daemon writes the received
+ packets to the local file system.
+
+The relay daemon writes trace files to
++$LTTNG_HOME/lttng-traces/__hostname__/__session__+ by default, where
++__hostname__+ is the host name of the target system and +__session__+
+is the tracing session name. Note that the env:LTTNG_HOME environment
+variable defaults to `$HOME` if not set. Use the
+opt:lttng-relayd(8):--output option of man:lttng-relayd(8) to write
+trace files to another base directory.
+
+
+[role="since-2.4"]
+[[lttng-live]]
+=== View events as LTTng emits them (noch:{LTTng} live)
+
+LTTng live is a network protocol implemented by the <<lttng-relayd,relay
+daemon>> (man:lttng-relayd(8)) to allow compatible trace viewers to
+display events as LTTng emits them on the target system while tracing is
+active.
+
+The relay daemon creates a _tee_: it forwards the trace data to both
+the local file system and to connected live viewers:
+
+[role="img-90"]
+.The relay daemon creates a _tee_, forwarding the trace data to both trace files and a connected live viewer.
+image::live.png[]
+
+To use LTTng live:
+
+. On the _target system_, create a <<tracing-session,tracing session>>
+ in _live mode_:
++
+--
+[role="term"]
+----
+$ lttng create my-session --live
+----
+--
++
+This spawns a local relay daemon.
+
+. Start the live viewer and configure it to connect to the relay
+ daemon. For example, with http://diamon.org/babeltrace[Babeltrace]:
++
+--
+[role="term"]
+----
+$ babeltrace --input-format=lttng-live \
+ net://localhost/host/hostname/my-session
+----
+--
++
+Replace:
++
+--
+* `hostname` with the host name of the target system.
+* `my-session` with the name of the tracing session to view.
+--
+
+. Configure the tracing session as usual with the man:lttng(1)
+ command-line tool, and <<basic-tracing-session-control,start tracing>>.
+
+You can list the available live tracing sessions with Babeltrace:
+
+[role="term"]
+----
+$ babeltrace --input-format=lttng-live net://localhost
+----
+
+You can start the relay daemon on another system. In this case, you need
+to specify the relay daemon's URL when you create the tracing session
+with the opt:lttng-create(1):--set-url option. You also need to replace
+`localhost` in the procedure above with the host name of the system on
+which the relay daemon is running.
+
+See man:lttng-create(1) and man:lttng-relayd(8) for the complete list of
+command-line options.
+
+
+[role="since-2.3"]
+[[taking-a-snapshot]]
+=== Take a snapshot of the current sub-buffers of a tracing session
+
+The normal behavior of LTTng is to append full sub-buffers to growing
+trace data files. This is ideal to keep a full history of the events
+that occurred on the target system, but it can
+represent too much data in some situations. For example, you may wish
+to trace your application continuously until some critical situation
+happens, in which case you only need the latest few recorded
+events to perform the desired analysis, not multi-gigabyte trace files.
+
+With the man:lttng-snapshot(1) command, you can take a snapshot of the
+current sub-buffers of a given <<tracing-session,tracing session>>.
+LTTng can write the snapshot to the local file system or send it over
+the network.
+
+[role="img-100"]
+.A snapshot is a copy of the current sub-buffers, which are not cleared after the operation.
+image::snapshot.png[]
+
+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,tracing session rotation>>
+feature (available since LTTng{nbsp}2.11).
+
+To take a snapshot:
+
+. Create a tracing session in _snapshot mode_:
++
+--
+[role="term"]
+----
+$ lttng create my-session --snapshot
+----
+--
++
+The <<channel-overwrite-mode-vs-discard-mode,event record loss mode>> of
+<<channel,channels>> created in this mode is automatically set to
+_overwrite_ (flight recorder mode).
+
+. Configure the tracing session as usual with the man:lttng(1)
+ command-line tool, and <<basic-tracing-session-control,start tracing>>.
+
+. **Optional**: When you need to take a snapshot,
+ <<basic-tracing-session-control,stop tracing>>.
++
+You can take a snapshot when the tracers are active, but if you stop
+them first, you are sure that the data in the sub-buffers does not
+change before you actually take the snapshot.
+
+. Take a snapshot:
++
+--
+[role="term"]
+----
+$ lttng snapshot record --name=my-first-snapshot
+----
+--
++
+LTTng writes the current sub-buffers of all the current tracing
+session's channels to trace files on the local file system. Those trace
+files have `my-first-snapshot` in their name.
+
+There is no difference between the format of a normal trace file and the
+format of a snapshot: viewers of LTTng traces also support LTTng
+snapshots.
+
+By default, LTTng writes snapshot files to the path shown by
+`lttng snapshot list-output`. You can change this path or decide to send
+snapshots over the network using either:
+
+. An output path or URL that you specify when you
+ <<creating-destroying-tracing-sessions,create the tracing session>>.
+. A snapshot output path or URL that you add using
+ `lttng snapshot add-output`.
+. An output path or URL that you provide directly to the
+ `lttng snapshot record` command.
+
+Method{nbsp}3 overrides method{nbsp}2, which overrides method 1. When
+you specify a URL, a relay daemon must listen on a remote system (see
+<<sending-trace-data-over-the-network,Send trace data over the
+network>>).
+
+
+[role="since-2.11"]
+[[session-rotation]]
+=== Archive the current trace chunk (rotate a tracing session)
+
+The <<taking-a-snapshot,snapshot user guide>> shows how you can dump
+a tracing session's current sub-buffers to the file system or send them
+over the network. When you take a snapshot, LTTng does not clear the
+tracing session's ring buffers: 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],
+_tracing 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 tracing session's creation 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 tracing session rotation
+is called the _current trace chunk_. When this current trace chunk is
+written to the file system or sent over the network, it is called a
+_trace chunk archive_. Therefore, a tracing session rotation _archives_
+the current trace chunk.
+
+[role="img-100"]
+.A tracing session rotation operation _archives_ the current trace chunk.
+image::rotation.png[]
+
+A trace chunk archive is a self-contained LTTng trace and is not managed
+anymore by LTTng: you can read it, modify it, move it, or remove it.
+
+There are two methods to perform a tracing session rotation:
+immediately or automatically.
+
+To perform an immediate tracing session rotation:
+
+. <<creating-destroying-tracing-sessions,Create a tracing session>>
+ in _normal mode_ or _network streaming mode_
+ (only those two creation modes support tracing session rotation):
++
+--
+[role="term"]
+----
+$ lttng create my-session
+----
+--
+
+. <<enabling-disabling-events,Create one or more event rules>>
+and <<basic-tracing-session-control,start tracing>>:
++
+--
+[role="term"]
+----
+$ lttng enable-event --kernel sched_'*'
+$ lttng start
+----
+--
+
+. When needed, immediately rotate the current tracing session:
++
+--
+[role="term"]
+----
+$ lttng rotate
+----
+--
++
+The cmd:lttng-rotate command prints the path to the created trace
+chunk archive. See man:lttng-rotate(1) to learn about the format
+of trace chunk archive directory names.
++
+You can perform other immediate rotations while the tracing session is
+active. It is guaranteed that all the trace chunk archives do not
+contain overlapping trace data. You can also perform an immediate
+rotation once the tracing session is
+<<basic-tracing-session-control,stopped>>.
+
+. When you are done tracing,
+ <<creating-destroying-tracing-sessions,destroy the current tracing
+ session>>:
++
+--
+[role="term"]
+----
+$ lttng destroy
+----
+--
++
+The tracing session destruction operation creates one last trace
+chunk archive from the current trace chunk.
+
+An automatic tracing session rotation is a rotation which LTTng
+performs automatically based on one of the following conditions:
+
+* A timer with a configured period times out.
+* The total size of the flushed part of the current trace chunk
+ becomes greater than or equal to a configured value.
+
+To configure a future, automatic tracing session rotation, you need
+to set a _rotation schedule_.
+
+To set a rotation schedule:
+
+. <<creating-destroying-tracing-sessions,Create a tracing session>>
+ in _normal mode_ or _network streaming mode_
+ (only those two creation modes support tracing session rotation):
++
+--
+[role="term"]
+----
+$ lttng create my-session
+----
+--
+
+. <<enabling-disabling-events,Create one or more event rules>>:
++
+--
+[role="term"]
+----
+$ lttng enable-event --kernel sched_'*'
+----
+--
+
+. Set a tracing session rotation schedule:
++
+--
+[role="term"]
+----
+$ lttng enable-rotation --timer=12s
+----
+--
++
+In this example, we set a rotation schedule so that LTTng performs a
+tracing session rotation every 12{nbsp}seconds.
++
+See man:lttng-enable-rotation(1) to learn more about other ways to set a
+rotation schedule.
+
+. <<basic-tracing-session-control,Start tracing>>:
++
+--
+[role="term"]
+----
+$ lttng start
+----
+--
++
+LTTng performs tracing session rotations automatically while the tracing
+session is active thanks to the rotation schedule.
+
+. When you are done tracing,
+ <<creating-destroying-tracing-sessions,destroy the current tracing
+ session>>:
++
+--
+[role="term"]
+----
+$ lttng destroy
+----
+--
++
+The tracing session destruction operation creates one last trace chunk
+archive from the current trace chunk.
+
+You can use man:lttng-disable-rotation(1) to unset an a tracing session
+rotation schedule.
+
+NOTE: man:lttng-rotate(1) and man:lttng-enable-rotation(1) list
+limitations regarding those two commands.
+
+
+[role="since-2.6"]
+[[mi]]
+=== Use the machine interface
+
+With any command of the man:lttng(1) command-line tool, you can set the
+opt:lttng(1):--mi option to `xml` (before the command name) to get an
+XML machine interface output, for example:
+
+[role="term"]
+----
+$ lttng --mi=xml enable-event --kernel --syscall open
+----
+
+A schema definition (XSD) is
+https://github.com/lttng/lttng-tools/blob/stable-2.11/src/common/mi-lttng-3.0.xsd[available]
+to ease the integration with external tools as much as possible.
+
+
+[role="since-2.8"]
+[[metadata-regenerate]]
+=== Regenerate the metadata of an LTTng trace
+
+An LTTng trace, which is a http://diamon.org/ctf[CTF] trace, has both
+data stream files and a metadata file. This metadata file contains,
+amongst other things, information about the offset of the clock sources
+used to timestamp <<event,event records>> when tracing.
+
+If, once a <<tracing-session,tracing session>> is
+<<basic-tracing-session-control,started>>, a major
+https://en.wikipedia.org/wiki/Network_Time_Protocol[NTP] correction
+happens, the trace's clock offset also needs to be updated. You
+can use the `metadata` item of the man:lttng-regenerate(1) command
+to do so.
+
+The main use case of this command is to allow a system to boot with
+an incorrect wall time and trace it with LTTng before its wall time
+is corrected. Once the system is known to be in a state where its
+wall time is correct, it can run `lttng regenerate metadata`.
+
+To regenerate the metadata of an LTTng trace:
+
+* Use the `metadata` item of the man:lttng-regenerate(1) command:
++
+--
+[role="term"]
+----
+$ lttng regenerate metadata
+----
+--
+
+[IMPORTANT]
+====
+`lttng regenerate metadata` has the following limitations:
+
+* Tracing session <<creating-destroying-tracing-sessions,created>>
+ in non-live mode.
+* User space <<channel,channels>>, if any, are using
+ <<channel-buffering-schemes,per-user buffering>>.
+====
+
+
+[role="since-2.9"]
+[[regenerate-statedump]]
+=== Regenerate the state dump of a tracing session
+
+The LTTng kernel and user space tracers generate state dump
+<<event,event records>> when the application starts or when you
+<<basic-tracing-session-control,start a tracing session>>. An analysis
+can use the state dump event records to set an initial state before it
+builds the rest of the state from the following event records.
+http://tracecompass.org/[Trace Compass] is a notable example of an
+application which uses the state dump of an LTTng trace.
+
+When you <<taking-a-snapshot,take a snapshot>>, it's possible that the
+state dump event records are not included in the snapshot because they
+were recorded to a sub-buffer that has been consumed or overwritten
+already.
+
+You can use the `lttng regenerate statedump` command to emit the state
+dump event records again.
+
+To regenerate the state dump of the current tracing session, provided
+create it in snapshot mode, before you take a snapshot:
+
+. Use the `statedump` item of the man:lttng-regenerate(1) command:
++
+--
+[role="term"]
+----
+$ lttng regenerate statedump
+----
+--
+
+. <<basic-tracing-session-control,Stop the tracing session>>:
++
+--
+[role="term"]
+----
+$ lttng stop
+----
+--
+
+. <<taking-a-snapshot,Take a snapshot>>:
++
+--
+[role="term"]
+----
+$ lttng snapshot record --name=my-snapshot
+----
+--
+
+Depending on the event throughput, you should run steps 1 and 2
+as closely as possible.
+
+NOTE: To record the state dump events, you need to
+<<enabling-disabling-events,create event rules>> which enable them.
+LTTng-UST state dump tracepoints start with `lttng_ust_statedump:`.
+LTTng-modules state dump tracepoints start with `lttng_statedump_`.
+
+
+[role="since-2.7"]
+[[persistent-memory-file-systems]]
+=== Record trace data on persistent memory file systems
+
+https://en.wikipedia.org/wiki/Non-volatile_random-access_memory[Non-volatile random-access memory]
+(NVRAM) is random-access memory that retains its information when power
+is turned off (non-volatile). Systems with such memory can store data
+structures in RAM and retrieve them after a reboot, without flushing
+to typical _storage_.
+
+Linux supports NVRAM file systems thanks to either
+http://pramfs.sourceforge.net/[PRAMFS] or
+https://www.kernel.org/doc/Documentation/filesystems/dax.txt[DAX]{nbsp}+{nbsp}http://lkml.iu.edu/hypermail/linux/kernel/1504.1/03463.html[pmem]
+(requires Linux{nbsp}4.1+).
+
+This section does not describe how to operate such file systems;
+we assume that you have a working persistent memory file system.
+
+When you create a <<tracing-session,tracing session>>, you can specify
+the path of the shared memory holding the sub-buffers. If you specify a
+location on an NVRAM file system, then you can retrieve the latest
+recorded trace data when the system reboots after a crash.
+
+To record trace data on a persistent memory file system and retrieve the
+trace data after a system crash:
+
+. Create a tracing session with a sub-buffer shared memory path located
+ on an NVRAM file system:
++
+--
+[role="term"]
+----
+$ lttng create my-session --shm-path=/path/to/shm
+----
+--
+
+. Configure the tracing session as usual with the man:lttng(1)
+ command-line tool, and <<basic-tracing-session-control,start tracing>>.
+
+. After a system crash, use the man:lttng-crash(1) command-line tool to
+ view the trace data recorded on the NVRAM file system:
++
+--
+[role="term"]
+----
+$ lttng-crash /path/to/shm
+----
+--
+
+The binary layout of the ring buffer files is not exactly the same as
+the trace files layout. This is why you need to use man:lttng-crash(1)
+instead of your preferred trace viewer directly.
+
+To convert the ring buffer files to LTTng trace files:
+
+* Use the opt:lttng-crash(1):--extract option of man:lttng-crash(1):
++
+--
+[role="term"]
+----
+$ lttng-crash --extract=/path/to/trace /path/to/shm
+----
+--
+
+
+[role="since-2.10"]
+[[notif-trigger-api]]
+=== Get notified when a channel's buffer usage is too high or too low
+
+With LTTng's $$C/C++$$ notification and trigger API, your user
+application can get notified when the buffer usage of one or more
+<<channel,channels>> becomes too low or too high. You can use this API
+and enable or disable <<event,event rules>> during tracing to avoid
+<<channel-overwrite-mode-vs-discard-mode,discarded event records>>.
+
+.Have a user application get notified when an LTTng channel's buffer usage is too high.
+====
+In this example, we create and build an application which gets notified
+when the buffer usage of a specific LTTng channel is higher than
+75{nbsp}%. We only print that it is the case in the example, but we
+could as well use the API of <<liblttng-ctl-lttng,`liblttng-ctl`>> to
+disable event rules when this happens.
+
+. Create the application's C source file:
++
+--
+[source,c]
+.path:{notif-app.c}
+----
+#include <stdio.h>
+#include <assert.h>
+#include <lttng/domain.h>
+#include <lttng/action/action.h>
+#include <lttng/action/notify.h>
+#include <lttng/condition/condition.h>
+#include <lttng/condition/buffer-usage.h>
+#include <lttng/condition/evaluation.h>
+#include <lttng/notification/channel.h>
+#include <lttng/notification/notification.h>
+#include <lttng/trigger/trigger.h>
+#include <lttng/endpoint.h>
+
+int main(int argc, char *argv[])
+{
+ int exit_status = 0;
+ struct lttng_notification_channel *notification_channel;
+ struct lttng_condition *condition;
+ struct lttng_action *action;
+ struct lttng_trigger *trigger;
+ const char *tracing_session_name;
+ const char *channel_name;
+
+ assert(argc >= 3);
+ tracing_session_name = argv[1];
+ channel_name = argv[2];
+
+ /*
+ * Create a notification channel. A notification channel
+ * connects the user application to the LTTng session daemon.
+ * This notification channel can be used to listen to various
+ * types of notifications.
+ */
+ notification_channel = lttng_notification_channel_create(
+ lttng_session_daemon_notification_endpoint);
+
+ /*
+ * Create a "high buffer usage" condition. In this case, the
+ * condition is reached when the buffer usage is greater than or
+ * equal to 75 %. We create the condition for a specific tracing
+ * session name, channel name, and for the user space tracing
+ * domain.
+ *
+ * The "low buffer usage" condition type also exists.
+ */
+ condition = lttng_condition_buffer_usage_high_create();
+ lttng_condition_buffer_usage_set_threshold_ratio(condition, .75);
+ lttng_condition_buffer_usage_set_session_name(
+ condition, tracing_session_name);
+ lttng_condition_buffer_usage_set_channel_name(condition,
+ channel_name);
+ lttng_condition_buffer_usage_set_domain_type(condition,
+ LTTNG_DOMAIN_UST);
+
+ /*
+ * Create an action (get a notification) to take when the
+ * condition created above is reached.
+ */
+ action = lttng_action_notify_create();
+
+ /*
+ * Create a trigger. A trigger associates a condition to an
+ * action: the action is executed when the condition is reached.
+ */
+ trigger = lttng_trigger_create(condition, action);
+
+ /* Register the trigger to LTTng. */
+ lttng_register_trigger(trigger);
+
+ /*
+ * Now that we have registered a trigger, a notification will be
+ * emitted everytime its condition is met. To receive this
+ * notification, we must subscribe to notifications that match
+ * the same condition.
+ */
+ lttng_notification_channel_subscribe(notification_channel,
+ condition);
+
+ /*
+ * Notification loop. You can put this in a dedicated thread to
+ * avoid blocking the main thread.
+ */
+ for (;;) {
+ struct lttng_notification *notification;
+ enum lttng_notification_channel_status status;
+ const struct lttng_evaluation *notification_evaluation;
+ const struct lttng_condition *notification_condition;
+ double buffer_usage;
+
+ /* Receive the next notification. */
+ status = lttng_notification_channel_get_next_notification(
+ notification_channel, ¬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 is not consuming the
+ * notifications fast enough.
+ */
+ continue;
+ case LTTNG_NOTIFICATION_CHANNEL_STATUS_CLOSED:
+ /*
+ * The notification channel has been closed by the
+ * session daemon. This is typically caused by a session
+ * daemon shutting down.
+ */
+ goto end;
+ default:
+ /* Unhandled conditions or errors. */
+ exit_status = 1;
+ goto end;
+ }
+
+ /*
+ * A notification provides, amongst other things:
+ *
+ * * The condition that caused this notification to be
+ * emitted.
+ * * The condition evaluation, which provides more
+ * specific information on the evaluation of the
+ * condition.
+ *
+ * The condition evaluation provides the buffer usage
+ * value at the moment the condition was reached.
+ */
+ notification_condition = lttng_notification_get_condition(
+ notification);
+ notification_evaluation = lttng_notification_get_evaluation(
+ notification);
+
+ /* We're subscribed to only one condition. */
+ assert(lttng_condition_get_type(notification_condition) ==
+ LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH);
+
+ /*
+ * Get the exact sampled buffer usage from the
+ * condition evaluation.
+ */
+ lttng_evaluation_buffer_usage_get_usage_ratio(
+ notification_evaluation, &buffer_usage);
+
+ /*
+ * At this point, instead of printing a message, we
+ * could do something to reduce the channel's buffer
+ * usage, like disable specific events.
+ */
+ printf("Buffer usage is %f %% in tracing session \"%s\", "
+ "user space channel \"%s\".\n", buffer_usage * 100,
+ tracing_session_name, channel_name);
+ lttng_notification_destroy(notification);
+ }
+
+end:
+ lttng_action_destroy(action);
+ lttng_condition_destroy(condition);
+ lttng_trigger_destroy(trigger);
+ lttng_notification_channel_destroy(notification_channel);
+ return exit_status;
+}
+----
+--
+
+. Build the `notif-app` application, linking it to `liblttng-ctl`:
++
+--
+[role="term"]
+----
+$ gcc -o notif-app notif-app.c -llttng-ctl
+----
+--
+
+. <<creating-destroying-tracing-sessions,Create a tracing session>>,
+ <<enabling-disabling-events,create an event rule>> matching all the
+ user space tracepoints, and
+ <<basic-tracing-session-control,start tracing>>:
++
+--
+[role="term"]
+----
+$ lttng create my-session
+$ lttng enable-event --userspace --all
+$ lttng start
+----
+--
++
+If you create the channel manually with the man:lttng-enable-channel(1)
+command, you can control how frequently are the current values of the
+channel's properties sampled to evaluate user conditions with the
+opt:lttng-enable-channel(1):--monitor-timer option.
+
+. Run the `notif-app` application. This program accepts the
+ <<tracing-session,tracing session>> name and the user space channel
+ name as its two first arguments. The channel which LTTng automatically
+ creates with the man:lttng-enable-event(1) command above is named
+ `channel0`:
++
+--
+[role="term"]
+----
+$ ./notif-app my-session channel0
+----
+--
+
+. In another terminal, run an application with a very high event
+ throughput so that the 75{nbsp}% buffer usage condition is reached.
++
+In the first terminal, the application should print lines like this:
++
+----
+Buffer usage is 81.45197 % in tracing session "my-session", user space
+channel "channel0".
+----
++
+If you don't see anything, try modifying the condition in
+path:{notif-app.c} to a lower value (0.1, for example), rebuilding it
+(step{nbsp}2) and running 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's value.
++
+The last value of a `ctf_enum_value()` entry is its +__value__+
+parameter.
++
+The last value of a `ctf_enum_range()` entry is its +__end__+ parameter.
++
+If `ctf_enum_auto()` is the first entry in the list, its integral
+value is 0.
+
+Use the `ctf_enum()` <<lttng-modules-tp-fields,field definition macro>>
+to use a defined enumeration as a tracepoint field.
+
+.Define an enumeration with `LTTNG_TRACEPOINT_ENUM()`.
+====
+[source,c]
+----
+LTTNG_TRACEPOINT_ENUM(
+ my_enum,
+ TP_ENUM_VALUES(
+ ctf_enum_auto("AUTO: EXPECT 0")
+ ctf_enum_value("VALUE: 23", 23)
+ ctf_enum_value("VALUE: 27", 27)
+ ctf_enum_auto("AUTO: EXPECT 28")
+ ctf_enum_range("RANGE: 101 TO 303", 101, 303)
+ ctf_enum_auto("AUTO: EXPECT 304")
+ )
+)
+----
+====
+
+
+[role="since-2.7"]
+[[lttng-modules-tp-fields]]
+==== Tracepoint fields macros (for `TP_FIELDS()`)
+
+[[tp-fast-assign]][[tp-struct-entry]]The available macros to define
+tracepoint fields, which must be listed within `TP_FIELDS()` in
+`LTTNG_TRACEPOINT_EVENT()`, are:
+
+[role="func-desc growable",cols="asciidoc,asciidoc"]
+.Available macros to define LTTng-modules tracepoint fields
+|====
+|Macro |Description and parameters
+
+|
++ctf_integer(__t__, __n__, __e__)+
+
++ctf_integer_nowrite(__t__, __n__, __e__)+
+
++ctf_user_integer(__t__, __n__, __e__)+
+
++ctf_user_integer_nowrite(__t__, __n__, __e__)+
+|
+Standard integer, displayed in base{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 does not need to be null-terminated.
+
++__t__+::
+ Array element C type (always `char`).
+
++__n__+::
+ Field name.
+
++__e__+::
+ Argument expression.
+
++__s__+::
+ Number of elements.
+
+|
++ctf_sequence(__t__, __n__, __e__, __T__, __E__)+
+
++ctf_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+
+
++ctf_user_sequence(__t__, __n__, __e__, __T__, __E__)+
+
++ctf_user_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+
+|
+Dynamically-sized array of integers.
+
+The type of +__E__+ must be unsigned.
+
++__t__+::
+ Array element C type.
+
++__n__+::
+ Field name.
+
++__e__+::
+ Argument expression.
+
++__T__+::
+ Length expression C type.
+
++__E__+::
+ Length expression.
+
+|
++ctf_sequence_hex(__t__, __n__, __e__, __T__, __E__)+
+
++ctf_user_sequence_hex(__t__, __n__, __e__, __T__, __E__)+
+|
+Dynamically-sized array of integers, displayed in base{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 does not need to be null-terminated.
+
+The type of +__E__+ must be unsigned.
+
+The behaviour is undefined if +__e__+ is `NULL`.
+
++__t__+::
+ Sequence element C type (always `char`).
+
++__n__+::
+ Field name.
+
++__e__+::
+ Argument expression.
+
++__T__+::
+ Length expression C type.
+
++__E__+::
+ Length expression.
+|====
+
+Use the `_user` versions when the argument expression, `e`, is
+a user space address. In the cases of `ctf_user_integer*()` and
+`ctf_user_float*()`, `&e` must be a user space address, thus `e` must
+be addressable.
+
+The `_nowrite` versions omit themselves from the session trace, but are
+otherwise identical. This means the `_nowrite` fields won't be written
+in the recorded trace. Their primary purpose is to make some
+of the event context available to the
+<<enabling-disabling-events,event filters>> without having to
+commit the data to sub-buffers.
+
+
+[[glossary]]
+== Glossary
+
+Terms related to LTTng and to tracing in general:
+
+Babeltrace::
+ The http://diamon.org/babeltrace[Babeltrace] project, which includes
+ the cmd:babeltrace command, libraries, and Python bindings.
+
+[[def-buffering-scheme]]<<channel-buffering-schemes,buffering scheme>>::
+ A layout of <<def-sub-buffer,sub-buffers>> applied to a given channel.
+
+<<channel,channel>>::
+ An entity which is responsible for a set of
+ <<def-ring-buffer,ring buffers>>.
++
+<<event,Event rules>> are always attached to a specific channel.
+
+clock::
+ A source of time for a <<def-tracer,tracer>>.
+
+<<lttng-consumerd,consumer daemon>>::
+ A process 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 <<tracing-session,tracing session>>'s
+ <<def-sub-buffer,sub-buffers>> and the stream files produced since the
+ latest event amongst:
++
+* The creation of the tracing session.
+* The last <<session-rotation,tracing session rotation>>, if any.
+
+<<channel-overwrite-mode-vs-discard-mode,discard mode>>:: The event
+ record loss mode in which the <<def-tracer,tracer>> _discards_ new
+ event records when there's no
+ <<def-sub-buffer,sub-buffer>> space left to store them.
+
+[[def-event]]event::
+ The consequence of 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 kernel kprobe.
++
+An event is said to _occur_ at a specific time. Different actions can
+be taken upon the occurrence of an event, like record the event's payload
+to a <<def-sub-buffer,sub-buffer>>.
+
+[[def-event-name]]event name::
+ The name of an event, which is also the name of the event record.
+ This is also called the _instrumentation point name_.
+
+[[def-event-record]]event record::
+ A record, in a <<def-trace,trace>>, of the payload of an event
+ which occured.
+
+[[def-event-record-loss-mode]]<<channel-overwrite-mode-vs-discard-mode,event record loss mode>>::
+ The mechanism by which event records of a given <<channel,channel>>
+ are lost (not recorded) when there is no <<def-sub-buffer,sub-buffer>>
+ space left to store them.
+
+<<event,event rule>>::
+ Set of conditions which must be satisfied for one or more occuring
+ events to be recorded.
+
+`java.util.logging`::
+ Java platform's
+ https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[core logging facilities].
+
+<<instrumenting,instrumentation>>::
+ The use of LTTng probes to make a piece of software traceable.
+
+[[def-instrumentation-point]]instrumentation point::
+ A point in the execution path of a piece of software that, when
+ reached by this execution, can emit an <<def-event,event>>.
+
+instrumentation point name::
+ See _<<def-event-name,event name>>_.
+
+log4j::
+ A http://logging.apache.org/log4j/1.2/[logging library] for Java
+ developed by the Apache Software Foundation.
+
+log level::
+ Level of severity of a log statement or user space
+ instrumentation point.
+
+LTTng::
+ The _Linux Trace Toolkit: next generation_ project.
+
+<<lttng-cli,cmd:lttng>>::
+ A command-line tool provided by the LTTng-tools project which you
+ can use to send and receive control messages to and from a
+ session daemon.
+
+LTTng analyses::
+ The https://github.com/lttng/lttng-analyses[LTTng analyses] project,
+ which is a set of analyzing programs that are used to obtain a
+ higher level view of an LTTng <<def-trace,trace>>.
+
+cmd:lttng-consumerd::
+ The name of the consumer daemon program.
+
+cmd:lttng-crash::
+ A utility provided by the LTTng-tools project which can convert
+ <<def-ring-buffer,ring buffer>> files (usually
+ <<persistent-memory-file-systems,saved on a persistent memory file system>>)
+ to <<def-trace,trace>> files.
+
+LTTng Documentation::
+ This document.
+
+<<lttng-live,LTTng live>>::
+ A communication protocol between the <<lttng-relayd,relay daemon>> and
+ live viewers which makes it possible to see <<def-event-record,event
+ records>> "live", as they are received by the relay daemon.
+
+<<lttng-modules,LTTng-modules>>::
+ The https://github.com/lttng/lttng-modules[LTTng-modules] project,
+ which contains the Linux kernel modules to make the Linux kernel
+ <<def-instrumentation-point,instrumentation points>> available for
+ LTTng tracing.
+
+cmd:lttng-relayd::
+ The name of the <<lttng-relayd,relay daemon>> program.
+
+cmd:lttng-sessiond::
+ The name of the <<lttng-sessiond,session daemon>> program.
+
+LTTng-tools::
+ The https://github.com/lttng/lttng-tools[LTTng-tools] project, which
+ contains the various programs and libraries used to
+ <<controlling-tracing,control tracing>>.
+
+<<lttng-ust,LTTng-UST>>::
+ The https://github.com/lttng/lttng-ust[LTTng-UST] project, which
+ contains libraries to instrument
+ <<def-user-application,user applications>>.
+
+<<lttng-ust-agents,LTTng-UST Java agent>>::
+ A Java package provided by the LTTng-UST project to allow the
+ LTTng instrumentation of `java.util.logging` and Apache log4j{nbsp}1.2
+ logging statements.
+
+<<lttng-ust-agents,LTTng-UST Python agent>>::
+ A Python package provided by the LTTng-UST project to allow the
+ LTTng instrumentation of Python logging statements.
+
+<<channel-overwrite-mode-vs-discard-mode,overwrite mode>>::
+ The <<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 <<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 <<channel,channel>>.
+
+<<lttng-relayd,relay daemon>>::
+ A process which is responsible for receiving the <<def-trace,trace>>
+ data sent by a distant <<lttng-consumerd,consumer daemon>>.
+
+[[def-ring-buffer]]ring buffer::
+ A set of <<def-sub-buffer,sub-buffers>>.
+
+rotation::
+ See _<<def-tracing-session-rotation,tracing session rotation>>_.
+
+<<lttng-sessiond,session daemon>>::
+ A process which receives control commands from you and orchestrates
+ the <<def-tracer,tracers>> and various LTTng daemons.
+
+<<taking-a-snapshot,snapshot>>::
+ A copy of the current data of all the <<def-sub-buffer,sub-buffers>>
+ of a given <<tracing-session,tracing session>>, saved as
+ <<def-trace,trace>> files.
+
+[[def-sub-buffer]]sub-buffer::
+ One part of an LTTng <<def-ring-buffer,ring buffer>> which contains
+ <<def-event-record,event records>>.
+
+timestamp::
+ The time information attached to an
+ <<def-event,event>> when it is emitted.
+
+[[def-trace]]trace (_noun_)::
+ A set of:
++
+* One http://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>>.
+
+trace (_verb_)::
+ The action of recording the <<def-event,events>> emitted by an
+ application or by a system, or to initiate such recording by
+ controlling a tracer.
+
+[[def-trace-chunk]]trace chunk::
+ A self-contained trace which is part of a <<tracing-session,tracing
+ session>>. Each <<session-rotation, tracing session rotation>>
+ produces a trace chunk archive.
+
+trace chunk archive::
+ The result of a <<session-rotation, tracing session rotation>>. A
+ trace chunk archive is not managed by LTTng, even if its containing
+ <<tracing-session,tracing 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 LTTng-UST.
+
+tracepoint definition::
+ The definition of a single tracepoint.
+
+tracepoint name::
+ The name of a tracepoint.
+
+tracepoint provider::
+ A set of functions providing tracepoints to an instrumented user
+ application.
++
+Not to be confused with a _tracepoint provider package_: many tracepoint
+providers can exist within a tracepoint provider package.
+
+tracepoint provider package::
+ One or more tracepoint providers compiled as an
+ 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 software which records emitted <<def-event,events>>.
+
+<<domain,tracing domain>>::
+ A namespace for <<def-event,event>> sources.
+
+<<tracing-group,tracing group>>::
+ The Unix group in which a Unix user can be to be allowed to trace the
+ Linux kernel.
+
+[[def-tracing-session-rotation]]<<tracing-session,tracing session>>::
+ A stateful dialogue between you and a <<lttng-sessiond,session
+ daemon>>.
+
+<<session-rotation,tracing session rotation>>::
+ The action of archiving the
+ <<def-current-trace-chunk,current trace chunk>> of a
+ <<tracing-session,tracing session>>.
+
+[[def-user-application]]user application::
+ An application running in user space, as opposed to a Linux kernel
+ module, for example.
projects / lttng-docs.git / commitdiff
