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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | ENVIRONMENT VARIABLES | FILES | EXIT STATUS | BUGS | RESOURCES | COPYRIGHTS | THANKS | AUTHORS | SEE ALSO | COLOPHON |
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LTTNG-ENABLE-EVENT(1) LTTng Manual LTTNG-ENABLE-EVENT(1)
lttng-enable-event - Create or enable LTTng event rules
Create or enable Linux kernel event rules:
lttng [GENERAL OPTIONS] enable-event --kernel
[--probe=SOURCE | --function=SOURCE | --syscall]
[--filter=EXPR] [--session=SESSION]
[--channel=CHANNEL] EVENT[,EVENT]...
Create or enable an "all" Linux kernel event rule:
lttng [GENERAL OPTIONS] enable-event --kernel --all [--syscall]
[--filter=EXPR] [--session=SESSION] [--channel=CHANNEL]
Create or enable application event rules:
lttng [GENERAL OPTIONS] enable-event
(--userspace | --jul | --log4j | --python)
[--filter=EXPR] [--exclude=EVENT[,EVENT]...]
[--loglevel=LOGLEVEL | --loglevel-only=LOGLEVEL]
[--session=SESSION] [--channel=CHANNEL] (--all | EVENT[,EVENT]...)
The lttng enable-event command can create a new event rule, or
enable one or more existing and disabled ones.
An event rule created by lttng enable-event is a set of conditions
that must be satisfied in order for an actual event to be emitted
by an LTTng tracer when the execution of an application or the
Linux kernel reaches an event source (tracepoint, system call,
dynamic probe). Event sources can be listed with the lttng-list(1)
command.
The lttng-disable-event(1) command can be used to disable existing
event rules.
Event rules are always assigned to a channel when they are
created. If the --channel option is omitted, a default channel
named channel0 is used (and created automatically if it does not
exist for the specified domain in the selected tracing session).
If the --session option is omitted, the chosen channel is picked
from the current tracing session.
Events can be enabled while tracing is active (use lttng-start(1)
to make a tracing session active).
Event source types
Four types of event sources are available in the Linux kernel
tracing domain (--kernel option):
Tracepoint (--tracepoint option; default)
A Linux kernel tracepoint, that is, a static instrumentation
point placed in the kernel source code. Standard tracepoints
are designed and placed in the source code by developers and
record useful payload fields.
Dynamic probe (--probe option)
A Linux kernel kprobe, that is, an instrumentation point
placed dynamically in the compiled kernel code. Dynamic probe
events do not record any payload field.
Function probe (--function option)
A Linux kernel kretprobe, that is, two instrumentation points
placed dynamically where a function is entered and where it
returns in the compiled kernel code. Function probe events do
not record any payload field.
System call (--syscall option)
A Linux kernel system call. Two instrumentation points are
statically placed where a system call function is entered and
where it returns in the compiled kernel code. System call
event sources record useful payload fields.
The application tracing domains (--userspace, --jul, --log4j, or
--python options) only support tracepoints. In the cases of the
JUL, Apache log4j, and Python domains, the event names correspond
to logger names.
Understanding event rule conditions
When creating an event rule with lttng enable-event, conditions
are specified using options. The logical conjunction (logical AND)
of all those conditions must be true when an event source is
reached by an application or by the Linux kernel in order for an
actual event to be emitted by an LTTng tracer.
Any condition that is not explicitly specified on creation is
considered a don’t care.
For example, consider the following commands:
$ lttng enable-event --userspace hello:world
$ lttng enable-event --userspace hello:world --loglevel=TRACE_INFO
Here, two event rules are created. The first one has a single
condition: the tracepoint name must match hello:world. The second
one has two conditions:
• The tracepoint name must match hello:world, and
• The tracepoint’s defined log level must be at least as severe
as the TRACE_INFO level.
In this case, the second event rule is pointless because the first
one is more general: it does not care about the tracepoint’s log
level. If an event source matching both event rules is reached by
the application’s execution, only one event is emitted.
The available conditions for the Linux kernel domain are:
• Tracepoint/system call name (EVENT argument with --tracepoint
or --syscall options) or dynamic probe/function name/address
(--probe or --function option’s argument) which must match
event source’s equivalent.
You can use * characters at any place in the tracepoint or
system call name as wildcards to match zero or more
characters. To use a literal * character, use \*.
• Filter expression (--filter option) executed against the
dynamic values of event fields at execution time that must
evaluate to true. See the Filter expression section below for
more information.
The available conditions for the application domains are:
• Tracepoint name (EVENT with --tracepoint option) which must
match event source’s equivalent.
You can use * characters at any place in the tracepoint name
as wildcards to match zero or more characters. To use a
literal * character, use \*. When you create an event rule
with a tracepoint name containing a wildcard, you can exclude
specific tracepoint names from the match with the --exclude
option.
• Filter expression (--filter option) executed against the
dynamic values of event fields at execution time that must
evaluate to true. See the Filter expression section below for
more information.
• Event’s log level that must be at least as severe as a given
log level (--loglevel option) or match exactly a given log
level (--loglevel-only option).
When using lttng enable-event with a set of conditions that does
not currently exist for the chosen tracing session, domain, and
channel, a new event rule is created. Otherwise, the existing
event rule is enabled if it is currently disabled (see
lttng-disable-event(1)).
The --all option can be used alongside the --tracepoint or
--syscall options. When this option is used, no EVENT argument
must be specified. This option defines a single event rule
matching all the possible events of a given tracing domain for the
chosen channel and tracing session. It is the equivalent of an
EVENT argument named * (wildcard).
Filter expression
A filter expression can be specified with the --filter option when
creating a new event rule. If the filter expression evaluates to
true when executed against the dynamic values of an event’s fields
when tracing, the filtering condition passes.
Note
Make sure to single-quote the filter expression when running
the command from a shell, as filter expressions typically
include characters having a special meaning for most shells.
The filter expression syntax is similar to C language conditional
expressions (expressions that can be evaluated by an if
statement), albeit with a few differences:
• C integer and floating point number constants are supported,
as well as literal strings between double quotes ("). You can
use * characters at any place in a literal string as wildcards
to match zero or more characters. To use a literal *
character, use \*.
Examples: 32, -0x17, 0755, 12.34, "a \"literal string\"",
"src/*/*.h".
• The dynamic value of an event field is read by using its name
as a C identifier.
The dot and square bracket notations are available, like in
the C language, to access nested structure and array/sequence
fields. Only a constant, positive integer number can be used
within square brackets. If the index is out of bounds, the
whole filter expression evaluates to false (the event is
discarded).
An enumeration field’s value is an integer.
When the expression’s field does not exist, the whole filter
expression evaluates to false.
Examples: my_field, target_cpu, seq[7],
msg.user[1].data[2][17].
• The dynamic value of a statically-known context field is read
by prefixing its name with $ctx.. Statically-known context
fields are context fields added to channels without the $app.
prefix using the lttng-add-context(1) command.
When the expression’s statically-known context field does not
exist, the whole filter expression evaluates to false.
Examples: $ctx.prio, $ctx.preemptible, $ctx.perf:cpu:stalled-
cycles-frontend.
• The dynamic value of an application-specific context field is
read by prefixing its name with $app. (follows the format
used to add such a context field with the lttng-add-context(1)
command).
When the expression’s application-specific context field does
not exist, the whole filter expression evaluates to false.
Example: $app.server:cur_user.
The following precedence table shows the operators which are
supported in a filter expression. In this table, the highest
precedence is 1. Parentheses are supported to bypass the default
order.
Important
Unlike the C language, the lttng enable-event filter
expression syntax’s bitwise AND and OR operators (& and |)
take precedence over relational operators (<, <=, >, >=, ==,
and !=). This means the filter expression 2 & 2 == 2 is true
while the equivalent C expression is false.
┌────────────┬──────────┬─────────────────┬───────────────┐
│ Precedence │ Operator │ Description │ Associativity │
├────────────┼──────────┼─────────────────┼───────────────┤
│ │ │ │ │
│ 1 │ - │ Unary minus │ Right-to-left │
├────────────┼──────────┼─────────────────┼───────────────┤
│ │ │ │ │
│ 1 │ + │ Unary plus │ Right-to-left │
├────────────┼──────────┼─────────────────┼───────────────┤
│ │ │ │ │
│ 1 │ ! │ Logical NOT │ Right-to-left │
├────────────┼──────────┼─────────────────┼───────────────┤
│ │ │ │ │
│ 1 │ ~ │ Bitwise NOT │ Right-to-left │
├────────────┼──────────┼─────────────────┼───────────────┤
│ │ │ │ │
│ 2 │ << │ Bitwise left │ Left-to-right │
│ │ │ shift │ │
├────────────┼──────────┼─────────────────┼───────────────┤
│ │ │ │ │
│ 2 │ >> │ Bitwise right │ Left-to-right │
│ │ │ shift │ │
├────────────┼──────────┼─────────────────┼───────────────┤
│ │ │ │ │
│ 3 │ & │ Bitwise AND │ Left-to-right │
├────────────┼──────────┼─────────────────┼───────────────┤
│ │ │ │ │
│ 4 │ ^ │ Bitwise XOR │ Left-to-right │
├────────────┼──────────┼─────────────────┼───────────────┤
│ │ │ │ │
│ 5 │ | │ Bitwise OR │ Left-to-right │
├────────────┼──────────┼─────────────────┼───────────────┤
│ │ │ │ │
│ 6 │ < │ Less than │ Left-to-right │
├────────────┼──────────┼─────────────────┼───────────────┤
│ │ │ │ │
│ 6 │ <= │ Less than or │ Left-to-right │
│ │ │ equal to │ │
├────────────┼──────────┼─────────────────┼───────────────┤
│ │ │ │ │
│ 6 │ > │ Greater than │ Left-to-right │
├────────────┼──────────┼─────────────────┼───────────────┤
│ │ │ │ │
│ 6 │ >= │ Greater than or │ Left-to-right │
│ │ │ equal to │ │
├────────────┼──────────┼─────────────────┼───────────────┤
│ │ │ │ │
│ 7 │ == │ Equal to │ Left-to-right │
├────────────┼──────────┼─────────────────┼───────────────┤
│ │ │ │ │
│ 7 │ != │ Not equal to │ Left-to-right │
├────────────┼──────────┼─────────────────┼───────────────┤
│ │ │ │ │
│ 8 │ && │ Logical AND │ Left-to-right │
├────────────┼──────────┼─────────────────┼───────────────┤
│ │ │ │ │
│ 9 │ || │ Logical OR │ Left-to-right │
└────────────┴──────────┴─────────────────┴───────────────┘
The arithmetic operators are NOT supported.
All integer constants and fields are first casted to signed 64-bit
integers. The representation of negative integers is two’s
complement. This means that, for example, the signed 8-bit integer
field 0xff (-1) becomes 0xffffffffffffffff (still -1) once casted.
Before a bitwise operator is applied, all its operands are casted
to unsigned 64-bit integers, and the result is casted back to a
signed 64-bit integer. For the bitwise NOT operator, it is the
equivalent of this C expression:
(int64_t) ~((uint64_t) val)
For the binary bitwise operators, it is the equivalent of those C
expressions:
(int64_t) ((uint64_t) lhs >> (uint64_t) rhs)
(int64_t) ((uint64_t) lhs << (uint64_t) rhs)
(int64_t) ((uint64_t) lhs & (uint64_t) rhs)
(int64_t) ((uint64_t) lhs ^ (uint64_t) rhs)
(int64_t) ((uint64_t) lhs | (uint64_t) rhs)
If the right-hand side of a bitwise shift operator (<< and >>) is
not in the [0, 63] range, the whole filter expression evaluates to
false.
Note
Although it is possible to filter the process ID of an event
when the pid context has been added to its channel using, for
example, $ctx.pid == 2832, it is recommended to use the PID
tracker instead, which is much more efficient (see
lttng-track(1)).
Filter expression examples:
msg_id == 23 && size >= 2048
$ctx.procname == "lttng*" && (!flag || poel < 34)
$app.my_provider:my_context == 17.34e9 || some_enum >= 14
$ctx.cpu_id == 2 && filename != "*.log"
eax_reg & 0xff7 == 0x240 && x[4] >> 12 <= 0x1234
Log levels
Tracepoints and log statements in applications have an attached
log level. Application event rules can contain a log level
condition.
With the --loglevel option, the event source’s log level must be
at least as severe as the option’s argument. With the --loglevel-
only option, the event source’s log level must match the option’s
argument.
The available log levels are:
User space domain (--userspace option)
Shortcuts such as system are allowed.
• TRACE_EMERG (0)
• TRACE_ALERT (1)
• TRACE_CRIT (2)
• TRACE_ERR (3)
• TRACE_WARNING (4)
• TRACE_NOTICE (5)
• TRACE_INFO (6)
• TRACE_DEBUG_SYSTEM (7)
• TRACE_DEBUG_PROGRAM (8)
• TRACE_DEBUG_PROCESS (9)
• TRACE_DEBUG_MODULE (10)
• TRACE_DEBUG_UNIT (11)
• TRACE_DEBUG_FUNCTION (12)
• TRACE_DEBUG_LINE (13)
• TRACE_DEBUG (14)
java.util.logging domain (--jul option)
Shortcuts such as severe are allowed.
• JUL_OFF (INT32_MAX)
• JUL_SEVERE (1000)
• JUL_WARNING (900)
• JUL_INFO (800)
• JUL_CONFIG (700)
• JUL_FINE (500)
• JUL_FINER (400)
• JUL_FINEST (300)
• JUL_ALL (INT32_MIN)
Apache log4j domain (--log4j option)
Shortcuts such as severe are allowed.
• LOG4J_OFF (INT32_MAX)
• LOG4J_FATAL (50000)
• LOG4J_ERROR (40000)
• LOG4J_WARN (30000)
• LOG4J_INFO (20000)
• LOG4J_DEBUG (10000)
• LOG4J_TRACE (5000)
• LOG4J_ALL (INT32_MIN)
Python domain (--python option)
Shortcuts such as critical are allowed.
• PYTHON_CRITICAL (50)
• PYTHON_ERROR (40)
• PYTHON_WARNING (30)
• PYTHON_INFO (20)
• PYTHON_DEBUG (10)
• PYTHON_NOTSET (0)
General options are described in lttng(1).
Domain
One of:
-j, --jul
Create or enable event rules in the java.util.logging (JUL)
domain.
-k, --kernel
Create or enable event rules in the Linux kernel domain.
-l, --log4j
Create or enable event rules in the Apache log4j domain.
-p, --python
Create or enable event rules in the Python domain.
-u, --userspace
Create or enable event rules in the user space domain.
Target
-c CHANNEL, --channel=CHANNEL
Create or enable event rules in the channel named CHANNEL
instead of the default channel name channel0.
-s SESSION, --session=SESSION
Create or enable event rules in the tracing session named
SESSION instead of the current tracing session.
Event source type
One of:
--function=SOURCE
Linux kernel kretprobe. Only available with the --kernel
domain option. SOURCE is one of:
• Function address (0x prefix supported)
• Function symbol
• Function symbol and offset (SYMBOL+OFFSET format)
--probe=SOURCE
Linux kernel kprobe. Only available with the --kernel domain
option. SOURCE is one of:
• Address (0x prefix supported)
• Symbol
• Symbol and offset (SYMBOL+OFFSET format)
--syscall
Linux kernel system call. Only available with the --kernel
domain option.
--tracepoint
Linux kernel or application tracepoint (default).
Log level
One of:
--loglevel=LOGLEVEL
Add log level condition to the event rule: the event source’s
defined log level must be at least as severe as LOGLEVEL. See
the Log levels section above for the available log levels.
Only available with application domains.
--loglevel-only=LOGLEVEL
Add log level condition to the event rule: the event source’s
defined log level must match LOGLEVEL. See the Log levels
section above for the available log levels. Only available
with application domains.
Filtering and exclusion
-x EVENT[,EVENT]..., --exclude=EVENT[,EVENT]...
Exclude events named EVENT from the event rule. This option
can be used when the command’s EVENT argument contains at
least one wildcard star (*) to exclude specific names. EVENT
can also contain wildcard stars. To use a literal , character,
use \,. Only available with the --userspace domain.
-f EXPR, --filter=EXPR
Add filter expression condition to the event rule. Expression
EXPR must evaluate to true when executed against the dynamic
values of event fields. See the Filter expression section
above for more information.
Shortcuts
-a, --all
Equivalent to an EVENT argument named * (wildcard) when also
using the --tracepoint (default) or --syscall option.
Program information
-h, --help
Show command help.
This option, like lttng-help(1), attempts to launch
/usr/bin/man to view the command’s man page. The path to the
man pager can be overridden by the LTTNG_MAN_BIN_PATH
environment variable.
--list-options
List available command options.
LTTNG_ABORT_ON_ERROR
Set to 1 to abort the process after the first error is
encountered.
LTTNG_HOME
Overrides the $HOME environment variable. Useful when the user
running the commands has a non-writable home directory.
LTTNG_MAN_BIN_PATH
Absolute path to the man pager to use for viewing help
information about LTTng commands (using lttng-help(1) or lttng
COMMAND --help).
LTTNG_SESSION_CONFIG_XSD_PATH
Path in which the session.xsd session configuration XML schema
may be found.
LTTNG_SESSIOND_PATH
Full session daemon binary path.
The --sessiond-path option has precedence over this
environment variable.
Note that the lttng-create(1) command can spawn an LTTng session
daemon automatically if none is running. See lttng-sessiond(8) for
the environment variables influencing the execution of the session
daemon.
$LTTNG_HOME/.lttngrc
User LTTng runtime configuration.
This is where the per-user current tracing session is stored
between executions of lttng(1). The current tracing session
can be set with lttng-set-session(1). See lttng-create(1) for
more information about tracing sessions.
$LTTNG_HOME/lttng-traces
Default output directory of LTTng traces. This can be
overridden with the --output option of the lttng-create(1)
command.
$LTTNG_HOME/.lttng
User LTTng runtime and configuration directory.
$LTTNG_HOME/.lttng/sessions
Default location of saved user tracing sessions (see
lttng-save(1) and lttng-load(1)).
/usr/local/etc/lttng/sessions
System-wide location of saved tracing sessions (see
lttng-save(1) and lttng-load(1)).
Note
$LTTNG_HOME defaults to $HOME when not explicitly set.
0
Success
1
Command error
2
Undefined command
3
Fatal error
4
Command warning (something went wrong during the command)
If you encounter any issue or usability problem, please report it
on the LTTng bug tracker <https://bugs.lttng.org/projects/lttng-
tools>.
• LTTng project website <https://lttng.org>
• LTTng documentation <https://lttng.org/docs>
• Git repositories <http://git.lttng.org>
• GitHub organization <http://github.com/lttng>
• Continuous integration <http://ci.lttng.org/>
• Mailing list <http://lists.lttng.org> for support and
development: lttng-dev@lists.lttng.org
• IRC channel <irc://irc.oftc.net/lttng>: #lttng on irc.oftc.net
This program is part of the LTTng-tools project.
LTTng-tools is distributed under the GNU General Public License
version 2 <http://www.gnu.org/licenses/old-
licenses/gpl-2.0.en.html>. See the LICENSE
<https://github.com/lttng/lttng-tools/blob/master/LICENSE> file
for details.
Special thanks to Michel Dagenais and the DORSAL laboratory
<http://www.dorsal.polymtl.ca/> at École Polytechnique de Montréal
for the LTTng journey.
Also thanks to the Ericsson teams working on tracing which helped
us greatly with detailed bug reports and unusual test cases.
LTTng-tools was originally written by Mathieu Desnoyers, Julien
Desfossez, and David Goulet. More people have since contributed to
it.
LTTng-tools is currently maintained by Jérémie Galarneau
<mailto:jeremie.galarneau@efficios.com>.
lttng-disable-event(1), lttng(1)
This page is part of the LTTng-Tools ( LTTng tools) project.
Information about the project can be found at ⟨http://lttng.org/⟩.
It is not known how to report bugs for this man page; if you know,
please send a mail to man-pages@man7.org. This page was obtained
from the project's upstream Git repository
⟨git://git.lttng.org/lttng-tools.git⟩ on 2019-11-19. (At that
time, the date of the most recent commit that was found in the
repository was 2019-11-14.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
LTTng 2.12.0-pre 10/29/2018 LTTNG-ENABLE-EVENT(1)
Pages that refer to this page: lttng(1), lttng-disable-event(1), lttng-enable-channel(1), lttng-track(1), lttng-ust(3), lttng_ust_tracef(3), lttng_ust_tracelog(3), babeltrace2-filter.lttng-utils.debug-info(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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