lttng-sessiond(8)
=================
:revdate: 7 October 2025
:daemon-bin-name: lttng-sessiond
:daemon-ini-section: sessiond


NAME
----
lttng-sessiond - LTTng session daemon


SYNOPSIS
--------
[verse]
*lttng-sessiond* [option:--background | option:--daemonize] [option:--sig-parent]
               [option:--config='PATH'] [option:--group='GROUP'] [option:--load='PATH']
               [option:--default-trace-format=(**ctf-1.8** | **ctf-2**)]
               [option:--agent-tcp-port='PORT'] [option:--pidfile='PATH']
               [option:--apps-sock='PATH'] [option:--client-sock='PATH']
               [option:--no-kernel | [option:--kmod-probes='PROBE'[,'PROBE']...]
                              [option:--extra-kmod-probes='PROBE'[,'PROBE']...]
                              [option:--kconsumerd-err-sock='PATH']
                              [option:--kconsumerd-cmd-sock='PATH']]
               [option:--ustconsumerd32-err-sock='PATH']
               [option:--ustconsumerd64-err-sock='PATH']
               [option:--ustconsumerd32-cmd-sock='PATH']
               [option:--ustconsumerd64-cmd-sock='PATH']
               [option:--consumerd32-path='PATH'] [option:--consumerd32-libdir='PATH']
               [option:--consumerd64-path='PATH'] [option:--consumerd64-libdir='PATH']
               [option:--event-notifier-error-buffer-size-kernel='SLOTS']
               [option:--event-notifier-error-buffer-size-userspace='SLOTS']
               [option:--quiet | [option:--verbose]... [option:--verbose-consumer]]


DESCRIPTION
-----------
include::common-intro.txt[]

An LTTng session daemon, `lttng-sessiond`, is a program which:

* Manages recording sessions (see man:lttng-concepts(7) to learn more
  about recording sessions).

* Controls the various components (like tracers and consumer daemons) of
  LTTng.

* Sends asynchronous notifications to user applications.

A session daemon receives commands from the man:lttng(1) command-line
tool, as well as from any user application linked with the LTTng control
library (`liblttng-ctl`).

Each Unix user may have its own independent running session daemon.
However, the man:lttng(1) tool must connect to the session daemon of the
`root` user (the root session daemon) to control Linux kernel tracing.

When you start `lttng-sessiond` as the `root` Unix user, a non-root Unix
user can connect to it if it's part of the Unix tracing group. By
default, the name of the tracing group is `tracing`. Override the
tracing group name with the option:--group option.

See the ``Session daemon connection'' section of man:lttng(1) to learn
how a user application connects to a session daemon.

A session daemon manages trace data consumer daemons, spawning them when
necessary. You do :not: need to manage the consumer daemons yourself.

By default, `lttng-sessiond` doesn't start as a daemon. Make it a daemon
with the option:--daemonize or option:--background option. With those
options, `lttng-sessiond` ensures the daemon is ready to receive client
commands before it exits. Make `lttng-sessiond` write its process ID
to a file with the option:--pidfile option.

NOTE: The LTTng project recommends that you start the session daemon at
boot time for stable and long-term tracing.

[NOTE]
====
For an unprivileged Unix user running `lttng-sessiond`, the maximum
number of file descriptors per process is usually 1024. This limits the
number of traceable applications, since, for each instrumented
application, there are two file descriptors per CPU as well as one
socket for bidirectional communication.

For the `root` user, the limit is usually 65,535.
====


include::common-daemon-cfg.txt[]

INI configuration file example:

[source,ini]
----
[sessiond]
daemonize=yes
default-trace-format=ctf-1.8
extra-kmod-probes=my-driver,other-module
----


[[load]]
Recording session configuration loading
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When the session daemon starts, it loads recording session
configurations from:

Without the option:--load option::
    In this order:
+
--
. All the files in `$LTTNG_HOME/.lttng/sessions/auto`.
+
`$LTTNG_HOME` defaults to `$HOME`.

. All the files in +{system_sessions_auto_dir}+.
--
+
`lttng-sessiond` only loads recording session configuration files from
the directories above if its UID and their UID are the same.

With the option:--load='PATH' option::
    'PATH' is a directory:::
        All the files in 'PATH'.

    'PATH' is a file:::
        The file 'PATH'.


[[options]]
OPTIONS
-------
General daemon configuration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
option:-b, option:--background::
    Start as a Unix daemon, but keep file descriptors (console) open.
+
With this option, `lttng-sessiond` ensures the daemon is ready to
receive client commands before it exits.
+
Use the option:--daemonize option instead to close the file descriptors.

option:-f 'PATH', option:--config='PATH'::
    Configure the daemon using the INI configuration file 'PATH' in
    addition to the default configuration files and the command-line
    options.
+
See the ``<<cfg,Daemon configuration>>'' section above.

option:-d, option:--daemonize::
    Start as a Unix daemon and close file descriptors (console).
+
With this option, `lttng-sessiond` ensures the daemon is ready to
receive client commands before it exits.
+
Use the option:--background option instead to keep the file descriptors
open.

option:--default-trace-format='FORMAT'::
    Set the default trace format of recording sessions to 'FORMAT'.
+
'FORMAT' is one of:
+
--
`ctf-1.8`:::
    https://diamon.org/ctf/v1.8.3/[CTF 1.8] (legacy).

`ctf-2`:::
    https://diamon.org/ctf/[CTF 2] (preferred).
--
+
When a user application creates a recording session using the
man:lttng-create(1) command or the liblttng-ctl API without explicitly
specifying a trace format, `lttng-sessiond` uses 'FORMAT'.
+
Default: `ctf-2`.
+
See also the `LTTNG_SESSIOND_DEFAULT_TRACE_FORMAT` environment variable.

option:-g 'GROUP', option:--group='GROUP'::
    Set the Unix tracing group to 'GROUP' instead of `tracing`.
+
This option is only meaningful when the `root` Unix user starts
`lttng-sessiond`.
+
Members of the Unix tracing group may connect to the root session daemon
and, therefore, control LTTng kernel tracing.

option:-l 'PATH', option:--load='PATH'::
    Load recording session configurations from 'PATH', either a
    directory or a file, instead of loading them from the default search
    directories.
+
See the ``<<load,Recording session configuration loading>>'' section
above.

option:-p 'PATH', option:--pidfile='PATH'::
    Write the process ID (PID) of the `lttng-sessiond`
    process to 'PATH'.
+
Such a file contains the textual PID (decimal) followed with a newline.

option:-S, option:--sig-parent::
    Send the `USR1` signal to the parent process to notify readiness.
+
You can also use the option:--daemonize or option:--background option,
in which case `lttng-sessiond` ensures the daemon is ready to receive
client commands before it exits.


Linux kernel tracing
~~~~~~~~~~~~~~~~~~~~
At most one of:

option:--extra-kmod-probes='PROBE'[,'PROBE']...::
    For each 'PROBE' argument, load the LTTng kernel probe module
    named ++lttng-probe-++__PROBE__++.ko++, in addition to loading the
    default LTTng kernel probe modules.
+
See also the `LTTNG_EXTRA_KMOD_PROBES` environment variable.

option:--kmod-probes='PROBE'[,'PROBE']...::
    Only load, for each 'PROBE' argument, the LTTng kernel probe
    module named ++lttng-probe-++__PROBE__++.ko++, instead of loading
    the default LTTng kernel probe modules.
+
See also the `LTTNG_KMOD_PROBES` environment variable.

option:--no-kernel::
    Disable Linux kernel tracing.


Paths and ports
~~~~~~~~~~~~~~~
option:--agent-tcp-port='PORT'::
    Listen on TCP port 'PORT' for agent application registration
    instead of a port within the range
    [{default_agent_tcp_port_range_begin},{nbsp}{default_agent_tcp_port_range_end}]).

option:-a 'PATH', option:--apps-sock='PATH'::
    Set the application Unix socket path to 'PATH'.
+
This option sets the application Unix socket path even if you set
the `LTTNG_UST_CTL_PATH` environment variable.

option:-c 'PATH', option:--client-sock='PATH'::
    Set the client Unix socket path to 'PATH'.

option:--consumerd32-libdir='PATH'::
    Set the 32-bit consumer daemon library directory to 'PATH'.
+
See also the `LTTNG_CONSUMERD32_LIBDIR` environment variable.

option:--consumerd32-path='PATH'::
    Set the 32-bit consumer daemon binary path to 'PATH'.
+
See also the `LTTNG_CONSUMERD32_BIN` environment variable.

option:--consumerd64-libdir='PATH'::
    Set the 64-bit consumer daemon library directory to 'PATH'.
+
See also the `LTTNG_CONSUMERD64_LIBDIR` environment variable.

option:--consumerd64-path='PATH'::
    Set the 64-bit consumer daemon binary path to 'PATH'.
+
See also the `LTTNG_CONSUMERD64_BIN` environment variable.

option:--kconsumerd-cmd-sock='PATH'::
    Set the command Unix socket path of the Linux kernel consumer daemon
    to 'PATH'.

option:--kconsumerd-err-sock='PATH'::
    Set the error Unix socket path of the Linux kernel consumer daemon
    to 'PATH'.

option:--ustconsumerd32-cmd-sock='PATH'::
    Set the Unix socket path of the 32-bit consumer daemon command to
    'PATH'.

option:--ustconsumerd64-cmd-sock='PATH'::
    Set the Unix socket path of the 64-bit consumer daemon command to
    'PATH'.

option:--ustconsumerd32-err-sock='PATH'::
    Set the Unix socket path of the 32-bit consumer daemon error to
    'PATH'.

option:--ustconsumerd64-err-sock='PATH'::
    Set the Unix socket path of the 64-bit consumer daemon error to
    'PATH'.


Buffer size of event notifier error counters
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
option:--event-notifier-error-buffer-size-kernel='SLOTS'::
    Set the size of the kernel event notifier error counter buffers to
    'SLOTS'{nbsp}slots.

option:--event-notifier-error-buffer-size-userspace='SLOTS'::
    Set the size of the user space event notifier error counter buffers
    to 'SLOTS'{nbsp}slots.

As of LTTng{nbsp}{lttng_version}, a _slot_ is a 32-bit counter, but this
may change in the future.


Verbosity
~~~~~~~~~
option:-q, option:--quiet::
    Suppress all messages, including warnings and errors.
+
You may :not: use this option with the option:--verbose and
option:--verbose-consumer options.

option:-v, option:--verbose::
    Increase verbosity.
+
Specify this option up to three times to get more levels of verbosity.
+
You may :not: use this option with the option:--quiet option.

option:--verbose-consumer::
    Increase the verbosity of the consumer daemons which this session
    daemon spawns.
+
You may :not: use this option with the option:--quiet option.


Program information
~~~~~~~~~~~~~~~~~~~
include::common-help-option.txt[]

option:-V, option:--version::
    Show version and quit.


EXIT STATUS
-----------
*0*::
    Success

*1*::
    Error

*3*::
    Fatal error


ENVIRONMENT
-----------
`LTTNG_ABORT_ON_ERROR`::
    Set to `1` to abort the process after the first error is
    encountered.

`LTTNG_APP_SOCKET_TIMEOUT`::
    Timeout (in seconds) of the application socket when
    sending/receiving commands.
+
After this period of time, `lttng-sessiond` unregisters the application.
+
Set to `0` or `-1` to set an infinite timeout.
+
Default: +{default_app_socket_rw_timeout}+.

`LTTNG_CONSUMERD32_BIN`::
    32-bit consumer daemon binary path.
+
The option:--consumerd32-path option overrides this environment
variable.

`LTTNG_CONSUMERD32_LIBDIR`::
    32-bit consumer daemon library directory path.
+
The option:--consumerd32-libdir option overrides this environment
variable.

`LTTNG_CONSUMERD64_BIN`::
    64-bit consumer daemon binary path.
+
The option:--consumerd64-path option overrides this environment
variable.

`LTTNG_CONSUMERD64_LIBDIR`::
    64-bit consumer daemon library directory path.
+
The option:--consumerd64-libdir option overrides this environment
variable.

`LTTNG_DEBUG_NOCLONE`::
    Set to `1` to disable the use of man:clone(2)/man:fork(2).
+
Setting this environment variable is considered insecure, but it's
required to allow debuggers to work with `lttng-sessiond` on some
operating systems.

`LTTNG_EXTRA_KMOD_PROBES`::
    Extra LTTng kernel probe modules to load.
+
See the option:--extra-kmod-probes option which overrides this
environment variable.

`LTTNG_KMOD_PROBES`::
    Exclusive LTTng kernel probe modules to load.
+
See the option:--kmod-probes option which overrides this environment
variable.

`LTTNG_NETWORK_SOCKET_TIMEOUT`::
    Socket connection, receive, and send timeout (milliseconds).
+
Set to `0` or `-1` to use the timeout of the operating system (default).

`LTTNG_RUNDIR`::
    The directory containing the control files instead of:
+
--
Session daemon of the `root` user (root session daemon)::
    The value of the `--with-lttng-system-rundir` option when
    configuring the build of the project (`/var/run/lttng` by default).

Other session daemon::
    `$LTTNG_HOME/.lttng`
--
+
This environment variable is particularly useful to run multiple root
session daemons.

`LTTNG_SESSION_CONFIG_XSD_PATH`::
    Recording session configuration XML schema definition (XSD) path.

`LTTNG_SESSIOND_DEFAULT_TRACE_FORMAT`::
    Default trace format of recording sessions.
+
The value is either `ctf-1.8` or `ctf-2`.
+
NOTE: The option:--default-trace-format option overrides this
environment variable.

`LTTNG_UST_CTL_PATH`::
    Colon-delimited paths of the directories where `lttng-sessiond`
    places:
+
--
* The application registration Unix socket.
+
NOTE: The option:--apps-sock option overrides this specific path.

* The "wait" shared memory files to wake up instrumented applications
  that are waiting for a session daemon to start.

* The agent port file.
+
This file contains the TCP port the agents must connect to
to enable LTTng tracing.
--
+
Any such directory must exist when you start `lttng-sessiond`.
+
As of LTTng{nbsp}{lttng_version}:
+
--
* `lttng-sessiond` ignores any path after the first colon (`:`) and
  prints a warning.

* You cannot escape a colon within a specific path.
--
+
When tracing an application, start it with the
`LTTNG_UST_APP_PATH` environment variable set to the same value as
this environment variable.
+
If not set, the default directory path is the effective control file
directory (see the `LTTNG_RUNDIR` environment variable).

FILES
-----
`$LTTNG_HOME/.lttng`::
    Unix user's LTTng runtime and configuration directory.

`$LTTNG_HOME/lttng-traces`::
    Default output directory of LTTng traces in local and snapshot
    modes.
+
Override this path with the nloption:--output option of the
man:lttng-create(1) command.

`$LTTNG_HOME/.lttng/sessions/auto`::
    Directory from which `lttng-sessiond` loads Unix user recording
    session configurations when starting.
+
See the ``<<load,Recording session configuration loading>>'' section
above to learn more.

+{system_sessions_auto_dir}+::
    Directory from which `lttng-sessiond` loads system-wide recording
    session configurations when starting.
+
See the ``<<load,Recording session configuration loading>>'' section
above to learn more.

`$LTTNG_HOME/.lttng/lttng.conf`::
    Unix user's LTTng daemon INI configuration file.
+
See the ``<<cfg,Daemon configuration>>'' section above to learn more.

+{system_lttng_conf}+::
    System-wide LTTng daemon INI configuration file.
+
See the ``<<cfg,Daemon configuration>>'' section above to learn more.

NOTE: `$LTTNG_HOME` defaults to `$HOME`.


include::common-footer.txt[]


SEE ALSO
--------
man:lttng(1),
man:lttng-concepts(7)
