sd_bus_default, sd_bus_default_user, sd_bus_default_system,
sd_bus_open, sd_bus_open_user, sd_bus_open_system,
sd_bus_open_system_remote, sd_bus_open_system_machine - Acquire a
connection to a system or user bus
sd_bus_default() acquires a bus connection object to the user bus
when invoked in user context, or to the system bus otherwise. The
connection object is associated with the calling thread. Each time
the function is invoked from the same thread, the same object is
returned, but its reference count is increased by one, as long as at
least one reference is kept. When the last reference to the
connection is dropped (using the sd_bus_unref(3) call), the
connection is terminated. Note that the connection is not
automatically terminated when the associated thread ends. It is
important to drop the last reference to the bus connection explicitly
before the thread ends, as otherwise, the connection will leak. Also,
queued but unread or unwritten messages keep the bus referenced, see
sd_bus_default_user() returns a user bus connection object associated
with the calling thread. sd_bus_default_system() is similar, but
connects to the system bus. Note that sd_bus_default() is identical
to these two calls, depending on the execution context.
sd_bus_open() creates a new, independent bus connection to the user
bus when invoked in user context, or the system bus otherwise.
sd_bus_open_user() is similar, but connects only to the user bus.
sd_bus_open_system() does the same, but connects to the system bus.
In contrast to sd_bus_default(), sd_bus_default_user(), and
sd_bus_default_system(), these calls return new, independent
connection objects that are not associated with the invoking thread
and are not shared between multiple invocations. It is recommended to
share connections per thread to efficiently make use the available
resources. Thus, it is recommended to use sd_bus_default(),
sd_bus_default_user() and sd_bus_default_system() to connect to the
user or system buses.
If the $DBUS_SESSION_BUS_ADDRESS environment variable is set (cf.
environ(7)), it will be used as the address of the user bus. This
variable can contain multiple addresses separated by ";". If this
variable is not set, a suitable default for the default user D-Bus
instance will be used.
If the $DBUS_SYSTEM_BUS_ADDRESS environment variable is set, it will
be used as the address of the system bus. This variable uses the same
syntax as $DBUS_SESSION_BUS_ADDRESS. If this variable is not set, a
suitable default for the default system D-Bus instance will be used.
sd_bus_open_system_remote() connects to the system bus on the
specified host using ssh(1). host consists of an optional user name
followed by the "@" symbol, and the hostname.
sd_bus_open_system_machine() connects to the system bus in the
specified machine, where machine is the name of a local container.
See machinectl(1) for more information about the "machine" concept.
Note that connections into local containers are only available to
privileged processes at this time.
These calls allocate a bus connection object and initiate the
connection to a well-known bus of some form. An alternative to using
these high-level calls is to create an unconnected bus object with
sd_bus_new(3) and to connect it with sd_bus_start(3).
The functions sd_bus_open(), sd_bus_open_user(),
sd_bus_open_system(), sd_bus_open_system_remote(), and
sd_bus_open_system_machine() return a new connection object and the
caller owns the sole reference. When not needed anymore, this
reference should be destroyed with sd_bus_unref(3).
The functions sd_bus_default(), sd_bus_default_user() and
sd_bus_default_system() do not necessarily create a new object, but
increase the connection reference of an existing connection object by
one. Use sd_bus_unref(3) to drop the reference.
Queued but unwritten/unread messages also keep a reference to their
bus connection object. For this reason, even if an application
dropped all references to a bus connection, it might not get
destroyed right away. Until all incoming queued messages are read,
and until all outgoing unwritten messages are written, the bus object
will stay alive. sd_bus_flush() may be used to write all outgoing
queued messages so they drop their references. To flush the unread
incoming messages, use sd_bus_close(), which will also close the bus
connection. When using the default bus logic, it is a good idea to
first invoke sd_bus_flush() followed by sd_bus_close() when a thread
or process terminates, and thus its bus connection object should be
The life cycle of the default bus connection should be the
responsibility of the code that creates/owns the thread the default
bus connection object is associated with. Library code should neither
call sd_bus_flush() nor sd_bus_close() on default bus objects unless
it does so in its own private, self-allocated thread. Library code
should not use the default bus object in other threads unless it is
clear that the program using it will life cycle the bus connection
object and flush and close it before exiting from the thread. In
libraries where it is not clear that the calling program will life
cycle the bus connection object, it is hence recommended to use
sd_bus_open_system() instead of sd_bus_default_system() and related
Returned errors may indicate the following problems:
The specified parameters are invalid.
Memory allocation failed.
The protocol version required to connect to the selected bus is
In addition, any further connection-related errors may be by
returned. See sd_bus_send(3).
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systemd 233 SD_BUS_DEFAULT(3)