Index · Directives systemd 228


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


#include <systemd/sd-bus.h>
int sd_bus_default(sd_bus **bus);
int sd_bus_default_user(sd_bus **bus);
int sd_bus_default_system(sd_bus **bus);
int sd_bus_open(sd_bus **bus);
int sd_bus_open_user(sd_bus **bus);
int sd_bus_open_system(sd_bus **bus);
int sd_bus_open_system_remote(sd_bus **bus,
 const char *host);
int sd_bus_open_system_machine(sd_bus **bus,
 const char *machine);


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 below.

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).

Reference ownership

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 freed.

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 calls.

Return Value

On success, these calls return 0 or a positive integer. On failure, these calls return a negative errno-style error code.


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 not supported.

In addition, any further connection-related errors may be by returned. See sd_bus_send(3).


sd_bus_open_user() and the other functions described here are available as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.

See Also

systemd(1), sd-bus(3), sd_bus_new(3), sd_bus_ref(3), sd_bus_unref(3), ssh(1), systemd-machined.service(8), machinectl(1)