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.
sd_bus_open_with_description(), sd_bus_open_user_with_description(), and sd_bus_open_system_with_description() are similar to sd_bus_open(), sd_bus_open_user(), and sd_bus_open_system(), but allow a description string to be set, see sd_bus_set_description(3). description may be NULL, in which case this function is equivalent to sd_bus_open(). This description string is used in log messages about the bus object, and including a "name" for the bus makes them easier to understand. Some messages are emitted during bus initialization, hence using this function is preferable to setting the description later with sd_bus_open_with_description(). The argument is copied internally and will not be referenced after the function returns.
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, optionally followed by a ":" and a port, optionally followed by a "/" and a machine name. If the machine name is given, a connection is created to the system bus in the specified container on the remote machine, and otherwise a connection to the system bus on the specified host is created.
Note that entering a container is a privileged operation, and will likely only work for the root user on the remote machine.
sd_bus_open_system_machine() connects to the system bus in the specified machine, where machine is the name of a local container, possibly prefixed by a user name and a separating "@". If the container name is specified as the special string ".host" the connection is made to the local system. This is useful to connect to the local system bus as specific user, e.g. "foobar@.host" to connect to the local system bus as local user "foobar". If the "@" syntax is used either the left-hand side or the right-hand side may be omitted (but not both) in which case the local user name or ".host" is implied. If the "@" syntax is not used the connection is always made as root user. See sd_bus_set_address(3) for a description of the address syntax, and machinectl(1) for more information about the "machine" concept. Note that connections into local containers are only available to privileged processes at this time.
sd_bus_open_user_machine() is similar to sd_bus_open_system_machine(), but connects to the user bus of the root user, or if the "@" syntax is used, of the specified user.
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_user_machine(), 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 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.
Normally, slot objects (as created by sd_bus_add_match(3) and similar calls) keep a reference to their bus connection object, too. Thus, as long as a bus slot object remains referenced its bus object will remain allocated too. Optionally, bus slot objects may be placed in "floating" mode. When in floating mode the life cycle of the bus slot object is bound to the bus object, i.e. when the bus object is freed the bus slot object is automatically unreferenced too. The floating state of a slot object may be controlled explicitly with sd_bus_slot_set_floating(3), though usually floating bus slot objects are created by passing NULL as the slot parameter of sd_bus_add_match() and related calls, thus indicating that the caller is not directly interested in referencing and managing the bus slot object.
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.
Returned errors may indicate the following problems:
In addition, other connection-related errors may be returned. See sd_bus_send(3).
These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.