Ubuntu Manpage: sd_bus_default, sd_bus_default_user, sd_bus_default_system, sd_bus

Provided by: libsystemd-dev_245.4-4ubuntu3.24_amd64

NAME

       sd_bus_default, sd_bus_default_user, sd_bus_default_system, sd_bus_open,
       sd_bus_open_with_description, sd_bus_open_user, sd_bus_open_user_with_description,
       sd_bus_open_system, sd_bus_open_system_with_description, sd_bus_open_system_remote,
       sd_bus_open_system_machine - Acquire a connection to a system or user bus

SYNOPSIS

       #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_with_description(sd_bus **bus, const char *description);

       int sd_bus_open_user(sd_bus **bus);

       int sd_bus_open_user_with_description(sd_bus **bus, const char *description);

       int sd_bus_open_system(sd_bus **bus);

       int sd_bus_open_system_with_description(sd_bus **bus, const char *description);

       int sd_bus_open_system_remote(sd_bus **bus, const char *host);

       int sd_bus_open_system_machine(sd_bus **bus, const char *machine);

DESCRIPTION

       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.

       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 prefereable 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. 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 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.

RETURN VALUE

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

   Errors
       Returned errors may indicate the following problems:

       -EINVAL
           The specified parameters are invalid.

       -ENOMEM
           Memory allocation failed.

       -ESOCKTNOSUPPORT
           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).

NOTES