sd_bus_default(3) — Linux manual page


SD_BUS_DEFAULT(3)            sd_bus_default            SD_BUS_DEFAULT(3)

NAME         top

       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_user_machine,
       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         top

       #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_user_machine(sd_bus **bus, const char *machine);

       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         top

       sd_bus_default() acquires a bus connection object to the user bus
       when invoked from within a user slice (any session under
       "user-*.slice", e.g.: "user@1000.service"), 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_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.
       "" 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.

RETURN VALUE         top

       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.

           The requested bus type is not available because of invalid
           environment (for example the user session bus is not
           available because $XDG_RUNTIME_DIR is not set).

           Memory allocation failed.

           The protocol version required to connect to the selected bus
           is not supported.

       In addition, other connection-related errors may be returned. See

NOTES         top

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

       The code described here uses getenv(3), which is declared to be
       not multi-thread-safe. This means that the code calling the
       functions described here must not call setenv(3) from a parallel
       thread. It is recommended to only do calls to setenv() from an
       early phase of the program when no other threads have been

HISTORY         top

       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(), and sd_bus_open_system_machine()
       were added in version 221.

       sd_bus_open_user_with_description(), and
       sd_bus_open_system_with_description() were added in version 239.

       sd_bus_open_user_machine() was added in version 248.

SEE ALSO         top

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

COLOPHON         top

       This page is part of the systemd (systemd system and service
       manager) project.  Information about the project can be found at
       ⟨⟩.  If you have
       a bug report for this manual page, see
       This page was obtained from the project's upstream Git repository
       ⟨⟩ on 2024-06-14.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2024-06-13.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

systemd 257~devel                                      SD_BUS_DEFAULT(3)

Pages that refer to this page: sd-bus(3)sd_bus_close(3)sd_bus_negotiate_fds(3)sd_bus_new(3)sd_bus_set_address(3)sd_bus_set_description(3)sd_bus_set_server(3)sd_bus_start(3)systemd.directives(7)systemd.index(7)systemd-machined.service(8)