sd_bus_default(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | REFERENCE OWNERSHIP | RETURN VALUE | NOTES | HISTORY | SEE ALSO | COLOPHON

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_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).

REFERENCE OWNERSHIP         top

       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.

   Errors
       Returned errors may indicate the following problems:

       -EINVAL
           The specified parameters are invalid.

       -ENOMEDIUM
           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).

       -ENOMEM
           Memory allocation failed.

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

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

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
       started.

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_with_description(),
       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

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       manager) project.  Information about the project can be found at
       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have a
       bug report for this manual page, see
       ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
       This page was obtained from the project's upstream Git repository
       ⟨https://github.com/systemd/systemd.git⟩ on 2025-02-02.  (At that
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systemd 258~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)