|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
NAME | SYNOPSIS | DESCRIPTION | REFERENCE OWNERSHIP | RETURN VALUE | NOTES | HISTORY | SEE ALSO | COLOPHON |
|
|
|
SD_BUS_DEFAULT(3) sd_bus_default SD_BUS_DEFAULT(3)
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
#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);
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).
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.
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).
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.
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.
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)
This page is part of the systemd (systemd system and service
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 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-08-11.) 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
man-pages@man7.org
systemd 258~rc2 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)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|