x SuSE Linux 13.1-RELEASE x
x SuSE Linux 13.1-RELEASEx
SD_BUS_DEFAULT(3) sd_bus_default SD_BUS_DEFAULT(3)
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_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
#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
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
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
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
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.
SEE ALSO
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)
systemd 254 SD_BUS_DEFAULT(3)
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