Ubuntu Manpage: sd_bus_default, sd_bus_default_user, sd_bus_default_system, sd_bus_open, sd_bus_open

Provided by: libsystemd-dev_237-3ubuntu10.57_amd64

NAME

       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, 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_user(sd_bus **bus);

       int sd_bus_open_system(sd_bus **bus);

       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.

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
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 also 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.

       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

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