sd_bus_process - Drive the connection
int sd_bus_process(sd_bus *bus, sd_bus_message **ret);
sd_bus_process() drives the connection between the client and the message bus. That is, it handles connecting, authentication, and message processing. When invoked pending I/O work is executed, and queued incoming messages are dispatched to registered callbacks. Each time it is invoked a single operation is executed. It returns zero when no operations were pending and positive if a message was processed. When zero is returned the caller should synchronously poll for I/O events before calling into sd_bus_process() again. For that either use the simple, synchronous sd_bus_wait(3) call, or hook up the bus connection object to an external or manual event loop using sd_bus_get_fd(3).
sd_bus_process() processes at most one incoming message per call. If the parameter ret is not NULL and the call processed a message, *ret is set to this message. The caller owns a reference to this message and should call sd_bus_message_unref(3) when the message is no longer needed. If ret is not NULL, progress was made, but no message was processed, *ret is set to NULL.
If progress was made, a positive integer is returned. If no progress was made, 0 is returned. If an error occurs, a negative errno-style error code is returned.
Returned errors may indicate the following problems:
An invalid bus object was passed.
The bus connection was allocated in a parent process and is being reused in a child process after fork().
The bus connection has been terminated already.
The bus connection has been terminated just now.
This function is already being called, i.e. sd_bus_process() has been called from a callback function that itself was called by sd_bus_process().
These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.