SD_EVENT_WAIT

sd_event_prepare() checks for pending events and arms necessary timers. If any events are ready to be processed ("pending"), it returns a positive, non-zero value, and the caller should process these events with sd_event_dispatch().

sd_event_dispatch() dispatches the highest priority event source that has a pending event. On success, sd_event_dispatch() returns either zero, which indicates that no further event sources may be dispatched and exiting of the event loop was requested via sd_event_exit(3); or a positive non-zero value, which means that an event source was dispatched and the loop returned to its initial state, and the caller should initiate the next event loop iteration by invoking sd_event_prepare() again.

In case sd_event_prepare() returned zero, sd_event_wait() should be called to wait for further events or a timeout. If any events are ready to be processed, it returns a positive, non-zero value, and the events should be dispatched with sd_event_dispatch(). Otherwise, the event loop returned to its initial state and the next event loop iteration should be initiated by invoking sd_event_prepare() again.

sd_event_get_state() may be used to determine the state the event loop is currently in. It returns one of the states described below.

sd_event_get_iteration() may be used to determine the current iteration of the event loop. It returns an unsigned 64bit integer containing a counter that increases monotonically with each iteration of the event loop, starting with 0. The counter is increased at the time of the sd_event_prepare() invocation.

All five functions take, as the first argument, the event loop object event that has been created with sd_event_new(). The timeout for sd_event_wait() is specified in usec in microseconds. (uint64_t) -1 may be used to specify an infinite timeout.  

STATE MACHINE

The event loop knows the following states, that may be queried with sd_event_get_state().

SD_EVENT_INITIAL

The initial state the event loop is in, before each event loop iteration. Use sd_event_prepare() to transition the event loop into the SD_EVENT_ARMED or SD_EVENT_PENDING states.

SD_EVENT_PREPARING

An event source is currently being prepared, i.e. the preparation handler is currently being executed, as set with sd_event_source_set_prepare(3). This state is only seen in the event source preparation handler that is invoked from the sd_event_prepare() call and is immediately followed by SD_EVENT_ARMED or SD_EVENT_PENDING.

SD_EVENT_ARMED

sd_event_prepare() has been called and no event sources were ready to be dispatched. Use sd_event_wait() to wait for new events, and transition into SD_EVENT_PENDING or back into SD_EVENT_INITIAL.

SD_EVENT_PENDING

sd_event_prepare() or sd_event_wait() have been called and there were event sources with events pending. Use sd_event_dispatch() to dispatch the highest priority event source and transition back to SD_EVENT_INITIAL, or SD_EVENT_FINISHED.

SD_EVENT_RUNNING

A regular event source is currently being dispatched. This state is only seen in the event source handler that is invoked from the sd_event_dispatch() call, and is immediately followed by SD_EVENT_INITIAL or SD_EVENT_FINISHED as soon the event source handler returns. Note that during dispatching of exit event sources the SD_EVENT_EXITING state is seen instead.

SD_EVENT_EXITING

Similar to SD_EVENT_RUNNING but is the state in effect while dispatching exit event sources. It is followed by SD_EVENT_INITIAL or SD_EVENT_FINISHED as soon as the event handler returns.

SD_EVENT_FINISHED

The event loop has exited. All exit event sources have run. If the event loop is in this state it serves no purpose anymore, and should be freed.

A simplified flow chart of the states and the calls to transition between them is shown below. Note that SD_EVENT_PREPARING, SD_EVENT_RUNNING and SD_EVENT_EXITING are not shown here.

          INITIAL -<---<---<---<---<---<---<---<---<---<---<---<---\
             |                                                     |
             |                                                     ^
             |                                                     |
             v                 ret == 0                            |
      sd_event_prepare() >--->--->--->--->- ARMED                  |
             |                                |                    ^
             | ret > 0                        |                    |
             |                                |                    |
             v                                v          ret == 0  |
          PENDING <---<---<---<---<---< sd_event_wait() >--->--->--+
             |           ret > 0                                   ^
             |                                                     |
             |                                                     |
             v                                                     |
      sd_event_dispatch() >--->--->--->--->--->--->--->--->--->--->/
             |                             ret > 0
             | ret == 0
             |
             v
          FINISHED
    

RETURN VALUE

On success, these functions return 0 or a positive integer. On failure, they return a negative errno-style error code. In case of sd_event_prepare() and sd_event_wait(), a positive, non-zero return code indicates that events are ready to be processed and zero indicates that no events are ready. In case of sd_event_dispatch(), a positive, non-zero return code indicates that the event loop returned to its initial state and zero indicates the event loop has exited. sd_event_get_state() returns a positive or zero state on success.  

Errors

Returned errors may indicate the following problems:

-EINVAL

The event parameter is invalid or NULL.

-EBUSY

The event loop object is not in the right state.

-ESTALE

The event loop is already terminated.

-ECHILD

The event loop has been created in a different process.

Other errors are possible, too.  

NOTES

These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.  

SEE ALSO

systemd(1), sd_event_new(3), sd_event_add_io(3), sd_event_add_time(3), sd_event_add_signal(3), sd_event_add_child(3), sd_event_add_inotify(3), sd_event_add_defer(3), sd_event_run(3), sd_event_get_fd(3), sd_event_source_set_prepare(3)