man sd_event_prepare (3): Low-level event loop operations

Other Alias

sd_event_wait, sd_event_dispatch, sd_event_get_state, SD_EVENT_INITIAL, SD_EVENT_PREPARING, SD_EVENT_ARMED, SD_EVENT_PENDING, SD_EVENT_RUNNING, SD_EVENT_EXITING, SD_EVENT_FINISHED

SYNOPSIS

#include <systemd/sd-event.h>

enum {
        SD_EVENT_INITIAL,
        SD_EVENT_PREPARING,
        SD_EVENT_ARMED,
        SD_EVENT_PENDING,
        SD_EVENT_RUNNING,
        SD_EVENT_EXITING,
        SD_EVENT_FINISHED,
};

int sd_event_prepare(sd_event *event);
int sd_event_wait(sd_event *event, uint64_t usec);
int sd_event_dispatch(sd_event *event);
int sd_event_get_state(sd_event *event);

DESCRIPTION

The low-level sd_event_prepare(), sd_event_wait() and sd_event_dispatch() functions may be used to execute specific phases of an event loop. See sd_event_run(3) and sd_event_loop(3) for higher-level functions that execute individual but complete iterations of an event loop or run it continuously.

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.

All four 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 milliseconds. (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_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.