NAME | SYNOPSIS | DESCRIPTION | STATE MACHINE | RETURN VALUE | ERRORS | NOTES | SEE ALSO | COLOPHON

SD_EVENT_WAIT(3)                sd_event_wait               SD_EVENT_WAIT(3)

NAME         top

       sd_event_wait, sd_event_prepare, sd_event_dispatch,
       sd_event_get_state, sd_event_get_iteration, SD_EVENT_INITIAL,
       SD_EVENT_PREPARING, SD_EVENT_ARMED, SD_EVENT_PENDING,
       SD_EVENT_RUNNING, SD_EVENT_EXITING, SD_EVENT_FINISHED - Low-level
       event loop operations

SYNOPSIS         top

       #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);

       int sd_event_get_iteration(sd_event *event, uint64_t *ret);

DESCRIPTION         top

       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.

       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         top

       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         top

       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         top

       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         top

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

SEE ALSO         top

       systemd(1), sd_event_new(3), sd_event_add_io(3),
       sd_event_add_time(3), sd_event_add_signal(3), sd_event_add_defer(3),
       sd_event_add_exit(3), sd_event_add_post(3), sd_event_run(3),
       sd_event_get_fd(3), sd_event_source_set_prepare(3)

COLOPHON         top

       This page is part of the systemd (systemd system and service manager)
       project.  Information about the project can be found at 
       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have a bug
       report for this manual page, see 
       ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.  This
       page was obtained from the project's upstream Git repository 
       ⟨https://github.com/systemd/systemd.git⟩ on 2017-09-15.  If you dis‐
       cover any rendering problems in this HTML version of the page, or you
       believe there is a better or more up-to-date source for the page, or
       you have corrections or improvements to the information in this
       COLOPHON (which is not part of the original manual page), send a mail
       to man-pages@man7.org

systemd 234                                                 SD_EVENT_WAIT(3)

Pages that refer to this page: sd-event(3)sd_event_get_fd(3)sd_event_run(3)systemd.directives(7)systemd.index(7)