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