Provided by: libsystemd-dev_245.4-4ubuntu3.24_amd64 
NAME
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
#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
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
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)