Provided by: libsystemd-dev_257.4-1ubuntu3.1_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 64-bit 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.
Added in version 229.
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.
Added in version 229.
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.
Added in version 229.
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.
Added in version 229.
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.
Added in version 229.
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.
Added in version 229.
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.
Added in version 229.
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, library or module instance.
Other errors are possible, too.
NOTES
Functions described here are available as a shared library, which can be compiled against and linked to
with the libsystemd pkg-config(1) file.
The code described here uses getenv(3), which is declared to be not multi-thread-safe. This means that
the code calling the functions described here must not call setenv(3) from a parallel thread. It is
recommended to only do calls to setenv() from an early phase of the program when no other threads have
been started.
HISTORY
sd_event_prepare(), sd_event_wait(), sd_event_dispatch(), and sd_event_get_state() were added in version
221.
sd_event_get_iteration() was added in version 231.
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)
systemd 257.4 SD_EVENT_WAIT(3)