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

SD_EVENT_PREPARING

SD_EVENT_ARMED

SD_EVENT_PENDING

SD_EVENT_RUNNING

SD_EVENT_EXITING

SD_EVENT_FINISHED

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.

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)