systemd Developer Tom Gundersen teg@jklm.no sd_event_wait 3 sd_event_wait sd_event_prepare sd_event_dispatch Run parts of libsystemd event loop #include <systemd/sd-event.h> int sd_event_prepare sd_event *event int sd_event_wait sd_event *event uint64_t timeout int sd_event_dispatch sd_event *event Functions described here form parts of an event loop. sd_event_prepare checks for pending events and arms necessary timers. If any events are ready to be processed, it returns a positive value, and the events should be processed with sd_event_dispatch. sd_event_dispatch runs a handler for one of the events from the sources with the highest priority. On success, sd_event_dispatch returns either 0, which means that the loop is finished, or a positive value, which means that the loop is again in the initial state and sd_event_prepare should be called again. In case sd_event_prepare returned 0, sd_event_wait should be called to wait for events or a timeout. If any events are ready to be processed, it returns a positive value, and the events should be processed with sd_event_dispatch. Otherwise, the loop is back in the initial state and sd_event_prepare should be called again. ┌──────────┐ │ initial ├──←←←←←←←←←←←←←←←←←←←─┐ └───┬──────┘ ↑ │ ↑ sd_event_prepare ┌─────────┐ ↑ ├ 0 →→→→→→→──┤ armed │ ↑ 1 └───┬─────┘ ↑ ↓ │ ↑ ↓ sd_event_wait ↑ ├───←←←←←←←─── 1 ┴─ 0 →→→→→→→─┘ ┌───┴──────┐ ↑ │ pending │ ↑ └───┬──────┘ ↑ │ ↑ sd_event_dispatch ↑ ↓ ↑ ├ 1 ──────────→→→→→→→─────────┘ 0 ↓ ┌───┴──────┐ │ finished │ └──────────┘ All three functions as the first argument take the event loop object event that is created with with sd_event_new. The timeout for sd_event_wait is specified with timeout in milliseconds. (uint64_t) -1 may be used to specify an infinite timeout. 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 value means that events are ready to be processed and 0 means that no events are ready. In case of sd_event_dispatch, a positive value means that the loop is again in the initial state and 0 means the loop is finished. For any of these functions, a negative return value means the loop must be aborted. Returned errors may indicate the following problems: -EINVAL Parameter event is 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. Functions described here are available as a shared library, which can be compiled and linked to with the libsystemd pkg-config1 file. systemd1, sd_event_new3, sd_event_run3, sd_event_add_io3, sd_event_add_time3, sd_event_add_signal3, sd_event_add_defer3, sd_event_add_exit3, sd_event_add_post3.