int xdf_add_event(struct xdf* xdf, int evttype,
double onset, double dur);
xdf_add_event() appends to the file referenced by the handle xdf opened for writing an event of type evttype at time onset lasting for a duration dur expressed in seconds. If the event has no notion of duration, dur should be set to 0. evttype should be a value returned by a successful call to xdf_add_evttype(3).
xdf_add_event() returns 0 in case of success. Otherwise -1 is returned and errno is set appropriately.
xdf is NULL or evttype has not been previously created by xdf_add_evttype(3)
The system is unable to allocate resources.
The file referenced by xdf has not been opened for writing or its file format does not support events.