Attempts to read an event from
the completion queue for the aio_context specified by ctx_id.
0 if no events are available and the timeout specified
by when has elapsed, where when == NULL specifies an infinite
timeout. Note that the timeout pointed to by when is relative and
will be updated if not NULL and the operation blocks. Will fail
with -ENOSYS if not implemented.
returns 0. Otherwise, -error is return, where
error is one of the Exxx values defined in the Errors section.
referenced data outside of the program's accessible address space.
refers to an unitialized aio context, the iocb pointed to by
contains an improperly initialized iocb.