used to tag "write pending" events
| std::READ | used to tag "read" events |
| std::READ_ERROR | used to tag "read error" events |
| std::WRITE_COMPLETED | used to tag "write completed" events |
| std::WRITE_ERROR | used to tag "write error" events |
| std::PROCESS_JOINED | used to tag "process joined" events |
| std::SIGNAL | used to tag "signal" events |