curl_multi_fdset(3) on macOS
The text c6bc63981637, as OS X 10.10.4, OS X 10.10.3, OS X 10.10.2, OS X 10.10.1, OS X 10.10, OS X 10.9.5, OS X 10.9.4, OS X 10.9.3, OS X 10.9.2, OS X 10.9.1, OS X 10.9, Mac OS X 10.8.5, Mac OS X 10.8.4, Mac OS X 10.8.3, Mac OS X 10.8.2, Mac OS X 10.8.1, Mac OS X 10.8 shipped it.
extracts file descriptor information from a multi handle