>>52Yeah, looking at the POSIX.1-2013 version of the spec, if you look at the History section for fclose, you see that the errno values for the errors that could occur during fflush were added a couple of years before POSIX.1-2008 was ratified.
musl advertises itself as supporting POSIX.1-2008, but maybe the musl developers erroneously were looking at the older IEEE 1003.1-2001 spec when implementing some of those functions.