This errno for "Operation not supported", another form of catch-all error. This one is for basic classes of operations mostly pertaining to either socket operations or vnode operations (vops), not specific ioctls like ENOTTY or device operations like ENODEV. For example, a socket type that doesn't have an accept function will return EOPNOTSUPP if accept() is called for it. Similarly, many of the default vnode operation descriptions and VFS operation descriptions will return this on a file system which does not implement a given operation.