The stream has been subject to an erroneous operation (possibly during input, output, or through the various Stream methods). See Section 14.5, “Errors during Input and Output”. You might be able to obtain additional information about the error with the description method.

The stream is known to be in such a state that the usual input or output operations attempted upon would raise the NOTREADY condition. (See Section 14.5, “Errors during Input and Output”.) For example, a simple input stream can have a defined length. An attempt to read that stream (with charIn or lineIn, perhaps) beyond that limit can make the stream unavailable until the stream has been closed (for example, with the close method) and then reopened.

The stream is known to be in such a state that the usual input or output operations might be attempted. This is the usual state for a stream, although it does not guarantee that any particular operation will succeed.

The state of the stream is unknown. This generally means that the stream is closed or has not yet been opened.