Problem:
 How is stderr opened: input only (not likely), output only (of course)
 or both (there's the rub)?

 Action:
 My preference: "stderr is opened for input and output, although it is
 expected that under normal circumstances it will not be used for input".
 I'm OK with "stderr is opened for output.  Implementations may open it
 for input as well, but a conforming application should not expect that."

Interactive Device

A terminal device.

Note: This definition is intended to align with the ISO C standard's use of "interactive device".

At program start-up, three streams shall be predefined and need not be opened explicitly: standard input (for reading conventional input), standard output (for writing conventional output), and standard error (for writing diagnostic output). When opened, the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device.

At program start-up, three streams shall be predefined and already open: stdin (standard input, for conventional input) for reading, stdout (standard output, for conventional output) for writing, and stderr (standard error, for diagnostic output) for writing. When opened, stderr shall not be fully buffered; stdin and stdout shall be fully buffered if and only if [CX]the file descriptor associated with the stream is determined not to be associated with an interactive device.[/CX]

The stderr stream is expected to be open for reading and writing.

These file descriptors are often all associated with a single open file description which has access mode O_RDWR (e.g., in the case of a terminal device for a login shell). However, the stderr, stdin, and stdout streams need not be opened for both reading and writing at program start-up in this case.