Answering questions that may arise related to the meaning of portions of an IEEE standard concerning specific applications.

IEEE Standards Interpretation for IEEE Std 1003.1™-2001 IEEE Standard Standard for Information Technology -- Portable Operating System Interface (POSIX®)

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Interpretation Request #71
Topic: read, thread blocked on O_NONBLOCK Relevant Sections: XSH read Page: 1190 Line: 37207

If a thread is blocked while attempting to read from a file descriptor that was opened with the O_NONBLOCK bit clear, can that thread be unblocked by another thread setting the O_NONBLOCK bit via fcntl()? Is an implementation that unblocks a previously blocked thread under these conditions conforming?

Issue an interpretation: "The standard clearly states that if, on entry to read, the thread finds O_NONBLOCK is not set (i.e. is clear), the thread shall block until a very specific event happens (data becomes available). It does not include the event that some other thread changed the O_NONBLOCK flag, and so the blocked thread should not wake up. Conforming implementations must conform to this."

Interpretation Response #71
The standard does not speak to this issue, and as such no conformance distinction can be made between alternative implementations based on this.

Rationale for Interpretation