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When opening a FIFO with O_RDONLY or O_WRONLY set:

• If O_NONBLOCK is set, an open() for reading-only returns shall return without delay. An open() for writing-only returns shall return an error if no process currently has the file open for reading.
• If O_NONBLOCK is clear, an open() for reading-only blocks shall block the calling thread until a thread opens the file for writing. An open() for writing-only blocks shall block the calling thread until a thread opens the file for reading.

When opening a block special or character special file that supports non-blocking opens:

• If O_NONBLOCK is set, the open() function returns shall return without blocking for the device to be ready or available. Subsequent behaviour behavior of the device is device-specific.
• If O_NONBLOCK is clear, the open() function blocks shall block the calling thread until the device is ready or available before returning.

Otherwise, the behavior of O_NONBLOCK is unspecified.

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