Why do flex scanners call fileno if it is not ANSI compatible?

Flex scanners call fileno() in order to get the file descriptor corresponding to yyin. The file descriptor may be passed to isatty() or read(), depending upon which %options you specified. If your system does not have fileno() support, to get rid of the read() call, do not specify %option read. To get rid of the isatty() call, you must specify one of %option always-interactive or %option never-interactive.

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