isatty(stdin) usage

[David J. Camp]

I am trying to use this invokation:

          if (isatty (stdin))

to detect whether a C program has been called interactively or with
redirection.  It does not work.  Can anyone suggest a better way, or
explain why it does not work?

I am using a Sun 3/260 running SunOS 4.0.1.
-David-
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