source: buchla-68k/orig/DOC/FILTER.TXT

        Function:    filter "Filter Command Line Files In Classic UNIX Style"

        Created:     Sat Aug 10 21:57:12 EDT 1985
        By:          Gary Perlman (Wang Institute, Tyngsboro, MA 01879 USA)

        Compilation: nothing unusual

        Tester:      $Compile: cc -DSTANDALONE -o filter %f

        Preconditions:
                The index of the first file operand has been determined.

        Postconditions:
                All files have been opened, processed, and closed.

        Returns:
                The return status (non-zero is bad) depends on the accessibility
                of files, the ability to open them, and the return statuses of
                the called function.

        Exceptions:
                If any file cannot be accessed, then none will be processed.
                During processing, if something goes wrong (a file that could
                be accessed cannot be opened, or the file processor returns a
                non-zero status), processing continues.

        Notes:
                "-" is the conventional name for the standard input.
                        It can only be read once.
                Fputs and putc are used to print error messages to avoid
                        loading fat fprintf just because filter used it.

        Some modifications might be useful but unpopular:
                If there is piped input (!isatty (fileno (stdin))),
                and the standard input is not read,
                then some information may be ignored,
                so a warning should be printed.
                Unfortunately, this would break things like vi filters.

                If there is not piped input,
                and the standard input is being read from the keyboard,
                then prompt the user for input with something like:
                        pgm: reading input from terminal
                This would avoid the problem of people forgetting to supply
                an input redirection.