/sys$common/syshlp/helplib.hlb CC, Run-time functions, isatty  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Returns 1 if the specified file descriptor is associated with a
  terminal, and 0 if it is not.

  Syntax:

       #include <unistd.h>

       int isatty(int file_descriptor);