VMS Help CC, Run-time functions, ctermid  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Returns a character string giving the equivalence string of
  SYS$COMMAND.  This is the name of the controlling terminal.  This
  function is nonreentrant.

  Syntax:

       #include <unistd.h>

       char *ctermid(char *str);