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

  Opens a file for reading, writing, or editing.  It positions the
  file at its beginning (byte 0).

  Syntax:

       #include <fcntl.h>

       int open(const char *file_spec, int flags, mode_t mode); (ANSI
       C)

       int open(const char *file_spec, int flags,...); (CEC C
       Extension)

  where the ...  represents optional file attribute arguments.  The
  file attribute arguments are the same as those used in the creat
  function.