/sys$common/syshlp/helplib.hlb FORTRAN, Statements, OPEN, IOSTAT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  IOSTAT = ios

  ios   Is an integer scalar memory reference.

  If no error exists, ios is defined as zero; if an error exists, ios
  is defined as a positive integer.  IOSTAT applies only to the OPEN
  statement in which it appears and not to later I/O operations on
  the logical unit that it opened.  However, you can use the IOSTAT
  parameter in later I/O statements to perform a similar function.

  Secondary operating system messages do not display when IOSTAT is
  specified.  To display these messages, remove IOSTAT or use a
  platform-specific method such as a OpenVMS condition handler.  (For
  more information, see your user manual.)