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

  READONLY

  Prohibits write access to the file.  Enables users with read access
  but not write access to access the file.

  The Fortran I/O system's default file access privileges are
  read-write, which can cause run-time I/O errors if the file
  protection does not permit write access.

  The READONLY keyword has no effect on the protection specified for
  a file.  Its main purpose is to allow a file to be read
  simultaneously by two or more programs.  For example, if you wish
  to open a file to read the file but want to allow others to read
  the same file while you have it open, specify the READONLY keyword.