/[NO]REENTRANCY

  Tells the VSI Fortran RTL whether the program will be relying on
  threaded or asynchronous (AST) reentrancy.

  /NOREENTRANCY specifies that the program will not be relying on
  threaded or asynchronous (AST) reentrancy.  So, the RTL will not
  guard against such interrupts inside the RTL.  This is the same as
  specifying /REENTRANCY=NONE (the default).

  ASYNC
    Specifies that the program may contain asynchronous (AST)
    handlers that could call the RTL.  This causes the RTL to guard
    against AST interrupts inside its own critical regions.

    This option is ignored for applications running on versions of
    OpenVMS earlier than Version 7.0.

  NONE
    Specifies that the program will not be relying on threaded or
    asynchronous (AST) reentrancy.  So, the RTL will not guard
    against such interrupts inside the RTL.  This is the default and
    is the same as specifying /NOREENTRANCY.

  THREADED
    Specifies that the program is multithreaded, such as programs
    using the DECthreads library.  This causes the RTL to use thread
    locking to guard its own critical regions.

    This option is ignored for applications running on versions of
    OpenVMS earlier than Version 7.0.