/REENTRANCY=option
     /REENTRANCY=TOLERANT (D)

  Controls the type of reentrancy that reentrant VSI C RTL routines
  will exhibit.

  (See also the decc$set_reentrancy RTL routine.)

  This qualifier is for use only with a module containing the main
  routine.

  The reentrancy level is set at run time according to the
  /REENTRANCY qualifier specified while compiling the module
  containing the main routine.

  You can specify one of the following options:

  AST               Uses the __TESTBITSSI builtin to perform simple
                    locking around critical sections of RTL code, and
                    may additionally disable asynchronous system
                    traps (ASTs) in locked region of codes.  This
                    type of locking should be used when AST code
                    contains calls to VSI C RTL I/O routines.

  MULTITHREAD        Designed to be used in conjunction with the
                    DECthreads product.  It performs DECthreads
                    locking and never disables ASTs.

  NONE              Gives optimal performance in the RTL, but does
                    absolutely no locking around critical sections of
                    RTL code.  It should only be used in a single
                    threaded environment when there is no chance that
                    the thread of execution will be interrupted by an
                    AST that would call the VSI C RTL.

  TOLERANT          Uses the __TESTBITSSI builtin to perform simple
                    locking around critical sections of RTL code, but
                    ASTs are not disabled.  This type of locking
                    should be used when ASTs are used and must be
                    delivered immediately.  This is also the default
                    reentrancy type.