DECC$THREAD_DATA_AST_SAFE

   The C RTL has a mode that allocates storage for thread-specific
   data allocated by threads at non-AST level separate for data
   allocated for ASTs. In this mode, each access to thread-
   specific data requires a call to LIB$AST_IN_PROG, which can add
   significant overhead when accessing thread-specific data in the
   C RTL.

   The alternate mode protects thread-specific data only if another
   function has it locked. This protects data that is in use within
   the C RTL, but does not protect the caller from an AST changing
   the data pointed to.

   This latter mode is now the C RTL default for the strtok, ecvt,
   and fcvt functions.

   You can select the legacy AST safe mode by enabling DECC$THREAD_
   DATA_AST_SAFE.