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.