efn

    OpenVMS usage:ef_number
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    Number of the event flag that is set when the service completes.
    If this argument is omitted, event flag 0 is used.

 flags

    OpenVMS usage:mask_longword
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    Flags specifying options for the service. The flags argument is
    a longword bit mask in which each bit corresponds to an option
    flag. The $DDTMDEF macro defines symbolic names for the option
    flag, described in $SET_DEFAULT_TRANS Option Flag. All undefined
    bits must be 0. If this argument is omitted, no flags are used.

    Table SYS-25 $SET_DEFAULT_TRANS Option Flag

    Flag Name      Description

    DDTM$M_SYNC    Specifies successful synchronous completion by
                   returning SS$_SYNCH. When SS$_SYNCH is returned,
                   the AST routine is not called, the event flag is
                   not set, and the I/O status block is not filled
                   in.

 iosb

    OpenVMS usage:io_status_block
    type:         quadword (unsigned)
    access:       write only
    mechanism:    by reference
    The I/O status block in which the completion status of the
    service is returned as a condition value.

    Refer to the VSI OpenVMS System Services Reference Manual to view
    the I/O status block diagram.

 astadr

    OpenVMS usage:ast_procedure
    type:         procedure entry mask
    access:       call without stack unwinding
    mechanism:    by reference
    The AST routine executed when the service completes, if SS$_
    NORMAL is returned in R0. The astadr argument is the address of
    the entry mask of this routine. The routine is executed in the
    same access mode as that of the caller of the $SET_DEFAULT_TRANS
    service.

 astprm

    OpenVMS usage:user_arg
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    The AST parameter passed to the AST routine specified by the
    astadr argument.

 new_tid

    OpenVMS usage:trans_id
    type:         octaword (unsigned)
    access:       read only
    mechanism:    by reference
    The identifier (TID) of the new default transaction for the
    calling process. If this argument is zero (the default) or if it
    specifies a zero TID (an octaword of zeros), the service clears
    the default transaction of the calling process.

 old_tid

    OpenVMS usage:trans_id
    type:         octaword (unsigned)
    access:       write only
    mechanism:    by reference
    An octaword in which the service returns the identifier (TID) of
    the calling process' previous transaction (the one that was set
    or cleared by this call to $SET_DEFAULT_TRANS).

    A zero TID is returned if the calling process did not have a
    default transaction prior to the call.