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    ADCMD_LOCK -- prevent audio channels from being stolen

    ADCMD_LOCK is a command for multiple audio channels.  For each
    selected channel (io_Unit), if the allocation key (ioa_AllocKey) is
    correct, ADCMD_LOCK locks the channel, preventing subsequent
    allocations (ADCMD_ALLOCATE or OpenDevice) from stealing the channel.
    Otherwise, ADCMD_LOCK returns an error (ADIOERR_NOALLOCATION) and will
    not lock any channels.

    Unlike setting the precedence (ADCMD_SETPREC, ADCMD_ALLOCATE or
    OpenDevice) to maximum (ADALLOC_MAXPREC) which would cause all
    subsequent allocations to fail, ADCMD_LOCK causes all higher
    precedence allocations, even no-wait (ADIOF_NOWAIT) allocations, to
    wait until the channels are un-locked.

    Locked channels can only be unlocked by freeing them (ADCMD_FREE),
    which clears the channel select bits (io_Unit).  ADCMD_LOCK does not
    reply the I/O request (mn_ReplyPort) until all the channels it locks
    are freed, unless a higher precedence allocation attempts to steal one
    the locked channels. If a steal occurs, ADCMD_LOCK replies and returns
    an error (ADIOERR_CHANNELSTOLEN).  If the lock is replied
    (mn_ReplyPort) with this error, the channels should be freed as soon
    as possible.  To avoid a possible deadlock, never make the freeing of
    stolen channels dependent on another allocations completion.

    ADCMD_LOCK is only asynchronous if the allocation key is correct, in
    which case it clears the quick flag (IOF_QUICK); otherwise, it is
    synchronous and only replies if the quick flag (IOF_QUICK) is clear.
    Do not use ADCMD_LOCK in interrupt code.

    mn_ReplyPort- pointer to message port that receives I/O request
                  if the quick flag (IOF_QUICK) is clear
    io_Device   - pointer to device node, must be set by (or copied from
                  I/O block set by) OpenDevice function
    io_Unit     - bit map of channels to lock (bits 0 thru 3 correspond to
                  channels 0 thru 3)
    io_Command  - command number for ADCMD_LOCK
    io_Flags    - flags, must be cleared
    ioa_AllocKey- allocation key, must be set by (or copied from I/O block
                  set by) OpenDevice function or ADCMD_ALLOCATE command

    io_Unit     - bit map of successfully locked channels (bits 0 thru 3
                  correspond to channels 0 thru 3) not freed (ADCMD_FREE)
    io_Flags    - IOF_QUICK flag cleared if the allocation key is correct
                  (no ADIOERR_NOALLOCATION error)
    io_Error    - error number:
                  0                    - no error
                  ADIOERR_NOALLOCATION - allocation key (ioa_AllocKey)
                                         does not match key for channel
                  ADIOERR_CHANNELSTOLEN- allocation attempting to steal
                                         locked channel

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