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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
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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