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rawrec/rawplay uses a fairly simple thread scheme to maximize audio
quality and dependibility, but its potentially quite confusing if you
have to try to figure out what's going on from the source, hence this
external description.
The record and play operations are largely symmetric, and what
asymmetry is present is due primarily to the limitations imposed by
working with standard input and output.
The main thread is the boss thread and manages two workers who do all
the actual work (just like rl).
One worker is responsible for moving data between the audio device
(kernel audio buffer) and the application ring buffer, and the other
for moving data between the application ring buffer and the file
system. These thread functions are called move_au and move_fd,
respectively. If possible, move_au runs at maximum priority, in order
to avoid any glitches between buffer segment read() or write()s.
move_fd runs at (maximum priority - 1).
When playing, the move_fd thread must fill the ring buffer segments
with data before that data can be sent to the audio device by move_au,
so move_fd starts first (using condition variable startup_next_cv) and
locks the first buffer segment, then signals move_au that it can start
(by incrementing the value associated with startup_next_cv to two).
Thereafter, when move_fd is finished filling a buffer segment, it
first locks the next segment (wrapping back to the beginning of the
ring if the current segment is the last), then releases the last
segment, so that move_au can start using it). move_au uses the same
system of locking the next buffer it needs before releasing the buffer
it has just finished with.
It is important that the first thread to start is not allowed to race
all the way around the ring buffer and grab the first segment again
before the second thread has a chance to get started. The wrap_ready
cv and flag protect this possibility by allowing the second thread to
signal the first when it is ok for the first to wrap around.
When recording, things work exactly the same way, except move_au
starts first (grabs the first segment) before signaling move_fd that
it can start).
This scheme ensures that: a. the right thread gets started on the
first buffer when the threads are launched, b. neither thread can ever
pass the other.
When playing data from standard input, there is always a chance that
there will not be a full segment's worth of data ready in the pipeline
for playing, so move_fd (which in this case is moveing data between
the standard input file descriptor and the ring buffer) records the
amount of data read for each segment, so that move_au doesn't play
random junk or old audio data left over from the last time around the
ring buffer. If move_au sees too many empty segments in a row ("too
many" is currently a magic constant), it gives up and the program
terminates with a diagnostic). Note that move_fd will already know
about the empty segments, but it doesn't terminate execution, since
move_fd may well be far ahead of move_au and there is no point in
quiting while there is still a lot of good data to be played.
When recording to standard output, the output pipe may break. If this
happens, move_fd terminates execution with a diagnostic.
There is currently some additional magic used to make signal handling
sort of work, but this code is hopefully temporary pending full
LinuxThreads signal handling correctness and doesn't really bear
describing.
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