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https://github.com/cookiengineer/audacity
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128 lines
5.6 KiB
C
128 lines
5.6 KiB
C
/* pmutil.h -- some helpful utilities for building midi
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applications that use PortMidi
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*/
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#ifdef __cplusplus
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extern "C" {
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#endif /* __cplusplus */
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typedef void PmQueue;
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/*
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A single-reader, single-writer queue is created by
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Pm_QueueCreate(), which takes the number of messages and
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the message size as parameters. The queue only accepts
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fixed sized messages. Returns NULL if memory cannot be allocated.
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This queue implementation uses the "light pipe" algorithm which
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operates correctly even with multi-processors and out-of-order
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memory writes. (see Alexander Dokumentov, "Lock-free Interprocess
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Communication," Dr. Dobbs Portal, http://www.ddj.com/,
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articleID=189401457, June 15, 2006. This algorithm requires
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that messages be translated to a form where no words contain
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zeros. Each word becomes its own "data valid" tag. Because of
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this translation, we cannot return a pointer to data still in
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the queue when the "peek" method is called. Instead, a buffer
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is preallocated so that data can be copied there. Pm_QueuePeek()
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dequeues a message into this buffer and returns a pointer to
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it. A subsequent Pm_Dequeue() will copy from this buffer.
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This implementation does not try to keep reader/writer data in
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separate cache lines or prevent thrashing on cache lines.
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However, this algorithm differs by doing inserts/removals in
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units of messages rather than units of machine words. Some
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performance improvement might be obtained by not clearing data
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immediately after a read, but instead by waiting for the end
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of the cache line, especially if messages are smaller than
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cache lines. See the Dokumentov article for explanation.
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The algorithm is extended to handle "overflow" reporting. To report
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an overflow, the sender writes the current tail position to a field.
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The receiver must acknowlege receipt by zeroing the field. The sender
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will not send more until the field is zeroed.
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Pm_QueueDestroy() destroys the queue and frees its storage.
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*/
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PmQueue *Pm_QueueCreate(long num_msgs, long bytes_per_msg);
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PmError Pm_QueueDestroy(PmQueue *queue);
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/*
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Pm_Dequeue() removes one item from the queue, copying it into msg.
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Returns 1 if successful, and 0 if the queue is empty.
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Returns pmBufferOverflow if what would have been the next thing
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in the queue was dropped due to overflow. (So when overflow occurs,
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the receiver can receive a queue full of messages before getting the
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overflow report. This protocol ensures that the reader will be
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notified when data is lost due to overflow.
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*/
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PmError Pm_Dequeue(PmQueue *queue, void *msg);
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/*
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Pm_Enqueue() inserts one item into the queue, copying it from msg.
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Returns pmNoError if successful and pmBufferOverflow if the queue was
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already full. If pmBufferOverflow is returned, the overflow flag is set.
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*/
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PmError Pm_Enqueue(PmQueue *queue, void *msg);
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/*
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Pm_QueueFull() returns non-zero if the queue is full
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Pm_QueueEmpty() returns non-zero if the queue is empty
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Either condition may change immediately because a parallel
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enqueue or dequeue operation could be in progress. Furthermore,
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Pm_QueueEmpty() is optimistic: it may say false, when due to
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out-of-order writes, the full message has not arrived. Therefore,
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Pm_Dequeue() could still return 0 after Pm_QueueEmpty() returns
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false. On the other hand, Pm_QueueFull() is pessimistic: if it
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returns false, then Pm_Enqueue() is guaranteed to succeed.
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Error conditions: Pm_QueueFull() returns pmBadPtr if queue is NULL.
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Pm_QueueEmpty() returns FALSE if queue is NULL.
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*/
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int Pm_QueueFull(PmQueue *queue);
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int Pm_QueueEmpty(PmQueue *queue);
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/*
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Pm_QueuePeek() returns a pointer to the item at the head of the queue,
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or NULL if the queue is empty. The item is not removed from the queue.
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Pm_QueuePeek() will not indicate when an overflow occurs. If you want
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to get and check pmBufferOverflow messages, use the return value of
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Pm_QueuePeek() *only* as an indication that you should call
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Pm_Dequeue(). At the point where a direct call to Pm_Dequeue() would
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return pmBufferOverflow, Pm_QueuePeek() will return NULL but internally
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clear the pmBufferOverflow flag, enabling Pm_Enqueue() to resume
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enqueuing messages. A subsequent call to Pm_QueuePeek()
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will return a pointer to the first message *after* the overflow.
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Using this as an indication to call Pm_Dequeue(), the first call
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to Pm_Dequeue() will return pmBufferOverflow. The second call will
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return success, copying the same message pointed to by the previous
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Pm_QueuePeek().
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When to use Pm_QueuePeek(): (1) when you need to look at the message
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data to decide who should be called to receive it. (2) when you need
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to know a message is ready but cannot accept the message.
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Note that Pm_QueuePeek() is not a fast check, so if possible, you
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might as well just call Pm_Dequeue() and accept the data if it is there.
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*/
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void *Pm_QueuePeek(PmQueue *queue);
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/*
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Pm_SetOverflow() allows the writer (enqueuer) to signal an overflow
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condition to the reader (dequeuer). E.g. when transfering data from
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the OS to an application, if the OS indicates a buffer overrun,
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Pm_SetOverflow() can be used to insure that the reader receives a
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pmBufferOverflow result from Pm_Dequeue(). Returns pmBadPtr if queue
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is NULL, returns pmBufferOverflow if buffer is already in an overflow
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state, returns pmNoError if successfully set overflow state.
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*/
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PmError Pm_SetOverflow(PmQueue *queue);
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#ifdef __cplusplus
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}
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#endif /* __cplusplus */
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