1 /* pmutil.h -- some helpful utilities for building midi
2 applications that use PortMidi
7 #endif /* __cplusplus */
12 A single-reader, single-writer queue is created by
13 Pm_QueueCreate(), which takes the number of messages and
14 the message size as parameters. The queue only accepts
15 fixed sized messages. Returns NULL if memory cannot be allocated.
17 This queue implementation uses the "light pipe" algorithm which
18 operates correctly even with multi-processors and out-of-order
19 memory writes. (see Alexander Dokumentov, "Lock-free Interprocess
20 Communication," Dr. Dobbs Portal, http://www.ddj.com/,
21 articleID=189401457, June 15, 2006. This algorithm requires
22 that messages be translated to a form where no words contain
23 zeros. Each word becomes its own "data valid" tag. Because of
24 this translation, we cannot return a pointer to data still in
25 the queue when the "peek" method is called. Instead, a buffer
26 is preallocated so that data can be copied there. Pm_QueuePeek()
27 dequeues a message into this buffer and returns a pointer to
28 it. A subsequent Pm_Dequeue() will copy from this buffer.
30 This implementation does not try to keep reader/writer data in
31 separate cache lines or prevent thrashing on cache lines.
32 However, this algorithm differs by doing inserts/removals in
33 units of messages rather than units of machine words. Some
34 performance improvement might be obtained by not clearing data
35 immediately after a read, but instead by waiting for the end
36 of the cache line, especially if messages are smaller than
37 cache lines. See the Dokumentov article for explanation.
39 The algorithm is extended to handle "overflow" reporting. To report
40 an overflow, the sender writes the current tail position to a field.
41 The receiver must acknowlege receipt by zeroing the field. The sender
42 will not send more until the field is zeroed.
44 Pm_QueueDestroy() destroys the queue and frees its storage.
47 PMEXPORT PmQueue *Pm_QueueCreate(long num_msgs, int32_t bytes_per_msg);
48 PMEXPORT PmError Pm_QueueDestroy(PmQueue *queue);
51 Pm_Dequeue() removes one item from the queue, copying it into msg.
52 Returns 1 if successful, and 0 if the queue is empty.
53 Returns pmBufferOverflow if what would have been the next thing
54 in the queue was dropped due to overflow. (So when overflow occurs,
55 the receiver can receive a queue full of messages before getting the
56 overflow report. This protocol ensures that the reader will be
57 notified when data is lost due to overflow.
59 PMEXPORT PmError Pm_Dequeue(PmQueue *queue, void *msg);
63 Pm_Enqueue() inserts one item into the queue, copying it from msg.
64 Returns pmNoError if successful and pmBufferOverflow if the queue was
65 already full. If pmBufferOverflow is returned, the overflow flag is set.
67 PMEXPORT PmError Pm_Enqueue(PmQueue *queue, void *msg);
71 Pm_QueueFull() returns non-zero if the queue is full
72 Pm_QueueEmpty() returns non-zero if the queue is empty
74 Either condition may change immediately because a parallel
75 enqueue or dequeue operation could be in progress. Furthermore,
76 Pm_QueueEmpty() is optimistic: it may say false, when due to
77 out-of-order writes, the full message has not arrived. Therefore,
78 Pm_Dequeue() could still return 0 after Pm_QueueEmpty() returns
79 false. On the other hand, Pm_QueueFull() is pessimistic: if it
80 returns false, then Pm_Enqueue() is guaranteed to succeed.
82 Error conditions: Pm_QueueFull() returns pmBadPtr if queue is NULL.
83 Pm_QueueEmpty() returns FALSE if queue is NULL.
85 PMEXPORT int Pm_QueueFull(PmQueue *queue);
86 PMEXPORT int Pm_QueueEmpty(PmQueue *queue);
90 Pm_QueuePeek() returns a pointer to the item at the head of the queue,
91 or NULL if the queue is empty. The item is not removed from the queue.
92 Pm_QueuePeek() will not indicate when an overflow occurs. If you want
93 to get and check pmBufferOverflow messages, use the return value of
94 Pm_QueuePeek() *only* as an indication that you should call
95 Pm_Dequeue(). At the point where a direct call to Pm_Dequeue() would
96 return pmBufferOverflow, Pm_QueuePeek() will return NULL but internally
97 clear the pmBufferOverflow flag, enabling Pm_Enqueue() to resume
98 enqueuing messages. A subsequent call to Pm_QueuePeek()
99 will return a pointer to the first message *after* the overflow.
100 Using this as an indication to call Pm_Dequeue(), the first call
101 to Pm_Dequeue() will return pmBufferOverflow. The second call will
102 return success, copying the same message pointed to by the previous
105 When to use Pm_QueuePeek(): (1) when you need to look at the message
106 data to decide who should be called to receive it. (2) when you need
107 to know a message is ready but cannot accept the message.
109 Note that Pm_QueuePeek() is not a fast check, so if possible, you
110 might as well just call Pm_Dequeue() and accept the data if it is there.
112 PMEXPORT void *Pm_QueuePeek(PmQueue *queue);
115 Pm_SetOverflow() allows the writer (enqueuer) to signal an overflow
116 condition to the reader (dequeuer). E.g. when transfering data from
117 the OS to an application, if the OS indicates a buffer overrun,
118 Pm_SetOverflow() can be used to insure that the reader receives a
119 pmBufferOverflow result from Pm_Dequeue(). Returns pmBadPtr if queue
120 is NULL, returns pmBufferOverflow if buffer is already in an overflow
121 state, returns pmNoError if successfully set overflow state.
123 PMEXPORT PmError Pm_SetOverflow(PmQueue *queue);
127 #endif /* __cplusplus */