Review: The Four Event Usage Models in .NET

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Review: The Four Event Usage Models
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The combination of auto- and manual-reset events with SetEvent and PulseEvent gives four distinct ways to use events Each combination is unique and each is useful, or even necessary, in some situations, and each model combination will be used in an example or exercise, either in this chapter or the next Warning: Events, if not used properly, can cause race conditions, deadlocks, and other subtle and difficult-to-diagnose errors 10 describes techniques that are almost always required if you are using events in any but the simplest situations Table 8-1 describes the four situations
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Table 8-1 Summary of Event Behavior
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Auto-Reset Event Manual-Reset Event
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SetEvent
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Exactly one thread is released If none is currently waiting on the event, the first thread to wait on it in the future will be released immediately The event is automatically reset
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All currently waiting threads are released The event remains signaled until reset by some thread
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PulseEvent Exactly one thread is released, but only if a thread is currently waiting on the event
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All currently waiting threads, if any, are released, and the event is then reset to nonsignaled
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An auto-reset event can be thought of as a door with a spring that slams the door shut, whereas a
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manual-reset event does not have a spring and will remain open Using this metaphor, PulseEvent opens the door and immediately shuts it after one (auto-reset) or all (manual-reset) waiting threads go through the door SetEvent opens the door and releases it
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Example: A Producer/Consumer System
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This example extends Program 8-1 so that the consumer can wait until there is an available message This eliminates the problem that requires the consumer to try again if a new message is not available The resulting program, Program 8-2, is called eventPC Notice that the solution uses a mutex rather than a CRITICAL_SECTION; there is no reason for this other than to illustrate mutex usage The use of an auto-reset event and SetEvent in the producer are, however, essential for correct operation to ensure that just one thread is released Also notice how the mutex and event are both associated with the message block data structure The mutex enforces the critical code section for accessing the data structure object, and the event is used to signal the fact that there is a new message Generalizing, the mutex ensures the object's invariants, and the event signals that the object is in a specified state This basic technique is used extensively in later chapters
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Program 8-2 eventPC: A Signaling Producer and Consumer
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/* /* /* /* /* 8 eventPCc */ Maintain two threads, a producer and a consumer */ The producer periodically creates checksummed data buffers, */ or "message blocks," signaling the consumer that a message */ is ready The consumer displays when prompted */
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#include "EvryThngh" #include <timeh> #define DATA_SIZE 256 typedef struct msg_block_tag { /* Message block */ volatile DWORD f_ready, f_stop; /* Ready state flag, stop flag */ volatile DWORD sequence; /* Message block sequence number */ volatile DWORD nCons, nLost; time_t timestamp; HANDLE mguard; /* Mutex to guard the message block structure */ HANDLE mready; /* "Message ready" event */ DWORD checksum; /* Message contents checksum */ DWORD data [DATA_SIZE]; /* Message contents */ } MSG_BLOCK; /* */ DWORD _tmain (DWORD argc, LPTSTR argv []) { DWORD Status, ThId; HANDLE produce_h, consume_h; /* Initialize the message block mutex and event (auto-reset) */ mblockmguard = CreateMutex (NULL, FALSE, NULL); mblockmready = CreateEvent (NULL, FALSE, FALSE, NULL); /* Create producer and consumer; wait until they terminate */ /* As in Program 91 */ CloseHandle (mblockmguard); CloseHandle (mblockmready);
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_tprintf (_T ("Producer and consumer threads terminated\n")); _tprintf (_T ("Produced: %d, Consumed: %d, Known Lost: %d\n"), mblocksequence, mblocknCons, mblocknLost); return 0; } DWORD WINAPI produce (void *arg) /* Producer thread -- create new messages at random intervals */ { srand ((DWORD)time(NULL)); /* Seed the random # generator */ while (!mblockf_stop) { /* Random delay */ Sleep (rand () / 10); /* Wait a long period for next message */ /* Get the buffer, fill it */ WaitForSingleObject (mblockmguard, INFINITE); __try { if (!mblockf_stop) { mblockf_ready = 0; MessageFill (&mblock); mblockf_ready = 1; mblocksequence++; SetEvent(mblockmready); /* Signal "message ready" */ } } __finally { ReleaseMutex (mblockmguard); } } return 0; } DWORD WINAPI consume (void *arg) { DWORD ShutDown = 0; CHAR command, extra; /* Consume the NEXT message when prompted by the user */ while (!ShutDown) { /* Only thread accessing stdin, stdout */ _tprintf (_T ("\n**Enter 'c' for consume; 's' to stop: ")); _tscanf ("%c%c", &command, &extra); if (command == 's') { WaitForSingleObject (mblockmguard, INFINITE); ShutDown = mblockf_stop = 1; ReleaseMutex (mblockmguard); } else if (command == 'c') { /* Get new buffer to consume */ WaitForSingleObject (mblockmready, INFINITE); WaitForSingleObject (mblockmguard, INFINITE); __try { if (!mblockf_ready) _leave; /* Wait for the event indicating a message is ready */ MessageDisplay (&mblock); mblocknCons++; mblocknLost = mblocksequence - mblocknCons; mblockf_ready = 0; /* No new messages are ready */ } __finally { ReleaseMutex (mblockmguard); } } else { _tprintf (_T ("Illegal command Try again\n")); } } return 0;
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Note: There is a possibility that the consumer, having received the message ready event, will not actually process the current message if the producer generates yet another message before the consumer acquires the mutex This behavior could cause a consumer to process a single message twice if it were not for the test at the start of the consumer's __try block This and similar issues will be addressed in 10
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