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Generation Code128 in Visual Studio .NET C h a pter 5: W i n d ows Ke r n e l Syn c h ro n i z a t i o n
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b e t o loop over the objects and wait o n each individually But this has drawbacks The performance drawback is obvious: there likely will be a con text switch for every single object, as it becomes signaled The functionality drawback is more subtle: if any of the objects' states are changed by waiting on them-as with mutexes, semaphores, and auto-reset events-the Win dows implementation ensures these changes only occur once all the objects have become signaled, not one by one This ensures that if a thread fails after some objects are signaled, but not others, there will be no state corruption Due to this, the FIFO ordering noted earlier is not strictly preserved for threads doing a wait-all If thread tl does a wait-all on objects A and B, and then A gets signaled, tl must still wait for B to become signaled before wak ing up In the meantime, some other thread t2 is still free to wait on A Instead of holding up t2's wait indefinitely while tl waits for B to also become signaled, Windows will let t2' s wait on a succeed ahead of tl ' s If that resets A's signal, tl will then have to wait for A to become signaled again This behavior also avoids deadlock: say tl waited on objects A and B, in that order, and t2 waited on the same objects in the reverse order, B and then A, the naIve one-at-a-time approach would lead to deadlock This C++ code sample shows a wait-any style wait with boilerplate code that handles the various return values including translating them into an array index
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c o n s t int c H a n d l e s = , HAN D L E wa i t H a n d l e s [ c Ha n d l e s ] ; II populate o u r a r ray with HANDL E s
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I I D o t h e wait ( po s s i bly bloc k i n g t h e t h read ) : DWORD dwWa it Ret Wa i t F o rMu l t i p leObj ect s ( c H a n d l e s , &wa itHandles [ a ] , FALS E , I N F I N I T E ) ; if ( dwWa itRet > = WAIT_OBJ ECT_a && dwWa itRet < WAIT_OB J ECT_a + c H a n d le s )
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{ HANDLE h S i g n a led = waitHand l e s [ dwWa it Ret - WAIT_OBJ ECT_a ] ; I I hSignaled i s a h a n d l e to t h e o b j e c t t h a t bec ame s i g n a l e d e l s e if ( dwWa itRet > = WAIT_ABANDON ED_a && dwWa itRet < WAIT_ABANDON ED_a + c H a n d l e s ) { HAN D L E hAbandoned = waitHand l e s [ dwWa itRet - WAIT_ABANDON ED_a ] ; I I hAba ndoned i s a h a n d l e to t h e mutex t h a t wa s a bandoned }
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T h e B a s i c s : S i g n a l i n g a n d Wa i t i n g
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e l s e if ( dwWa itRet
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WAIT_TIMEOUT )
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} e l s e if ( dwWa itRet
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WAI T_FAI L E D )
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DWORD dwE rror Get l a s t E rror ( ) ; II H a n d l e error condition
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Alertable Waits The Wa it F o r S i n g l e Ob j e c t E x and Wa i t F o rM u lt i p l e Obj e c t s E x APIs have a n extra parameter that we haven' t described yet: BOO l bAl e rt a b l e For the non-E x methods, this is effectively always FALS E But if you pass T R U E explicitly and the thread blocks, i t can be interrupted and wakened before the wait is satisfied by a Windows user-mode asyn chronous procedure call (APC) APCs are discussed later, but in summary An APC unblocks the thread so it can perform some interesting (but often unrelated) work instead of remaining in the wait state They are used by some Win32 infrastructure-like marshaling the bytes read from a file into a buffer after an asynchronous R e a d F i l e E x operation-without you neces sarily being aware of it If an APC interrupts the wait, the call will return even though objects haven't necessarily been signaled In such cases, the return value will be WAIT_IO_COMP l ETION In most cases, the caller should respond to a return value of WAIT_IO_COMP l E TION by reissuing the wait Restarting the wait is a little tricky because of timeouts: if a dwTimeout value other than I N F I N I T E was specified, we will need to manually decrement the number of milliseconds that elapsed since the start of our previous wait Otherwise, we'll possibly wait multiple times with the same original timeout, which would clearly be wrong (eg, if dwTimeout was 1 000, we could wait for 999 milliseconds, wake up due to an APC, wait again for 999 milliseconds, wake up due to an APC, and so forth) This demands some kind of time accounting, as the fol lowing code example illustrates:
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# i n c l u d e < st d io h > #def i n e _WI N 3 2_WINNT axa4aa # i n c l u d e <win dows h > DWORD DoS ingleWa it ( HAN D l E h , DWORD dwMi l l i second s , BOO l bAle rt a b l e )
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