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II E r ror or syn c h ronou s completion
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In this example, I/O happens completely asynchronously Once we notice a T R U E return value from GetOve r l a p p ed R e s u l t, we switch over to processing it Otherwise, there's a placeholder where "useful" work is done This might involve any sort of application specific bookkeeping, such as computing some background statistics, running a Windows message loop to process GUI message, dispatch COM RPC calls, APCs, and so forth You could even dispatch additional I / O requests If you find that there's no useful work to do, pass T R U E to the GetOve r l a pped Re s u lt function and it will block until the I/O completes A higher performance macro is available that inspects data on the OVE R LAP P E D object instead of making a function call This can be used instead of
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BOO l Ha sOve r l a p p e d I oCompleted ( l POVE R lAPPED l pOve r l a p p e d ) ;
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The polling approach generally has the benefit of being low overhead because there are no additional kernel objects to create and manage The code also looks like a synchronous I / O would have, so there isn't much restructuring of program logic needed A disadvantage of polling, however, is that there may be latency between the time an I / O completes and the time our loop gets around to noticing and processing it These delays can add up
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Method #3: Waiting on the Device Handle Directly The polling mecha nism shown above allows you to block waiting for I/O to complete by pass ing T R U E for the bwa it parameter to GetOve r l a p pe d R e s u l t This is often sufficient if you'd like to wait But as we saw in prior chapters, sometimes you need more flexibility over the way a thread waits Maybe you need to pump for GUI messages and run APCs Or maybe you'd like to use a time out so that if I/O doesn't complete quickly, you can go off and do some more application specific bookkeeping (or at least check if any needs to be done) Or perhaps you'd like to wait for multiple kernel objects simultaneously,
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with Wa i t F o rMu l t i p leObj e c t s, including the possibility of waiting on multiple outstanding asynchronous I / O operations All of this is simple to achieve by using the wait APIs to which you've grown accustomed The question then becomes: What HAN D L E should be used The h E vent field of the OV E R LAP P E D structure has probably piqued your interest But we'll get to that shortly For now, you can wait on the same device HAN D L E used to start the asynchronous operation itself The implementation of asynchronous I / O unsignals this HAN D L E before return ing from the function used to start execution and will later signal the HAN D L E once the I / O completes Notice that multiple threads may not use the same HAN D L E in this manner, since the signals will get jumbled up across threads in a way that makes it impossible to determine when I / O has actually finished For example, this code waits on the file HAN D L E to ensure that messages are pumped while we wait for I / O to finish rather than looping around and continuously polling for completion
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