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Notice here that we're passing a stack allocated array ( d a t a ) as the location where the read operation will put data from the read Recall from earlier that this data must last at least as long as the asynchronous I / O itself S o this technique, while applicable t o such a simple example, i s usu ally not going to work We'll continue using it as long as possible because it simplifies the example, but typically you'll need to resort to heap alloca tion and manual freeing of buffers If 1/ 0 completion is used, a completion packet will still be generated even though we are able to handle the I / O synchronously Additionally, the file HAND L E will be set by the OS (as we'll see later) If code has been written to handle the synchronous completion, these two things are unnecessary and can lead to performance degradation A new API was added to Windows Vista and Windows Server 2008 to allow suppression of these steps
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Two flags are available for the F l a g s argument, corresponding directly to the two unneeded steps mentioned above: F I L E_S K I P_COMP L E T ION_PORT_ON_SUCC E S S avoids queuing a packet to a port if the HAN D L E has been bound, and F I L E_S K I P_S E T_EV E N T_ON_HAND L E skips setting the file
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HAND L E If a custom HAN D L E was provided in the h E v e nt field of the OVE R LAP P E D structure, i t will still be set even if this flag was passed
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Method #2: Po lling with GetOverlappedResult Next to synchronous completion, the simplest rendezvous technique is to poll for completion Polling is the act of periodically checking whether the I / O has completed: if it hasn't, some useful application specific work can be done, and if it has finished, the I / O request can be processed accordingly This is done using the GetOv e r l a pped Re s u lt API shown earlier
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The following code snippet demonstrates how one might use polling to continue doing work while some asynchronous I / O is underway Syn chronous completion is omitted (see the previous code snippet)
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II Async 1/0 is s t i l l o c c u r r i n g We j u st loop I I around a n d keep doing some u s eful wo rk cont i n u e ;
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} I I ( Ha n d l e ot h e r t y p e s o f e r r o r s ) } I I Asy n c h ronou s 1 / 0 i s done - - j u st exit t h e loop brea k ; } brea k ; I I ( Ha n d l e ot h e r types o f e r ro r s ) } else
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