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to service multiple files, sockets, and / or manually posted packets Unless there are many, many requests going against a single device HAND L E , hav ing a port dedicated to each one adds unnecessary overhead Reuse is typically best After a port has been created like this, you can then call C re a t e loCom pletion Port and pass a HAN D L E to a pre-existing port as the E x i s t i n gCom p l et ion Port argument The OS will then use the existing port for the particular file HAND L E (or SOC K ET, as we will soon see) The device HAND L E supplied must be one that was opened for overlapped I /O This is how the legacy thread pool's B i n d loCom p l e t i o n C a l l b a c k, Vista thread pool's C reateT h r e a d po o l I o, and the CLR thread pool's B i n d H a n d l e functions are implemented The Com p l e t i o n Key is an opaque value that will be supplied to any thread that completes due to I / O completions posted to the particular file (which is irrelevant if a file is not specified) Unfortunately, there's no easy way to supply a callback to run when a thread waiting for work awakens (as with APCs above), but the Comp l e t i o n Key can be a handy way of pass ing a function pointer that is to be executed by the thread that wakes up This requires an application specific convention to be established As you may have guessed, this is exactly how the thread pools work: they have some internal convention for passing completion routines as function pointers and delegates around in the I / O completion registration The Numbe rOfCon c u r rentTh r e a d s indicates how many threads the OS should use for servicing packets Often this should be the number of processors-based on the logic outlined earlier-but doesn't necessarily need to be For example, if you have many ports in a single process, it may make sense to distribute the number of threads used more evenly This parameter is ignored if you don't pass NU L L for the E x i s t i ngComp let i o n Port So now that you've got a port created, what do you do with one You'll probably create threads (like the aforementioned thread pools) to wait for packets Waiting for a completion packet is done with the GetQu e u edCom pletionSt a t u s API
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BOOl WINAPI GetQue uedComplet ionSt at u s ( HANDLE Com p l et ionPort , lPDWORD IpNumbe rOfByt e s ,
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PU lONG_PTR IpCompletionKey , l POV E R lAPPED * I pOve r l a pped , DWORD dwMi l l i s e c on d s
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This function blocks until a new packet arrives and the thread is selec tively unblocked based on the runnable thread throttling logic in the OS You pass to it the Comp l e t i o n Port you'd like to wait on, a bunch of argu ments into which data associated with the completion packet will be placed, and a dwMi l l i s e c o n d s timeout The timeout works the same way as those you've seen previously, that is, I N F I N I T E ( - 1 ) to specify "no timeout," e to avoid blocking, or some other number of milliseconds otherwise The l pN u m b e rOfByt e s DWO RD receives the number of bytes associated with the completion, l pCom p l e t i o n Key is set to the key passed to the completion port creation routine, and the OVE R LAP P E D contains additional information about the completion The API returns FALS E if an error or a timeout occurs To differentiate between the two, call Get L a s t E r r o r and look for a return of WAIT_T I M E OUT Notice that GetQu e u edCom p l e t i o n S t at u s does not offer a way to pump for messages or to do an alertable wait This can cause some problems in systems that use APCs to take back control of threads, for example In such cases, you may need to rely on timeouts instead There is a GetQu e u edComp l et i o n S t a t u s E x method that was added in Windows Vista and Server 2008, which provides two additional useful fea tures when compared to its counterpart First, you can receive multiple completion entries at once This reduces performance overhead, due to fewer kernel transitions and internal locks being taken, and can be useful on heavily loaded server programs that can experience times during which I / Os are finishing faster than they can be processed Second, you can specify that the wait be alertable
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BOO l WINAPI GetQueuedCompletionSt a t u s E x ( HAN D L E CompletionPort , l POV E R lAPPED_ENTRY I pComp let ionPort E n t r ie s , U lONG u lCount , PU lONG u l N u m E n t r i e s Removed , DWORD dwMi l l i s e c o nd s , BOOl fAlert a b l e
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