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The most basic function that the thread pool performs is enabling you to queue a callback for execution, represented in native code by a function pointer and L PVOI D pair Submitting work to execute on a thread pool thread is fairly straightforward The simplest way to do so is with the TrySu bmitTh r e a d poolCa l l b a c k API
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BOO L WINAPI TryS u bmitThread poolC a l l b a c k ( PTP_S I M P L E_CAL L BAC K pfn s , PVOID p v , PTP_CAL L BAC K_ENVI RON p c be
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The pfn s argument is a pointer to a callback function that will be invoked on a thread running in the thread pool, and the pv argument is an optional state argument, passed as the callback's Cont ext argument
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VOID CAL LBAC K SimpleCa l l b a c k ( PTP_CAL L BACK_INSTANCE I n s t a n c e , PVOID Context )j
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The callback environment argument, p c be, allows you to control where, specifically, the work gets executed For now we will always pass NU L L and ignore callback environments completely, though they are quite useful and we will return to them later The thread pool supplies the I n st a n c e argument to the callback, which is just a pointer to an internally managed thread pool data structure; this structure can be used as an input argument to various other APIs that manage state associated with the callback (as we'll see later) After T ryS u b m itTh r e a d poolWo r k returns T R U E , the work has been enqueued into the work queue The callback threads monitor this queue for new work, running inside a loop that continuously dequeues and executes items as quickly as possible After our work item has been enqueued, any of the thread pool threads are apt to dequeue and execute the work Which particular one happens to run the work and the precise timing of its exe cution are determined by a combination of the queue contents and what threads are doing at that particular point in time The TryS ubmi tTh readpoolC a l l b a c k function can fail-hence the Try part of its name-in which case the function returns FALS E and Get L a st E r ro r can b e used t o retrieve failure details This i s usually caused b y insufficient memory to allocate the necessary internal data structures This should rarely happen except for low resource situations Nevertheless, it is possible and, thus, needs to be considered and handled Note that because all of the APIs in this section are new to Windows Vista, you will need to define _WI N 3 2_WINNT to be elxel6elel before importing W i n dows h to access them
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An Alternative Way to Submit Work There is an alternative way to sub mit work items to the pool It's a multi-step process instead of a single API
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call, but gives you two additional capabilities: you can submit the same work item object multiple times, and you can easily wait for the submitted work to finish The latter is a very useful feature, so you'll probably find yourself using this alternative approach quite often The first step is to call the C reateTh r e a d poolWo r k API
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PTP_WORK WINAPI C reateThread poolWork ( PTP_WOR K_CA L L BAC K pfnwk , PVOID p v , PTP_CAL L BAC K_ENVI RON p c be );
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You supply a function pointer representing the work to be done con currently, a PVO I D state argument, and, as with TryS u bm i t T h r e a d poolWo r k, an environment (for which we will pass NU L L for now) It gives back a pointer to a newly allocated TP _WOR K structure, which is then submitted for execution with the S u bmitTh r e a d poolWo r k function
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VOID WINAPI S u bmitTh readpoolWork ( PTP_WORK pwk ) ;
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Notice the pfnwk callback type is PTP_WOR K_CAL L BAC K rather than PTP_S IMP L E_CA L L BACK, as was taken by T ryS u bm i tTh r e a d poolCa U b a c k The only difference between them i s that you can now access the TP_WO RK object from inside the callback, whereas the TP _WO R K object was entirely hidden with the previous scheme
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VOI D CAL L BAC K WorkCa l l ba c k ( PTP_CAL L BAC K_INSTAN C E I n s t a n c e , PVOID Context , PTP_WORK Work );
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C r e ateTh r e a d poolWo r k will return N U L L if it wasn't able to allocate the TP_WOR K data structure Check Get L a s t E r r o r for failure details
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Somewhat cleverly, S u bmi tTh r e a d poolWo r k will not fail; this is because the internal data structures used to queue work rely on storage that has already been allocated by reusing memory in the TP _WOR K structure to link submissions together When I say it cannot fail, that's not entirely true: the API doesn't validate the pwk argument, so if you pass garbage to it, you're likely to see an AV or memory corruption
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