\nIn honor of National Puzzle Day,\nI leave you today with a puzzle\nbased on an actual customer problem.\n<\/P>\n
\nPart One<\/B>: The customer explains the problem.\n<\/P>\n \nWe have this DLL, and during its startup, it creates a thread\nwith the following thread procedure:\n<\/P>\n \nDuring process shutdown, the following function is called\nas part of \/\/ wait for it to stop\n WaitForSingleObject(ThreadCompleteEvent, INFINITE);<\/p>\n \/\/ Clean up\n CloseHandle(ShutdownEvent);\n ShutdownEvent = NULL;<\/p>\n CloseHandle(ThreadCompleteEvent);\n ThreadCompleteEvent = NULL;\n}\n<\/PRE>\n \nThe above function is hanging at the call to\n \nI checked, and nobody sets the \nI am completely stumped as to how this thread disappeared.\n<\/P>\n<\/BLOCKQUOTE>\n \nYou already know enough to explain how the thread disappeared.\n<\/P>\n \nPart Two<\/B>: After providing your explanation, the customer\ncame up with this solution.\n<\/P>\n \nThank you for your explanation.\nWe’ve made the following changes to fix the problem.\nAgain, thank you for your help.\n<\/P>\n void StopWorkerThread()\n{\n \/\/ tell the thread to stop\n SetEvent(ShutdownEvent);<\/p>\n \/\/ wait for the thread<\/FONT>\n WaitForSingleObject(ThreadHandle<\/FONT>, INFINITE);<\/p>\n \/\/ Clean up\n CloseHandle(ShutdownEvent);\n ShutdownEvent = NULL;\n}\n<\/PRE>\n<\/BLOCKQUOTE>\n \nCriticize this proposed solution.\n<\/P>\n \nPart Three<\/B>:\nEven though the proposed solution is flawed,\nexplain why it doesn’t cause a problem in practice.\n(I.e., explain why the customer is always lucky.)\n<\/P><\/p>\n","protected":false},"excerpt":{"rendered":" In honor of National Puzzle Day, I leave you today with a puzzle based on an actual customer problem. Part One: The customer explains the problem. We have this DLL, and during its startup, it creates a thread with the following thread procedure: DWORD CALLBACK ThreadFunction(void *) { HANDLE HandleArray[2]; HandleArray[0] = SetUpStuff(); if (HandleArray[0]) […]<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-19333","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":" In honor of National Puzzle Day, I leave you today with a puzzle based on an actual customer problem. Part One: The customer explains the problem. We have this DLL, and during its startup, it creates a thread with the following thread procedure: DWORD CALLBACK ThreadFunction(void *) { HANDLE HandleArray[2]; HandleArray[0] = SetUpStuff(); if (HandleArray[0]) […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/19333","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=19333"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/19333\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=19333"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=19333"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=19333"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}\n
\nDWORD CALLBACK ThreadFunction(void *)\n{\n HANDLE HandleArray[2];\n HandleArray[0] = SetUpStuff();\n if (HandleArray[0]) {\n HandleArray[1] = ShutdownEvent;\n while (WaitForMultipleObjects(2, HandleArray,\n FALSE, INFINITE) == WAIT_OBJECT_0) {\n ProcessStuff();\n }\n CleanUpStuff(HandleArray[0]);\n }\n SetEvent(ThreadCompleteEvent);\n FreeLibraryAndExitThread(ThisLibrary, 0);\n}\n<\/PRE>\nDLL_PROCESS_DETACH<\/CODE> handling:\n<\/P>\n\nvoid StopWorkerThread()\n{\n \/\/ tell the thread to stop\n SetEvent(ShutdownEvent);<\/p>\nWaitForSingleObject<\/CODE>.\nIf we break in, we see that the thread that is supposed\nto be running the ThreadFunction<\/CODE> is gone.\nI verified that the thread was successfully created,\nbut by the time we get around to waiting for it, it’s already gone.\n<\/P>\nThreadCompleteEvent<\/CODE>\nexcept the StopWorkerThread<\/cODE> function.\nI stepped through SetUpStuff<\/CODE>, and it succeeded.\nHowever, a breakpoint on CleanUpStuff<\/CODE> was never hit.\nNo exceptions were thrown either.\n<\/P>\n\n
\nDWORD CALLBACK ThreadFunction(void *)\n{\n HANDLE HandleArray[2];\n HandleArray[0] = SetUpStuff();\n if (HandleArray[0]) {\n HandleArray[1] = ShutdownEvent;\n while (WaitForMultipleObjects(2, HandleArray,\n FALSE, INFINITE) == WAIT_OBJECT_0) {\n ProcessStuff();\n }\n CleanUpStuff(HandleArray[0]);\n }\n \/\/ SetEvent(ThreadCompleteEvent);<\/STRIKE><\/FONT>\n FreeLibraryAndExitThread(ThisLibrary, 0);\n}<\/p>\n