\nRegardless of \n\nwhich interpretation you use<\/A>,\nit remains the case that posted messages\nare processed ahead of input messages.\nUnder the MSDN interpretation,\nposted messages and input messages all go into the message queue<\/I>,\nbut\n\nposted messages are pulled from the queue before input messages<\/A>.\nUnder the Raymond interpretation,\nposted messages and input messages are kept in separate queues,\nand the message retrieval functions will look first in the posted\nmessage queue before looking in the input queue.\n<\/P>\n \nLet’s run an experiment to see posted messages get processed\nahead of input messages.\nStart with the\n\nnew scratch program<\/A>\nand make these changes:\n<\/P>\n class RootWindow : public Window\n{\npublic:\n virtual LPCTSTR ClassName() { return TEXT(“Scratch”); }\n static RootWindow *Create();<\/p>\n void AppendText(LPCTSTR psz)\n {\n ListBox_SetCurSel(m_hwndChild,\n ListBox_AddString(m_hwndChild, psz));\n }<\/p>\n void AppendFormat(LPCTSTR pszFormat, …)\n {\n va_list ap;\n va_start(ap, pszFormat);\n TCHAR szMsg[256];\n StringCchVPrintf(szMsg, ARRAYSIZE(szMsg), pszFormat, ap);\n AppendText(szMsg);\n va_end(ap);\n }<\/p>\n void LogMessage(const MSG *pmsg)\n {\n AppendFormat(TEXT(“%d\\t%04x\\t%p\\t%p”),\n pmsg->time,\n pmsg->message,\n pmsg->wParam,\n pmsg->lParam);\n }<\/FONT><\/p>\n …\n};<\/p>\n LRESULT RootWindow::OnCreate()\n{\n m_hwndChild = CreateWindow(\n TEXT(“listbox”), NULL,\n LBS_HASSTRINGS | LBS_USETABSTOPS |\n WS_CHILD | WS_VISIBLE | WS_TABSTOP | WS_VSCROLL,\n 0, 0, 0,0, GetHWND(), (HMENU)1, g_hinst, 0);<\/FONT><\/FONT>\n return 0;\n}<\/p>\n int WINAPI WinMain(HINSTANCE hinst, HINSTANCE hinstPrev,\n LPSTR lpCmdLine, int nShowCmd)\n{\n …\n while (GetMessage(&msg, NULL, 0, 0)) {<\/p>\n switch (msg.message) {\n case WM_KEYDOWN:\n prw->AppendText(TEXT(“Sleeping”));\n UpdateWindow(prw->GetHWND());\n Sleep(1000);\n prw->AppendText(TEXT(“Posting”));\n PostMessage(prw->GetHWND(), WM_USER, 0, 0);\n break;\n case WM_KEYUP:\n case WM_USER:\n prw->LogMessage(&msg);\n break;\n }<\/FONT><\/p>\n TranslateMessage(&msg);\n DispatchMessage(&msg);\n …\n}\n<\/PRE>\n \nThis program creates a list box so we can display some output.\nIn the message loop, it sniffs at all the queued messages and\ndoes the following:\n<\/P>\n \nRun this program, and then tap the shift key.\n<\/P>\n \nThe window gets a \nThe \nThat’s because of the rule that says that\nposted messages are processed ahead of input messages.\n(Depending on how you want to look at it, you might say\nthat posted messages are “called out for preferential treatment”\nin the queue,\nor you might say that posted messages are placed in a different\nqueue from input messages, and the posted message queue has\nhigher priority.)\n<\/P>\n \nObserve also that the timestamp on the \nMake the following change to our program:\nChange the message we post from\n \nRun the program again, and again tap the shift key.\nObserve that the posted \nThis little demonstration also reinforces some other things we already knew.\nFor example, it once again shows that the input manager\n\ndoes not wiretap your posted messages<\/A>.\nIf you post a\n Regardless of which interpretation you use, it remains the case that posted messages are processed ahead of input messages. Under the MSDN interpretation, posted messages and input messages all go into the message queue, but posted messages are pulled from the queue before input messages. Under the Raymond interpretation, posted messages and input messages are […]<\/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-4203","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":" Regardless of which interpretation you use, it remains the case that posted messages are processed ahead of input messages. Under the MSDN interpretation, posted messages and input messages all go into the message queue, but posted messages are pulled from the queue before input messages. Under the Raymond interpretation, posted messages and input messages are […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/4203","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=4203"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/4203\/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=4203"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=4203"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=4203"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}\n#include <strsafe.h><\/FONT><\/p>\n
\n
WM_KEYUP<\/CODE> or\n WM_USER<\/CODE>, then it logs the message timestamp\n and some parameters.\nWM_KEYDOWN<\/CODE>,\n then it sleeps without processing messages for one second,\n and then posts a WM_USER<\/CODE> message\n to the main window (which ignores it).\n<\/UL>\nWM_KEYDOWN<\/CODE>\nfor the shift key.\nIt sleeps for one second (plenty of time for you to release\nthe shift key),\nand then posts a WM_USER<\/CODE> message.\n<\/P>\nWM_USER<\/CODE> and WM_KEYUP<\/CODE>\nmessages arrive, and observe via the log window\nthat they arrive out of order<\/I>.\nWM_USER<\/CODE> message arrived first!\n<\/P>\nWM_USER<\/CODE>\nmessage is greater than<\/I> the timestamp on the\nWM_KEYUP<\/CODE> message,\nbecause the key went up before the WM_USER<\/CODE>\nmessage was posted.\nTime has gone backward<\/I>.\n<\/P>\nWM_USER<\/CODE> to\nWM_KEYUP<\/CODE>:\n<\/P>\n\n PostMessage(hwnd, WM_KEYUP<\/FONT>, 0, 0);\n<\/PRE>\n
WM_KEYUP<\/CODE> message\nis processed ahead of the WM_KEYUP<\/CODE> input message.\n(You can see the difference because we posted the\nWM_KEYUP<\/CODE> message with wParam<\/CODE>\nand lParam<\/CODE> both zero,\nwhereas the\nWM_KEYUP<\/CODE> input message has information in\nthose parameters.)\n<\/P>\nWM_KEYUP<\/CODE> message,\nit is treated like a posted message not an input message.\nWe saw earlier that\n\nposting a keyboard message does not update internal input states<\/A>.\nThe keyboard shift states are not updated to match your prank call message.\nIf somebody calls GetQueueStatus<\/CODE>,\nthey will not be told that there is input waiting.\nIt will not wake a\nMsgWaitForMultipleObjects<\/CODE> function\nthat is waiting for QS_INPUT<\/CODE>.\nAnd as we saw here today,\nthe message gets processed out of order.\n<\/P><\/p>\n","protected":false},"excerpt":{"rendered":"