{"id":4043,"date":"2013-06-19T07:00:00","date_gmt":"2013-06-19T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2013\/06\/19\/attachthreadinput-is-like-taking-two-threads-and-pooling-their-money-into-a-joint-bank-account-where-both-parties-need-to-be-present-in-order-to-withdraw-any-money\/"},"modified":"2013-06-19T07:00:00","modified_gmt":"2013-06-19T07:00:00","slug":"attachthreadinput-is-like-taking-two-threads-and-pooling-their-money-into-a-joint-bank-account-where-both-parties-need-to-be-present-in-order-to-withdraw-any-money","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20130619-00\/?p=4043","title":{"rendered":"AttachThreadInput is like taking two threads and pooling their money into a joint bank account, where both parties need to be present in order to withdraw any money"},"content":{"rendered":"

\nConsider this code:\n<\/P>\n

\n\/\/ Code in italics is wrong\n   foregroundThreadId = ::GetWindowThreadProcessId(::GetForegroundWindow(), 0); \n   myThreadId = GetCurrentThreadId(); <\/p>\n
   if (foregroundThreadId != myThreadId) \n   { \n       AttachThreadInput(foregroundThreadId, myThreadId, TRUE); \n       BringWindowToTop(myWindowHandle);<\/I>\n<\/PRE>\n
\nIf you try to step over the\nAttach­Thread­Input<\/CODE>\ncall in the debugger,\nboth the debugger and the application being debugged will freeze.\nWhy is that?\n<\/P>\n
\nThis should look familiar because\nit’s basically\n\nthe same code that I warned you about several years ago<\/A>.\nThe code grabs the current foreground window and attaches\nits input state to the current thread.\nNow you’re in trouble.\n<\/P>\n
\nRemember dual-signature bank accounts?\nThese were bank accounts that required the signatures of both\naccount holders in order to make a withdrawal.\nIt can work out fine\nif the two parties trust each other with a shared bank account\nand can coordinate their actions so that when one of them needs money,\nit can go to the other and say,\n“Hey, can you sign this withdrawal slip?\nI need some money.”\n(Another use case for dual-signature bank accounts was a parent\nwanting to monitor their child’s spending.)\n<\/P>\n
\nAttach­Thread­Input<\/CODE>\ntells the window manager,\n“Please take these two threads and put all their money\nin a dual-signature bank account.”\n<\/P>\n
\nIn the case above, the code said,\n“See that random person being served by the bank teller<\/I>?\nPlease take all my money and all his money and\nput them into a dual-signature bank account.”\n<\/P>\n
\nAs you can imagine, this is a bad idea,\nboth for you and for the other person.\nYou cannot withdraw any money until you can somehow\ntrack down that random person and get him to sign the withdrawal form.\nAnd it’s not like you have any relationship with that\nperson—you don’t even know his name!—so the only chance\nyou have is to go down to the bank and hang out there hoping that\nthe other guy will show up to make a withdrawal as part of his normal\ncourse of business,\nand then you can say,\n“Hey, you there!\nSign this for me, will ya?”\n<\/P>\n
\nThe other person is in a similar predicament.\nWhen he goes to the bank to make a withdrawal, the teller will say,\n“I’m sorry, sir, but your money is in a dual-signature account,\nand your withdrawal slip has only one signature on it.”\nHe’s stuck doing the same thing that you do:\nWhenever he wants to withdraw money, he has to go to the bank\nand hang around hoping that you will show up eventually.\n<\/P>\n\n\n    \n    \n    
bank account<\/TD>\n    ↔<\/TD>\n    input queue\n<\/TR>\n
money<\/TD>\n    ↔<\/TD>\n    input\n<\/TR>\n
go to the bank<\/TD>\n    ↔<\/TD>\n    check the message queue\n<\/TR>\n<\/TABLE>\n
\nIn this case,\nwhat happened was that the code grabbed the debugger and said,\n“Okay, we now have a dual-signature bank account!”\nAnd now you’re stuck.\nThe debugger cannot withdraw any money\nbecause it is waiting for you to go to the bank.\nBut you can’t go to the bank because you’re broken into the debugger.\nResult: Nobody gets any money.\n<\/P>\n
\nThis is why you shouldn’t grab random people in the bank and unilaterally\ncreate dual-signature bank accounts with them.\n<\/P>\n
\nReminder<\/B>:\nAttaching input queues is not a Get Out of Jail Free card.\n\nIt’s a\nGet Into the Same Jail<\/I> card<\/A>.\n<\/P><\/p>\n","protected":false},"excerpt":{"rendered":"
Consider this code: \/\/ Code in italics is wrong foregroundThreadId = ::GetWindowThreadProcessId(::GetForegroundWindow(), 0); myThreadId = GetCurrentThreadId(); if (foregroundThreadId != myThreadId) { AttachThreadInput(foregroundThreadId, myThreadId, TRUE); BringWindowToTop(myWindowHandle); If you try to step over the Attach­Thread­Input call in the debugger, both the debugger and the application being debugged will freeze. Why is that? This should look familiar because […]<\/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-4043","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Consider this code: \/\/ Code in italics is wrong foregroundThreadId = ::GetWindowThreadProcessId(::GetForegroundWindow(), 0); myThreadId = GetCurrentThreadId(); if (foregroundThreadId != myThreadId) { AttachThreadInput(foregroundThreadId, myThreadId, TRUE); BringWindowToTop(myWindowHandle); If you try to step over the Attach­Thread­Input call in the debugger, both the debugger and the application being debugged will freeze. Why is that? This should look familiar because […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/4043","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=4043"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/4043\/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=4043"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=4043"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=4043"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}