{"id":28523,"date":"2007-01-03T10:00:15","date_gmt":"2007-01-03T10:00:15","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2007\/01\/03\/wait-but-why-can-i-getprocaddress-for-isdialogmessage\/"},"modified":"2007-01-03T10:00:15","modified_gmt":"2007-01-03T10:00:15","slug":"wait-but-why-can-i-getprocaddress-for-isdialogmessage","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20070103-15\/?p=28523","title":{"rendered":"Wait, but why can I GetProcAddress for IsDialogMessage?"},"content":{"rendered":"

\nOkay, so\n\nI explained<\/a>\nthat a lot of so-called functions are\nreally redirecting macros, function-like macros, intrinsic\nfunctions, and inline functions,\nand consequently, GetProcAddress<\/code> won’t actually\nget anything since the function doesn’t exist in the form of\nan exported function.\nBut why, then, can you GetProcAddress<\/code> for\nIsDialogMessage<\/code>?\n<\/p>\n

\nLet’s take a closer look at the exports from user32.dll<\/code>.\nHere’s the relevant excerpt.\n<\/p>\n

\n        417  1A0 0002C661 IsDialogMessage\n        418  1A1 0002C661 IsDialogMessageA\n        419  1A2 0001DFBC IsDialogMessageW\n<\/pre>\n
\nNotice that this function is exported three<\/strong> ways.\nThe last two are the ones you expect,\nIsDialogMessageA<\/code> for ANSI callers and\nIsDialogMessageW<\/code> for UNICODE callers.\nThat first one is the one you didn’t expect:\nIsDialogMessage<\/code> with no A or W suffix.\nBut notice that its entry point address is identical to that\nof IsDialogMessageA<\/code>.\nThe\nIsDialogMessage<\/code> entry point is just an alias for\nIsDialogMessageA<\/code>.\n<\/p>\n
\nThis phantom third function is hidden from C and C++ programs\nbecause any attempt to call\nIsDialogMessage<\/code> gets converted to\nIsDialogMessageA<\/code> or\nIsDialogMessageW<\/code> due to the redirection macro:\n<\/p>\n
\n#ifdef UNICODE\n#define IsDialogMessage  IsDialogMessageW\n#else\n#define IsDialogMessage  IsDialogMessageA\n#endif \/\/ !UNICODE\n<\/pre>\n
\n(Of course, you can play fancy games to remove the redirection\nmacros; I’m just talking about the non-fancy case.)\nIf nobody can call the function, then why does it exist?\n<\/p>\n
\nBecause of mistakes made long ago.\n<\/p>\n
\nIf you hunt through user32.dll<\/code> you’ll find a few\nother functions that follow a similar pattern of having\nthree versions, an A version, a W version, and a phantom\nundecorated version (which is an alias for the A version).\nAt one point long ago, the function existed only in an undecorated\nversion.\nThis turned out to have been a mistake, since there was a\ncharacter set dependency in the parameters (perhaps obvious,\nperhaps subtle).\nThe mistake was corrected by splitting the function into\nthe A and W versions you see today, but in order to maintain\ncompatibility with older programs that were written before\nthe mistake was recognized, the original undecorated function was left\nin the export table.\n<\/p>\n
\n\nWhen you don’t have a time machine<\/a>,\nyou have to live with your mistakes.\n<\/p>\n
\nIn a sense, these functions are vestigial organs of Win32.\n<\/p>\n
\nPostscript<\/b>:\nUnfortunately, like your appendix, which can get infected,\nthese vestigial organs can create a different sort of infection:\nIf you are using p\/invoke to call these functions and mistakenly\noverride the default name declaration with\nExactSpelling=true<\/code>, like so:\n<\/p>\n
\n[DllImport(\"user32.dll\", ExactSpelling=true)]\npublic static extern\nbool IsDialogMessage(IntPtr hWndDlg,\n                     [In] ref MSG msg);\n<\/pre>\n
\nthen you will in fact get the normally-inaccessible undecorated name,\nsince you specified that you wanted the exact spelling.\nThis highlights once again that you need to be alert when\ndoing interop programming:\nYou get what you ask for, which might not be what you actually wanted.<\/p>\n","protected":false},"excerpt":{"rendered":"
Okay, so I explained that a lot of so-called functions are really redirecting macros, function-like macros, intrinsic functions, and inline functions, and consequently, GetProcAddress won’t actually get anything since the function doesn’t exist in the form of an exported function. But why, then, can you GetProcAddress for IsDialogMessage? Let’s take a closer look at the […]<\/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":[2],"class_list":["post-28523","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-history"],"acf":[],"blog_post_summary":"
Okay, so I explained that a lot of so-called functions are really redirecting macros, function-like macros, intrinsic functions, and inline functions, and consequently, GetProcAddress won’t actually get anything since the function doesn’t exist in the form of an exported function. But why, then, can you GetProcAddress for IsDialogMessage? Let’s take a closer look at the […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/28523","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=28523"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/28523\/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=28523"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=28523"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=28523"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}