{"id":19693,"date":"2008-12-29T10:00:00","date_gmt":"2008-12-29T10:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2008\/12\/29\/undecorating-names-to-see-why-a-function-cant-be-found\/"},"modified":"2008-12-29T10:00:00","modified_gmt":"2008-12-29T10:00:00","slug":"undecorating-names-to-see-why-a-function-cant-be-found","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20081229-00\/?p=19693","title":{"rendered":"Undecorating names to see why a function can’t be found"},"content":{"rendered":"

\nHere’s a problem inspired by actual events.\n<\/P>\n

\n

\nWhen I build my project, it compiles fine, but it fails during the\nlink step with an unresolved external:\n<\/P>\n

\nprogram.obj : error LNK2001: unresolved external symbol\n“public: virtual wchar_t const * __thiscall\nUILibrary::PushButton::GetName(class UILibrary::StringHolder * *)”\n(?GetName@PushButton@UILibrary@@UAEPB_WPAPAVStringHolder@2@@Z)\n<\/PRE>\n
\nThe function I’m trying to call exists in the source code for\nuilibrary.lib<\/CODE>; I’m looking at it right now.\nAnd the definition in the source code matches the declaration\nin the header file:\n<\/P>\n
\nnamespace UILibrary {\n …\n class PushButton {\n public:\n  virtual LPCWSTR GetName(StringHolder **Holder);\n };\n …\n}\n<\/PRE>\n
\nWhy can’t the linker find it?\n(Other functions in uilibrary.lib<\/CODE> link just fine.)\n<\/P>\n<\/BLOCKQUOTE>\n
\nIn order to find something, you have to be looking in the right place,\nand the thing you’re looking for actually needs to be there.\n(And you have to be able to see it when it’s there.)\nThe first part, looking in the right place, appears to be addressed\nby the parenthetical:\nThe linker is definitely looking in uilibrary.lib<\/CODE>\nsince it managed to find other things in that library.\n<\/P>\n
\nLet’s look at the second step, then.\nIs the thing you’re looking for really there?\nI fired up a little hex editor on uilibrary.lib<\/CODE>,\nbut you could use strings<\/CODE> or, if you really want\nto get fancy, link \/dump \/headers<\/CODE>.\nI went looking for “GetName@PushButton” to see if the member\nfunction was actually in the library.\n<\/P>\n
\nAnd yup, the function is there.\nBut it looks slightly different:\n?GetName@PushButton@UILibrary@@UAEPBGPAPAVStringHolder@2@@Z<\/CODE>.\n(See if you can spot the difference.)\nAha, the symbol couldn’t be found because it indeed doesn’t exist!\nWhat does exist is something that superficially resembles the\nsymbol we want, but which has different decoration.\nWe ask the\n\nundname<\/CODE><\/A> program to convert this name\ninto something a bit more readable:\n<\/P>\n
\nC:\\> undname ?GetName@PushButton@UILibrary@@UAEPBGPAPAVStringHolder@2@@Z\npublic: virtual unsigned short const * __thiscall\nUILibrary::PushButton::GetName(class UILibrary::StringHolder * *)\n<\/PRE>\n
\nLooking carefully at the two functions, we see that the\ndifference is that the one that program.obj<\/CODE> \nis looking for has a return type of wchar_t const *<\/CODE>,\nwhereas the one in the library returns a\nunsigned short const *<\/CODE>.\n<\/P>\n
\nAt this point the answer is obvious.\nThe library was compiled with the \/Zc:wchar_t-<\/CODE>* flag,\nwhich disables wchar_t<\/CODE> as a native type.\nWhen that happens, the Windows header files gives the\nwchar_t<\/CODE> symbol the definition\ntypedef unsigned short wchar_t;<\/CODE>\nOn the other hand, the customer’s project was being compiled\nwithout that switch, in which case wchar_t<\/CODE> is\na native type and not an alias for unsigned short<\/CODE>.\n<\/P>\n
\nNow you know enough to solve this customer’s problem,\nwhich is very similar to the previous one:\n<\/P>\n
\n
\nWhen I build my project, it compiles fine, but it fails during the\nlink step with an unresolved external:\n<\/P>\n
\nprogram.obj : error LNK2019: unresolved external symbol\n“long __cdecl UILibrary::Initialize(bool)”\n(?Initialize@UILibrary@@YAJ_N@Z)\n<\/PRE>\n<\/BLOCKQUOTE>\n
\nThe function as it exists in the library undecorates as follows:\n<\/P>\n
\nlong __stdcall UILibrary::Initialize(bool)\n<\/PRE>\n
\nNote<\/B>\n<\/P>\n
\nThe undname<\/CODE> program and the \/Zc:wchar_t-<\/CODE>\nswitches are specific to the Microsoft Visual C++ compiler.\nNaturally, if you use a different compiler, you should use the\nutility or command line switch appropriate to your compiler.\nIn particular, if you use the Visual Studio development environment,\nI’m told (but have not tried it myself)\nthat the switch you’re looking for is called\n“Treat wchar_t as a built-in type” on the “C\/C++ Language”\nproperty page.\n<\/P><\/p>\n","protected":false},"excerpt":{"rendered":"
Here’s a problem inspired by actual events. When I build my project, it compiles fine, but it fails during the link step with an unresolved external: program.obj : error LNK2001: unresolved external symbol “public: virtual wchar_t const * __thiscall UILibrary::PushButton::GetName(class UILibrary::StringHolder * *)” (?GetName@PushButton@UILibrary@@UAEPB_WPAPAVStringHolder@2@@Z) The function I’m trying to call exists in the source code […]<\/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-19693","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Here’s a problem inspired by actual events. When I build my project, it compiles fine, but it fails during the link step with an unresolved external: program.obj : error LNK2001: unresolved external symbol “public: virtual wchar_t const * __thiscall UILibrary::PushButton::GetName(class UILibrary::StringHolder * *)” (?GetName@PushButton@UILibrary@@UAEPB_WPAPAVStringHolder@2@@Z) The function I’m trying to call exists in the source code […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/19693","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=19693"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/19693\/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=19693"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=19693"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=19693"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}