How can I resolve “error LNK2019: unresolved exter

问题:

This question already has an answer here:

• What is an undefined reference/unresolved external symbol error and how do I fix it? 32 answers

I've got this MFC application I'm working on that needs to have an embedded database. So I went hunting for a slick, fast "embeddable" database for it and stumbled accross SQLite.

I created a DB with it, and I created a static library project with Visual Studio 2008. the library project will be used in another main project.

In the library project, I created a class DBClass with a method AddFeedToDB(CFeed f). The library project uses the .lib file from codeproject (cppsqlite3.lib).

When compiling the static library, no error is detected, but when I try to use the library project file in the main project, I get these type of errors:

error LNK2019: unresolved external symbol "public:void __thiscall
   CppSQLite3DB::close(void)" (?close@CppSQLite3DB@@QAEXXZ 
   referenced in function "public: int __thiscall
   CTalkingFeedsDB::AddFeedToDB(class CFeed,char const*)" (?
   AddFeedToDB@CTalkingFeedsDB@@QAEHVCFeed@@PDB@Z

What am I missing?

回答1:

It happened to me more than once that I thought symbol XXX (i.e. ?close@CppSQLite3DB@@QAEXXZ) was in the import lib, while the actual symbol was __impXXX (i.e. __imp?close@CppSQLite3DB@@QAEXXZ).

The reason for the linker error is then to be found in the compilation step: the compiler will generate the ?close@CppSQLite3DB@@QAEXXZ symbol to be imported, where it should generate __imp?close@CppSQLite3DB@@QAEXXZ. This often means that the function declaration itself didn't have __declspec( dllimport ). Which may be caused by some preprocessor symbol not being defined. Or the __declspec not being there at all...

回答2:

I know it is already 2 years since this question... but i run in the same situation here. Added all the header files... added the lib directories.. and keep having this error. So i added manually the lib to the Configuration Properties -> Linker -> Input -> Aditional Dependencies and all works for me.

回答3:

Don't know if it is your case, but the imp prefix may mean that you are compiling a x64 library in a Win32 project.

回答4:

You either need to link the codeproject SQLite lib to your executable, or to include the sources files in your project directly. (Which one did you do ?)

回答5:

I would follow these steps:

1. think about what library or .obj file you expect the symbol to be exported by.

2. check whether it actually does export that very symbol (check character-wise). Sometimes, it's the calling convention differs.

3. check if the library you expect to contain the symbol is known to the linker - first check for the 'additional libraries', then check if the library is actually found (I mostly do this by using filemon.exe from sysinternals, and look for link.exe to open the lib file. )

After thinking a while, you may find that your library project will not export the sought for function. That function is in the database lib. You should add that lib to your main project. It's no use adding it to your static lib project.

回答6:

The compiler and linker will not link one library into another (unless one is a DLL). You need to specify both libraries (cppsqlite3.lib and your own static library) in your main project.