How do I link a DLL to my project? error LNK2019: unresolved external symbol

Question

I have a file foo.h that has various declarations for functions. All of these functions are implemented in a file foo.dll. However, when I include the .h file and try to use any of the functions, I get the error:

bar.obj : error LNK2019: unresolved external symbol SomeFunction

so obviously the function implementations aren't being found.

What do I have to do to help the compiler find the definitions in the DLL and associate them with the .h file?

I've seen some stuff about __declspec(dllexport) and __declspec(dllimport) but I still can't figure out how to use them.

Answer 1

You should have received at least three files from the DLL owner. The DLL which you'll need at runtime, the .h file with the declarations of the exported functions, you already have that. And a .lib file, the import library for the DLL. Which the linker requires so it knows how to add the functions to the program's import table.

You are missing the step where you told the linker that it needs to link the .lib file. It needs to be added to the linker's Input + Additional Dependencies setting of your project. Or most easily done by writing the linker instruction in your source code:

#include "foo.h"
#pragma comment(lib, "foo.lib")

Which works for MSVC, not otherwise portable but linking never is. Copy the .lib file to your project directory or specify the full path.

Answer 2

I just had a similar problem. The solution turned out to be that the DLL was 64 bit, and the simple app using it was 32. I had forgotten to change it to x64 in the Configuration Manager.