Error LNK2001 or LNK2019 Unresolved Symbol From DLL

Symptoms

When building with Microsoft C++ and coverage build enabled, you get error LNK2001 or LNK2019 on a function symbol F which is defined in a DLL but not exported from the DLL by name. The error does not occur with coverage build disabled. Your compile settings do not include any optimization option which expands inline functions.

error LNK2001: unresolved external symbol "F"
error LNK2019: unresolved external symbol "F" referenced in function "G"

Cause

There is an inline function G that calls F. With coverage build disabled, G is not expanded inline and so G is defined inside the DLL. With coverage build enabled, G is expanded inline and so G is defined outside the DLL. Function G might be a compiler-generated default constructor or destructor.

You can confirm this as the cause by reproducing the error with Coverage Build disabled and inline function expansion enabled.

Resolution

Use either of the resolutions below.

Export function F by name, for example using __declspec(dllexport)
Make function G not inline. For a compiler-generated default constructor or destructor, explicitly define the default constructor or destructor in the DLL. To locate function G, have the compiler generate assembly code and then search the assembly code for calls to F.

More Information

To enable inline function expansion with Visual Studio, select your project, choose menu command Project Properties, navigate to C/C++ Optimization, and set Inline Function Expansion to Any Suitable (/Ob2).

Updated: 10 Sep 2018