I received the above mentioned error when trying to build a piece of sample code in visual studio. The original sample code included a makefile but I didn't know how to do the linking that the makefile was doing within the visual studio project.
Note: This is a duplicate of this question: Error LNK2019: Unresolved External Symbol in Visual Studio
The author in that case was specifically asking about how to resolve in visual studio but was closed as a duplicate of a excellent, yet general description of the fundamentals of the problem, without getting specifics for Visual Studio.
Edit: I thought I was pretty clear on why this was a different question. This is a visual studio specific error message and I was looking for a visual studio specific solution. I did not need an explination on the linker, just a better understanding what this tool was and was not doing for me.