CppCon 2017

If you build software for Windows, you use DLLs, and it’s likely that you may build DLLs of your own. DLLs are the primary mechanism for packaging and encapsulating code on the Windows platform. But have you ever stopped to think about how DLLs work? What goes into a DLL when you build it, what happens when you link your program with a DLL, or how do DLLs get located and loaded at runtime? Many of us build and use DLLs without fully understanding them. In this session, we’ll give an in-depth introduction to DLLs and how they work.

We’ll begin by looking at what’s in a DLL—the kinds of things a DLL can contain and the basic data structures that are used—and the benefits and drawbacks of packaging code in a DLL. We’ll look at how DLLs are loaded, including the details of how the loader locates DLLs and maps them into the process; how dependencies are resolved among DLLs; and DLL lifetime and how DLLs get unloaded. We’ll also look at how DLLs get built, including what makes DLLs “special,” what goes into an import library, and how the linker uses import libraries. Finally, we’ll look at several other miscellaneous topics, including how DLLs interact with threads and thread-local storage, and mechanisms for solving or mitigating the dreaded “DLL hell.”