What is the use of .exp and what is the difference between .lib and .dll?

c dll linker static-libraries
Vineel Kumar Reddy picture Vineel Kumar Reddy · Apr 28, 2010 · Viewed 17.8k times · Source

During compilation and linking, what is use of .exp? What is the difference between .lib and .dll? I know that .lib will be used, while linking and .dll will be used when running the program. But what exactly is the difference between .lib and .dll?

Does .lib file not contain the code for the functions coming from .dll files? What is the need for using two separate files?

Please clarify.

Answer

Jerry Coffin picture Jerry Coffin · Apr 28, 2010

In the case of an import library for a DLL, the .lib file does not contain any actual code at all. It basically contains just a list of the functions in the associated DLL -- enough for the linker to embed a reference to that DLL into something linked with the library, but not much else.

A .exp file is an export file -- basically just about the same as a .lib file. It's used (at least primarily) when you have a circular dependency. For example, assume you have a DLL that acts as a plug-in for an executable. The executable supplies some exported functions for use by plug-in DLLs, but also needs to be able to call some functions in the plug-ins as well (e.g. to load and initialize a plug-in).

The DLL won't link until the executable is built to provide a .lib file -- but the executable won't link until the DLL is built to provide a .lib file. To break the dependency, you run the linker against the executable, which fails (because it can't find a .lib file for the DLL), but will produce a .exp file. You then link the DLL against the .exp file for the executable. You can then re-run link to produce the executable, using the .lib file for the DLL.

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