What OS and how do you build? I fail to see what this could have to do with zlib. Looks to me like a regular linking problem due to missing object unit or maybe due to dllimport/dllexport issues. If you are building a shared lib, QUAZIP_BUILD should be defined (done automatically by qmake), if linking to the shared QuaZIP, it should NOT be defined. If linking statically (either a static lib or just using the source code), then QUAZIP_STATIC should be defined (done automatically in case of a static lib build, but you still need to define it when using the resulting lib).