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DLLs versus shared objects

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John Lakos wrote the book on Large-Scale C++ Software Design more than 10 years ago, but it remains a must read for any serious C++ developer today.

It doesn't go much into the language. For instance there isn't anything inside regarding dynamic casts and virtual inheritance. Neither will it tell you how to calculate the factorial at compile time using compile time recursive templates.

What it does, is talk about the very real issues faced when a project gets big and complicated, a realty in today's world of cloud computing, online services, distributed applications and data centers. These issues lie between the abstraction presented by language and the underlying hardware as complexities of scale and lead us to start subdividing the software into static libraries and shared objects.

This book's standing in the industry shows that the issues are taken seriously, are largely platform independent and should be understood. But what about the Windows-specific issues - DLLs versus shared objects?

DLLs on Windows are not 100 per cent analogous to shared objects in the Unix world. In a shared object all symbols are exported unless steps are taken to prevent this and, while a shared object knows its dependences in terms of external symbols and libraries, the linker doesn't give an error if a needed externally defined symbol is not present at link time.

Dynamic loaders and real code

If the same symbol occurs in different libraries, as can happen with templates, then warnings result at link time and the duplicate symbols are merged by the dynamic loader. Data is also managed differently. Symbols naming static data are exported like any other and multiple definitions are merged to unambiguously name a single piece of memory at runtime.

DLLs on the other hand have a different model. Symbols that are intended for external use must be marked so that the linker knows to put them in the export table. This is what the declaration specifier __declspec(dllexport) is for and there are some divergences from Unix shared-object behavior as a result of this difference.

External code finds the symbol via a corresponding import declaration and linking with the import library of the DLL. The import library is linked into a client-portable executable as any other static library, but when called its code will invoke the dynamic loader and call the real code via pointers to functions obtained from GetProcAddress. We'll see why this is important in a moment, for now just remember that code inside the DLL is not passing by this import library.

Internal code accesses its siblings, both exported and otherwise, by offset addresses relative to the DLL base address. This implies the first important difference, linking a DLL on Windows is a full blown link operation, all symbols are resolved and replaced with offset addresses and errors are given if something is not found. In fact when linking a DLL you are creating a real image in portable executable format and this takes considerably more time than linking a static library which is essentially just an archive.

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