Build and manage large-scale C++ on Windows

DLLs versus shared objects

HP ProLiant Gen8: Integrated lifecycle automation

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.

The Power of One eBook: Top reasons to choose HP BladeSystem

More from The Register

next story
Apple fanbois SCREAM as update BRICKS their Macbook Airs
Ragegasm spills over as firmware upgrade kills machines
HIDDEN packet sniffer spy tech in MILLIONS of iPhones, iPads – expert
Don't panic though – Apple's backdoor is not wide open to all, guru tells us
NO MORE ALL CAPS and other pleasures of Visual Studio 14
Unpicking a packed preview that breaks down ASP.NET
Captain Kirk sets phaser to SLAUGHTER after trying new Facebook app
William Shatner less-than-impressed by Zuck's celebrity-only app
Do YOU work at Microsoft? Um. Are you SURE about that?
Nokia and marketing types first to get the bullet, says report
Microsoft takes on Chromebook with low-cost Windows laptops
Redmond's chief salesman: We're taking 'hard' decisions
Cheer up, Nokia fans. It can start making mobes again in 18 months
The real winner of the Nokia sale is *drumroll* ... Nokia
EU dons gloves, pokes Google's deals with Android mobe makers
El Reg cops a squint at investigatory letters
prev story


Designing a Defense for Mobile Applications
Learn about the various considerations for defending mobile applications - from the application architecture itself to the myriad testing technologies.
How modern custom applications can spur business growth
Learn how to create, deploy and manage custom applications without consuming or expanding the need for scarce, expensive IT resources.
Reducing security risks from open source software
Follow a few strategies and your organization can gain the full benefits of open source and the cloud without compromising the security of your applications.
Boost IT visibility and business value
How building a great service catalog relieves pressure points and demonstrates the value of IT service management.
Consolidation: the foundation for IT and business transformation
In this whitepaper learn how effective consolidation of IT and business resources can enable multiple, meaningful business benefits.