Friday, June 13, 2014

The Great C Runtime (CRT) Refactoring

James McNellis (via Hacker News):

For the past seven releases of Visual Studio (2002, 2003, 2005, 2008, 2010, 2012, and 2013), the Visual C++ libraries have been versioned and each versioned set of libraries is independent of other versioned sets of libraries. For example, a C++ program built with Visual C++ 2010 using the DLL runtime libraries will depend on msvcr100.dll and msvcp100.dll, while a C++ program built with Visual C++ 2013 will depend on msvcr120.dll and msvcp120.dll.


Starting with Visual Studio “14,” we will stop releasing new versions of the CRT with each release of Visual Studio. Whereas before we would have released msvcr140.dll in this forthcoming release, then msvcr150.dll in the next release, we will instead release one new CRT in Visual Studio “14” then update that version in-place in subsequent releases, maintaining backwards compatibility for existing programs.


The “best” example of this maintainability problem could be found in the old implementation of the printf family of functions. The CRT provides 142 different variations of printf, but most of the behavior is the same for all of the functions, so there are a set of common implementation functions that do the bulk of the work. These common implementation functions were all defined in output.c in the CRT sources(1). This 2,696 line file had 223 conditionally compiled regions of code (#ifdef, #else, etc.), over half of which were in a single 1,400 line function. This file was compiled 12 different ways to generate all of the common implementation functions. Even with the large number of tests that we have for these functions, the code was exceedingly brittle and difficult to modify.

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