The source code of the C Run-Time library (CRT) of Visual C++ can be found in the directory:
C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\crt\src
Exploring the code, one sees that most of the C functions end up calling the Win32 API. The C++ library is licensed from Dinkumware. So, one can see the name of P. J. Plauger who created these files at their bottom. There are a few assembly files too (*.asm), which are mostly for memory and string operations.