%VCToolsInstallDir%\crt\src\vcruntime

Another way to locate this directory is also: %VCINSTALLDIR%Tools\MSVC\%VCToolsVersion%\crt\src\vcruntime.

mcrtexe.cpp contains mainCRTStartup which calls main.

dll_dllmain_cpp contains _DllMainCRTStartup.

%VCToolsInstallDir%\crt\src\vcruntime exe_winmain_cpp[exe_winmain.cpp] defines the entry point WinMainCRTStartup().

gs_cookie.c defines the buffer overrun security cookie.

gs_support.c defines the __security_init_cookie() function which is called at startup to initialize the global buffer overrun security cookie (defined in gs_cookie.c) if a compilation unit is compiled with /GS.