crtdll.c & dllcrt0.c
_DllMainCRTStartup is in crtdll.c (when your DLL library is dynamically linked to CRT) or dllcrt0.c (when your DLL library is statically linked to CRT). In both files you will find _CRT_INIT, called from _DllMainCRTStartup just before user entry point (DllMain) is executed. In _CRT_INIT in dllcrt0.c you will find call to _mtinit, which is defined in tidtable.c. Here you will find "__tlsindex = FLS_ALLOC(&_freefls)". But I think that's all irrelevant in your case, as your library B (being static library) does not have an entry point.