Roland wrote:
Now in your module B you have a (global) TSS variable. A calls into B. When should the TSS be initialized? On first use? I am afraid that is not trivially
You do not need to worry about local thread data for CRT, at least if you use MSVC. Here is explanation: your library B contains function DllMain, and it's called just when library got loaded, right? Wrong. If you linked with CRT (no matter which one - statically or dynamically linked, multi- or single-threaded, with debug symbols or without), your entry point is actually _DllMainCRTStartup (use dumpbin.exe to see it). You may find this function in CRT sources (if you installed it together with MSVC). In multi-threaded, statically linked version of CRT this function will indirectly call TlsAlloc (or actually FlsAlloc - if present in "kernel32.dll", at least in CRT delivered with MSVC71). Index for CRT local storage has been initialized, what remains is CRT data for each thread. You are concerned that thread started with CreateThread (or whatever other API different than "blessed" _beginthread) might not contain valid CRT data in it's local storage. It's true, but CRT runtime will receive DLL_THREAD_ATTACH notifications. It will arrive to _DllMainCRTStartup if you linked CRT statically, otherwise to dynamic CRT runtime library (BTW, it's entry point is called _CRTDLL_INIT). There is still possibility that you disabled thread notifications in your library *and* linked CRT statically. That might be a problem, but if you grep for _getptd function (again, it's in sources of CRT in your MSVC directory) you will understand there's no problem at all. This function is the only way that other CRT functions use to access local thread storage. It will create and initialize CRT data if current thread does not contain it yet. Thus all CRT functions requiring access to local thread data will actually gain it without problem - it will be created no later than on first access. Now about release. If you used dynamically linked CRT library, that's no problem at all. It will receive DLL_THREAD_DETACH and execute _freeptd in order to free CRT thread data. If you used statically linked CRT runtime, this notification will arrive to _DllMainCRTStartup (which will execute _freeptd). There is a problem however if you disable thread notifications in your library *and* use statically linked CRT. This will result in memory leak in CRT, documented here : http://msdn.microsoft.com/library/en-us/dllproc/base/createthread.asp (see Remarks, bottom of page) When your library got unloaded from memory, TlsFree (or FlsFree) is called indirectly in entry point of your library, ie. _DllMainCRTStartup (if you used statically linked CRT). In case of dynamic CRT library, it's calling TlsAlloc and TlsFree (or FlsAlloc and FlsFree) in its own entry point on DLL_PROCESS_ATTACH and DLL_PROCESS_DETTACH - you may find it in crtlib.c in your MSVC directory. B.