I have a Dllmain that allocates Thread local storage when a thread attaches to this DLL. Code as below:

BOOL APIENTRY DllMain(HMODULE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved)
{
    LPVOID lpvData; 
    BOOL fIgnore; 

    switch (ul_reason_for_call)
    {
    case DLL_PROCESS_ATTACH:
        onProcessAttachDLL();
        // Allocate a TLS index.
        if ((dwTlsIndex = TlsAlloc()) == TLS_OUT_OF_INDEXES) 
            return FALSE; 
        // how can it jump to next case???
    case DLL_THREAD_ATTACH:
        // Initialize the TLS index for this thread.
        lpvData = (LPVOID) LocalAlloc(LPTR, MAX_BUFFER_SIZE); 
        if (lpvData != NULL) 
            fIgnore = TlsSetValue(dwTlsIndex, lpvData);  
        break; 
    ...

}

I know that for the main thread, the DLL_THREAD_ATTACH is not entered, as per Microsoft Documentation. However, the above code worked. I am using VC2005. When I entered the debugger, I saw that after it entered DLL_THREAD_ATTACH case when ul_reason_for_call = 1! How can that happen? If I add `break' at the end of DLL_PROCESS_ATTACH block, the DLL failed to work.

How can this happen?