Re: Can a windows DLL have more than one process attached?

On Tue, Nov 27, 2001 at 01:31:40PM -0500, Tom Lane wrote:
> In reviewing some recent patches I notice the following code added to
> src/interfaces/libpq/libpqdll.c:
> 
> BOOL WINAPI
> DllMain(HINSTANCE hinstDLL, DWORD fdwReason,
>         LPVOID lpReserved)
> {
>     switch (fdwReason)
>     {
>         case DLL_PROCESS_ATTACH:
>             ...
>             if (netmsgModule == NULL)
>                 netmsgModule = LoadLibraryEx("netmsg.dll", NULL, LOAD_LIBRARY_AS_DATAFILE);
>             break;
>         case DLL_PROCESS_DETACH:
>             if (netmsgModule != NULL)
>                 FreeLibrary(netmsgModule);
>             ...
>             break;
>     }
> }
> 
> where netmsgModule is
> 
> static HINSTANCE netmsgModule = NULL;
> 
> This sure looks to me like it will fail miserably if more than one
> process can attach to the DLL concurrently: won't the first one to
> detach release the netmsg library, breaking access to it for all the
> remaining processes?
> 
> I don't know enough about Windows to know if there's really a problem
> here, but the code looks fishy to me.  I'd expect to need a reference
> count.  Comments anyone?

I'm not much of a WinAPI coder, either, but ISTR something from the Wine
project, and found this:

http://www.allapi.net/apilist/apifunction.php?apifunction=FreeLibrary
FreeLibrary
The FreeLibrary function decrements the reference count of theloaded dynamic-link library (DLL) module.

So it's doing the reference counting.

'Offical' reference at:

http://msdn.microsoft.com/library/en-us/dllproc/dll_3cs9.asp?frame=false

However, there's another problem. According to the above, and:

http://msdn.microsoft.com/library/en-us/dllproc/dll_4abc.asp?frame=true

It is not safe to call FreeLibrary or LoadLibraryEx from DllMain.

Which points to:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dllproc/dll_8asu.asp
Warning  On attach, the body of your DLL entry-point functionshould perform only simple initialization tasks, such
assettingup thread local storage (TLS), creating objects, andopening files. You must not call LoadLibrary  in the
entry-pointfunction,because you may create dependency loops in the DLL loadorder. This can result in a DLL being used
beforethe systemhas executed its initialization code. Similarly, you must notcall the FreeLibrary  function in the
entry-pointfunction ondetach, because this can result in a DLL being used after thesystem has executed its termination
code.


Any real windows coders know if this is a problem?

Ross