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
