Issue with RegConnectRegistry connecting to 64 bit machines

Question

I'm seeing a weird thing when connecting to the performance registry on 64 bit editions of Windows. The whole program stalls and callstacks becomes unreadable. After a long timeout, the connection attempts aborts and everything goes back to normal.

The only solution is to make sure that only one thread at the time queries the remote registry, unless the remote machine is a 32 bit Windows XP, 2003, 2000 , then you can use as many threads as you like.

Have anyone a technical explanation why this might be happening ? I've spent 2-3 days searching the web without coming up with anything.

Here is a test program, run it first with one thread (connecting to a 64 bit Windows), then remove the comment in tmain and run it with 4 threads. Running it with one thread works as expected, running with 4, returns ERROR_BUSY (dwRet == 170) after stalling for a while.

Remember to set a remote machine correctly in RegConnectRegistry before running the program.

#define TOTALBYTES    8192
#define BYTEINCREMENT 4096

void PerfmonThread(void *pData)
{
    DWORD BufferSize = TOTALBYTES;
    DWORD cbData;
    DWORD dwRet;

    PPERF_DATA_BLOCK PerfData = (PPERF_DATA_BLOCK) malloc( BufferSize );
    cbData = BufferSize;

    printf("\nRetrieving the data...");

    HKEY hKey;
    DWORD dwAccessRet = RegConnectRegistry(L"REMOTE_MACHINE",HKEY_PERFORMANCE_DATA,&hKey);

    dwRet = RegQueryValueEx( hKey,L"global",NULL,NULL,(LPBYTE) PerfData, &cbData );
    while( dwRet == ERROR_MORE_DATA )
    {
        // Get a buffer that is big enough.

        BufferSize += BYTEINCREMENT;
        PerfData = (PPERF_DATA_BLOCK) realloc( PerfData, BufferSize );
        cbData = BufferSize;

        printf(".");
        dwRet = RegQueryValueEx( hKey,L"global",NULL,NULL,(LPBYTE) PerfData,&cbData );
    }
    if( dwRet == ERROR_SUCCESS )
        printf("\n\nFinal buffer size is %d\n", BufferSize);
    else 
        printf("\nRegQueryValueEx failed (%d)\n", dwRet);

    RegCloseKey(hKey);
}

int _tmain(int argc, _TCHAR* argv[])
{
    _beginthread(PerfmonThread,0,NULL);
/*  _beginthread(PerfmonThread,0,NULL);
    _beginthread(PerfmonThread,0,NULL);
    _beginthread(PerfmonThread,0,NULL);
*/

    while(1)
    {

        Sleep(2000);
    }
}

Answer 1

This is not really an answer, but a suggestion. Even though you are only querying the registry (not writing), I'm wondering if you are producing some kind of dead-lock with the multiple threads.

Lacking a Windows development or testing environment, take this suggestion for what its worth: perhaps you could use mutexes around the registry calls... that may relieve any deadlock situation, if that is indeed the problem.

Good luck.

Answer 2

I think it must be an environmental issue. I just tried this from 32-bit Windows XP Professional to 64-bit Windows 7 Ultimate and it worked fine. Occasionally on a thread or two a call to RegQueryValueEx would fail with either ERROR_BUSY or ERROR_NOT_READY, but I never experienced any long delays. In case anybody else tries to test this, I ran into a snag; the account you are using must be a member of the Performance Monitor Users group in order to remotely access HKEY_PERFORMANCE_DATA. Also ensure the Remote Registry Service is running.