Question

I am using windows named pipe example. When I run sample programs to create pipe, write something and receive it in client program everything is just fine. When I move client code into dll, which runs in windows service it just won’t receive sent bytes.

Server’s code is as follows:

ThreadParams * params = reinterpret_cast<ThreadParams*>(args);
CString * connectionString = params->connectString;

HANDLE hPipe;
DWORD dwBytesRead;
TCHAR buf[1024];
int len;

hPipe = CreateNamedPipe(PIPE_NAME,  // Name
                        PIPE_ACCESS_OUTBOUND | WRITE_OWNER, // OpenMode
                        PIPE_TYPE_BYTE | PIPE_READMODE_BYTE | PIPE_WAIT, // PipeMode
                        2, // MaxInstances
                        1024, // OutBufferSize
                        1024, // InBuffersize
                        2000, // TimeOut
                        NULL); // Security
if (hPipe == INVALID_HANDLE_VALUE)
{
    Globals::WriteLog("Could not create the pipe",1);
    exit(1);
}

Globals::WriteLog("connect...",1);
ConnectNamedPipe(hPipe, NULL);
Globals::WriteLog("...connected",1);

swprintf(buf, connectionString->GetBuffer());
len = wcslen(buf);

if (!WriteFile(hPipe, buf, len*sizeof(TCHAR), &dwBytesRead, NULL))
    Globals::WriteLog("WriteFile failed",1);
else
    wprintf(L"written %d bytes\n",dwBytesRead);

DisconnectNamedPipe(hPipe);

CloseHandle(hPipe);

And client:

    CString finalResult = _T("");

HANDLE      hOut = INVALID_HANDLE_VALUE;
TCHAR       buf[1024];
DWORD       len;
DWORD       dwWritten;

Global::WriteLog("pwrite: waiting for the pipe...",1);
if (WaitNamedPipe(PIPE_NAME, NMPWAIT_WAIT_FOREVER) == 0)
{
    Global::WriteLog("WaitNamedPipe failed. error=%d",1,GetLastError());
    goto cleanup;
}
Global::WriteLog("the pipe is ready",1);

hOut = CreateFile(PIPE_NAME,
    GENERIC_READ,
    0,
    NULL, OPEN_EXISTING,
    FILE_ATTRIBUTE_NORMAL,
    NULL);
if (hOut == INVALID_HANDLE_VALUE)
{
    Global::WriteLog("CreateFile failed with error %d",1,GetLastError());
    goto cleanup;
}
Global::WriteLog("Opened the pipe",1);

for (;;)
{
    if (!ReadFile(hOut, buf, sizeof(buf), &dwWritten, NULL))
    {
        Global::WriteLog("ReadFile failed -- probably EOF. Read %d bytes.",1,dwWritten);
        goto cleanup;
    }
    else
        break;
}

finalResult = CString(buf);
Global::WriteLog("String from pipe:%S",1,buf);
cleanup:
if(hOut != INVALID_HANDLE_VALUE)
    CloseHandle(hOut);

Server’s code runs in a thread if that changes anything (I’ve tested it with this thread version in sample program and there was no problem with this).

Why does it not work?

Thanks in advance

Answer 1

Okay, it seems I figured it out. It seems I didn’t understood documentation properly.

At server’s side WriteFile function doesn’t block until the string is read. My program simply wrote data and then closed handle – pipe. Client didn’t catch the message and threw error stating that there is no process on the other side of pipe.

Also from client I removed for(;;) loop.

To wait for read operation on client side to complete I added

FlushFileBuffers(hPipe);

after successful write operation.

Hope that help somebody