Windows NT Systems Programming

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RPC Audio Message Server

(source code here)

Objectives

• RPC to Audio Message Server
• Context Handles for Client State
• Dialog Box as the main window
• Enabling/disabling buttons
• Use of Threads to keep User Interface Alive
• Streaming Audio between machines

Context Handles (chapter 8 pp. 451-461)

Problem: How to keep state of client/server connection between RPC calls?

• Could store with connection information (server only keeps)
  • How does the RPC server know which connection is being used? Could connect on named pipes, TCP/IP or other. port number is chosen at random for TCP connection.
  • What is network connection goes down - would like to reestablish over a new connection
• Could keep entire state on client and pass this to server each time (WEB Browser Cookies) - client only keeps
• Could pass in as one of the arguments of the RPC a ticket which maps to the state information.
  Both client and server keep information.

WIN32 RPC use the later approach.

• Each RPC contains an additional 32 bit argument called the context handle
• Usually this is a pointer IN THE SERVER ADDRESS SPACE to a structure containing the client state information.
• Need to Initialize and Uninitialize the client state information.

An RPC File Server

• Store/Retrieve Audio information in the RPC file server.
• Here is the IDL file (audMsg.idl)

[
  uuid(c262dfb0-54af-11d1-a98f-00609752bfb9), // Used uuidgen -i to generate this number
  version(1.0),
  endpoint ("ncacn_ip_tcp:[44533]")
]
interface audioRPC // Provides a set of RPC calls
{
	typedef [context_handle] void *audioContext; // This is a 32 bit pointer to the client state in the server

	void openRecord([in , string] char*, [out, ref] audioContext * ac);
	void saveAudio([in] audioContext ac, [in] int size, [in, size_is(size)] char* buf);
	void closeRecord([in, out,ref] audioContext *ac);
	void openPlayBack([in, string] char*, [out, ref] audioContext * ac);
	void closePlayBack([in, out,ref] audioContext *ac);
	int getAudio([in] audioContext ac, [in] int size, [out, size_is(size)] char buf[]); 
}

• F1: A reference pointer has the following characteristics: 
  · Always points to valid storage; never has the value NULL.
   A reference pointer can always be dereferenced. 
  · Never changes during a call. A reference pointer always points
   to the same storage on the client before and after the call. 
  · Does not allocate new memory on the client.
   Data returned from the server is written into existing storage
   specified by the value of the reference pointer before the call. 
  · Does not cause aliasing. Storage pointed to by a reference pointer
   cannot be reached from any other name in the function. 
  A reference pointer cannot be used as the type of a pointer returned by a function. 

• F2: Determines length using normal string length function

• F3: Pointer is passed back after opening Audio file

Server (AudioRPC.cpp)

#include <windows.h>
#include "audMsg.h" // generated by the IDL file - contains the stubs

typedef struct
{
  HANDLE audioFile;
} audioContextStruct;

// function to initialize sum context
void openRecord(unsigned char* fileName, audioContext *contextHandle)
{
  audioContextStruct *tmp;

  tmp=(audioContextStruct *) midl_user_allocate(sizeof(audioContextStruct));

  	tmp->audioFile = CreateFile((const char*)fileName, GENERIC_WRITE,
		0,0, CREATE_ALWAYS, 0, 0);
  

  *contextHandle=(audioContext)tmp;
}

void openPlayBack(unsigned char* fileName, audioContext *contextHandle)
{
  audioContextStruct *tmp;

  tmp=(audioContextStruct *)
    midl_user_allocate(sizeof(audioContextStruct));

  	tmp->audioFile = CreateFile((const char*)fileName, GENERIC_READ,
		0,0, OPEN_EXISTING, 0, 0);
  

  *contextHandle=(audioContext)tmp;
}


void saveAudio(audioContext contextHandle, int size, unsigned char* buf)
{
	DWORD numWrite;

	WriteFile(((audioContextStruct *) contextHandle)->audioFile, buf, size, &numWrite, 0);
  
}

int getAudio(audioContext contextHandle,
  int size, unsigned char* buf)
{
	DWORD numWrite;

	ReadFile(((audioContextStruct *) contextHandle)->audioFile, buf, size, &numWrite, 0);
	return numWrite;
  
}
// function to uninitialize sum context
void closeRecord(audioContext *contextHandle)
{
 	CloseHandle(((audioContextStruct *) *contextHandle)->audioFile);
 	 midl_user_free(*contextHandle);

  	*contextHandle=NULL;
}

void closePlayBack(audioContext *contextHandle)
{
	CloseHandle(((audioContextStruct *) *contextHandle)->audioFile);
  midl_user_free(*contextHandle);

  *contextHandle=NULL;
}

// function which gets called if client
// looses contact with server
void __RPC_USER audioContext_rundown
  (audioContext contextHandle)
{
	midl_user_free(*contextHandle);
}

• F4: Allocates a structure to contain the client's information (which audio file is opened for that client)
• F5: Passes context pointer back to client
• F6: Deallocates the client's structure.
• F7: Called if network link is lost

Server.cpp

 if (RpcServerUseAllProtseqsIf(1,
    audioRPC_v1_0_s_ifspec,
    NULL))
  if (RpcServerRegisterIf(audioRPC_v1_0_s_ifspec,
    NULL, NULL))
  if (RpcServerListen(1, 5, FALSE))
 

Client.h

// Main Window is a dialog box

class CMsgApp : public CWinApp
{
public:
	virtual BOOL InitInstance(); // everybody overrides this one
};

Client.cpp

CMsgApp msgApp;  

// Init the application and the main window
BOOL CMsgApp::InitInstance()
{
	CMsgDlg dlg;
	m_pMainWnd = &dlg;
	dlg.DoModal();
	
	// Since the dialog has been closed, return FALSE so that we exit the
	//  application, rather than start the application's message pump.
	return FALSE;

}

CMsgDlg.h

class CMsgDlg : public CDialog
{
public:
	CMsgDlg(CWnd* pParentWnd = NULL) :
	  CDialog(IDD_MSGDLG, pParentWnd) {}
	  virtual BOOL OnInitDialog();

private:
	HANDLE audioThread;
protected:
	afx_msg void OnRecord();
	afx_msg void OnPlay();
	afx_msg void OnStop();
	DECLARE_MESSAGE_MAP()
};

CMsgDlg.cpp

BOOL CMsgDlg::OnInitDialog()
{
	UCHAR *stringBinding;
	
	CDialog::OnInitDialog();
	RpcStringBindingCompose(NULL,
		(UCHAR *) "ncacn_ip_tcp", (UCHAR *) "localhost",
		(UCHAR *) "44533", NULL,
		&stringBinding);
	
	RpcBindingFromStringBinding(stringBinding,
		&audioRPCHandle);
	
	RpcStringFree(&stringBinding);
	stopRecord = CreateEvent(0, FALSE, FALSE, 0); // so that Record/Stop buttons can control recording
	ResetEvent(stopRecord);
	return TRUE;
}


void CMsgDlg::OnRecord()
// Saves the message
{
	DWORD threadid;
	void record(void*);
	CString msgName;
	char* msgName2;
	
	GetDlgItem(IDC_STOP)->EnableWindow(true);
	GetDlgItem(IDC_RECORD)->EnableWindow(false);
	GetDlgItem(IDC_PLAY)->EnableWindow(false);
	GetDlgItemText(IDC_MSG_ID, msgName);
	msgName2 = new char[sizeof(msgName)]+1;
	strcpy(msgName2, msgName);
	
	audioThread = CreateThread(NULL, 0,
		(LPTHREAD_START_ROUTINE) record, msgName2, 0,
		&threadid);   
	
}

void CMsgDlg::OnStop()
{
	GetDlgItem(IDC_STOP)->EnableWindow(false);
	GetDlgItem(IDC_RECORD)->EnableWindow(true);
	GetDlgItem(IDC_PLAY)->EnableWindow(true);
	SetEvent(stopRecord);
	GetDlgItem(IDC_STOP)->EnableWindow(FALSE);
}

void CMsgDlg::OnPlay()
// Saves the message
{
	DWORD threadid;
	void playBack(void*);
	int i = 0;
	CString msgName;
	char* msgName2;
	
	GetDlgItem(IDC_STOP)->EnableWindow(true);
	GetDlgItem(IDC_RECORD)->EnableWindow(false);
	GetDlgItem(IDC_PLAY)->EnableWindow(false);
	GetDlgItemText(IDC_MSG_ID, msgName);
	msgName2 = new char[sizeof(msgName)]+1;
	strcpy(msgName2, msgName);
	
	audioThread = CreateThread(NULL, 0,
		(LPTHREAD_START_ROUTINE) playBack, msgName2, 0,
		&threadid);   
}

• F8: Enables Stop Button and disables the Record and Play buttons

Recording Thread

void record(void * msgName) // this is a thread function which is passed a pointer to a string
{
	DWORD threadid;
	HANDLE recordHandle;
	openRecord((unsigned char*)msgName,&ac);
	msgApp.m_pMainWnd->SetWindowText("Start Recording");
	calledBack = CreateEvent(0, FALSE, FALSE, 0);
	ResetEvent(calledBack);
	recordHandle = CreateThread(NULL, 0,
		(LPTHREAD_START_ROUTINE) Recorder, NULL, 0,
		&threadid);
	openWaveInDev();
	WaitForSingleObject(recordHandle, INFINITE);
	waveInClose(hWaveIn);
	CloseHandle(calledBack);
	CloseHandle(recordHandle);
	closeRecord(&ac);
	msgApp.m_pMainWnd->SetWindowText("Done Recording");
	
}

• F9: RPC to open Audio file for recording
• F10: Event used to synchronize callback and thread
• F11: RPC to Close Audio file on server

Record Thread

UINT Recorder(void *p)
{
	
	while (true)
	{
		WaitForSingleObject(calledBack, INFINITE); // wait till device has called back
		if (!(WaveHeader.dwFlags & WHDR_DONE))
		{
			continue;
		}
		saveAudio(ac, WaveHeader.dwBytesRecorded, (unsigned char*) WaveHeader.lpData);
		if (waveInUnprepareHeader(hWaveIn, &WaveHeader,
			sizeof(WAVEHDR)))
		{
			break;
		}
		WaveHeader.dwFlags = 0;
		
		if (waveInPrepareHeader(hWaveIn, &WaveHeader,
			sizeof(WAVEHDR)))
		{
			break;
		}
		if (waveInAddBuffer(hWaveIn, &WaveHeader,
			sizeof(WAVEHDR)))
		{
			break;
		}
		if(WaitForSingleObject(stopRecord, 0) != WAIT_TIMEOUT)
			break;
		
		
	}
	waveInReset(hWaveIn);
	return 0;
}

• F12: RPC to transfer data buffer to server
• F13: Checks to see if user clicked STOP button (TIMEOUT of 0 will return imediately in every case)

What Next?

• Audio only messages
• MultiMedia Messages (embed format tags like HTML)
• Might be useful to separate into two threads a function which makes the RPC call to get/send data and a function which sets up the audio device so this can be overlapped. (RPC calls can have significant overhead.)
• Could manage a buffer pool of audio data buffers to smooth over network transients
• Could connect playback to record on two instantiations of the client to get two way interactive communications
  RPC is not the best model here because of its asymmetry OR IS IT?

Copyright chris wild 1997.
For problems or questions regarding this web contact [Dr. Wild].
Last updated: November 05, 1997.