Interprocess communication through Named pipes

Threads and processes are very similar but they differ in the way they share resources. Processes are normally independent and have its own address space. Now the question comes when 2 independent processes want to communicate how do they do that. There are quite a few options for them to establish a good amount of communication protocol. Some of them are listed below.

1. Message passing.
2. Through Socket communication.
3. Named piped communication.

I will be mainly focusing on Named piped Communication.

We start a server in a low priority thread.

Code:

CPipeSampleDlg *pPipeDlg = (CPipeSampleDlg*)lpData;

while(TRUE) 
{ 
	BOOL fConnected,fSuccess; 
	CHAR chRequest[MAX_PATH]; 
	DWORD cbBytesRead; 
	
	// If Pipe server is stopped break from the loop.
	if(pPipeDlg->m_PipeHandle == NULL)
		break;

	fConnected = ConnectNamedPipe(pPipeDlg->m_PipeHandle, NULL) ? TRUE : (GetLastError() == ERROR_PIPE_CONNECTED); 
	
	if (fConnected) 
	{ 
		fSuccess = ReadFile (pPipeDlg->m_PipeHandle, // handle to pipe 
			chRequest, // buffer to receive data 
			MAX_PATH, // size of buffer 
			&cbBytesRead, // number of bytes read 
			NULL); // not overlapped I/O 
		
		chRequest[cbBytesRead] = '\0';
		CString msg;
		msg.Format("Received string : %s\r\n",chRequest);
		pPipeDlg->ShowLastError(msg);

		FlushFileBuffers(pPipeDlg->m_PipeHandle); 
		DisconnectNamedPipe(pPipeDlg->m_PipeHandle); 
	}
} 
return 0;

The thread waits for a client to connect to it as passes some data to the server.

Code:

CString szPipeName;
szPipeName = _T("\\\\.\\pipe\\PipeTest");

BOOL bret = FALSE;

//first we will try to connect to the exising PIPE
bret = ::WaitNamedPipe((LPCSTR)szPipeName,PROVIDER_WAIT_TIME);
//the named 
if(bret == FALSE)
{
	ShowLastError();
	return;
}
else
{
	//the PIPE is available so we will do the rest of the work in a thread
	ShowLastError("PIPE is available for connection");
	BOOL flg;
	DWORD dwWrite;

	CString szPipeUpdate;
	
	HANDLE hPipeHandle = ::CreateFile((LPCSTR)szPipeName,GENERIC_WRITE,0,NULL,
		OPEN_EXISTING,0,NULL);
	
	szPipeUpdate = _T("WM_USER_MESSAGE");
	
	flg = WriteFile(hPipeHandle, szPipeUpdate, strlen(szPipeUpdate), &dwWrite, NULL);

	if (FALSE == flg)
		ShowLastError("Sending string : " + szPipeUpdate + " Failed");
	else
		ShowLastError("Sending string : " + szPipeUpdate);

	CloseHandle(hPipeHandle);
}

The client connects to the pipe creates a file to be wriiten. Writes the file and returns. It does not wait for the server to complete the process with the data. This is asynchronous mode of tranfer.

I have used the name of the pipe as "\\\\.\\pipe\\PipeTest" which connects to the server on the local machine but it can be used to specify some other name of the server or even the ip address where the server is running like "\\\\127.0.0.1\\pipe\\PipeTest"

I have similar code in VB with same API's for you to see.
Server code

Code:

Dim ovl As OVERLAPPED
Dim fConnected, fSuccess As Boolean
Dim iConnectionStatus As Long
Dim chRequest As String
Dim cbBytesRead As Long

Do
	DoEvents
	
	If PipeHandle = 0 Then
		Exit Do
	End If
	
	iConnectionStatus = ConnectNamedPipe(PipeHandle, ovl)
	
	If iConnectionStatus <> 0 Then
		fConnected = True
	Else
		If Err.LastDllError = ERROR_PIPE_CONNECTED Then
			fConnected = True
		Else
			fConnected = False
		End If
	End If
	
	If fConnected = True Then
		chRequest = Space(MAX_PATH)
		fSuccess = ReadFile(PipeHandle, chRequest, MAX_PATH, cbBytesRead, ovl)
	 
		If Not fSuccess Or cbBytesRead = 0 Then
		   Exit Do
		End If
	
		ShowLastStatus "Recieved : " + chRequest
		FlushFileBuffers ByVal PipeHandle
		DisconnectNamedPipe ByVal PipeHandle
	End If
Loop

Client code

Code:

Dim bret As Boolean
bret = False
   
bret = WaitNamedPipe(szPipeName, PROVIDER_WAIT_TIME)
If bret = False Then
	ShowLastStatus
Else
	ShowLastStatus "PIPE is available for connection"
	Dim flg As Boolean
	Dim dwWrite As Long
	Dim szPipeUpdate As String
	Dim hPipeHandle As Long
	Dim sa As SECURITY_ATTRIBUTES
	Dim ovl As OVERLAPPED
			
	hPipeHandle = CreateFile(szPipeName, GENERIC_WRITE, 0, sa, OPEN_EXISTING, 0, 0)
	
	szPipeUpdate = "HI FROM CLIENT"
	
	flg = WriteFile(hPipeHandle, szPipeUpdate, Len(szPipeUpdate), dwWrite, ovl)

	If False = flg Then
		ShowLastStatus "Sending string : " + szPipeUpdate + " Failed"
	Else
		ShowLastStatus "Sending string : " + szPipeUpdate
	End If
End If

API's

Code:

'Constants
Public Const PROVIDER_WAIT_TIME = 30
Public Const PIPE_ACCESS_DUPLEX = &H3
Public Const FILE_FLAG_OVERLAPPED = &H40000000
Public Const PIPE_TYPE_MESSAGE = &H4
Public Const PIPE_WAIT = &H0
Public Const MAX_PATH = 260
Public Const INVALID_HANDLE_VALUE = -1
Public Const FORMAT_MESSAGE_ALLOCATE_BUFFER = &H100
Public Const FORMAT_MESSAGE_FROM_SYSTEM = &H1000
Public Const FORMAT_MESSAGE_IGNORE_INSERTS = &H200
Public Const LANG_NEUTRAL = &H0
Public Const GENERIC_WRITE = &H40000000
Public Const OPEN_EXISTING = 3
Public Const ERROR_PIPE_CONNECTED = 535&

'Types
Public Type SECURITY_ATTRIBUTES
        nLength As Long
        lpSecurityDescriptor As Long
        bInheritHandle As Long
End Type
Public Type OVERLAPPED
        Internal As Long
        InternalHigh As Long
        offset As Long
        OffsetHigh As Long
        hEvent As Long
End Type

'Functions
Public Declare Function CreateNamedPipe Lib "kernel32" Alias "CreateNamedPipeA" (ByVal lpName As String, ByVal dwOpenMode As Long, ByVal dwPipeMode As Long, ByVal nMaxInstances As Long, ByVal nOutBufferSize As Long, ByVal nInBufferSize As Long, ByVal nDefaultTimeOut As Long, lpSecurityAttributes As SECURITY_ATTRIBUTES) As Long
Public Declare Function FormatMessage Lib "kernel32" Alias "FormatMessageA" (ByVal dwFlags As Long, lpSource As Any, ByVal dwMessageId As Long, ByVal dwLanguageId As Long, ByVal lpBuffer As String, ByVal nSize As Long, Arguments As Long) As Long
Public Declare Function ConnectNamedPipe Lib "kernel32" (ByVal hNamedPipe As Long, lpOverlapped As OVERLAPPED) As Long
Public Declare Function ReadFile Lib "kernel32" (ByVal hFile As Long, lpBuffer As Any, ByVal nNumberOfBytesToRead As Long, lpNumberOfBytesRead As Long, lpOverlapped As OVERLAPPED) As Long
Public Declare Function FlushFileBuffers Lib "kernel32" (ByVal hFile As Long) As Long
Public Declare Function DisconnectNamedPipe Lib "kernel32" (ByVal hNamedPipe As Long) As Long
Public Declare Function WaitNamedPipe Lib "kernel32" Alias "WaitNamedPipeA" (ByVal lpNamedPipeName As String, ByVal nTimeOut As Long) As Long
Public Declare Function CreateFile Lib "kernel32" Alias "CreateFileA" (ByVal lpFileName As String, ByVal dwDesiredAccess As Long, ByVal dwShareMode As Long, lpSecurityAttributes As SECURITY_ATTRIBUTES, ByVal dwCreationDisposition As Long, ByVal dwFlagsAndAttributes As Long, ByVal hTemplateFile As Long) As Long
Public Declare Function WriteFile Lib "kernel32" (ByVal hFile As Long, lpBuffer As Any, ByVal nNumberOfBytesToWrite As Long, lpNumberOfBytesWritten As Long, lpOverlapped As OVERLAPPED) As Long
Public Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long

I am not a VB expert and if I you have any suggestion feel free to post a comment.

No need to copy and paste the code as I have attached the VB and VC Source codes for you to download.

 

The data can be anything and not necessarily be string.

 

Read the file in the binary mode into a buffer and send the buffer.

 

Refer to the following articles for that.

[thread=2977]File read write in plain C[/thread].
[thread=1323]File Handling in C - File Pointers[/thread].
[thread=1268]File Splitter and Merger[/thread].

 

If I have the time I can try but you can assume it may take ages as well.

 

[comment]You can always assume that[/comment]

 

That will corrupt the final output file and send them as bytes.