Named Pipe Transaction Functions in Visual Studio .NET

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Named Pipe Transaction Functions
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Figure 11-2 shows a typical client configuration in which the client does the following:
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Opens an instance of the pipe, creating a long-lived connection to the server and consuming a pipe instance Repetitively sends requests and waits for responses Closes the connection
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The common WriteFile, ReadFile sequence could be regarded as a single client transaction, and Windows provides such a function for message pipes BOOL TransactNamedPipe ( HANDLE hNamedPipe, LPVOID lpWriteBuf, DWORD cbWriteBuf, LPVOID lpReadBuf, DWORD cbReadBuf, LPDWORD lpcbRead, LPOVERLAPPED lpOverlapped)
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The parameter usage is clear because this function combines WriteFile and ReadFile on the named pipe handle Both the output and input buffers are specified, and *lpcbRead gives the message length Overlapped operations ( 14) are possible More typically, the function waits for the response transactNamedPipe is convenient, but, as in Figure 11-2, it requires a permanent connection, which [3] limits the number of clients Note that transactNamedPipe is more than a mere convenience compared with WriteFile and ReadFile and can provide some performance advantages One experiment shows throughput enhancements ranging from 57 percent (small messages) to 24 percent (large messages) CallNamedPipe is the second client convenience function: BOOL CallNamedPipe ( LPCTSTR lpPipeName, LPVOID lpWriteBuf, DWORD cbWriteBuf, LPVOID lpReadBuf, DWORD cbReadBuf, LPDWORD lpcbRead, DWORD dwTimeOut)
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CallNamedPipe does not require a permanent connection; instead it makes a temporary connection by combining the following complete sequence: CreateFile
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WriteFile ReadFile CloseHandle into a single function The benefit is better pipe utilization at the cost of per-request connection overhead The parameter usage is similar to that of transactNamedPipe except that a pipe name, rather than a handle, is used to specify the pipe CallNamedPipe is synchronous (there is no overlapped structure) It specifies a time-out period, in milliseconds, for the connection but not for the transaction There are three special values for dwTimeOut:
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NMPWAIT_NOWAIT NMPWAIT_WAIT_FOREVER NMPWAIT_USE_DEFAULT_WAIT, which uses the default time-out period specified by CreateNamedPipe
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Peeking at Named Pipe Messages
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In addition to reading a named pipe using ReadFile, you can also determine whether there is actually a message to read using PeekNamedPipe This can be used to poll the named pipe (an inefficient operation), determine the message length so as to allocate a buffer before reading, or look at the incoming data so as to prioritize its processing BOOL PeekNamedPipe ( HANDLE hPipe, LPVOID lpBuffer, DWORD cbBuffer, LPDWORD lpcbRead, LPDWORD lpcbAvail, LPDWORD lpcbMessage)
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PeekNamedPipe nondestructively reads any bytes or messages in the pipe, but it does not block; it returns immediately Test *lpcbAvail to determine whether there is data in the pipe; if there is, *lpcbAvail will be greater than 0 In this case, lpBuffer and lpcbRead can be NULL If a buffer is specified with lpBuffer and cbBuffer, then *lpcbMessage will tell whether there are leftover message bytes that could not fit into the buffer, allowing you to allocate a large buffer before reading from the named pipe This value is 0 for a byte mode pipe Again, PeekNamedPipe reads nondestructively, so a subsequent ReadFile is required to remove messages or bytes from the pipe
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The UNIX FIFO is similar to a named pipe, thus allowing communication between unrelated processes There are limitations compared with Windows named pipes
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FIFOs are half-duplex FIFOs are limited to a single machine FIFOs are still byte-oriented, so it is easiest to use fixed-size records in client/server applications Nonetheless, individual read and write operations are atomic
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A server using FIFOs must use a separate FIFO for each client's response, although all clients can send requests to a single, well-known FIFO A common practice is for the client to include a FIFO name in a connect request mkfifo is the UNIX function that is a limited version of CreateNamedPipe If the clients and server are to be networked, use sockets or a similar transport mechanism Sockets are full-duplex, but there must still be one separate connection per client
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