Named Pipes in .NET

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Named Pipes
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Named pipes have several features that make them an appropriate general-purpose mechanism for [2] implementing IPC-based applications, including networked file access and client/server systems, although anonymous pipes remain a good choice for simple byte-stream IPC, such as the preceding example, where communication is within a single system Named pipe features (some are optional) include the following This statement requires a major qualification Windows Sockets ( 12) is the preferred API for most networking applications and higher-level protocols (http, ftp, and so on), especially where TCP/IPbased interoperability with non-Windows systems is required Many developers prefer to limit named pipe usage to IPC within a single system or to communication within Windows networks
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Named pipes are message-oriented, so the reading process can read varying-length messages precisely as sent by the writing process Named pipes are bidirectional, so two processes can exchange messages over the same pipe There can be multiple, independent instances of pipes with the same name For example, several clients can communicate concurrently with a single server system using pipes with the same name Each client can have its own named pipe instance, and the server can respond to a client using the same instance The pipe name can be accessed by systems on a network Named pipe communication is the same whether the two processes are on the same machine or on different machines Several convenience and connection functions simplify named pipe request/response interaction and client/server connection
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Named pipes are generally preferable to anonymous pipes, although Program 11-1 and Figure 11-1 did illustrate a situation in which anonymous pipes are useful Named pipes should be used any time your communication channel needs to be bidirectional, message oriented, or available to multiple client processes The upcoming examples could not be implemented using anonymous pipes without a great deal of difficulty
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Using Named Pipes
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CreateNamedPipe creates the first instance of a named pipe and returns a handle The function also specifies the maximum number of pipe instances and, hence, the number of clients that can be supported simultaneously Normally, the creating process is regarded as the server Client processes, possibly on other systems, open the pipe with CreateFile Figure 11-2 shows an illustrative client/server relationship, and the pseudocode shows one scheme for using named pipes Notice that the server creates multiple instances of the same pipe, each of which can support a client The server also creates a thread for each named pipe instance, so that each client has a dedicated thread and named pipe instance Figure 11-2, then, shows how to implement the multithreaded server model, first shown in Figure 7-1
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Figure 11-2 Clients and Servers Using Named Pipes
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Creating Named Pipes
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Only Windows NT (that is, as always, Version 40 and above) systems can act as named pipe servers; Windows 9x systems can only be clients Here is the specification of the CreateNamedPipe function HANDLE CreateNamedPipe ( LPCTSTR lpName, DWORD dwOpenMode, DWORD dwPipeMode, DWORD nMaxInstances, DWORD nOutBufferSize, DWORD nInBufferSize, DWORD nDefaultTimeOut, LPSECURITY_ATTRIBUTES lpSecurityAttributes)
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Parameters
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lpName indicates the pipe name, which must be of the form: \\\pipe\[path]pipename
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The period () stands for the local machine; thus, it is not possible to create a pipe on a remote machine dwOpenMode specifies one of the following flags
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PIPE_ACCESS_DUPLEX This flag is equivalent to the combination of GENERIC_READ and GENERIC_WRITE PIPE_ACCESS_INBOUND Data flow is from the client to the server only, equivalent to GENERIC_READ PIPE_ACCESS_OUTBOUND This flag is equivalent to GENERIC_WRITE
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The mode can also specify FILE_FLAG_WRITE_THROUGH (not used with message pipes) and FILE_FLAG_OVERLAPPED (overlapped operations are discussed in 14) dwPipeMode has three mutually exclusive flag pairs They indicate whether writing is message-oriented or byte-oriented, whether reading is by messages or blocks, and whether read operations block
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PIPE_TYPE_BYTE and PIPE_TYPE_MESSAGE indicate whether data is written to the pipe as a stream of bytes or messages Use the same type value for all pipe instances with the same name PIPE_READMODE_BYTE and PIPE_READMODE_MESSAGE indicate whether data is read as a stream of bytes or messages PIPE_READMODE_MESSAGE requires PIPE_TYPE_MESSAGE PIPE_WAIT and PIPE_NOWAIT determine whether ReadFile will block Use PIPE_WAIT because there are better ways to achieve asynchronous I/O
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nMaxInstances determines the number of pipe instances and, therefore, the number of simultaneous clients As Figure 11-2 shows, this same value must be used for every CreateNamedPipe call for a given pipe Use the value PIPE_UNLIMITED_INSTANCES to have the OS base the number on available system resources nOutBufferSize and nInBufferSize give the sizes, in bytes, of the input and output buffers used for the named pipes Specify 0 to get default values nDefaultTimeOut is a default time-out period (in milliseconds) for the WaitNamedPipe function, which is discussed in an upcoming section This situation, in which the create function specifies a time-out for a related function, is unique The return value in case of error is INVALID_HANDLE_VALUE because pipe handles are similar to file handles If you inadvertently attempt to create a named pipe on Windows 9x, which cannot act as a named pipe server, the return value will be NULL, possibly causing confusion lpSecurityAttributes operates as in all the other create functions The first CreateNamedPipe call actually creates the named pipe rather than just an instance Closing the last handle to an instance will delete the instance (usually, there is only one handle per instance) Deleting the last instance of a named pipe will delete the pipe itself, making the pipe name available for reuse
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