Named Pipe Client

A named pipe client uses the CreateFile function to open a handle to a named pipe. If the pipe exists but all of its instances are busy, CreateFile returns zero and the GetLastError function returns ERROR_PIPE_BUSY. When this happens, the named pipe client uses the WaitNamedPipe function to wait for an instance of the named pipe to become available.

The CreateFile function fails if the access specified is incompatible with the access specified (duplex, outbound, or inbound) when the server created the pipe. For a duplex pipe, the client can specify read, write, or read/write access; for an outbound pipe (write-only server), the client must specify read-only access; and for an inbound pipe (read-only server), the client must specify write-only access.

The handle returned by CreateFile defaults to byte-read mode, blocking-wait mode, overlapped mode disabled, and write-through mode disabled. The pipe client can use CreateFile to enable overlapped mode by specifying FILE_FLAG_OVERLAPPED or to enable write-through mode by specifying FILE_FLAG_WRITE_THROUGH. The client can use the SetNamedPipeHandleState function to enable nonblocking mode by specifying PIPE_WAIT or to enable message-read mode by specifying PIPE_READMODE_MESSAGE.

The following example shows a pipe client that opens a named pipe, sets the pipe handle to message-read mode, uses the WriteFile function to send a request to the server, and uses the ReadFile function to read the server's reply. This pipe client can be used with any of the message-type servers shown in the previous examples. With a byte-type server, however, this pipe client fails when it calls SetNamedPipeHandleState to change to message-read mode. Because the client is reading from the pipe in message-read mode, it is possible for the ReadFile operation to return zero after reading a partial message. This happens when the message is larger than the read buffer. In this situation, GetLastError returns ERROR_MORE_DATA, and the client can read the remainder of the message using additional calls to ReadFile.

#include <windows.h> 
 
DWORD main(int argc, char *argv[]) 
{ 
   HANDLE hPipe; 
   LPVOID lpvMessage; 
   CHAR chBuf[512]; 
   BOOL fSuccess; 
   DWORD cbRead, cbWritten, dwMode; 
   LPTSTR lpszPipename = "\\\\.\\pipe\\mynamedpipe"; 
 
// Try to open a named pipe; wait for it, if necessary. 
 
   while (1) 
   { 
      hPipe = CreateFile( 
         lpszPipename,   // pipe name 
         GENERIC_READ |  // read and write access 
         GENERIC_WRITE, 
         0,              // no sharing 
         NULL,           // no security attributes
         OPEN_EXISTING,  // opens existing pipe 
         0,              // default attributes 
         NULL);          // no template file 
 
   // Break if the pipe handle is valid. 
 
      if (hPipe != INVALID_HANDLE_VALUE) 
         break; 
 
      // Exit if an error other than ERROR_PIPE_BUSY occurs. 
 
      if (GetLastError() != ERROR_PIPE_BUSY) 
         MyErrExit("Could not open pipe"); 
 
      // All pipe instances are busy, so wait for 20 seconds. 
 
      if (! WaitNamedPipe(lpszPipename, 20000) ) 
         MyErrExit("Could not open pipe"); 
   } 
 
// The pipe connected; change to message-read mode. 
 
   dwMode = PIPE_READMODE_MESSAGE; 
   fSuccess = SetNamedPipeHandleState( 
      hPipe,    // pipe handle 
      &dwMode,  // new pipe mode 
      NULL,     // don't set maximum bytes 
      NULL);    // don't set maximum time 
   if (!fSuccess) 
      MyErrExit("SetNamedPipeHandleState"); 
 
// Send a message to the pipe server. 
 
   lpvMessage = (argc > 1) ? argv[1] : "default message"; 
 
   fSuccess = WriteFile( 
      hPipe,                  // pipe handle 
      lpvMessage,             // message 
      strlen(lpvMessage) + 1, // message length 
      &cbWritten,             // bytes written 
      NULL);                  // not overlapped 
   if (! fSuccess) 
      MyErrExit("WriteFile"); 
 
   do 
   { 
   // Read from the pipe. 
 
      fSuccess = ReadFile( 
         hPipe,    // pipe handle 
         chBuf,    // buffer to receive reply 
         512,      // size of buffer 
         &cbRead,  // number of bytes read 
         NULL);    // not overlapped 
 
      if (! fSuccess && GetLastError() != ERROR_MORE_DATA) 
         break; 
 
      // Reply from the pipe is written to STDOUT. 
 
      if (! WriteFile(GetStdHandle(STD_OUTPUT_HANDLE), 
         chBuf, cbRead, &cbWritten, NULL)) 
      {
         break; 
      }
 
   } while (! fSuccess);  // repeat loop if ERROR_MORE_DATA 
 
   CloseHandle(hPipe); 
 
   return 0; 
}