Named Pipe Client Process

A client process 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 FALSE and the GetLastError function returns ERROR_PIPE_BUSY. When this happens, the client process uses the WaitNamedPipe function to wait for an instance of the pipe to be available.

CreateFile 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 (read/write), the client can specify read, write, or read/write access; for an outbound pipe (server write only), the client must specify read only; and for an inbound pipe (server read only), the client must specify write only.

The handle returned by CreateFile defaults to byte-read mode, blocking-wait mode, overlapped mode disabled, and write-through mode disabled. The client process 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 client process that opens a named pipe, sets the pipe handle to message-read mode, uses WriteFile to send a request to the server, and uses ReadFile to read the server's reply. This client process can be used with any of the message-type servers shown in the previous sections. With a byte-type server, however, this client process 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 FALSE 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 remainder of the message is read by 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 max. bytes

NULL); // don't set max. 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");

{

// 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;

}

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