重新连接到命名管道时出现问题

Question

I have a named pipe server and client. (Doing this in VC++).

Server does

1. CreateNamedPipe
2. ConnectNamedPipe
3. WriteFile
4. Disconnect
5. Repeat from 2 to 4

Client does

1. CreateFile
2. ReadFile

The order of execution is as follows,

1. Server -- CreateNamedPipe
2. Client -- CreateFile
3. Server -- ConnectNamedPipe (should return immediately as the client is already connected)
4. Server -- WriteFile
5. Client -- ReadFile
6. Server -- DisconnectNamedPipe
7. Client -- CloseHandle
8. goto 2

This works fine for the first time. However problem occurs when client tries to connects for the second time. When the client tries to connect (CreateFile) for the second time before the server did ConnectNamedPipe (but after disconnectnamedpipe), it gets ERROR_PIPE_BUSY. It works if client calls createfile after the server calls ConnectNamedPipe.

Is there anyway that i can get client connected (CreateFile) before server called ConnectNamedPipe (after DisconnectNamedPipe)?

Server code:

pipe_handle.pipe = CreateNamedPipe(TEXT("\\\\.\\pipe\\testpipe1"),
                PIPE_ACCESS_OUTBOUND |
                FILE_FLAG_OVERLAPPED,        // read/write access
                PIPE_TYPE_MESSAGE |           // message type pipe
                PIPE_READMODE_MESSAGE |       // message-read mode
                PIPE_WAIT,                    // blocking mode
                PIPE_UNLIMITED_INSTANCES,     // max. instances
                BUFFER_SIZE,                  // output buffer size
                BUFFER_SIZE,                  // input buffer size
                2000,              // client time-out
                NULL);

if (pipe_handle.pipe == INVALID_HANDLE_VALUE) {
    std::cout << "Error while creating pipe" << std::endl;
    return -1;
}
std::cout <<"Connecting to named pipe" << std::endl;

std::cout<< "Somebody connected to named pipe" << std::endl;

int ac;

for (ac=0; ac<2; ac++) {

    char a[25];
    // Wait for some input. This helps me to start the client in other terminal.
    cin >> a;
    cout << "Connecting..." << endl;

    ConnectNamedPipe(pipe_handle.pipe, 0);

    cout << "Connect pipe returned." << endl;

    // Wait for some input.
    cin >> a;
    string message = "Test message";
    DWORD bytes_written;

    if (!WriteFile(pipe_handle.pipe, message.c_str(), message.size(),
                   &bytes_written, NULL)) {

        DWORD er = GetLastError();
        char errs[200];
        sprintf(errs, "Error : %ld", er);
        std::cout << "Error communicating to client.";
        std::cout << errs;
    }
    std::cout << "Written to pipe";
    FlushFileBuffers(pipe_handle.pipe);
    if (!DisconnectNamedPipe(pipe_handle.pipe)) {
        std::cout << "Disconnect failed"<< GetLastError() << endl;
    } else {
        std::cout << "Disconnect successful"<<endl;
    }
}

Client Code:

while (1) { 

    std::cout << "Returned" << std::endl;
    hPipe = CreateFile( 
              lpszPipename,   // pipe name 
              GENERIC_READ, 
              0,              // no sharing 
              NULL,           // default security attributes
              OPEN_EXISTING,  // opens existing pipe 
              FILE_FLAG_OVERLAPPED,              // 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) {
        std::cout<< "Could not open pipe " << GetLastError() << std::endl; 
        return -1;
    }

    // All pipe instances are busy, so wait for sometime.

    if ( ! WaitNamedPipe(lpszPipename, NMPWAIT_USE_DEFAULT_WAIT)) { 
        std::cout<<  "Could not open pipe: wait timed out." << std::endl; 
    } 
}

OVERLAPPED ol1;

memset(&ol1, 0, sizeof(ol1));
ol1.Offset = 0;
ol1.OffsetHigh = 0;
ol1.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);

HANDLE events[1];
events[0] = ol1.hEvent;
cbToWrite = (lstrlen(message)+1)*sizeof(TCHAR);

DWORD bytes_to_read = 2000;
char * buf = reinterpret_cast<char *>(malloc(bytes_to_read));
DWORD bytes_read;

std::cout << "Waiting for read" << std::endl;
bool a = ReadFile(hPipe, buf, bytes_to_read, &bytes_read, &ol1);


if ( ! fSuccess) {
    std::cout << "WriteFile to pipe failed. GLE " << GetLastError() << std::endl; 
}
std::cout << "Waiting for multiple objects" << std::endl;
WaitForMultipleObjects(1, events, FALSE, INFINITE);
std::cout << "multiple objects returned" << std::endl;
printf("\nMessage sent to server");
CancelIo(hPipe);
CloseHandle(hPipe);

Answer 1

If you get ERROR_PIPE_BUSY on the CreateFile() call in the client, you need to call WaitNamedPipe() and then retry when it returns. If you get a return of zero from WaitNamedPipe() that means it timed out without the pipe becoming available. You'll never see that happen if you pass NMPWAIT_WAIT_FOREVER as the timeout.

You also need to keep in mind that the pipe may become busy again between the time WaitNamedPipe() returns and you call CreateFile(); therefore, you need to do it in a loop. Like this:

while (true)
{
    hPipe = CreateFile(pipeName,
                       GENERIC_READ | GENERIC_WRITE,
                       0,
                       0,
                       OPEN_EXISTING,
                       FILE_ATTRIBUTE_NORMAL,
                       0);
    if (hPipe == INVALID_HANDLE_VALUE)
    {
        if (GetLastError() == ERROR_PIPE_BUSY)
        {
            if (!WaitNamedPipe(pipeName, NMPWAIT_USE_DEFAULT_WAIT))
                continue;   // timeout, try again
        }
        else
            return false;   // error
    }
    else
        break;   // success
}

EDIT:

I simplified your code and now it works fine. Working server and client follow.

Server:

#include <windows.h>
#include <stdio.h>

int main(void)
{
    HANDLE pipe;
    const DWORD BUFFER_SIZE = 1024;

    pipe = CreateNamedPipe("\\\\.\\pipe\\testpipe1",
                                  PIPE_ACCESS_OUTBOUND |
                                  FILE_FLAG_OVERLAPPED,          // read/write access
                                  PIPE_TYPE_MESSAGE |             // message type pipe
                                  PIPE_READMODE_MESSAGE |         // message-read mode
                                  PIPE_WAIT,                          // blocking mode
                                  PIPE_UNLIMITED_INSTANCES,   // max. instances
                                  BUFFER_SIZE,                        // output buffer size
                                  BUFFER_SIZE,                        // input buffer size
                                  2000,                 // client time-out
                                  NULL);

    if (pipe == INVALID_HANDLE_VALUE)
    {
        printf("Error while creating pipe\n");
        return -1;
    }
    printf("Connecting to named pipe\n");

    int ac;

    for (ac=0; ac<2; ac++)
    {
        // Wait for some input. This helps me to start the client in other terminal.
        printf("Connecting...\n");

        ConnectNamedPipe(pipe, 0);

        printf("Connect pipe returned.\n");

        // Wait for some input.
        char * message = "Test message";
        DWORD bytes_written;

        if (!WriteFile(pipe, message, strlen(message)+1, &bytes_written, NULL))
        {

            DWORD er = GetLastError();
            char errs[200];
            sprintf_s(errs, "Error : %ld", er);
            printf("Error communicating to client.\n");
            printf(errs);
        }
        printf("Written to pipe\n");
        FlushFileBuffers(pipe);
        if (!DisconnectNamedPipe(pipe))
        {
            printf("Disconnect failed %d\n", GetLastError());
        }
        else
        {
            printf("Disconnect successful\n");
        }
    }
}

Client:

#include <windows.h>
#include <stdio.h>

int main(void)
{
    HANDLE hPipe;

    while (1)
    {

        printf("Returned\n");
        hPipe = CreateFile("\\\\.\\pipe\\testpipe1",
                                GENERIC_READ, 
                                0,                   // no sharing 
                                NULL,                // default 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)
        {
            printf("Could not open pipe %d\n", GetLastError()); 
            return -1;
        }

        // All pipe instances are busy, so wait for sometime.

        if ( ! WaitNamedPipe("\\\\.\\pipe\\testpipe1", NMPWAIT_USE_DEFAULT_WAIT))
        {
            printf("Could not open pipe: wait timed out.\n"); 
        }
    }


    char *message = "hello";
    DWORD cbToWrite = (strlen(message)+1)*sizeof(message[0]);

    DWORD bytes_to_read = 2000;
    char * buf = reinterpret_cast<char *>(malloc(bytes_to_read));
    DWORD bytes_read;

    printf("Waiting for read\n");
    bytes_read = 0;
    ReadFile(hPipe, buf, bytes_to_read, &bytes_read, 0);

    if (bytes_read <= 0)
    {
        printf("ReadFile from pipe failed. GLE \n"); 
    }
    else
        printf("Read %d bytes: %s\n", bytes_read, buf);

    CloseHandle(hPipe);
    return 0;
}

Answer 2

On the Server side when you decide to break the connection you must use chain:

1) CloseHandle (Pipe);

2) DisconnectNamedPipe (Pipe);