命名管道问题

Question

I am trying to learn how named pipes work, and created 2 consoles to test the connectivity between server and client. Client will send a message to the server and the server will display the message, but instead of the message, it returns a value of "nullptr" as shown in the error exception break from VS.

below are my codes, do enlighten me if you found any problem with my code, and I am still learning..

Server.cpp

#include "cust_ostream.hpp"
#include <Windows.h>
#include <iostream>
#include <conio.h>

using namespace std;

int main()
{
    LPVOID buffer = NULL;
    DWORD readbyte;

    cout << "---Named Pipe Server Test---" << endl << endl;
    cout << "Creating named pipe: \\\\.\\pipe\\mypipe" << endl;

    HANDLE hPipe = CreateNamedPipeA("\\\\.\\pipe\\mypipe", PIPE_ACCESS_DUPLEX, PIPE_TYPE_MESSAGE | PIPE_READMODE_MESSAGE | PIPE_WAIT,
        PIPE_UNLIMITED_INSTANCES, 1024, 1024, 0, NULL);

    if (!hPipe || hPipe == INVALID_HANDLE_VALUE)
    {
        cout << "Pipe creation failed." << endl;
        return 0;
    }

    cout << "Connecting pipe to client..." << endl;

    BOOL connect = ConnectNamedPipe(hPipe, NULL);

    if (!connect)
    {
        cout << "Connect named pipe failed" << endl;
    }

    cout << "Success! Reading pipe message from client..." << endl;

    ReadFile(hPipe, buffer, sizeof(buffer), &readbyte, NULL);

    c_cout << "Pipe message = " << *(int *)buffer << endl;

    _getch();

    return 0;
}

cust_ostream.hpp

#include <Windows.h>
#include <iostream>
#include <sstream>

using namespace std;

#define endl "\n"

class cust_ostream
{
public:
    ~cust_ostream()
    {
        cout << m_buffer.str();
    }

    template <typename T>
    cust_ostream &operator<<(T const &value)
    {
        m_buffer << value;
        return *this;
    }

private:
    ostringstream m_buffer;
};

#define c_cout cust_ostream()

and my client

#include <Windows.h>
#include <iostream>
#include <conio.h>

using namespace std;

int main()
{
    LPVOID data;
    DWORD writebyte;

    int i = 2;

    cout << "---Named Pipe Client---" << endl << endl;
    cout << "Creating pipe file: \\\\.\\pipe\\mypipe" << endl;

    HANDLE pipe = CreateFileA("\\\\.\\pipe\\mypipe", GENERIC_READ | GENERIC_WRITE, 0, NULL, OPEN_EXISTING, 0, NULL);

    if (!pipe || pipe == INVALID_HANDLE_VALUE)
    {
        cout << "Pipe client failed." << endl;
        return 0;
    }

    cout << "Pipe connected to server, sending data..." << endl;

    WriteFile(pipe, &i, sizeof(i), &writebyte, NULL);

    _getch();

    return 0;
}

Answer 1

You need to wait for the NamedPipe to have a ConnectPipeReady event on it. As it stands, you are trying to create the pipe without actually seeing if it was succesfull. See the MSDN documentation for Named Pipes here: https://msdn.microsoft.com/en-ca/library/windows/desktop/aa365592(v=vs.85).aspx

Specifically, this block:

while (1) 
{ 
  hPipe = CreateFile( 
     lpszPipename,   // pipe name 
     GENERIC_READ |  // read and write access 
     GENERIC_WRITE, 
     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) 
  {
     _tprintf( TEXT("Could not open pipe. GLE=%d\n"), GetLastError() ); 
     return -1;
  }

  // All pipe instances are busy, so wait for 20 seconds. 

  if ( ! WaitNamedPipe(lpszPipename, 20000)) 
  { 
     printf("Could not open pipe: 20 second wait timed out."); 
     return -1;
  } 
}

Also you shouldn't use #define endl "\\n" , use std::endl

Answer 2

You have initialized your buffer as NULL which means that by default its length is zero. Now when you use the sizeof operator in your read function in server (to retrieve the message received by server from client), what happens is that you are asking the sizeof operator in Read function to read 0 bytes! which means that nothing will be read.

To solve this, you can declare a char array of size 100 or a size of a message which you are sure that won't be exceeded by client. Like if you are assuming that message by client is not going to be longer than lets say 60 characters, then you can create your char buffer to be of size 100 just to make sure that you do accommodate all the message by client.

And one more thing, if problem still persists, instead of using sizeof in read, use 100 or whatever the size of of your char buffer array. This should solve your problem.