Question

How can I send string data from Java to a C++ console application under Windows? I am trying to do this:

BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(proc.getOutputStream()));
String o = ...;
proc.getOutputStream().write(o.getBytes());

But I never see it on the C++ side when I do this:

ReadFile(stdin_h,buf, sizeof(buf), &bytes, 0)

ReadFile never returns.

What follows is further elaboration and sample code.

---

I have written a simple C++ console (Win32) application which reads from STDIN and performs actions based on input.

Now I want to write a Java application to “drive” the C++ application. The Java applicaton should:

1. Start the C++ application using Runtime.exec()
2. Write string data to the C++ app’s STDIN
3. Repeat until it’s time to die.

My Java application seems to be working, but the C++ application never receives any data on STDIN.

Here is the C++ application:

int main()
{
    ofstream f("c:\\temp\\hacks.txt");

    HANDLE stdin_h = GetStdHandle(STD_INPUT_HANDLE);
    DWORD file_type = GetFileType(stdin_h);
    if( file_type != FILE_TYPE_CHAR )   
        return 42;

    f << "Pipe" << endl;
    for( bool cont = true; cont; )
    {
        char buf[64*1024] = {};
        DWORD bytes = 0;
        if( ReadFile(stdin_h,buf, sizeof(buf), &bytes, 0) )
        {
            string in(buf,bytes);
            cout << "Got " << in.length() << " bytes: '" << in << "'" << endl;
            f << "Got " << in.length() << " bytes: '" << in << "'" << endl;
            if( in.find('Q') )
                cont = false;
        }
        else
        {
            cout << "Err " << GetLastError() << " while reading file" << endl;
            f << "Err " << GetLastError() << " while reading file" << endl;
        }
    }
}

And here is the Java side:

public static void main(String[] args) {
    Runtime rt =Runtime.getRuntime();
    try {
        Process proc = rt.exec("c:\\dev\\hacks\\x64\\debug\\hacks.exe");

        BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(proc.getOutputStream()));

        int a = 0;
        while(a < 5)
        {
            String o = (a == 4 ? "Q\n" : "A\n");
            proc.getOutputStream().write(o.getBytes());
            System.out.println("Wrote '" + o + "'");
            ++a;
        }
        try {
            proc.waitFor();

            // TODO code application logic here
        } catch (InterruptedException ex) {
            Logger.getLogger(Java_hacks.class.getName()).log(Level.SEVERE, null, ex);
        }
    } catch (IOException ex) {
        Logger.getLogger(Java_hacks.class.getName()).log(Level.SEVERE, null, ex);
    }
}

The Java side seems to be working correctly, but I’m not ever receiving the strings on the C++ side.

Am I doing something wrong here? How can I send string data from Java to a C++ console application under Windows?

Answer 1

Why are you not flushing the output stream on the Java side after writing 5 strings?

proc.getOutputStream().flush();