Read and write to the same unnamed pipe(boost::process::pstream) in child process

Question

I am trying to implement reading and writing in the child process from/to the same pipe. Instead of pipe I'm using boost::process::pstream and redirecting stdin and stdout in child process to that stream.

Simple example. I send a message from the main process to the child one, read it from the child process, and send it back to the main. The problem is that I'm receiving a garbage(empty string) in the main process back from the child process. What am I doing wrong?

The code example below works correct if I use separate streams for the input/output of the child process...

int main(int argc, char **argv) {
    if (argc == 1)
    {
        bp::pstream p;
        p << "What?" << endl;
        bp::child c(argv[0], "OPTIONAL", bp::std_in=p, 
            bp::std_out=p, bp::std_err=stderr);
        c.wait();

        string x;
        getline(p, x);
        cout << "Received from child process:" << x << endl;
        system("pause");
    }
    else {
        string x;
        getline(cin, x);
        cerr << "Received from main process:" << x << endl;
        cout << x << endl;
    }
}

The output is following:

Received from main process:What?
Received from child process:
Press any key to continue . . .