Read Write issues with Pseudo Terminal in Linux

Question

I am writing a C++ program that would interact with an external process. The external process is written in C# and runs on mono. Note that I cannot modify the C# code as it is not a program written by me.

In this regard, I first set out by using pipes, which of course as I later realized is fully buffered and hence I faced a lot of sync issues. Essentially the external process had to flush its output after every write and this was not possible.

The next thing that I was about to try out was files, but however I found out that using pseudo-terminals would be more apt in my case. Here is some sample code that I have written:

int main()
{
    int fdm, fds, rc, pid;
    bool rValue;
    /* Setup Master pty*/
    rValue = rValue && (fdm = posix_openpt(O_RDWR)) >= 0 &&
             (rc = grantpt(fdm)) == 0 && (rc = unlockpt(fdm) == 0);
    if (rValue) {
        /* Open Slave pty */
        fds = open(ptsname(fdm), O_RDWR);
        pid = fork();
        if(pid < 0)
            perror("fork failed");
        else if(pid == 0) //child
        {
            close(fdm); //close master
            struct termios slave_orig_term_settings;
            struct termios new_term_settings;
            tcgetattr(slaveTTY, &slave_orig_term_settings);
            new_term_settings = slave_orig_term_settings;
            cfmakeraw(&new_term_settings);
            tcsetattr(slaveTTY, TCSANOW, &new_term_settings);

            //redirect I/O of this process
            close(0);
            close(1);
            close(2);
            dup(slaveTTY);
            dup(slaveTTY);
            dup(slaveTTY);

            close(slaveTTY);

            setsid();
            ioctl(0, TIOCSCTTY, 1);

            //launch the external process and replace its image in this process
            execve(argv[0],...);
        }
        else
        {
            close(fds); //close slave
            //Perform some interaction
            write(something using fdm);
            //Assume fdsets declared and set somewhere here
            select(fdm +1,&fdset,NULL,NULL,NULL);
            int readBytes = read(someting using fds);
        }
    }
    return EXIT_SUCCESS;
}

Assume that the fdset and fdclr for select are being taken care of.

The following issues are being observed in the parent process:

1. Sometimes read returns with readBytes > 0 but there is nothing present in the buffer
2. Sometimes whatever has been written to the terminal is read back
3. Some garbage values such as ^]]49]1R are being dumped on the terminal (this is the actual terminal ie my output window)

PS: When the external process is written in C/C++, this issue is not occuring. Only when I run a C# program in mono.

Answer 1

I think pexpect in python is a good choice if you don't have to do that in C++ , it will save you a lot of time. And also you can use python freeze tools like pyinstaller to convert your python script to standalone binary.