Question

Why CTRL + M gives an ASCII value of 10 (decimal value). It should actually give 13. I am connecting to Amazon EC2 linux instance through putty. I execute the below program

import java.io.IOException;
import java.io.InputStream;
import java.io.ByteArrayOutputStream;

public class NumbersConsole {

private static String ttyConfig;

public static void main(String[] args) {

        try {
                setTerminalToCBreak();

                int i=0;
                while (true) {

                        //System.out.println( ""+ i++ );

                        if ( System.in.available() != 0 ) {
                                int c = System.in.read();
                    System.out.println(c);
                                if ( c == 13 ) {
                                        break;
                                }
                        }

                } // end while
        }
        catch (IOException e) {
                System.err.println("IOException");
        }
        catch (InterruptedException e) {
                System.err.println("InterruptedException");
        }
        finally {
            try {
                stty( ttyConfig.trim() );
             }
             catch (Exception e) {
                 System.err.println("Exception restoring tty config");
             }
        }

}

private static void setTerminalToCBreak() throws IOException, InterruptedException {

    ttyConfig = stty("-g");

    // set the console to be character-buffered instead of line-buffered
    stty("-icanon min 1");

    // disable character echoing
    stty("-echo");
}

/**
 *  Execute the stty command with the specified arguments
 *  against the current active terminal.
 */
private static String stty(final String args)
                throws IOException, InterruptedException {
    String cmd = "stty " + args + " < /dev/tty";

    return exec(new String[] {
                "sh",
                "-c",
                cmd
            });
}

/**
 *  Execute the specified command and return the output
 *  (both stdout and stderr).
 */
private static String exec(final String[] cmd)
                throws IOException, InterruptedException {
    ByteArrayOutputStream bout = new ByteArrayOutputStream();

    Process p = Runtime.getRuntime().exec(cmd);
    int c;
    InputStream in = p.getInputStream();

    while ((c = in.read()) != -1) {
        bout.write(c);
    }

    in = p.getErrorStream();

    while ((c = in.read()) != -1) {
        bout.write(c);
    }

    p.waitFor();

    String result = new String(bout.toByteArray());
    return result;
}

}

and when I give the input as (CTRL + M), I am getting displayed a value of 10. But I am expecting a value of 13. Please let me know if I am missing anything??

Answer 1

The translation of CR to LF is handled by the tty driver. You’re calling setTerminalToCBreak(), which manipulates the tty settings (I think it disables the erase, kill, werase, and rprnt special characters).

The icrnl setting, which is enabled by default, causes carriage return (CR) to be translated to newline (LF). Disabling that should let you see CR characters directly. Setting raw mode changes a number of flags, including turning off icrnl. (Figuring out how to do that in Java is left as an exercise.)

But beware of doing this. The Enter or Return key typically sends a CR character. Translating it to LF is what allows it to mark the end of a line. If you turn off that translation, you might break that behavior unless you handle CR yourself.

For more information on tty settings, man tty or follow this link.