The meaning of disabling local echo in terminal

Question

I have a c application running on Linux OS's. This app gets keyboard keys from a terminal and sends them to remote server.
The code below opens the terminal:

    // save old terminal attributes
if (tcgetattr(0, &ttyold) != 0) {
    fprintf(stderr, "Failed getting terminal attributes\n");
    goto out;
}

ttynew = ttyold;

ttynew.c_iflag = 0;
ttynew.c_oflag = 0;

// disable canonical mode (don't buffer by line)
ttynew.c_lflag &= ~ICANON;

// disable local echo
ttynew.c_lflag &= ~ECHO;

ttynew.c_cc[VMIN] = 1;
ttynew.c_cc[VTIME] = 1;

// set new terminal attributes 
if (tcsetattr(0, TCSANOW, &ttynew) != 0) {
    fprintf(stderr, "Failed setting terminal attributes\n");
    goto out;

I did not write this app, I'm just trying to understand this code. I don't understand why the previous engeneer disabled the echo? the data that has to be sent isn't secret. What else could be the meaning of this? performance? disable buffering?
In addition, I'll be happy to get an explanation of "ttynew.c_lflag &= ~ICANON;" code.

Thanks in advance.

Best idea is to ask him.. – VoidPointer Jul 23 '13 at 08:32 — VoidPointer, Jul 23 '13 at 08:32

score 1 · Accepted Answer · answered Jul 23 '13 at 07:22

1

If the receiving end doesn't echo, you need to enable this. If the receiving end echoes, you disable it, otherwise you end up with seeing everything double.

Here everything is explained:

In canonical mode: Input is made available line by line. An input line is available when one of the line delimiters is typed (NL, EOL, EOL2; or EOF at the start of line).

Basically, data is sent after EOL, rather than per character.

answered Jul 23 '13 at 07:22

Bart Friederichs

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I don't quite understand you: when I am disabling the echo, the charcters are not visible in the local terminal (in the client). why is it related to the recieving side? I'm sending the data by TCP protocol... – RRR Jul 23 '13 at 07:33
Dont understand your answer about the echo – RRR Jul 23 '13 at 07:44
I think it is only applicable to terminal emulation. You could try switching it on and see what happens. On the other hand, if it works, don't fix it. – Bart Friederichs Jul 23 '13 at 12:09

The meaning of disabling local echo in terminal

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