Compare the canonical vs. noncanonical input modes of the TTY line driver

termios.c

#include <stdio.h>
#include <termios.h>
#include <unistd.h>

#define MAX_BUFSIZE 4096

int main(int argc, char *argv[])
{
    struct termios old_termios;
    struct termios new_termios;
    char *tty_devfile;
    char buf[MAX_BUFSIZE];
    int bytes_read;

    if ( argc < 2 ) {
        dprintf(2, "Pass argument 1 for canonical mode, 0 for noncanonical\n");
        return 1;
    }

    if ( !isatty(0) ) {
        dprintf(2, "STDIN is not a terminal device\n");
        return 1;
    }

    if ( (tty_devfile = ttyname(0)) ) {
        dprintf(2, "Using terminal device %s\n", tty_devfile);
    }

    if ( tcgetattr(0, &old_termios) == -1 ) {
        perror("Error while getting old terminal settings");
        return 1;
    }

    new_termios = old_termios;

    switch ( argv[1][0] ) {
    case '0':
        // If MIN > 0 then a pending read is not satisfied until MIN bytes are received. A program that uses this case
        // to read record-based terminal I/O may block indefinitely in the read operation. If MIN = 0 and no characters
        // are available, read is nonblocking and returns a value of zero, having read no data.
        new_termios.c_lflag &= ~(ICANON | ECHO);
        new_termios.c_cc[VMIN] = 0;
        new_termios.c_cc[VTIME] = 0;
        break;

    case '1':
        new_termios.c_lflag &= ICANON | ECHO | ECHOE;
        new_termios.c_cc[VMIN] = 0;
        new_termios.c_cc[VTIME] = 0;
        break;

    default:
        dprintf(2, "Invalid argument\n");
        return 1;
    }

    if ( tcsetattr(0, TCSADRAIN, &new_termios) == -1 ) {
        perror("Error while setting new terminal settings");
        return 1;
    }

    while ( (bytes_read = read(0, buf, sizeof(buf) - 1)) != -1 ) {
        if ( bytes_read > 0 ) {
            buf[bytes_read] = '\0';

            if ( buf[0] == 'q' ) {
                dprintf(2, "Exiting...\n");
                break;
            }

            dprintf(2, "Bytes read: %d\n", bytes_read);
            dprintf(2, "Buffer content: %s\n", buf);
        }
    }

    if ( tcsetattr(0, TCSADRAIN, &old_termios) == -1 ) {
        perror("Error while resetting old terminal settings");
        return 1;
    }

    return 0;
}