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In the above answers the getchar() function blocks the ros's while loop, If you want the loop to run at a particular rate and to get inputs when it's available replace the getch() function definition with the below.

You can change the timeout.tv_sec, timeout.tv_sec to change the time to wait for user input. filedesc is 0 for taking input from keyboard, most probably it's similar to stdin but not sure.

char getch()
    fd_set set;
    struct timeval timeout;
    int rv;
    char buff = 0;
    int len = 1;
    int filedesc = 0;
    FD_SET(filedesc, &set);

    timeout.tv_sec = 0;
    timeout.tv_usec = 1000;

    rv = select(filedesc + 1, &set, NULL, NULL, &timeout);

    struct termios old = {0};
    if (tcgetattr(filedesc, &old) < 0)
    old.c_lflag &= ~ICANON;
    old.c_lflag &= ~ECHO;
    old.c_cc[VMIN] = 1;
    old.c_cc[VTIME] = 0;
    if (tcsetattr(filedesc, TCSANOW, &old) < 0)
        ROS_ERROR("tcsetattr ICANON");

    if(rv == -1)
    else if(rv == 0)
        read(filedesc, &buff, len );

    old.c_lflag |= ICANON;
    old.c_lflag |= ECHO;
    if (tcsetattr(filedesc, TCSADRAIN, &old) < 0)
        ROS_ERROR ("tcsetattr ~ICANON");
    return (buff);

If you require an example code visit here: keyboard_non_blocking_input_node.cpp

This link helped in implementing this:

I Hope This Helps!!!!!