how can I change backspace to '\b'?

Question

I see a question in The C Programming Language. It's like this: Write a program to copy its input to its output, replacing each tab by \t , each backspace by \b , and each backslash by \ . This makes tabs and backspaces visible in an unambiguous way.

Then I write a program like this:

#include<stdio.h>

int main(void)
{
    int c;
    while((c=getchar())!=EOF)
    {
        if(c=='\t')
        {
            putchar('\\');
            putchar('t');
        }
        if(c=='\b')
        {
            putchar('\\');
            putchar('b');
        }
        if(c=='\\')
        {

            putchar('\\');
            putchar('\\');
        }
        else{
            putchar(c);
        }
    }
    return 0;
}

But when I input backspace, I can't get '\b', so how can I get the output '\b'? I.e., I mean, how can I output backspace?

Answer 1

Keyboard input is preprocessed by the operating system. Most characters are fed directly as input to your program, but some are handled specially.

On UNIX-like systems, standard input is usually line-buffered. The system reads a whole line of text and doesn't pass it on to your program until you press Enter. While reading that line, the system processes Backspace itself; rather than adding a backspace character to the buffer, it erases the most recent character. Your program never sees the '\b' character.

To enter a literal backspace character that can be seen by your program, on a UNIX-like system you can precede it with Ctrl-V. And depending on your tty and terminal emulator settings, typing the Backspace key might transmit an ASCII DEL character rather than backspace. To make sure your program sees a backspace character, type Ctrl-V Ctrl-H.

On non-UNIX-like systems (most likely Windows), there's probably a way to do something similar, but I don't know what it is.

You can also run your program with input from a file. Getting a literal backspace character into the input file is left as an exercise (it depends on the workings of your text editor).