[c] What do \t and \b do?

I expect this simple line of code

printf("foo\b\tbar\n");

to replace "o" with "\t" and to produce the following output

fo     bar

(assuming that tab stop occurs every 8 characters). On the contrary I get

foo    bar

It seems that my shell interprets \b as "move the cursors one position back" and \t as "move cursor to the next tab stop". Is this behaviour specific to the shell in which I'm running the code? Should I expect different behaviour on different systems?

No, that's more or less what they're meant to do.

In C (and many other languages), you can insert hard-to-see/type characters using \ notation:

• \a is alert/bell
• \b is backspace/rubout
• \n is newline
• \r is carriage return (return to left margin)
• \t is tab

You can also specify the octal value of any character using \0nnn, or the hexadecimal value of any character with \xnn.

• EG: the ASCII value of _ is octal 137, hex 5f, so it can also be typed \0137 or \x5f, if your keyboard didn't have a _ key or something. This is more useful for control characters like NUL (\0) and ESC (\033)

As someone posted (then deleted their answer before I could +1 it), there are also some less-frequently-used ones:

• \f is a form feed/new page (eject page from printer)
• \v is a vertical tab (move down one line, on the same column)

On screens, \f usually works the same as \v, but on some printers/teletypes, it will go all the way to the next form/sheet of paper.

\t is the tab character, and is doing exactly what you're anticipating based on the action of \b - it goes to the next tab stop, then gets decremented, and then goes to the next tab stop (which is in this case the same tab stop, because of the \b.

This behaviour is terminal-specific and specified by the terminal emulator you use (e.g. xterm) and the semantics of terminal that it provides. The terminal behaviour has been very stable for the last 20 years, and you can reasonably rely on the semantics of \b.

The C standard (actually C99, I'm not up to date) says:

Alphabetic escape sequences representing nongraphic characters in the execution character set are intended to produce actions on display devices as follows:

\b (backspace) Moves the active position to the previous position on the current line. [...]

\t (horizontal tab) Moves the active position to the next horizontal tabulation position on the current line. [...]

Both just move the active position, neither are supposed to write any character on or over another character. To overwrite with a space you could try: puts("foo\b \tbar"); but note that on some display devices - say a daisy wheel printer - the o will show the transparent space.