fputc('\b',file);

hai /prog i nid u halp!
why do \n \t \r works on fputc and \b don't do backspace, but print other character? How do I backspace with fputc?
thanks a lot!

\n, \t, and \r all result in a character being written to the file. \b prints the backspace character to the file. To make it work like a backspace, you need to interpret it yourself.
If you have a seekable output stream, you could seek back one character.