Answer: dup2(8,8) doesn't duplicate the file descriptor, and then close(8) closes the file descriptor we just opened. So we don't actually have the desired file open on fd 8 after all. (The call dup2(8,8) in fact does nothing. But if you assumed it closed fd 8, well, that's a reasonable guess and the answer is the same, that in the end the file "foo" is not actually open as desired.)

And how do you fix this bug?

Answer: Put the dup2 and close inside an "if fd != 8":

    if (fd != 8) {
	dup2(fd, 8);
	close(fd);
    }