How can a Unix program display output on screen even when stdout and stderr are redirected?

Question

I was running a program (valgrind, actually) on my Ubuntu machine, and had redirected both stdout and stderr to different files. I was surprised to see a short message appea

Answer 1

The message is most probably from GCC's stack protector feature or from glib itself. If it's from GCC, it is output using the fail() function, which directly opens /dev/tty:

fd = open (_PATH_TTY, O_WRONLY);

_PATH_TTY is not really standard, but SingleUnix actually demands that /dev/tty exists.