TIL: File descriptor 3
There are three standard file descriptions, STDIN, STDOUT, and STDERR. They are assigned to 0, 1, and 2 respectively.
File descriptor 3, 4 and so on can be accessed at:
I found it when looking at this Patchset. Chromium open
--remote-debugging-pipe at file descriptor 3 for output and file descriptor 4 for input.