Question

I found out the next issue in this simple code:

let () =
    print_endline "Hello";
    print_endline (Unix.getlogin ())

Running in the normal case, with ./a.out gives:

Hello
ricardo

But running like ./a.out </dev/null makes Unix.getlogin fail:

Hello
Fatal error: exception Unix.Unix_error(20, "getlogin", "")

Any idea why this happens?

Answer 1

Redirecting the input of a program overrides its controlling terminal. Without a controlling terminal, there is no login to be found:

$ tty
/dev/pts/2
$ tty < /dev/null
not a tty

You can, however, still find a user’s name (perhaps) by getting the user’s id (getuid) and looking up his passwd entry (related docs) (getpwuid), then finding his username in it.