root@poseidon:~# lsof /dev/ttyS4
lsof: WARNING: can't stat() fuse.gvfs-fuse-daemon file system /home/henrik/.gvfs
      Output information may be incomplete.
COMMAND  PID   USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
screen  3051 henrik    6u   CHR   4,68      0t0 1165 /dev/ttyS4
screen  3510   root    6u   CHR   4,68      0t0 1165 /dev/ttyS4
screen  3589   root    6u   CHR   4,68      0t0 1165 /dev/ttyS4
screen  3791   root    6u   CHR   4,68      0t0 1165 /dev/ttyS4