The /proc/PID/fd directory on procfs-enabled systems contains information about file descriptors opened by a process.
% ls -l proc/self/fd total 0 lrwx—— 1 daniel daniel 64 Apr 2 10:22 0 -> /dev/pts/0 lrwx—— 1 daniel daniel 64 Apr 2 10:22 1 -> /dev/pts/0 lrwx—— 1 daniel daniel 64 Apr 2 10:22 2 -> dev/pts/0 lr-x—— 1 daniel daniel 64 Apr 2 10:22 3 -> /proc/2517738/fd