Write a program that lists all processes that have a particular file pathname open. This can be achieved by inspecting the contents of all of the /proc/PID/fd/* symbolic links. This will require nested loops employing readdir(3) to scan all /proc/PID directories, and then the contents of all /proc/PID/fd entries within each /proc/PID directory. To read the contents of a /proc/PID/fd/n symbolic link requires the use of readlink(), described in Section 18.5.