Each process is represented by a unique identifier, which is called as the process id (PID), at a given time, PID is 17 guaranteed to be unique. The kernel allocates process ids to processes in a strictly linear fashion. For e.g if pid 17 is the highest number currently allocated, the next process would be allocated the PID 18, even though the process with PID 17 is no longer running. The kernel doesn't reuse process id values until the value in /proc/sys/kernel/pid_max is allocated.