How Windows/Linux assign PID to processes?

Question

Q1 : I want to know how Windows/Linux assign a PID to a process when that process began? Is it a delimited random number? or it is fixed for a file on a compute? does it have a special algorithm? and ...

Q2 : Is it possible to change PID of a process after it started?

Q3 : Is it possible to prevent a file to run(I mean is it possible to restrict OS to don't assign any PID to a file)?

Answer 1

This answer is Windows-specific.

PIDs are not random. PIDs (and thread IDs; they're in the same "number space") are managed through a multilevel table structure in exactly the same way as are handles to kernel objects - although PIDs are not handles. Like handle values, PIDs are aggressively re-used, but for all practical purposes they are unpredictable.

Q2: No.

Q3: No, because the PID to be assigned to a new process is unknown. Try it. Run an instance of notepad, then close it. Sometime later, run another instance of notepad. Almost certainly your two notepads will have different PIDs.