In Linux, the PID (Process IDentification) is a unique number assigned to each running process in the system. PIDs are generated and managed by the kernel. The way Linux generates PIDs is as follows:
- When the system starts up, the first process that is spawned is the init process, which is always assigned PID 1.
- When a new process is created, the kernel assigns it a unique PID. The assignment is done sequentially, starting from the next available number.
- The kernel maintains a variable called "next_pid" which keeps track of the next available PID. Each time a new process is created, the "next_pid" is incremented, and the incremented value is assigned as the PID for the process.
- The kernel ensures that no two processes have the same PID at the same time, and PIDs are not recycled or reused while a process is still running.
- The maximum value of a PID is defined by a system parameter called "PID_MAX". The default value is usually 32768, but it can be changed by modifying kernel parameters.
It's important to note that when a process exits, its PID becomes available for reuse by a new process. This allows the Linux kernel to manage and track processes efficiently.