Process Identifier - Unix-like

Unix-like

In Unix-like operating systems, new processes are created by the fork system call. The PID is returned to the parent enabling it to refer to the child in further function calls. The parent may, for example, wait for the child to terminate with the waitpid function, or terminate the process with kill.

There are two tasks with specially distinguished process IDs: swapper or sched has process ID 0 and is responsible for paging, and is actually part of the kernel rather than a normal user-mode process. Process ID 1 is usually the init process primarily responsible for starting and shutting down the system. Originally, process ID 1 was not specifically reserved for init by any technical measures: it simply had this ID as a natural consequence of being the first process invoked by the kernel. More recent Unix systems typically have additional kernel components visible as 'processes', in which case PID 1 is actively reserved for the init process to maintain consistency with older systems.

Process IDs are usually allocated on a sequential basis, beginning at 0 and rising to a maximum value which varies from system to system. Once this limit is reached, allocation restarts at 300 and again increases. In Mac OS X and HP-UX, allocation restarts at 100. However, for this and subsequent passes any PIDs still assigned to processes are skipped. Some consider this to be a potential security vulnerability in that it allows information about the system to be extracted, or messages to be covertly passed between processes. As such, implementations that are particularly concerned about security may choose a different method of PID assignment. On some systems, like MPE/iX, the lowest available PID is used, sometimes in an effort to minimize the number of process information kernel pages in memory.

The current process ID is provided by a getpid system call, or as a variable $$ in shell. The process ID of a parent process is obtainable by a getppid system call.

Under Linux, the maximum process ID is given by the pseudo-file /proc/sys/kernel/pid_max.