![]() ![]() 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. ![]() As such, implementations that are particularly concerned about security may choose a different method of PID assignment. 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. However, for this and subsequent passes any PIDs still assigned to processes are skipped. In Mac OS X and HP-UX, allocation restarts at 100. Once this limit is reached, allocation restarts at 300 and again increases. Under Unix, process IDs are usually allocated on a sequential basis, beginning at 0 and rising to a maximum value which varies from system to system. ![]() 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. 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. Process ID 1 is usually the init process primarily responsible for starting and shutting down the system. In Unix-like operating systems, 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. The parent may, for example, wait for the child to terminate with the waitpid() function, or terminate the process with kill(). The PID is returned to the parent enabling it to refer to the child in further function calls. In Unix-like operating systems, new processes are created by the fork() system call. This number may be used as a parameter in various function calls allowing processes to be manipulated, such as adjusting the process's priority or killing it altogether. In computing, the process identifier (normally referred to as the process ID or just PID) is a number used by most operating system kernels (such as that of UNIX, Mac OS X or Microsoft Windows) to (temporarily) uniquely identify a process. The first one you launch is 1, the second one is 2, and so forth. The job id is a sequential number assigned to the processes associated with the current shell you are running. ![]()
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