In computing, the process identifier (normally referred to as the process ID or just PID) is a number used by some operating system kernels (such as that of UNIX, Mac OS X or Microsoft Windows) to uniquely identify a process. 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 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().
In Unix-like operating systems, there are two tasks with specially distinguished process IDs: swapper or sched has process ID zero 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.
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. Once this limit is reached, allocation restarts at zero and again increases. 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. [1]
Under Windows you can get the current process ID using GetCurrentProcessId function.[2]
See also
- pidof
- User identifier (Unix) (UID)
- Group identifier (Unix) (GID)
References
- ^ "comp.unix.aix Frequently Asked Questions". http://www.faqs.org/faqs/aix-faq/part2/section-20.html. Retrieved on 2008-07-21.
- ^ "GetCurrentProcessId Function". http://msdn.microsoft.com/en-us/library/ms683180(VS.85).aspx. Retrieved on 2009-05-20.
This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
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