Process identifier
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In
computing Computing is any goal-oriented activity requiring, benefiting from, or creating computer, computing machinery. It includes the study and experimentation of algorithmic processes, and the development of both computer hardware, hardware and softw ...
, the process identifier (a.k.a. process ID or PID) is a number used by most
operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...
kernels—such as those of
Unix Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...
,
macOS macOS, previously OS X and originally Mac OS X, is a Unix, Unix-based operating system developed and marketed by Apple Inc., Apple since 2001. It is the current operating system for Apple's Mac (computer), Mac computers. With ...
and
Windows Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...
—to uniquely identify an active
process A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic. Things called a process include: Business and management * Business process, activities that produce a specific s ...
. 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.


Unix-like

In
Unix-like A Unix-like (sometimes referred to as UN*X, *nix or *NIX) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Uni ...
operating systems, new processes are created by the fork()
system call In computing, a system call (syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, accessing a hard disk drive ...
. The PID is returned to the
parent process In computing, a parent process is a process that has created one or more child processes. Unix-like systems In Unix-like operating systems, every process except (the swapper) is created when another process executes the fork() system call. T ...
, 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: PID 0 is used for ''swapper'' or ''sched'', which is part of the kernel and is a process that runs on a CPU core whenever that CPU core has nothing else to do. Linux also calls the threads of this process ''idle tasks''. In some APIs, PID 0 is also used as a special value that always refers to the calling thread, process, or process group. Process ID 1 is usually the
init In Unix-based computer operating systems, init (short for ''initialization'') is the first process started during booting of the operating system. Init is a daemon process that continues running until the system is shut down. It is the direc ...
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, in the first place, 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
macOS macOS, previously OS X and originally Mac OS X, is a Unix, Unix-based operating system developed and marketed by Apple Inc., Apple since 2001. It is the current operating system for Apple's Mac (computer), Mac computers. With ...
and
HP-UX HP-UX (from "Hewlett Packard Unix") is a proprietary software, proprietary implementation of the Unix operating system developed by Hewlett Packard Enterprise; current versions support HPE Integrity Servers, based on Intel's Itanium architect ...
, 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. On
Linux Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...
, the maximum process ID is given by the pseudo-file /proc/sys/kernel/pid_max.


Pidfile

Some processes, for example, the moc music player and the
MySQL MySQL () is an Open-source software, open-source relational database management system (RDBMS). Its name is a combination of "My", the name of co-founder Michael Widenius's daughter My, and "SQL", the acronym for Structured Query Language. A rel ...
daemon, write their PID to a documented file location, to allow other processes to look it up.


Microsoft Windows

On the
Windows Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...
family of operating systems, one can get the current process's ID using the GetCurrentProcessId() function of the
Windows API The Windows API, informally WinAPI, is the foundational application programming interface (API) that allows a computer program to access the features of the Microsoft Windows operating system in which the program is running. Programs can acces ...
, and ID of other processes using GetProcessId(). Internally, process ID is called a ''client ID'', and is allocated from the same namespace as thread IDs, so these two never overlap. The
System Idle Process In Windows NT operating systems, the System Idle Process contains one or more kernel threads which run when no other runnable thread can be scheduled on a CPU. In a multiprocessor system, there is one idle thread associated with each CPU core. ...
is given process ID 0. The System Process is given the process ID 8 on
Windows 2000 Windows 2000 is a major release of the Windows NT operating system developed by Microsoft, targeting the server and business markets. It is the direct successor to Windows NT 4.0, and was Software release life cycle#Release to manufacturing (RT ...
and 4 on
Windows XP Windows XP is a major release of Microsoft's Windows NT operating system. It was released to manufacturing on August 24, 2001, and later to retail on October 25, 2001. It is a direct successor to Windows 2000 for high-end and business users a ...
and
Windows Server 2003 Windows Server 2003, codenamed "Whistler Server", is the sixth major version of the Windows NT operating system produced by Microsoft and the first server version to be released under the Windows Server brand name. It is part of the Windows NT ...
. On the
Windows NT family Windows NT is a proprietary graphical operating system produced by Microsoft as part of its Windows product line, the first version of which, Windows NT 3.1, was released on July 27, 1993. Originally made for the workstation, office, and ser ...
of operating systems, process and thread identifiers are all multiples of 4, but it is not part of the specification.


See also

*
User identifier Unix-like operating systems identify a user by a value called a user identifier, often abbreviated to user ID or UID. The UID, along with the group identifier (GID) and other access control criteria, is used to determine which system resources a us ...
(UID) *
Group identifier In Unix-like systems, multiple users can be put into '' groups''. POSIX and conventional Unix file system permissions are organized into three classes, ''user'', ''group'', and ''others''. The use of groups allows additional abilities to be dele ...
(GID) * Thread identifier (TID) *
Handle (computing) In computer programming, a handle is an abstract reference to a resource that is used when application software references blocks of memory or objects that are managed by another system like a database or an operating system. A resource handle ...
* Program Segment Prefix (PSP)


References

{{DEFAULTSORT:Process Identifier Process (computing)