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In computing, a process is an instance of a computer program that is being sequentially executed. A computer program itself is just a passive collection of instructions, while a process is the actual execution of those instructions. Several processes may be associated with the same program; for example, opening up several windows of the same program typically means more than one process is being executed. In the computing world, processes are formally defined by the operating system(s)(OS) running them and so may differ in detail from one OS to another.
A single computer processor executes only one instruction at a time, one after the other. To allow users to run several programs at once (e.g., so that processor time is not wasted waiting for input from a resource), single-processor computer systems can perform time-sharing. Time-sharing allows processes to switch between being executed and waiting (to continue) to be executed. In most cases this is done very rapidly, providing the illusion that several processes are executing 'at once'. (This is known as concurrency or multiprogramming.) Using more than one physical processor on a computer, permits true simultaneous execution of more than one stream of instructions from different processes, but time-sharing is still typically used to allow more than one process to run at a time. (Concurrency is the term generally used to refer to several independent processes sharing a single processor; simultaneity is used to refer to several processes, each with their own processor.) Different processes may share the same set of instructions in memory (to save storage), but this is not known to any one process. Each execution of the same set of instructions is known as an instance— a completely separate instantiation of the program.
For security reasons most modern operating systems prevent direct communication between 'independent' processes, providing strictly mediated and controlled interprocess communication functionality. However, a process may split itself into multiple 'daughter' subprocesses or threads that execute in parallel, running different instructions on much of the same resources and data (or, as noted, the same instructions on logically different resources and data). This is useful when, for example, it is desirable to have it appear that various 'events' within the same process are occurring more or less at the same time. This allows a spell check to be performed in a word processor while the user is typing and/or text is being added from a (remote?) file.
Representation
In general, a computer system process consists of (or is said to 'own') the following resources:
* An image of the executable machine code associated with a program.
* Memory (typically some region of virtual memory); which includes the executable code, process-specific data (input and output), a call stack (to keep track of active subroutines and/or other events), and a heap to hold intermediate computation data generated during run time.
* Operating system descriptors of resources that are allocated to the process, such as file descriptors (Unix terminology) or handles (Windows), and data sources and sinks.
* Security attributes, such as the process owner and the process' set of permissions (allowable operations).
* Processor state (context), such as the content of registers, physical memory addressing, etc. The state is typically stored in computer registers when the process is executing, and in memory otherwise.
The operating system holds most of this information about active processes in data structures called process control blocks (PCB).
Any subset of resources, but typically at least the processor state, may be associated with each of the process' threads in operating systems that support threads or 'daughter' processes.
The operating system keeps its processes separated and allocates the resources they need so that they are less likely to interfere with each other and cause system failures (e.g., deadlock or thrashing). The operating system may also provide mechanisms for inter-process communication to enable processes to interact in safe and predictable ways.
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Process management in multitasking operating systems
A multitasking* operating system may just switch between processes to give the appearance of many processes executing concurrently or simultaneously, though in fact only one process can be executing at any one time on a single-core CPU (unless using multi-threading or other similar technology).
It is usual to associate a single process with a main program, and 'daughter' ('child') processes with any spin-off, parallel processes, which behave like asynchronous subroutines. A process is said to own resources, of which an image of its program (in memory) is one such resource. (Note, however, that in multiprocessing systems, many processes may run off of, or share, the same reentrant program at the same location in memory— but each process is said to own its own image of the program.)
Processes are often called tasks in embedded operating systems. The sense of 'process' (or task) is 'something that takes up time', as opposed to 'memory', which is 'something that takes up space'. (Historically, the terms 'task' and 'process' were used interchangeably, but the term 'task' seems to be dropping from the computer lexicon.)
The above description applies to both processes managed by an operating system, and processes as defined by process calculi.
If a process requests something for which it must wait, it will be blocked. When the process is in the Blocked State, it is eligible for swapping to disk, but this is transparent in a virtual memory system, where blocks of memory values may be really on disk and not in main memory at any time. Note that even unused portions of active processes/tasks (executing programs) are eligible for swapping to disk. All parts of an executing program and its data do not have to be in physical memory for the associated process to be active.
*Tasks and processes refer essentially to the same entity. And, although they have somewhat different terminological histories, they have come to be used as synonyms. Today, the term process is generally preferred over task, except when referring to 'multitasking', since the alternative term, 'multiprocessing', is too easy to confuse with multiprocessor (which is a computer with two or more CPUs).
Process states
Processes go through various process states which determine how the process is handled by the operating system kernel. The specific implementations of these states vary in different operating systems, and the names of these states are not standardised, but the general high-level functionality is the same.
When a process is created, it needs to wait for the process scheduler (of the operating system) to set its status to "waiting" and load it into main memory from secondary storage device (such as a hard disk or a CD-ROM). Once the process has been assigned to a processor by a short-term scheduler, a context switch is performed (loading the process into the processor) and the process state is set to "running" - where the processor executes its instructions. If a process needs to wait for a resource (such as waiting for user input, or waiting for a file to become available), it is moved into the "blocked" state until it no longer needs to wait - then it is moved back into the "waiting" state. Once the process finishes execution, or is terminated by the operating system, it is moved to the "terminated" state where it waits to be removed from main memory.
Inter-process communication
Processes can communicate with each other via Inter-process communication (IPC). This is possible for both processes running on the same machine and on different machines. The subject is a difficult one to discuss concisely, because it differs considerably from one operating system (OS) to another. However, a useful way to approach it is to consider the general mechanisms used in one form or another by most OS and to recognize that any given OS will only employ some subset of that universe.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.
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