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In Unix-like operating systems, new processes are created by the fork() system call. The PID is returned to the parent process, 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. [1] Linux also calls the threads of this process idle tasks. [2] In some APIs, PID 0 is also used as a special value that always refers to the calling thread, process, or process group. [3] [4] 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, in the first place, are usually allocated on a sequential basis, [5] 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 and HP-UX, allocation restarts at 100. [6] 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. [7] 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, [8] or as a variable $$ in shell. The process ID of a parent process is obtainable by a getppid() system call. [9]
On Linux, the maximum process ID is given by the pseudo-file /proc/sys/kernel/pid_max. [10]
Some processes, for example, the moc music player and the MySQL daemon, write their PID to a documented file location, to allow other processes to look it up.[ citation needed ]
On the Windows family of operating systems, one can get the current process's ID using the GetCurrentProcessId() function of the Windows API, [11] and ID of other processes using GetProcessId(). [12] 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 is given process ID 0. The System Process is given the process ID 8 on Windows 2000 and 4 on Windows XP and Windows Server 2003. [13] On the Windows NT family of operating systems, process and thread identifiers are all multiples of 4, but it is not part of the specification. [14]
In computing, a system call 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, creation and execution of new processes, and communication with integral kernel services such as process scheduling. System calls provide an essential interface between a process and the operating system.
In UNIX computing, the system load is a measure of the amount of computational work that a computer system performs. The load average represents the average system load over a period of time. It conventionally appears in the form of three numbers which represent the system load during the last one-, five-, and fifteen-minute periods.
A kernel panic is a safety measure taken by an operating system's kernel upon detecting an internal fatal error in which either it is unable to safely recover or continuing to run the system would have a higher risk of major data loss. The term is largely specific to Unix and Unix-like systems. The equivalent on Microsoft Windows operating systems is a stop error, often called a "blue screen of death".
A background process is a computer process that runs behind the scenes and without user intervention. Typical tasks for these processes include logging, system monitoring, scheduling, and user notification.
In computing, scheduling is the action of assigning resources to perform tasks. The resources may be processors, network links or expansion cards. The tasks may be threads, processes or data flows.
In computing, particularly in the context of the Unix operating system and its workalikes, fork is an operation whereby a process creates a copy of itself. It is an interface which is required for compliance with the POSIX and Single UNIX Specification standards. It is usually implemented as a C standard library wrapper to the fork, clone, or other system calls of the kernel. Fork is the primary method of process creation on Unix-like operating systems.
RTLinux is a hard realtime real-time operating system (RTOS) microkernel that runs the entire Linux operating system as a fully preemptive process. The hard real-time property makes it possible to control robots, data acquisition systems, manufacturing plants, and other time-sensitive instruments and machines from RTLinux applications. The design was patented. Despite the similar name, it is not related to the Real-Time Linux project of the Linux Foundation.
A child process in computing is a process created by another process. This technique pertains to multitasking operating systems, and is sometimes called a subprocess or traditionally a subtask.
In computing, a parent process is a process that has created one or more child processes.
In Unix and Unix-like computer operating systems, a file descriptor is a process-unique identifier (handle) for a file or other input/output resource, such as a pipe or network socket.
top is a task manager or system monitor program, found in many Unix-like operating systems, that displays information about CPU and memory utilization.
In most Unix and Unix-like operating systems, the ps program displays the currently-running processes. The related Unix utility top provides a real-time view of the running processes.
Signals are standardized messages sent to a running program to trigger specific behavior, such as quitting or error handling. They are a limited form of inter-process communication (IPC), typically used in Unix, Unix-like, and other POSIX-compliant operating systems.
In computing, kill is a command that is used in several popular operating systems to send signals to running processes.
The proc filesystem (procfs) is a special filesystem in Unix-like operating systems that presents information about processes and other system information in a hierarchical file-like structure, providing a more convenient and standardized method for dynamically accessing process data held in the kernel than traditional tracing methods or direct access to kernel memory. Typically, it is mapped to a mount point named /proc at boot time. The proc file system acts as an interface to internal data structures about running processes in the kernel. In Linux, it can also be used to obtain information about the kernel and to change certain kernel parameters at runtime (sysctl).
In computer programming, DLL injection is a technique used for running code within the address space of another process by forcing it to load a dynamic-link library. DLL injection is often used by external programs to influence the behavior of another program in a way its authors did not anticipate or intend. For example, the injected code could hook system function calls, or read the contents of password textboxes, which cannot be done the usual way. A program used to inject arbitrary code into arbitrary processes is called a DLL injector.
The Linux booting process involves multiple stages and is in many ways similar to the BSD and other Unix-style boot processes, from which it derives. Although the Linux booting process depends very much on the computer architecture, those architectures share similar stages and software components, including system startup, bootloader execution, loading and startup of a Linux kernel image, and execution of various startup scripts and daemons. Those are grouped into 4 steps: system startup, bootloader stage, kernel stage, and init process.
A process is a program in execution, and an integral part of any modern-day operating system (OS). The OS must allocate resources to processes, enable processes to share and exchange information, protect the resources of each process from other processes and enable synchronization among processes. To meet these requirements, The OS must maintain a data structure for each process, which describes the state and resource ownership of that process, and which enables the operating system to exert control over each process.
cgroups is a Linux kernel feature that limits, accounts for, and isolates the resource usage of a collection of processes.
Namespaces are a feature of the Linux kernel that partition kernel resources such that one set of processes sees one set of resources, while another set of processes sees a different set of resources. The feature works by having the same namespace for a set of resources and processes, but those namespaces refer to distinct resources. Resources may exist in multiple namespaces. Examples of such resources are process IDs, host-names, user IDs, file names, some names associated with network access, and Inter-process communication.
pid 0 == swapper == idle task