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 Windows NT) to uniquely identify a process.
In Unix-like operating systems, the PID of a newly created child process is returned by the fork() system call to the parent.
The PID can be passed to process control functions like waitpid() or kill() to perform actions on the given process, and if the operating system has procfs support the files in /proc/pid/ can be queried for information about the process.
In Unix-like operating systems, there are two tasks with specially distinguished process IDs: the idle task has process ID zero, and never exits. Another specially distinguished task on Unix-like operating systems is the init process, with process ID 1, which does nothing but wait around for its child processes to die.
This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.
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