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Concept
Parent process
Summary
In computing, a parent process is a process that has created one or more child processes.
In Unix-like operating systems, every process except process 0 (the swapper) is created when another process executes the fork() system call. The process that invoked fork is the parent process and the newly created process is the child process. Every process (except process 0) has one parent process, but can have many child processes.
The operating system kernel identifies each process by its process identifier. Process 0 is a special process that is created when the system boots; after forking a child process (process 1), process 0 becomes the swapper process (sometimes also known as the "idle task"). Process 1, known as , is the ancestor of every other process in the system.
In the Linux kernel, in which there is a very slim difference between processes and POSIX threads, there are two kinds of parent processes, namely real parent and parent. Parent is the process that receives the SIGCHLD signal on child's termination, whereas real parent is the thread that actually created this child process in a multithreaded environment. For a normal process, both these two values are same, but for a POSIX thread which acts as a process, these two values may be different.
Zombie process
The operating system maintains a table that associates every process, by means of its process identifier (generally referred to as "pid") to the data necessary for its functioning. During a process's lifetime, such data might include memory segments designated to the process, the arguments it's been invoked with, environment variables, counters about resource usage, user-id, group-id and group set, and maybe other types of information.
When a process terminates its execution, either by calling exit (even if implicitly, by executing a return command from the main function) or by receiving a signal that causes it to terminate abruptly, the operating system releases most of the resources and information related to that process, but still keeps the data about resource utilization and the termination status code, because a parent process might be interested in knowing if that child executed successfully (by using standard functions to decode the termination status code) and the amount of system resources it consumed during its execution.
Official source
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