On Unix and Unix-like computer operating systems, a zombie process or defunct process is a process that has completed execution but still has an entry in the process table, this entry being still needed to allow the process that started the zombie process to read its exit status. Unix (officially trademarked as UNIX, sometimes also written as Unix with Small caps) is a computer A Unix-like (sometimes shortened to *nix) Operating system is one that behaves in a manner similar to a Unix system while not necessarily conforming An operating system (commonly abbreviated OS and O/S) is the software component of a Computer system that is responsible for the management and coordination In computing a process is an instance of a Computer program that is being sequentially executed by a computer system that has the ability to run several computer A Computer process terminates its execution by making an exit System call. In computing a process is an instance of a Computer program that is being sequentially executed by a computer system that has the ability to run several computer The exit status of a process in Computer programming is an small number passed from a Child process to a Parent process when it is done executing The term zombie process derives from the common definition of zombie—an undead person. zombie is a reanimated human corpse Stories of zombies originated in the Afro-Caribbean spiritual belief system of Vodou, which told of the people being controlled Undead is a collective name for fictional beings that are deceased yet behave as if alive In the term's colorful metaphor, the child process has died but has not yet been reaped. Death is the termination of the biological functions that define living Organisms It refers both to a specific In English Death is often given the name the " Grim Reaper " and shown as a skeletal figure carrying a large Scythe, and wearing a midnight black gown robe
When a process ends, all of the memory and resources associated with it are deallocated so they can be used by other processes. However, the process's entry in the process table remains. The parent can read the child's exit status by executing the wait system call, at which stage the zombie is removed. In modern Computer Operating systems a process (or task) may wait on another process to complete its execution In Computing, a system call is the mechanism used by an application program to request service from the Kernel. The wait call may be executed in sequential code, but it is commonly executed in a handler for the SIGCHLD signal, which the parent is sent whenever a child has died. In computer programming an event handler is an asynchronous callback subroutine that handles inputs received in a program On POSIX -compliant platforms SIGCHLD is the signal sent by Computer programs when a child process terminates. A signal is a limited form of Inter-process communication used in Unix, Unix-like, and other POSIX -compliant operating systems
After the zombie is removed, its process ID and entry in the process table can then be reused. In Computing, the process identifier (normally referred to as the process ID or just PID) is a number used by some Operating system kernels However, if a parent fails to call wait, the zombie will be left in the process table. In some situations this may be desirable, for example if the parent creates another child process it ensures that it will not be allocated the same process ID. As a special case, under Linux, if the parent explicitly ignores the SIGCHLD (sets the handler to SIG_IGN, rather than simply ignoring the signal by default), all child exit status information will be discarded and no zombie processes will be left. Linux (commonly pronounced ˈlɪnəks On POSIX -compliant platforms SIGCHLD is the signal sent by Computer programs when a child process terminates.
A zombie process is not the same as an orphan process. An orphan process is a computer process whose Parent process has finished or terminated. An orphan process is a process that is still executing, but whose parent has died. They do not become zombie processes; instead, they are adopted by init (process ID 1), which waits on its children. Adoption is the act of legally placing a child with a Parent or parents other than those to whom they were born init (short for " Initialization " is the program on Unix and Unix-like systems that spawns all other processes
Zombies can be identified in the output from the Unix ps command by the presence of a "Z" in the STAT column. In most Unix-like operating systems the ps program displays the currently-running processes. Zombies that exist for more than a short period of time typically indicate a bug in the parent program. As with other leaks, the presence of a few zombies isn't worrisome in itself, but may indicate a problem that would grow serious under heavier loads. In Computer science, a memory leak is a particular type of unintentional memory consumption by a Computer program where the program fails to release memory Since there is no memory allocated to zombie processes except for the process table entry itself, the primary concern with many zombies is not running out of memory, but rather running out of process ID numbers.
To remove zombies from a system, the SIGCHLD signal can be sent to the parent manually, using the kill command. On POSIX -compliant platforms SIGCHLD is the signal sent by Computer programs when a child process terminates. A signal is a limited form of Inter-process communication used in Unix, Unix-like, and other POSIX -compliant operating systems In Unix and Unix-like Operating systems kill is a command used to send simple messages to processes running on the system If the parent process still refuses to reap the zombie, the next step would be to remove the parent process. When a process loses its parent, init becomes its new parent. init (short for " Initialization " is the program on Unix and Unix-like systems that spawns all other processes Init periodically executes the wait system call to reap any zombies with init as parent. init (short for " Initialization " is the program on Unix and Unix-like systems that spawns all other processes In modern Computer Operating systems a process (or task) may wait on another process to complete its execution In Computing, a system call is the mechanism used by an application program to request service from the Kernel. init (short for " Initialization " is the program on Unix and Unix-like systems that spawns all other processes
See also
External links
- Zombie process usenet post
- UNIX FAQ Question 3.13: How do I get rid of zombie processes that persevere?
- Zombies(5) (from UNIX System Concepts man pages)
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