Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s, a zombie process or defunct process is a process
Process (computing)
In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...
On many computer operating systems, a computer process terminates its execution by making an exit system call. More generally, an exit in a multithreading environment means that a thread of execution has stopped running. The operating system reclaims resources that were used by the process...
but still has an entry in the process table. This entry is still needed to allow the process that started the (now zombie) process to read its exit status
Exit status
The exit status or return code of a process in computer programming is a small number passed from a child process to a parent process when it has finished executing a specific procedure or delegated task...
. The term zombie process derives from the common definition of zombie
Zombie
Zombie is a term used to denote an animated corpse brought back to life by mystical means such as witchcraft. The term is often figuratively applied to describe a hypnotized person bereft of consciousness and self-awareness, yet ambulant and able to respond to surrounding stimuli...
Undead is a collective name for fictional, mythological, or legendary beings that are deceased and yet behave as if alive. Undead may be incorporeal, such as ghosts, or corporeal, such as vampires and zombies...
person. In the term's metaphor, the child process has "died
Death
Death is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....
" but has not yet been "reaped". Also, unlike normal processes, the kill command has no effect on a zombie process.
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 waitsystem call
System call
In computing, a system call is how a program requests a service from an operating system's kernel. This may include hardware related services , creating and executing new processes, and communicating with integral kernel services...
, at which stage the zombie is removed. The wait call may be executed in sequential code, but it is commonly executed in a handler for the SIGCHLD
SIGCHLD
On POSIX-compliant platforms, SIGCHLD is the signal sent to a process when a child process terminates. The symbolic constant for SIGCHLD is defined in the header file signal.h. Symbolic signal names are used because signal numbers can vary across platforms....
A signal is a limited form of inter-process communication used in Unix, Unix-like, and other POSIX-compliant operating systems. Essentially it is an asynchronous notification sent to a process in order to notify it of an event that occurred. When a signal is sent to a process, the operating system...
, which the parent receives whenever a child has died.
After the zombie is removed, its process ID and entry in the process table can then be reused. 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. On modern UNIX-like systems (that comply with SUSv3 specification in this respect), the following special case applies: if the parent explicitly ignores SIGCHLD by setting its handler to SIG_IGN (rather than simply ignoring the signal by default) or has the SA_NOCLDWAIT flag set, all child exit status information will be discarded and no zombie processes will be left.
An orphan process is a computer process whose parent process has finished or terminated, though itself remains running.In a Unix-like operating system any orphaned process will be immediately adopted by the special init system process. This operation is called re-parenting and occurs automatically...
. 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
Init
init is a program for Unix-based computer operating systems that spawns all other processes. It runs as a daemon and typically has PID 1. The boot loader starts the kernel and the kernel starts init...
(process ID 1), which waits on its children.
Zombies can be identified in the output from the Unix ps
Ps (Unix)
In most Unix-like operating systems, the ps program displays the currently-running processes. A related Unix utility named top provides a real-time view of the running processes....
In computing, a command is a directive to a computer program acting as an interpreter of some kind, in order to perform a specific task. Most commonly a command is a directive to some kind of command line interface, such as a shell....
by the presence of a “Z” in the “STAT” column. Zombies that exist for more than a short period of time typically indicate a bug in the parent program, or just an uncommon decision to reap children (see example). If the parent program is no longer running, zombie processes typically indicate a bug in the operating system. As with other leaks
Memory leak
A memory leak, in computer science , occurs when a computer program consumes memory but is unable to release it back to the operating system. In object-oriented programming, a memory leak happens when an object is stored in memory but cannot be accessed by the running code...
, the presence of a few zombies is not worrisome in itself, but may indicate a problem that would grow serious under heavier loads. 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
Signal (computing)
A signal is a limited form of inter-process communication used in Unix, Unix-like, and other POSIX-compliant operating systems. Essentially it is an asynchronous notification sent to a process in order to notify it of an event that occurred. When a signal is sent to a process, the operating system...
can be sent to the parent manually, using the kill command. 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 periodically executes the wait system call to reap any zombies with init as parent.
Examples
Synchronously waiting for specific child processes in a (specific) order may leave zombies present longer than the above-mentioned “short period of time”. It is not necessarily a program bug.
include
include
include
int main(void)
{
pid_t pids[10];
int i;
for (i = 9; i >= 0; --i) {
pids[i] = fork;
if (pids[i]
0) {
sleep(i+1);
_exit(0);
}
}
for (i = 9; i >= 0; --i)
waitpid(pids[i], NULL, 0);
