On Unix operating systems, a zombie process or defunct process is a process that has completed execution but still has an entry in the process table, allowing the process that started it to read its exit status. In the term's colorful metaphor, the child process has died but has not yet been reaped. Wikibooks has more about this subject: Guide to UNIX Unix or UNIX is a computer operating system originally developed in the 1960s and 1970s by a group of AT&T Bell Labs employees including Ken Thompson, Dennis Ritchie, and Douglas McIlroy. ... In computing, a process is a running instance of a program, including all variables and other state. ... The exit status of a process in computer programming is a small number passed from a child process to a parent process when it is done executing a specific task delegated. ... Death is the cessation of physical life in a living organism or the state of the organism after that event. ... Death, personified is an anthropomorphic figure or a fictional character who has existed in mythology and popular culture since the earliest days of storytelling. ...

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 is sent a SIGCHLD signal indicating that a child has died; the handler for this signal will typically execute the wait system call, which reads the exit status and removes the zombie. The zombie's process ID and entry in the process table can then be reused. In UNIX, a process can have children. ... A signal is an asynchronous event transmitted between one process and another. ... An event handler is a part of a computer program created to tell the program how to act in response to a specific event (e. ... In computing, a system call, or software interrupt is the mechanism used by an application program to request service from the operating system. ... 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 or Windows NT) to uniquely identify a process. ...

A zombie process is not the same as an orphan process. Orphan processes don't become zombie processes; instead, they are adopted by init (process ID 1), which waits on its children. A orphan process is a computer process whose Parent process has finished or terminated. ...

The term zombie process derives from the common definition of zombie—an undead person. A zombie, at twilight, in a sugarcane field in Haiti A zombie is traditionally an undead person in the Caribbean spiritual belief system of voodoo. ... Undead is a collective name for all types of supernatural entities who were once alive in the normal sense, died, and then continued to exist in the world of the living, in forms spiritual (as ghosts) or corporeal (as animated corpses). ...

Zombies can be identified in the output from the Unix ps command 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. 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. Memory leak is often thought of as a failure to release unused memory by a computer program. ...