waitsystem call, at which stage the zombie is removed. The
waitcall may be executed in sequential code, but it is commonly executed in a handler for the SIGCHLD signal, 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. 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.
A zombie process is not the same as an orphan process. 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.
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. 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. 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.