zombie process การใช้

• Because the child is still consuming system resources but not executing it is known as a zombie process.
• The term " zombie process " derives from the common definition of zombie  an undead person.
• If the parent program is no longer running, zombie processes typically indicate a bug in the operating system.
• Zombie processes should not be confused with orphan processes : an orphan process is a process that is still executing, but whose parent has died.
• In the term's metaphor, the child process has " died " but has not yet been " kill command has no effect on a zombie process.
• Zombie objects are somewhat analogous to zombie processes, in that they have undergone a termination state change and are close to deallocation, but the details are significantly different.
• Another common example is a parent process failing to call wait on a child process, which leaves the completed child process as a zombie process, leaking a process table entry.
• These do not remain as zombie processes; instead, ( like all orphaned processes ) they are adopted by init ( process ID 1 ), which wait s on its children.
• Such a terminated process whose data has not been collected is called a " zombie process ", or simply a " zombie ", in the UNIX parlance.
• Orphan processes are an opposite situation to zombie processes, referring to the case in which a parent process terminates before its child processes, which are said to become " orphaned ".
• Zombie processes might pose problems on systems with limited resources or that have limited-size process tables, as the creation of new, active processes might be prevented by the lack of resources still used by long lasting zombies.
• Since there is no memory allocated to zombie processes  the only system memory usage is for the process table entry itself  the primary concern with many zombies is not running out of memory, but rather running out of process table entries, concretely process ID numbers.
• In some situations this may be desirable  the parent process wishes to continue holding this resource  for example if the parent creates another child process it ensures that it will not be allocated the same PID . 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.