A zombie process, also known as a defunct process, is a type of process in a computer’s operating system that has completed its execution but still has an entry in the process table. Zombie processes occur when a child process finishes executing but its parent process hasn’t yet retrieved the exit status or other information about the child process. As a result, the child process becomes a zombie process, and its resources are kept allocated in the system’s process table.
Here’s how the lifecycle of a zombie process typically occurs:
- Process Creation: When a parent process creates a child process using a system call (e.g.,
fork()), a new process is created. The parent process continues its execution, while the child process starts its execution.
- Child Process Completion: The child process completes its execution and exits. At this point, it still retains some information in the process table, including its process ID (PID) and exit status.
- Zombie State: The child process enters the zombie state because its exit status and other information are still needed by the parent process.
- Parent Process Action: The parent process should retrieve the exit status of the child process using the
waitpid()system calls. This action releases the resources associated with the child process, allowing it to be completely removed from the process table.
If the parent process fails to retrieve the exit status of the child process—for example, if the parent process terminates unexpectedly or doesn’t properly handle the child’s termination—the zombie process remains in the system’s process table. While zombie processes themselves do not consume system resources beyond a process table entry, having too many zombie processes can eventually exhaust the available process table slots, leading to issues in the overall functioning of the system.
To prevent or handle zombie processes, developers and system administrators should ensure that parent processes appropriately wait for their child processes to complete and retrieve their exit status. This can involve using the
waitpid() system calls, or taking other actions depending on the programming language and operating system being used. Properly managing zombie processes is crucial for maintaining system stability and resource availability.