Zombie Process

Zombie processes, also known as "zombies," are a state of terminated processes in operating systems. They occur when a child process has finished its execution but has not yet been fully removed from the process table, and its exit status has not been collected by its parent process. Zombie processes consume a minimal amount of system resources but still occupy an entry in the process table until they are properly reaped. Here's an explanation of zombie processes, an example, and how they are resolved in an operating system:

Example of Zombie Process:

Consider a simple example in a Unix-like operating system:

1. A parent process (Process A) creates a child process (Process B) using the fork() system call.

2. Process B performs some computation and then exits.

3. However, before Process A has a chance to collect the exit status of Process B using the wait() or waitpid() system call, Process A either terminates prematurely or doesn't wait for its child to finish.

In this scenario, Process B becomes a zombie because it has terminated but its exit status hasn't been collected by its parent process, Process A.

Effect of Zombie Process

The major effect of a zombie process is the potential consumption of process table resources. Zombie processes occupy an entry in the process table until they are properly reaped by their parent process or adopted by the init process (or its equivalent on different operating systems). Here's a more specific explanation of this major effect:

Process Table Resource Consumption:

• Zombie processes remain in the process table until their exit status is collected by their parent process or the init process. This means that for each zombie process, an entry in the process table is occupied.

• The process table has a finite capacity, which can vary depending on the operating system and its configuration. When too many zombie processes accumulate and occupy process table entries, it can potentially prevent the creation of new processes.

• If the process table becomes full due to a large number of zombie processes, the operating system may be unable to create new processes, leading to resource allocation issues and potential service disruptions.

Resolution of Zombie Processes:

Zombie processes can be resolved by following these steps:

1. Parent Process Reaps Child's Exit Status:

   • The responsibility for reaping the exit status of child processes lies with their parent process. The parent should use system calls like wait() or waitpid() to retrieve the exit status.

2. Init Process Reaps Orphans:

   • If the parent process fails to reap the exit status of a child process and exits before doing so, the child process becomes an orphan.

   • Orphan processes are adopted by the init process (or its equivalent on different operating systems) automatically.

   • The init process periodically checks the status of orphaned child processes and reaps them when they terminate.

3. Resource Cleanup:

   • When the exit status of a zombie process is collected (reaped), its entry in the process table is removed, and system resources associated with the process are released.

   • This includes releasing memory, file descriptors, and other resources held by the zombie process.

4. No Active Execution:

   • Zombie processes do not consume CPU time or actively execute code. They only occupy a process table entry until reaped.

It's important to note that while zombie processes themselves don't consume significant resources, having too many of them can eventually fill up the process table, potentially preventing new processes from being created. Therefore, it's crucial for the parent processes to properly reap the exit status of their child processes to prevent the accumulation of zombies.

In summary, zombie processes are terminated child processes whose exit status has not yet been collected by their parent processes. The parent processes are responsible for using system calls to reap the exit status, or if they fail to do so, orphaned processes are adopted and reaped by the init process. Resolving zombie processes is essential to prevent resource leakage and ensure efficient process management in an operating system.