What is Zombie process vs Orphan process in C

Zombie Processes

A zombie process refers to a process that has completed execution but still retains an entry in the process table. Such a process has finished its work and exited normally, but its parent process has not called wait() or waitpid() to retrieve the child process's termination status, so it still occupies a slot in the process table. Processes in this state are termed "zombie" processes.

Example

For example, in a Unix system, when a child process completes its task, it sends a SIGCHLD signal to the parent process. If the parent process does not handle this signal correctly (typically by calling wait() to read the child process's exit status), the child process's process descriptor and related resources are not fully released, resulting in a zombie process. If numerous zombie processes exist in the system, they may exhaust system resources and degrade performance.

Orphan Processes

An orphan process is one where the parent process has ended or exited abnormally, while the child process continues running. These orphan processes are adopted by the init process (the process with PID 1) and become its child processes. The init process periodically calls wait() to clean up terminated child processes, ensuring no zombie processes remain.

Example

Suppose a parent process creates a child process, and then the parent process terminates for some reason (e.g., due to an exception or abnormal exit). At this point, the child process continues running but has no parent process, so it becomes an orphan process. Due to Unix system design, the init process automatically becomes the new parent of this orphan process and handles its exit status.

Summary

Overall, zombie processes and orphan processes represent two distinct process states closely tied to their lifecycle and system resource management. System administrators and programmers must manage these processes properly to avoid wasting or exhausting system resources.

What is a Zombie Process?

A zombie process refers to a program that has completed execution in Unix-like systems, but its parent process has not yet reaped it (i.e., called functions such as wait() to retrieve the termination status of the child process). Consequently, it still occupies a slot in the process table, displayed as 'Z (zombie)' state. Zombie processes do not consume system resources such as CPU and memory, but they occupy a process ID. If numerous zombie processes exist in the system, it may lead to exhaustion of process IDs, preventing the creation of new processes.

Example: Assume a parent process creates a child process to execute a computational task. After the child process completes its task and exits, if the parent process does not call wait() or waitpid() to check the child process's status, the child process's process descriptor and status information remain retained in the system, forming a zombie process.

What is an Orphan Process?

An orphan process occurs when the parent process terminates before the child process. At this point, the child process's parent becomes the init process (the process with PID 1 in Unix-like systems). The init process automatically adopts these orphan processes and is responsible for cleaning up their termination status to prevent them from becoming zombie processes.

Example: Suppose a server process creates a child process while handling client requests. If the server process abnormally terminates for some reason and the child process does not terminate simultaneously, it becomes an orphan process. Due to the init process in Unix systems periodically calling wait() to clean up terminated child processes, this orphan process is eventually terminated normally and does not become a zombie process.

Summary

Zombie processes and orphan processes are common phenomena in process management, primarily resulting from inadequate handling of parent-child relationships and process termination states. System administrators and developers should ensure that wait() or similar functions are properly called in the parent process, and abnormal termination scenarios are correctly handled, to prevent numerous zombie or orphan processes from consuming system resources.