Create zombie process

In operating systems, a zombie process refers to a process that has completed execution but still occupies a position in the process table. It occurs when the child process has terminated, but its parent process has not yet called wait or waitpid to check its status. Consequently, although the child process has ended, its process descriptor and associated resources persist, resulting in resource wastage.

How to Create Zombie Processes?

An example of creating a zombie process can be demonstrated using C language in Unix or Linux systems. Here is a simple example:

c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main() {
    pid_t pid = fork();

    if (pid == -1) {
        // In case of fork failure
        perror("fork failed");
        exit(EXIT_FAILURE);
    } else if (pid > 0) {
        // Parent process
        printf("I am the parent process. My child's PID is %d\n", pid);
        sleep(20); // Waiting for 20 seconds; during this period, the child process has terminated, but the parent process has not called wait/waitpid
        printf("Parent process ends.\n");
    } else {
        // Child process
        printf("I am the child process and I am about to exit.\n");
        exit(0); // Child process exits normally
    }

    return 0;
}

In this program:

1. Use fork() to create a child process. If fork() succeeds, the parent process receives a non-zero PID of the child, while the child process receives 0.
2. The child process terminates its execution by calling exit(0).
3. The parent process continues running after the child process exits. Since the parent process does not call wait() or waitpid() to check the child's status, the child process becomes a zombie process until the parent process terminates or collects the status.

Why Avoid Zombie Processes?

Although zombie processes consume no system resources beyond the process identifier, a large number of them can exhaust the system's process ID resources. The number of concurrently existing processes in the system is limited; if many zombie processes occupy process IDs, new processes may fail to be created, impacting system performance.

How to Handle Zombie Processes?

The most common approach is for the parent process to call wait() or waitpid() functions, which reclaim the child process's status information and release resources. Another method is to set the child process's parent to the init process (the process with PID 1), so any zombie processes managed by the init process are automatically cleared.

By implementing these measures, zombie processes can be effectively managed and prevented from arising, maintaining system health and performance.