Zombie Processes

Orphan process: The father dies first, and the son is adopted by process 1.

Zombie process: The son dies first, and the father doesn't care.

Killing Zombie Processes

1. Use top to check if there are zombie processes
2. Query where the zombie processes are through commands

ps -A -ostat,ppid,pid,cmd | grep -e '^[Zz]'
# Command explanation:

# -A parameter lists all processes

# -o custom output fields, we set the display fields to stat (status), ppid (parent process id), pid (process id), cmd (command) these four parameters

# Because processes with status z or Z are zombie processes, we use grep to capture processes with stat status zZ

Examples

Orphan Process

By controlling the sleep time, the father dies first, and the son is adopted by the orphanage (process 1). You can check through ps that the parent process of a.out becomes 1.

#include <stdio.h>  
#include <stdlib.h>  
#include <unistd.h>  
#include <sys/wait.h>  
  
int main() {  
    pid_t pid = fork(); // Create child process  
  
    if (pid < 0) {  
        // fork failed  
        perror("fork failed");  
        exit(EXIT_FAILURE);  
    } else if (pid == 0) {  
        // Child process  
        printf("I am the child process with PID %d\n", getpid());  
        sleep(2000); // Let the child process run for a while, at this time the father is already dead.
        printf("Child process exiting...\n");  
        exit(EXIT_SUCCESS); // Child process exits  
    } else {  
        // Parent process exits directly
        printf("I am the parent process with PID %d and my child has PID %d\n", getpid(), pid);  
    }  
    return 0;  
}

Zombie Process

By controlling the sleep time, the child process dies first. However, the parent process does not reclaim the child process.

#include <stdio.h>  
#include <stdlib.h>  
#include <unistd.h>  
#include <sys/wait.h>  
  
int main() {  
    pid_t pid = fork(); // Create child process  
  
    if (pid < 0) {  
        // fork failed  
        perror("fork failed");  
        exit(EXIT_FAILURE);  
    } else if (pid == 0) {  
        // Child process  
        printf("I am the child process with PID %d\n", getpid());  
        sleep(2); // Let the child process run for a while  
        printf("Child process exiting...\n");  
        exit(EXIT_SUCCESS); // Child process exits  
    } else {  
        // Parent process  
        printf("I am the parent process with PID %d and my child has PID %d\n", getpid(), pid);  
        // Note: The parent process does not call wait() or waitpid() here, so the child process will become a zombie process  
        sleep(10000);  
    }  
    return 0;  
}

Query zombie processes

$ ps -A -ostat,ppid,pid,cmd | grep -e '^[Zz]'
Z+    933981  933982 [a.out] <defunct>
$ ps -ef | grep a.out
root      933981  933844  0 11:11 pts/4    00:00:00 ./a.out
root      933982  933981  0 11:11 pts/4    00:00:00 [a.out] <defunct>
root      934096  933897  0 11:13 pts/5    00:00:00 grep --color=auto a.out

Reclaiming the Child

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/wait.h>

// SIGCHLD signal handler function
void sigchld_handler(int signum) {
    // Use waitpid non-blocking to reclaim all terminated child processes
    while (waitpid(-1, NULL, WNOHANG) > 0);
}

int main() {
    // Set up SIGCHLD signal handler
    struct sigaction sa;
    sa.sa_handler = sigchld_handler;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART; // Automatically restart interrupted system calls
    if (sigaction(SIGCHLD, &sa, NULL) == -1) {
        perror("sigaction");
        exit(EXIT_FAILURE);
    }

    // Create child process
    pid_t pid = fork();
    if (pid < 0) {
        perror("fork");
        exit(EXIT_FAILURE);
    }

    if (pid == 0) {
        // Child process
        printf("Child process: PID = %d\n", getpid());
        sleep(10); // Simulate child process executing some tasks
        printf("Child process ending: PID = %d\n", getpid());
        exit(EXIT_SUCCESS);
    } else {
        // Parent process
        printf("Parent process: PID = %d, Child process PID = %d\n", getpid(), pid);
        // Parent process continues to execute other tasks
        for (int i = 0; i < 5; ++i) {
            printf("Parent process is working...\n");
            sleep(10000);
        }
        // Cleanup work before parent process ends
        printf("Parent process ending: PID = %d\n", getpid());
    }
    return 0;
}