Which signal: SIGTERM or SIGKILL can generate zombie processes?

SIGTERM cannot directly spawn zombie processes. It sends a notification to the process to terminate properly, giving it a chance to clean up resources. If the process ignores SIGTERM, it will not terminate.

SIGKILL cannot spawn zombie processes. It immediately kills the process without giving it a chance to terminate properly or spawn child processes.

A zombie process occurs when a child process terminates, but its parent does not call the wait() or waitpid() function to collect its exit status. The parent process is responsible for collecting the exit status of its child processes. Regardless of how the child process terminates (by SIGTERM, SIGKILL, or other reasons), it becomes a zombie until the parent "collects" it or the parent itself terminates.

Therefore, zombies are spawned by the parent process that does not handle the termination of its child processes, regardless of which signal caused the child's termination.

Example in C demonstrating zombie creation:

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

int main() {
    pid_t pid = fork(); // Create a child process

    if (pid == 0) {
        // Child process code
        printf("Child process (PID: %d) is terminating.\n", getpid());
        exit(0); // Child process terminates
    } else if (pid > 0) {
        // Parent process code
        printf("Parent process (PID: %d) created child (PID: %d).\n", getpid(), pid);
        // Parent does not call wait() or waitpid(), creating a zombie when the child terminates

        printf("Parent process is sleeping. Child should become a zombie.\n");
        sleep(60); // Parent sleeps to allow child to terminate and become zombie
    } else {
        // Fork error
        perror("Fork error");
        return 1;
    }

    return 0;
}

In this example, the child process terminates, but the parent does not call wait(), leading to a zombie. The child's termination was not directly caused by SIGTERM or SIGKILL, but the principle is the same: the parent did not collect the status.

If the child was killed by SIGKILL, it would also become a zombie if the parent did not collect its status. SIGKILL simply forcibly terminates the process, but its exit status still remains in the process table for the parent until it is collected.