CLONE_VM Example

This post we will write a code, which will give us an observation what changes when CLONE_VM is added and when it is removed.

Code:

	#define _GNU_SOURCE
	#include <stdio.h>
	#include <sched.h>
	#include <stdlib.h>
	#include <string.h>
	#include <unistd.h>
	#include <sys/wait.h>
	#define STACK_SIZE 65536
	int global_value = 0;
	char *heap;
	static int child_func(void *arg)
	{
	char *buf = (char *)arg;
	printf("Child sees buf = %s\n", buf);
	printf("Child sees global value = %d\n", global_value);
	printf("Child see heap = %s\n", heap);
	strcpy(buf, "hello from child");
	global_value = 10;
	strcpy(heap, "bye");
	return 0;
	}
	int main(int argc, char *argv[])
	{
	//Allocate stack for child task
	char *stack = malloc(STACK_SIZE);
	unsigned long flags = 0;
	char buf[256];
	int status;
	if (!stack) {
	perror("Failed to allocate memory\n");
	exit(1);
	}
	heap = malloc(1024);
	if (!heap) {
	perror("Failed to allocate memory\n");
	exit(2);
	}
	if (argc == 2 && !strcmp(argv[1], "vm"))
	flags |= CLONE_VM;
	strcpy(buf, "Hello from Parent");
	strcpy(heap, "Hey");
	global_value = 5;
	if (clone(child_func, stack + STACK_SIZE, flags | SIGCHLD, buf) == -1) {
	perror("clone");
	exit(1);
	}
	if (wait(&status) == -1) {
	perror("Wait");
	exit(1);
	}
	printf("Child exited with status:%d\t", status);
	printf("buf:%s\t global_value=%d\n",
	buf, global_value);
	printf("Parent heap:%s\n", heap);
	return 0;
	}

view raw clone_vm.c hosted with ❤ by GitHub

Output:

Notes:

1. Modifications to Local variable 'buf' by child reflected when CLONE_VM is added
2. Modifications to Global variable 'global_value" by child reflected when CLONE_VM is added
3. Modifications to Heap variable 'heap' by child reflected when CLONE_VM is added.