CLONE_FILES example

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

Code:

	#define _GNU_SOURCE
	#include <stdio.h>
	#include <sys/types.h>
	#include <fcntl.h>
	#include <sched.h>
	#include <stdlib.h>
	#include <string.h>
	#include <unistd.h>
	#include <sys/wait.h>
	#define STACK_SIZE 65536
	int fd;
	static int child_func(void *arg)
	{
	close(fd);
	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;
	fd = open("file.txt", O_RDWR);
	if (fd == -1) {
	perror("Failed to open file\n");
	exit(1);
	}
	if (!stack) {
	perror("Failed to allocate memory\n");
	exit(1);
	}
	if (argc == 2 && !strcmp(argv[1], "files"))
	flags |= CLONE_FILES;
	if (clone(child_func, stack + STACK_SIZE, flags | SIGCHLD, NULL) == -1) {
	perror("clone");
	exit(1);
	}
	if (wait(&status) == -1) {
	perror("Wait");
	exit(1);
	}
	printf("Child exited with status:%d\n", status);
	status = read(fd, buf, 100);
	if (status < 0) {
	perror("Read Failed\n");
	exit(1);
	}
	printf("Parent read:%s\n", buf);
	close(fd);
	return 0;
	}

view raw clone_files.c hosted with ❤ by GitHub

Output:

Notes:

1. You can see from the code when we passed CLONE_FILES, when the child close file descriptor, it was closed in the parent process file descriptor too.

2. So, any operations which are performed on file descriptor (read, write, fcntl) by one process will affect the other process too