cow.c

#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <sys/mman.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <err.h>

#define BUFFER_SIZE	(100 * 1024 * 1024)
#define PAGE_SIZE	4096
#define COMMAND_SIZE	4096

static char command[COMMAND_SIZE];

static void child_fn(char *p) {
	printf("*** child ps info before memory access ***:\n");
	fflush(stdout);
	snprintf(command, COMMAND_SIZE,
		 "ps -o pid,comm,vsz,rss,min_flt,maj_flt | grep '^ *%d'",
		 getpid());
	system(command);
	printf("*** free memory info before memory access ***:\n");
	fflush(stdout);
	system("free");

	int i;
	for (i = 0; i < BUFFER_SIZE; i += PAGE_SIZE)
		p[i] = 0;

	printf("*** child ps info after memory access ***:\n");
	fflush(stdout);
	system(command);

	printf("*** free memory info after memory access ***:\n");
	fflush(stdout);
	system("free");

	exit(EXIT_SUCCESS);
}

static void parent_fn(void) {
	wait(NULL);

	exit(EXIT_SUCCESS);
}

int main(void)
{
	char *p;
	p = malloc(BUFFER_SIZE);
	if (p == NULL)
		err(EXIT_FAILURE, "malloc() failed");

	int i;
	for (i = 0; i < BUFFER_SIZE; i += PAGE_SIZE)
		p[i] = 0;

	printf("*** free memory info before fork ***:\n");
	fflush(stdout);
	system("free");

	pid_t ret;
	ret = fork();
	if (ret == -1)
		err(EXIT_FAILURE, "fork() failed");

	if (ret == 0)
		child_fn(p);
	else
		parent_fn();

	err(EXIT_FAILURE, "shouldn't reach here");
}