#include <stdio.h>
#include <pthread.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/mman.h>

#define MAX_THREADS	256

static char *mapping;

static void *masher(void *v)
{
	int id = (int)v;
	unsigned offset = id * getpagesize();

	printf("started thread %d\n", id);

	for(;;) {
		mprotect(mapping+offset, getpagesize(), PROT_READ);
		mprotect(mapping+offset, getpagesize(), PROT_READ | PROT_WRITE);
	}

	return NULL;
}

int main(int argc, char **argv)
{
	int i;
	int nthreads = 4;
	pthread_t threads[MAX_THREADS];

	if (argc == 2) {
		int t = atoi(argv[1]);
		if (t != 0)
			nthreads = t;
	}
	if (nthreads > MAX_THREADS)
		nthreads = MAX_THREADS;

	printf("creating %d threads...\n", nthreads);

	mapping = mmap(0, getpagesize() * nthreads, PROT_NONE,
		       MAP_POPULATE|MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
	if (mapping == (char *)-1) {
		perror("mmap failed");
		return 1;
	}

	for(i = 0; i < nthreads; i++) {
		int ret;
		ret = pthread_create(&threads[i], NULL, masher, (void *)i);
		if (ret) {
			printf("pthread create %d failed: %s\n", i, strerror(ret));
			return 1;
		}
	}

	for(i = 0; i < nthreads; i++) {
		void *ret;
		pthread_join(threads[i], &ret);
	}

	return 0;
}