#include <sys/mman.h>
#include <sys/wait.h>

#include <assert.h>
#include <err.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

static void
cow(void)
{
	pid_t child;

	child = fork();
	switch (child) {
	case -1:		/* error */
		err(1, "fork");
	case 0:			/* child */
		_Exit(0);
	default:		/* parent */
		if (waitpid(child, NULL, 0) == -1)
			err(1, "waitpid");
	}
}

int
main(void)
{
	const size_t PAGE_SIZE = sysconf(_SC_PAGESIZE);
	void *p, *q;

	p = mmap(/*hint*/NULL, /*size*/PAGE_SIZE,
	    PROT_MPROTECT(PROT_READ|PROT_WRITE),
	    MAP_ANON|MAP_PRIVATE, /*fd*/-1, /*offset*/0);
	if (p == MAP_FAILED)
		err(1, "mmap");

	q = mremap(/*oldp*/p, PAGE_SIZE, /*newp*/NULL, PAGE_SIZE,
	    MAP_REMAPDUP);
	if (q == MAP_FAILED)
		err(1, "mmap");

	if (mprotect(p, PAGE_SIZE, PROT_WRITE) == -1)
		err(1, "mprotect w");
	if (mprotect(q, PAGE_SIZE, PROT_READ) == -1)
		err(1, "mprotect r");

	assert(*((const char *)q) == 0);
	*((char *)p) = 1;
	assert(*((const char *)q) == 1);
	cow();
	assert(*((const char *)q) == 1);
	*((char *)p) = 2;
	assert(*((const char *)q) == 2);

	return 0;
}