watching a write using posix and linux virtual memory api.c

#include <errno.h>
#include <fcntl.h>
#include <linux/fs.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <unistd.h>

int main(void) {
	struct page_region pr;
	char *region = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_SHARED, -1, 0);

	if (region == MAP_FAILED) {
		puts("mmap failed");
		exit(1);
	}

	int page_map = open("/proc/self/pagemap", O_RDONLY);

	if (page_map < 0) {
		printf("page_map was %d\n", page_map);
		exit(1);
	}

	struct pm_scan_arg a = {
		.size = sizeof (struct pm_scan_arg),
		.flags = 0,
		.start = (uintptr_t)region,
		.end = (uintptr_t)region + 4096,
		.vec = (uintptr_t)&pr,
		.vec_len = 1,
		.max_pages = 0,
		.category_inverted = 0,
		.category_mask = 0,
		.category_anyof_mask = PAGE_IS_WRITTEN,
		.return_mask = PAGE_IS_WRITTEN,
	};

	*(int *)region = 123;

	int ioctl_result = ioctl(page_map, PAGEMAP_SCAN, &a);

	perror("ioctl");
	printf("%d, %d\n", ioctl_result, errno);
	printf("%d\n", *(int *)region);
	printf("0x%llx\n", pr.start);

	if (close(page_map) < 0) {
		puts("close failed");
		exit(1);
	}

	return 0;
}