Testing APFS mtime granularity

apfs-timestamp-granularity.c

#include <stdio.h>
#include <sys/stat.h>
#include <time.h>
#include <mach/clock_types.h>
#include <fcntl.h>
#include <unistd.h>

static const char s_filePath[] = "./testfile"; //"/Volumes/Macintosh HD/Users/phil/Desktop/test/testfile";

int main(int argc, const char * argv[])
{
	struct stat stat_buf = {};
	int fd = open(s_filePath, O_WRONLY | O_CREAT | O_TRUNC, 0644);
	if (fd < 0)
	{
		perror("open() failed");
		return 1;
	}
	for (unsigned j = 0; j < 100; ++j)
	{
		// warmup
		for (unsigned i = 0; i < 1000000; ++i)
		{
			pwrite(fd, "\0", 1, i);
		}
		fstat(fd, &stat_buf);
		uint64_t mtime_ns = stat_buf.st_mtimespec.tv_nsec + stat_buf.st_mtimespec.tv_sec * NSEC_PER_SEC;
		uint64_t new_mtime_ns;
		unsigned written = 0;
		do
		{
			pwrite(fd, "\0", 1, written);
			++written;
			fstat(fd, &stat_buf);
			new_mtime_ns = stat_buf.st_mtimespec.tv_nsec + stat_buf.st_mtimespec.tv_sec * NSEC_PER_SEC;
		} while (new_mtime_ns == mtime_ns);
		printf("%u write()s until mtime change\ndelta =%13llu\n%20llu\n%20llu\n", written, new_mtime_ns - mtime_ns, new_mtime_ns, mtime_ns);
	}
	
	return 0;
}