(*nix) Merge two dirs w/o copying

mvdir.c

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
#include <dirent.h>

#ifdef O_PATH
#define LEAST_OPEN_PRIV O_PATH   /* Linux >= 2.6.39 */
#elif defined(O_SEARCH) && !defined(O_EXEC)
#define LEAST_OPEN_PRIV O_SEARCH /* POSIX.1-2008 */
#else
#define LEAST_OPEN_PRIV O_EXEC   /* some FreeBSD filesystem drivers allow VEXEC for directory search */
#endif

static int movex(int olddirfd, const char *oldpath, int newdirfd, const char *newpath)
{
	int res, err, err2, oldfd, newfd;

	while ((res = renameat(olddirfd, oldpath, newdirfd, newpath)) < 0 &&
			((err = errno) == ENOTEMPTY || err == EEXIST))
	{
		DIR *dir;
		struct dirent *ent;

		newfd = openat(newdirfd, newpath, LEAST_OPEN_PRIV | O_CLOEXEC | O_NOFOLLOW | O_DIRECTORY);
		if (newfd < 0)
		{
			err2 = errno;
			if (err2 == ENOENT || err2 == ENOTDIR) continue;
			errno = err2;
			break;
		}

		oldfd = openat(olddirfd, oldpath, O_RDONLY | O_CLOEXEC | O_NOFOLLOW | O_DIRECTORY);
		if (oldfd < 0)
		{
			err2 = errno;
			close(newfd);
			if (err2 == ENOTDIR) continue;
			errno = err2;
			break;
		}

		dir = fdopendir(oldfd);
		if (dir == NULL)
		{
			err2 = errno;
			close(oldfd);
			close(newfd);
			errno = err2;
			break;
		}

		res = 0;
		while ((ent = readdir(dir)) != NULL)
		{
			const char *n = ent->d_name;
			if (n[0] == '.')
			{
				char ch1 = n[1];
				if (ch1 == '\0' || (ch1 == '.' && n[2] == '\0'))
				{
					continue;
				}
			}
			res = movex(oldfd, ent->d_name, newfd, ent->d_name);
			if (res < 0)
			{
				if (errno != ENOENT) break;
				res = 0;
			}
		}
		err = errno;
		closedir(dir);
		close(newfd);

		if (res >= 0 && unlinkat(olddirfd, oldpath, AT_REMOVEDIR) < 0)
		{
			if (errno == ENOENT || errno == ENOTEMPTY || errno == ENOTDIR) continue;
			fprintf(stderr, "<%d>/%s: unlinkat(%d, AT_REMOVEDIR): %s\n", olddirfd, oldpath, oldfd, strerror(errno));
		}
		errno = err;
		break;
	}

	if (res != 0)
	{
		fprintf(stderr, "move <%d>/%s to <%d>/%s failed: %s\n", olddirfd, oldpath, newdirfd, newpath, strerror(errno));
	}
	return res;
}

int main(int argc, char **argv)
{
	if (argc != 3)
	{
		fprintf(stderr, "usage: %s <old> <new>\n", argv[0]);
		return EXIT_FAILURE;
	}
	return movex(AT_FDCWD, argv[1], AT_FDCWD, argv[2]) < 0 ? EXIT_FAILURE : 0;
}