[mort96]

How about fd of the file you wanna rename, dirfd of the directory you want to open it in, and name of the new file? You could then represent a "rename within the same directory" as: dfd = opendir(...); fd = openat(dfd, "a"); rename2(fd, dfd, "b");

I can't think of a case this API doesn't cover, but maybe there is one.

[masklinn]

The file may have been renamed or deleted since the fd was opened, and it might have been legitimate and on purpose, but there’s no way to tell what trying to resolve the fd back to a path will give you.

And you need to do that because nothing precludes having multiple entries to the same inode in the same directory, so you need to know specifically what the source direntry is, and a direntry is just a name in the directory file.