unlink(2) returns EISDIR (on Linux at least) if you try to unlink a directory. I'm not sure if you can still do it, but long ago you could unlink(2) a directory

unlink(2) returns EISDIR (on Linux at least) if you try to unlink a directory. I'm not sure if you can still do it, but long ago you could unlink(2) a directory as root, but it was not a good idea since it did not recursively free up the contents back to the file system's raw storage. To remove a directory on Linux you have to empty it out (recursively, using unlink(2) on files and rmdir(2) on empty directories) and then rmdir(2) the top level directory. So not a single system call.