The description uses the term ``a file descriptor'' rather than ``the file descriptor''. The implication intended is that an implementation that does not use an fd for opendir() could still open() the directory to implement the dirfd() function. Such a descriptor must be closed later during a call to closedir().

If it is necessary to allocate an fd to be returned by dirfd(), it should be done at the time of a call to opendir().

On an implementation where reading from a directory stream does not use a file descriptor, opendir() need not allocate one to be returned by dirfd(). The implementation can instead delay the allocation of a suitable file descriptor until the first time dirfd() is called for the stream. A file descriptor allocated by dirfd() must be closed by closedir().