PRO/5 provides access to a directory only on host systems supporting such access. A directory is a list of file names. Other information may be kept in the directory but that will differ between host systems. For the sake of portability, PRO/5 provides access to a directory file as a read-only SERIAL file where each record contains only a file name with no special characters or terminators added. The file names are not necessarily kept in any special order. While not guaranteed, random access is "legal" only if it causes forward movement through the file. Directories may not be locked, nor may a record in a directory file be locked. Directories are accessed through a READ RECORD only, because within a directory, there is no concept of a field.