DOS Filesystem

The DOS Filesystem, fs-dos.so, provides transparent access to DOS disks, so you can treat DOS filesystems as though they were POSIX filesystems. This transparency allows processes to operate on DOS files without any special knowledge or work on their part.

The structure of the DOS filesystem on disk is old and inefficient, and lacks many desirable features. Its only major virtue is its portability to DOS and Windows environments. You should choose this filesystem only if you need to transport DOS files to other machines that require it. Consider using the Power-Safe filesystem alone if DOS file portability isn't an issue or in conjunction with the DOS filesystem if it is.

If there's no DOS equivalent to a POSIX feature, fs-dos.so, with either return an error or a reasonable default. For example, an attempt to create a link() will result in the appropriate errno being returned. On the other hand, if there's an attempt to read the POSIX times on a file, fs-dos.so will treat any of the unsupported times the same as the last write time.

DOS version support

The fs-dos.so program supports hard disk partitions from DOS version 2.1 to Windows 98 with long filenames.

DOS text files

DOS terminates each line in a text file with two characters (CR/LF), while POSIX (and most other) systems terminate each line with a single character (LF). Note that fs-dos.so makes no attempt to translate text files being read. Most utilities and programs aren't affected by this difference.

Note also that some very old DOS programs may use a Ctrl–Z (^Z) as a file terminator. This character is also passed through without modification.

QNX-to-DOS filename mapping

In DOS, a filename can't contain any of the following characters:

/ \ [ ] : * | + = ; , ?
  

An attempt to create a file that contains one of these invalid characters will return an error. DOS (8.3 format) also expects all alphabetical characters to be uppercase, so fs-dos.so maps these characters to uppercase when creating a filename on disk. But it maps a filename to lowercase by default when returning a filename to a QNX Neutrino application, so that QNX Neutrino users and programs can always see and type lowercase (via the sfn=sfn_mode option).

Handling filenames

You can specify how you want fs-dos.so to handle long filenames (via the lfn=lfn_mode option):

• Ignore them—display/create only 8.3 filenames.
• Show them—if filenames are longer than 8.3 or if mixed case is used.
• Always create both short and long filenames.

If you use the ignore option, you can specify whether or not to silently truncate filename characters beyond the 8.3 limit.

International filenames

The DOS filesystem supports DOS “code pages” (international character sets) for locale filenames. Short 8.3 names are stored using a particular character set (typically the most common extended characters for a locale are encoded in the 8th-bit character range). All the common American as well as Western and Eastern European code pages (437, 850, 852, 866, 1250, 1251, 1252) are supported. If you produce software that must access a variety of DOS/Windows hard disks, or operate in non-US-English countries, this feature offers important portability—filenames will be created with both a Unicode and locale name and are accessible via either name.

Note: The DOS filesystem supports international text in filenames only. No attempt is made to be aware of data contents, with the sole exception of Windows “shortcut” (.LNK) files, which will be parsed and translated into symbolic links if you've specified that option (lnk=lnk_mode).

DOS volume labels

DOS uses the concept of a volume label, which is an actual directory entry in the root of the DOS filesystem. To distinguish between the volume label and an actual DOS directory, fs-dos.so reports the volume label according to the way you specify its vollabel option. You can choose to:

• Ignore the volume label.
• Display the volume label as a name-special file.
• Display the volume label as a name-special file with an equal sign (=) as the first character of the volume name (the default).

DOS-QNX permission mapping

DOS doesn't support all the permission bits specified by POSIX. It has a READ_ONLY bit in place of separate READ and WRITE bits; it doesn't have an EXECUTE bit. When a DOS file is created, the DOS READ_ONLY bit is set if all the POSIX WRITE bits are off. When a DOS file is accessed, the POSIX READ bit is always assumed to be set for user, group, and other.

Since you can't execute a file that doesn't have EXECUTE permission, fs-dos.so has an option (exe=exec_mode) that lets you specify how to handle the POSIX EXECUTE bit for executables.

File ownership

Although the DOS file structure doesn't support user IDs and group IDs, fs-dos.so (by default) doesn't return an error code if an attempt is made to change them. An error isn't returned because a number of utilities attempt to do this and failure would result in unexpected errors. The approach taken is “you can change anything to anything since it isn't written to disk anyway.”

The posix= options let you set stricter POSIX checks and enable POSIX emulation. For example, in POSIX mode, an error of EINVAL is flagged for attempts to do any of the following:

• Set the user ID or group ID to something other than the default (root).
• Remove an r (read) permission.
• Set an s (set ID on execution) permission.

If you set the posix option to emulate (the default) or strict, you get the following benefits:

• The . and .. directory entries are created in the root directory.
• The directory size is calculated.
• The number of links in a directory is calculated, based on its subdirectories.