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File Attributes

Each file is stored in a directory, and uses a directory entry that describes its characteristics such as its name and size. The directory entry also contains a pointer to where the file is stored on disk. One of the characteristics stored for each file is a set of file attributes that give the operating system and application software more information about the file and how it is intended to be used.

The use of attributes is "voluntary". What this means is that any software program can look in the directory entry to discern the attributes of a file, and based on them, make intelligent decisions about how to treat the file. For example, a file management program's delete utility, seeing a file marked as a read-only system file, would be well-advised to at least warn the user before deleting it. However, it doesn't have to. Any programmer that knows what it is doing can override the attributes of a file, and certainly, the writers of viruses do this as a matter of course!

That said, most operating systems assign definite meanings to the attributes stored for files, and will alter their behavior according to what they see. If at a DOS prompt you type "DIR" to list the files in the directory, by default you will not see any files that have the "hidden" attribute set. You have to type "DIR /AH" to see the hidden files.

A file can have more than one attribute attached to it, although only certain combinations really make any sense. The attributes are stored in a single byte, with each bit of the byte representing a specific attribute (actually, only six bits are used of the eight in the byte). Each bit that is set to a one means that the file has that attribute turned on. (These are sometimes called attribute bits or attribute flags). This method is a common way that a bunch of "yes/no" parameters are stored in computers to save space. The following are the attributes and the bits they use in the attribute byte:

The attribute bits are summed to form the attribute byte. So, the attribute byte for a hidden, read-only directory would be 00010011, which is simply the codes for those three attributes from the table above, added together. Here is a more detailed description of what these attributes mean (or more accurately, how they are normally used). Note that each of the attributes below apply equally to files and directories (except for the directory attribute of course!):

• Read-Only: Most software, when seeing a file marked read-only, will refuse to delete or modify it. This is pretty straight-forward. For example, DOS will say "Access denied" if you try to delete a read-only file. On the other hand, Windows Explorer will happily munch it. Some will choose the middle ground: they will let you modify or delete the file, but only after asking for confirmation.
• Hidden: This one is pretty self-explanatory as well; if the file is marked hidden then under normal circumstances it is hidden from view. DOS will not display the file when you type "DIR" unless a special flag is used, as shown in the earlier example.
• System: This flag is used to tag important files that are used by the system and should not be altered or removed from the disk. In essence, this is like a "more serious" read-only flag and is for the most part treated in this manner.
• Volume Label: Every disk volume can be assigned an identifying label, either when it is formatted, or later through various tools such as the DOS command "LABEL". The volume label is stored in the root directory as a file entry with the label attribute set.
• Directory: This is the bit that differentiates between entries that describe files and those that describe subdirectories within the current directory. In theory you can convert a file to a directory by changing this bit. Of course in practice, trying to do this would result in a mess--the entry for a directory has to be in a specific format.
• Archive: This is a special bit that is used as a "communications link" between software applications that modify files, and those that are used for backup. Most backup software allows the user to do an incremental backup, which only selects for backup any files that have changed since the last backup. This bit is used for this purpose. When the backup software backs up ("archives") the file, it clears the archive bit (makes it zero). Any software that modifies the file subsequently, is supposed to set the archive bit. Then, the next time that the backup software is run, it knows by looking at the archive bits which files have been modified, and therefore which need to be backed up. Again, this use of the bit is "voluntary"; the backup software relies on other software to use the archive bit properly; some programs could modify the file without setting the archive attribute, but fortunately most software is "well-behaved" and uses the bit properly. Still, you should not rely on this mechanism absolutely to ensure that your critical files are backed up.

Most of the attributes for files can be modified using the DOS ATTRIB command, or by looking at the file's properties through the Windows 95 Windows Explorer or other similar file navigation tools.

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