Extended Attributes

File attributes are information about a file that is maintained by the operating system outside the file's overt storage area. The FAT file system supports only a few simple attributes (read only, system, hidden, and archive) that are actually stored as bit flags in the file's directory entry these attributes are inspected or modified by special function calls and are not accessible through the normal file open, read, and write calls. The HPF'S supports the same attributes as the FAT file system for historical reasons, but it also supports a new form of file- associated, highly generalized information called Extended Attributes (EAs). Each EA is conceptually similar to an environment variable, taking the form (name=value) except that the value portion can be either a null- tenninated (ASCIIZ) string or binary data. In OS/2 1.2, each file or direc-tory can have a maximum of 64Kb of EAs attached to it. This limit may be lifted in a later release of OS/2. The storage method for EAs can vary. If the EAs associated with a given file or directory are small enough, they will be stored right in the Fnode. If the total size of the EAs is too large, they are stored outside the Fnode in sector runs, and a B+ Tree of allocation sectors can be created to describe the runs. If a single EA gets too large, it can be pushed outside the Fnode into a B+ Tree of its own.

The kernel API functions DosQFileInfo and DosSetFileInfo have been expanded with new information levels that allow application programs to manipulate extended attributes for files. The new functions DosQPathInfo and DosSetPathInfo are used to read or write the EAs associated with arbitrary path names. An application program can either ask for the value of a specific EA (supplying a name to be matched) or can obtain all of the EAs for the file or directory at once. Although application programs can begin to take advantage of EAs as soon as the HPFS is released, support for EAs is an essential component in Microsoft's long-range plans for object-oriented file systems. Information of almost any type can be stored in EAs, ranging from the name of the application that owns the file to names of dependent files to icons to executable code. As the HPFS evolves, its facilities for manipulating EAs are likely to become much more sophisticated. It's easy to imagine, for example, that in future versions the API might be extended with EA functions that are analogous to DosFindFirst and DosFindNext and EA data might get organized into B-Trees. I should note here that in addition to EAs, the LAN Manager version of HPFS will support another class of fil-associated information called Access Control Lists (ACLs). ACLs have the same general appearance as EAs and are manipulated in a similar manner, but they are used to store access rights, passwords, and other information of interest in a networking multi user environment.

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