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af_intro(3) Attribute Filesystem (AtFS) af_intro(3)

NAME

AtFS - introduction to AtFS library functions and error codes

SYNOPSIS

#include <atfs.h>

int af_errno

DESCRIPTION

The following manual pages (named af*) describe the library functions of the Attribute File System (AtFS).

AtFS is an extension to the UNIX file system interface that allows the storage of files as complex of content data and an arbitrary number of associated attributes. Attributes are either standard attributes or user defined attributes (also called application defined attributes). A data complex consisting of contents data and attributes, is called Attributed Software Object (ASO). In the following they are also referred to as object versions or simply versions. AtFS has a built-in version control system that manages object histories. A object history consists of an optional busy version and a number of saved versions. The busy version is represented by an ordinary alterable UNIX file. It can be accessed via AtFS and UNIX file system operations. Saved versions come into being by making a copy of the current state of the busy version. They can be stored as source versions or as derived versions.

Source versions are typically composed manually (e.g. by use of a text editor). AtFS stores source versions as immutable objects. Once saved, they cannot be modified any longer. Saved versions are stored in archive files, residing in a subdirectory named AtFS. AtFS maintains two archive files for each history of source versions - one holding the attributes and the other holding the data. To save disk space, the versions in an archive are stored as deltas, that means, only differences between two successive versions are stored.

Derived versions are typically derived automatically (e.g. by a compiler) from a source version and thus be reproducible at any time. They are kept in a derived object cache a data store of limited size that is administered in a cache fashion. When space runs short, old versions are cleaned out of the cache according to a defined policy. Check af_cache(3) for details.

AtFS makes no assumptions whether a copy of a busy object shall be stored as source object or as derived object. The application has to decide that by calling the appropriate function (af_saverev - manual page af_version(3) or af_savecache - manual page af_cache(3)).

The main data types for AtFS applications are:

•
The object key that uniquely identifies an ASO (version). The structure of this type can be different in different implementations of AtFS. Consequently, application programs should handle this type as opaque type and should not access single fields.
•
A set descriptor represents a set of object keys. A set descriptor contains information about the size of the set and a list of object keys in the set.
•
The user identification represents a user. As AtFS realizes a simple network user concept, it does not identify users by their UNIX user id, but rather by the user name and the domain where this name is valid. See af_afuser (manual page af_misc(3)) for details.
•
An attribute buffer is capable to hold all attributes of a software object (standard attributes and user defined attributes). Attribute buffers have two different purposes. First, they can hold a retrieve pattern, i.e. they may be (partially) filled with desired attribute values and then be passed as argument to a retrieve operation (af_find and af_cachefind - manual page af_retrieve(3)). Second, an attribute buffer is used to return all attributes of an identified ASO on demand.

There are several ways for an AtFS application to get an object key pointing to a specific object version. The most important is the function af_getkey (manual page af_retrieve(3)), that returns a key for an explicitly named version. After having retrieved a key, the data for that object version remains cached in memory as long as the application does not explicitly give the key back. The function af_dropkey (manual page af_retrieve(3)) gives a key back and releases the object version. A retrieved set of keys has also to be given back by use of af_dropset (manual page af_retrieve(3)). af_dropall (manual page af_retrieve(3)) sets all reference counters for cached object versions to zero, that means, it gives all formerly retrieved keys and sets back. In the case, that any attribute, or the contents data of a cached object version is modified on disk by another application, the data in the cache is automatically updated. In that case, a warning is written to the error log file.

For handling sets of keys, AtFS provides a full set algebra with functions for adding and deleting single keys, sorting, building subsets, intersections, unions and differences of sets.

The built-in version control functionality features a status mode for versions. Each object version has a certain state, one of busy, saved, proposed, published, accessed and frozen. The already known busy version always has the state busy, while when previously referencing a saved versions we meant a version that can have any state from saved to frozen. AtFS does not enforce any specific semantics with the version state. It is rather a help for applications to implement a classification for versions. Another part of the version control functionality is a locking facility. Adding a new version to an existing object history always requires a lock on the most recent version or the generation to which the version shall be added. Locks can be used for implementing a synchronization mechanism for concurrent updates to one object history.

A user defined attribute (or application defined attribute) has the general form name[=value [value [...]]]. It has a name and a (possibly empty) list of values. The name may consist of any characters (even non-alphanumeric) but an equal sign (=). The equal sign is the delimiter between name and value. The attribute value may not contain Ctrl-A (\001) characters. Although AtFS promises the storage of an arbitrary number of user defined attributes, the current implementation limits their number to 255.

Most of the AtFS calls have one or more error returns. An error is indicated by a return value that is either -1 or a nil pointer, depending on the return type of the function. If one of the functions returns with an error, the variable af_errno is set to indicate the appropriate error number. af_errno is not cleared upon successful calls.

ERROR CODES

The following is a complete collection of the error numbers defined in atfs.h. The first list contains return values indicating common error conditions like implausible arguments passed to an AtFS function or permission problems. The error codes listed in the second list point to serious trouble, which can either be an internal error in AtFS or a corrupted archive file. The occurrence of an serious problem is recorded in an error protocol (/usr/adm/AtFSerrlog). On machines without syslog(3) switched on for AtFS, the error protocol is located in a /tmp/AtFSerrlog.

A called system library function or system call returned with an error condition. See errno for a more precise error specification.
An attempt was made to perform an operation (e.g. open a file) without appropriate permissions.
One of the archive files you are operating on has been modified by another process since your process has read it from disk. In this case, AtFS refuses to store your changes because this would destroy the modifications made by the other process. In order to make your desired changes happen, you have to rerun your application.
Some AtFS-operations cannot be performed on ASOs which have the state AF_BUSY.
Some AtFS-operations (eg. af_lock - manual page af_lock(3), af_newgen - manual page af_version(3)) cannot be performed on ASOs stored in derived object caches.
An invalid object key (eg. nil pointer) was passed to an AtFS function.
An invalid set descriptor (eg. nil pointer) was passed to an AtFS function.
An invalid user structure or user id was passed to an AtFS operation.
An attempt was made to set a version number, that contradicts the AtFS version numbering philosophy, to an ASO. You cannot change the version number of an ASO into a version number that is "smaller" than the one given by the system.
This error code is set when an error occurs that does not fit in any of the other error categories. See your error logging file (/usr/adm/AtFSerrlog of /tmp/AtFSerrlog) for a detailed description of the error. af_perror (manual page af_error(3)) also gives a diagnostic message explaining the error.
The function af_setattr (manual page af_attrs(3)) requires a change mode. This error condition complains about an invalid mode given to this function.
There is no place where AtFS can create it's archive files. Either a global archive path should be defined (see af_setarchpath - manual page af_misc(3)) or a subdirectory named AtFS should be present and writable.
A specified key that shall be removed from a keyset does not exist in the concerning set.
A specified position in a keyset (keysets are organized as arrays of object keys) lies beyond the bounds of the concerning set.
A revision - uniquely identified by a set of attributes (eg. af_getkey - manual page af_retrieve(3)) - does not exist in the current search space.
Some AtFS operations (eg. af_setbusy - manual page af_version(3)) require the key of a busy ASO as input parameter. If you pass key of a non-busy ASO, the function sets this error code.
An attempt was made to restore an object that in not stored in a derived object cache.
An AtFS operation cannot be performed because the specified ASO is either not locked (see af_lock(3)) or it is locked by another user.
With this error number AtFS refuses to generate versions of non-regular UNIX files such as directories, special files and sockets.
Typically, this error occurs if an operation requiring at least one saved revision (eg. af_newgen - manual page af_version(3)) is applied on a versionless file.
A user defined attribute with the given name does not exist.
An attempt was made to open a non-busy version with write access.
The Attribute File System's built in revision-states model allows only well defined revision state changes.
Some AtFS operations can only be performed on ASOs with a specific version state.

The error codes indicating real trouble:

Some error occurred during invocation of the delta processor.
The data in the archive file are corrupted. This may have happened by editing the archive file or by malfunction of an AtFS operation. Try atfsrepair ti fix it.
Please inform your local trouble shooting service, go to your favorite pub and order a beers. Cheers !
Archive file lost.

LIST OF FUNCTIONS

Name Appears on Page Description

af_abort	af_transact.3	abort transaction
af_access	af_history.3	test existence of history
af_afname	af_misc.3	get ASO name from UNIX path
af_afpath	af_misc.3	get ASO syspath from UNIX path
af_aftype	af_misc.3	get ASO type from UNIX path
af_afuser	af_misc.3	get AtFS user from UNIX uid
af_allattrs	af_attrs.3	get all attributes of ASO
af_cachefind	af_retrieve.3	find derived objects by attributes
af_cachenames	af_history.3	return list of names in derived object cache
af_cachesize	af_cache.3	define object cache size policy
af_chauthor	af_protect.3	change author of ASO
af_chmod	af_protect.3	change protection of ASO
af_chowner	af_protect.3	change owner of ASO
af_cleanup	af_error.3	do cleanup after error
af_close	af_files.3	close ASO contents
af_commit	af_transact.3	commit transaction
af_copyset	af_sets.3	copy sets
af_crkey	af_files.3	create object key for UNIX-file
af_diff	af_sets.3	build difference between two sets of object keys
af_dropall	af_retrieve.3	drop all accessed object keys
af_dropkey	af_retrieve.3	drop object key
af_dropset	af_retrieve.3	drop set of object keys
af_errmsg	af_error.3	return AtFS- or system error message
af_errno	af_error.3	global error code variable
af_establish	af_files.3	establish ASO as UNIX file
af_find	af_retrieve.3	find ASOs by attributes
af_freeattr	af_attrs.3	free memory associated with attribute value
af_freeattrbuf	af_attrs.3	free memory associated with attribute buffer
af_getkey	af_retrieve.3	get key by unique attributes
af_histories	af_history.3	return list of history names matching pattern
af_initattrs	af_retrieve.3	initialize attribute buffer
af_initset	af_sets.3	initialize set
af_intersect	af_sets.3	build intersection between two sets of object keys
af_isstdval	af_attrs.3	check if attribute is from a standard attribute
af_lock	af_lock.3	set lock on ASO 
af_newgen	af_version.3	increase generation number of ASO
af_nrofkeys	af_sets.3	return number of keys in set of object keys
af_open	af_files.3	open ASO contents
af_perror	af_error.3	report AtFS- or system error
af_predsucc	af_version.3	get successor or predecessor of version
af_restore	af_files.3	restore derived ASO
af_retattr	af_attrs.3	return value of attribute as string
af_retnumattr	af_attrs.3	return value of numeric attribute
af_rettimeattr	af_attrs.3	return value of time attribute
af_retuserattr	af_attrs.3	return value of user attribute
af_rm	af_files.3	remove ASO
af_rnote	af_note.3	return note attribute
af_savecache	af_cache.3	save derived object to cache
af_saverev	af_version.3	save busy version of source object
af_setaddkey	af_sets.3	add key to set of object keys
af_setarchpath	af_misc.3	set path for location of archives
af_setattr	af_attrs.3	set user defined attribute
af_setbusy	af_version.3	set version busy
af_setgkey	af_sets.3	get key from set of object keys
af_setposrmkey	af_sets.3	remove key (id. by position) from set of object keys
af_setrmkey	af_sets.3	remove key from set of object keys
af_snote	af_note.3	modify note attribute
af_sortset	af_sets.3	sort set of object keys
af_sstate	af_version.3	set version state
af_subset	af_sets.3	build subset of set of object keys
af_svnum	af_version.3	set version number
af_testlock	af_lock.3	see if ASO is locked
af_transaction	af_transact.3	begin transaction
af_union	af_sets.3	build union of two sets of object keys
af_unlock	af_lock.3	remove lock from ASO
af_version	af_misc.3	return identification of current AtFS version

FILES

/var/adm/AtFSerrlog, /tmp/AtFSerrlog

SEE ALSO

intro(2), intro(3)

Andreas Lampen and Axel Mahler
An Object Base for Attributed Software Objects
Proceedings of the Autumn '88 EUUG Conference (Cascais, October 3-7)
European UNIX System User Group, London 1988

For an outlook on a totally new implementation of the AtFS idea, have a look at:
Andreas Lampen
Advancing Files to Attributed Software Objects
Proceedings of USENIX Technical Conference (Dallas TX, January 21-25, 1991)
USENIX Association, Berkeley CA, January 1991

BUGS

The built in archive file locking to prevent concurrent updates is very simple and my lead to confusing situations. It may happen that changes will not be saved to disk. See the error conditions AF_EARCHANGED and AF_EARLOCKED for more details.

AUTHOR

Andreas Lampen, Tech. Univ. Berlin (Andres.Lampen@cs.tu-berlin.de)

Technische Universitaet Berlin
Sekr. FR 5-6
Franklinstr. 28/29
D-10587 Berlin, Germany

Fri Jun 25 14:32:56 1993 AtFS-1.71