
LIBARCHIVE_INTERNALS(3)  BSD Library Functions Manual  LIBARCHIVE_INTERNALS(3)

NAME
     libarchive_internals — description of libarchive internal interfaces

OVERVIEW
     The libarchive library provides a flexible interface for reading and
     writing streaming archive files such as tar and cpio.  Internally, it
     follows a modular layered design that should make it easy to add new ar‐
     chive and compression formats.

GENERAL ARCHITECTURE
     Externally, libarchive exposes most operations through an opaque, object‐
     style interface.  The archive_entry(3) objects store information about a
     single filesystem object.	The rest of the library provides facilities to
     write archive_entry(3) objects to archive files, read them from archive
     files, and write them to disk.  (There are plans to add a facility to
     read archive_entry(3) objects from disk as well.)

     The read and write APIs each have four layers: a public API layer, a for‐
     mat layer that understands the archive file format, a compression layer,
     and an I/O layer.	The I/O layer is completely exposed to clients who can
     replace it entirely with their own functions.

     In order to provide as much consistency as possible for clients, some
     public functions are virtualized.	Eventually, it should be possible for
     clients to open an archive or disk writer, and then use a single set of
     code to select and write entries, regardless of the target.

READ ARCHITECTURE
     From the outside, clients use the archive_read(3) API to manipulate an
     archive object to read entries and bodies from an archive stream.	Inter‐
     nally, the archive object is cast to an archive_read object, which holds
     all read‐specific data.  The API has four layers: The lowest layer is the
     I/O layer.  This layer can be overridden by clients, but most clients use
     the packaged I/O callbacks provided, for example, by
     archive_read_open_memory(3), and archive_read_open_fd(3).	The compres‐
     sion layer calls the I/O layer to read bytes and decompresses them for
     the format layer.	The format layer unpacks a stream of uncompressed
     bytes and creates archive_entry objects from the incoming data.  The API
     layer tracks overall state (for example, it prevents clients from reading
     data before reading a header) and invokes the format and compression
     layer operations through registered function pointers.  In particular,
     the API layer drives the format‐detection process: When opening the ar‐
     chive, it reads an initial block of data and offers it to each registered
     compression handler.  The one with the highest bid is initialized with
     the first block.  Similarly, the format handlers are polled to see which
     handler is the best for each archive.  (Prior to 2.4.0, the format bid‐
     ders were invoked for each entry, but this design hindered error recov‐
     ery.)

   I/O Layer and Client Callbacks
     The read API goes to some lengths to be nice to clients.  As a result,
     there are few restrictions on the behavior of the client callbacks.

     The client read callback is expected to provide a block of data on each
     call.  A zero‐length return does indicate end of file, but otherwise
     blocks may be as small as one byte or as large as the entire file.  In
     particular, blocks may be of different sizes.

     The client skip callback returns the number of bytes actually skipped,
     which may be much smaller than the skip requested.  The only requirement
     is that the skip not be larger.  In particular, clients are allowed to
     return zero for any skip that they don’t want to handle.  The skip call‐
     back must never be invoked with a negative value.

     Keep in mind that not all clients are reading from disk: clients reading
     from networks may provide different‐sized blocks on every request and
     cannot skip at all; advanced clients may use mmap(2) to read the entire
     file into memory at once and return the entire file to libarchive as a
     single block; other clients may begin asynchronous I/O operations for the
     next block on each request.

   Decompresssion Layer
     The decompression layer not only handles decompression, it also buffers
     data so that the format handlers see a much nicer I/O model.  The decom‐
     pression API is a two stage peek/consume model.  A read_ahead request
     specifies a minimum read amount; the decompression layer must provide a
     pointer to at least that much data.  If more data is immediately avail‐
     able, it should return more: the format layer handles bulk data reads by
     asking for a minimum of one byte and then copying as much data as is
     available.

     A subsequent call to the consume() function advances the read pointer.
     Note that data returned from a read_ahead() call is guaranteed to remain
     in place until the next call to read_ahead().  Intervening calls to
     consume() should not cause the data to move.

     Skip requests must always be handled exactly.  Decompression handlers
     that cannot seek forward should not register a skip handler; the API
     layer fills in a generic skip handler that reads and discards data.

     A decompression handler has a specific lifecycle:
     Registration/Configuration
	     When the client invokes the public support function, the decom‐
	     pression handler invokes the internal
	     __archive_read_register_compression() function to provide bid and
	     initialization functions.	This function returns NULL on error or
	     else a pointer to a struct decompressor_t.  This structure con‐
	     tains a void * config slot that can be used for storing any cus‐
	     tomization information.
     Bid     The bid function is invoked with a pointer and size of a block of
	     data.  The decompressor can access its config data through the
	     decompressor element of the archive_read object.  The bid func‐
	     tion is otherwise stateless.  In particular, it must not perform
	     any I/O operations.

	     The value returned by the bid function indicates its suitability
	     for handling this data stream.  A bid of zero will ensure that
	     this decompressor is never invoked.  Return zero if magic number
	     checks fail.  Otherwise, your initial implementation should
	     return the number of bits actually checked.  For example, if you
	     verify two full bytes and three bits of another byte, bid 19.
	     Note that the initial block may be very short; be careful to only
	     inspect the data you are given.  (The current decompressors
	     require two bytes for correct bidding.)
     Initialize
	     The winning bidder will have its init function called.  This
	     function should initialize the remaining slots of the struct
	     decompressor_t object pointed to by the decompressor element of
	     the archive_read object.  In particular, it should allocate any
	     working data it needs in the data slot of that structure.	The
	     init function is called with the block of data that was used for
	     tasting.  At this point, the decompressor is responsible for all
	     I/O requests to the client callbacks.  The decompressor is free
	     to read more data as and when necessary.
     Satisfy I/O requests
	     The format handler will invoke the read_ahead, consume, and skip
	     functions as needed.
     Finish  The finish method is called only once when the archive is closed.
	     It should release anything stored in the data and config slots of
	     the decompressor object.  It should not invoke the client close
	     callback.

   Format Layer
     The read formats have a similar lifecycle to the decompression handlers:
     Registration
	     Allocate your private data and initialize your pointers.
     Bid     Formats bid by invoking the read_ahead() decompression method but
	     not calling the consume() method.	This allows each bidder to
	     look ahead in the input stream.  Bidders should not look further
	     ahead than necessary, as long look aheads put pressure on the
	     decompression layer to buffer lots of data.  Most formats only
	     require a few hundred bytes of look ahead; look aheads of a few
	     kilobytes are reasonable.	(The ISO9660 reader sometimes looks
	     ahead by 48k, which should be considered an upper limit.)
     Read header
	     The header read is usually the most complex part of any format.
	     There are a few strategies worth mentioning: For formats such as
	     tar or cpio, reading and parsing the header is straightforward
	     since headers alternate with data.  For formats that store all
	     header data at the beginning of the file, the first header read
	     request may have to read all headers into memory and store that
	     data, sorted by the location of the file data.  Subsequent header
	     read requests will skip forward to the beginning of the file data
	     and return the corresponding header.
     Read Data
	     The read data interface supports sparse files; this requires that
	     each call return a block of data specifying the file offset and
	     size.  This may require you to carefully track the location so
	     that you can return accurate file offsets for each read.  Remem‐
	     ber that the decompressor will return as much data as it has.
	     Generally, you will want to request one byte, examine the return
	     value to see how much data is available, and possibly trim that
	     to the amount you can use.  You should invoke consume for each
	     block just before you return it.
     Skip All Data
	     The skip data call should skip over all file data and trailing
	     padding.  This is called automatically by the API layer just
	     before each header read.  It is also called in response to the
	     client calling the public data_skip() function.
     Cleanup
	     On cleanup, the format should release all of its allocated mem‐
	     ory.

   API Layer
     XXX to do XXX

WRITE ARCHITECTURE
     The write API has a similar set of four layers: an API layer, a format
     layer, a compression layer, and an I/O layer.  The registration here is
     much simpler because only one format and one compression can be regis‐
     tered at a time.

   I/O Layer and Client Callbacks
     XXX To be written XXX

   Compression Layer
     XXX To be written XXX

   Format Layer
     XXX To be written XXX

   API Layer
     XXX To be written XXX

WRITE_DISK ARCHITECTURE
     The write_disk API is intended to look just like the write API to
     clients.  Since it does not handle multiple formats or compression, it is
     not layered internally.

GENERAL SERVICES
     The archive_read, archive_write, and archive_write_disk objects all con‐
     tain an initial archive object which provides common support for a set of
     standard services.  (Recall that ANSI/ISO C90 guarantees that you can
     cast freely between a pointer to a structure and a pointer to the first
     element of that structure.)  The archive object has a magic value that
     indicates which API this object is associated with, slots for storing
     error information, and function pointers for virtualized API functions.

MISCELLANEOUS NOTES
     Connecting existing archiving libraries into libarchive is generally
     quite difficult.  In particular, many existing libraries strongly assume
     that you are reading from a file; they seek forwards and backwards as
     necessary to locate various pieces of information.  In contrast,
     libarchive never seeks backwards in its input, which sometimes requires
     very different approaches.

     For example, libarchive’s ISO9660 support operates very differently from
     most ISO9660 readers.  The libarchive support utilizes a work‐queue
     design that keeps a list of known entries sorted by their location in the
     input.  Whenever libarchive’s ISO9660 implementation is asked for the
     next header, checks this list to find the next item on the disk.  Direc‐
     tories are parsed when they are encountered and new items are added to
     the list.	This design relies heavily on the ISO9660 image being opti‐
     mized so that directories always occur earlier on the disk than the files
     they describe.

     Depending on the specific format, such approaches may not be possible.
     The ZIP format specification, for example, allows archivers to store key
     information only at the end of the file.  In theory, it is possible to
     create ZIP archives that cannot be read without seeking.  Fortunately,
     such archives are very rare, and libarchive can read most ZIP archives,
     though it cannot always extract as much information as a dedicated ZIP
     program.

SEE ALSO
     archive(3), archive_entry(3), archive_read(3), archive_write(3),
     archive_write_disk(3)

HISTORY
     The libarchive library first appeared in FreeBSD 5.3.

AUTHORS
     The libarchive library was written by Tim Kientzle ⟨kientzle@acm.org⟩.

BSD			       January 26, 2011 			   BSD
