plucky (3) libarchive_internals.3.gz

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
     archive 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 format 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.  Internally, 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 compression 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 archive, 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 bidders were invoked for each entry, but this design hindered error recovery.)

   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 callback 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.

   Decompression 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 decompression 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 available, 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 decompression 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
             contains a void * config slot that can be used for storing any customization 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
             function 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.  Remember 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 memory.

   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
     registered 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 contain 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.  Directories are parsed when they are encountered and new items are
     added to the list.  This design relies heavily on the ISO9660 image being optimized 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_entry(3), archive_read(3), archive_write(3), archive_write_disk(3), libarchive(3)

HISTORY

     The libarchive library first appeared in FreeBSD 5.3.

AUTHORS

     The libarchive library was written by Tim Kientzle <kientzle@acm.org>.