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.

   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 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(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>.