Provided by: libarchive-dev_3.1.2-7ubuntu2.8_amd64 
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>.
Debian January 26, 2011 LIBARCHIVE_INTERNALS(3)