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<p>LIBARCHIVE_INTERNALS(3) BSD Library Functions Manual
LIBARCHIVE_INTERNALS(3)</p>

<p style="margin-top: 1em"><b>NAME</b></p>

<p style="margin-left:6%;"><b>libarchive_internals</b>
&mdash; description of libarchive internal interfaces</p>

<p style="margin-top: 1em"><b>OVERVIEW</b></p>

<p style="margin-left:6%;">The <b>libarchive</b> 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.</p>

<p style="margin-top: 1em"><b>GENERAL ARCHITECTURE</b></p>

<p style="margin-left:6%;">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.)</p>

<p style="margin-left:6%; margin-top: 1em">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.</p>

<p style="margin-left:6%; margin-top: 1em">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.</p>

<p style="margin-top: 1em"><b>READ ARCHITECTURE</b></p>

<p style="margin-left:6%;">From the outside, clients use
the archive_read(3) API to manipulate an <b>archive</b>
object to read entries and bodies from an archive stream.
Internally, the <b>archive</b> object is cast to an
<b>archive_read</b> 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 <b>archive_entry</b> 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.)</p>

<p style="margin-left:6%; margin-top: 1em"><b>I/O Layer and
Client Callbacks</b> <br>
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.</p>

<p style="margin-left:6%; margin-top: 1em">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.</p>

<p style="margin-left:6%; margin-top: 1em">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&rsquo;t want to handle. The skip callback must never be
invoked with a negative value.</p>

<p style="margin-left:6%; margin-top: 1em">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.</p>


<p style="margin-left:6%; margin-top: 1em"><b>Decompresssion
Layer</b> <br>
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.</p>

<p style="margin-left:6%; margin-top: 1em">A subsequent
call to the <b>consume</b>() function advances the read
pointer. Note that data returned from a <b>read_ahead</b>()
call is guaranteed to remain in place until the next call to
<b>read_ahead</b>(). Intervening calls to <b>consume</b>()
should not cause the data to move.</p>

<p style="margin-left:6%; margin-top: 1em">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.</p>

<p style="margin-left:6%; margin-top: 1em">A decompression
handler has a specific lifecycle:</p>

<p>Registration/Configuration</p>

<p style="margin-left:17%;">When the client invokes the
public support function, the decompression handler invokes
the internal <b>__archive_read_register_compression</b>()
function to provide bid and initialization functions. This
function returns <b>NULL</b> on error or else a pointer to a
<b>struct decompressor_t</b>. This structure contains a
<i>void * config</i> slot that can be used for storing any
customization information.</p>

<p>Bid</p>

<p style="margin-left:17%; margin-top: 1em">The bid
function is invoked with a pointer and size of a block of
data. The decompressor can access its config data through
the <i>decompressor</i> element of the <b>archive_read</b>
object. The bid function is otherwise stateless. In
particular, it must not perform any I/O operations.</p>

<p style="margin-left:17%; margin-top: 1em">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.)</p>

<p>Initialize</p>

<p style="margin-left:17%;">The winning bidder will have
its init function called. This function should initialize
the remaining slots of the <i>struct decompressor_t</i>
object pointed to by the <i>decompressor</i> element of the
<i>archive_read</i> object. In particular, it should
allocate any working data it needs in the <i>data</i> 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.</p>

<p>Satisfy I/O requests</p>

<p style="margin-left:17%;">The format handler will invoke
the <i>read_ahead</i>, <i>consume</i>, and <i>skip</i>
functions as needed.</p>

<p>Finish</p>

<p style="margin-left:17%; margin-top: 1em">The finish
method is called only once when the archive is closed. It
should release anything stored in the <i>data</i> and
<i>config</i> slots of the <i>decompressor</i> object. It
should not invoke the client close callback.</p>

<p style="margin-left:6%; margin-top: 1em"><b>Format
Layer</b> <br>
The read formats have a similar lifecycle to the
decompression handlers:</p>

<p>Registration</p>

<p style="margin-left:17%;">Allocate your private data and
initialize your pointers.</p>

<p>Bid</p>

<p style="margin-left:17%; margin-top: 1em">Formats bid by
invoking the <b>read_ahead</b>() decompression method but
not calling the <b>consume</b>() 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.)</p>

<p>Read header</p>

<p style="margin-left:17%;">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.</p>

<p>Read Data</p>

<p style="margin-left:17%;">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.</p>

<p>Skip All Data</p>

<p style="margin-left:17%;">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 <b>data_skip</b>() function.</p>

<p>Cleanup</p>

<p style="margin-left:17%;">On cleanup, the format should
release all of its allocated memory.</p>

<p style="margin-left:6%; margin-top: 1em"><b>API Layer</b>
<br>
XXX to do XXX</p>

<p style="margin-top: 1em"><b>WRITE ARCHITECTURE</b></p>

<p style="margin-left:6%;">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.</p>

<p style="margin-left:6%; margin-top: 1em"><b>I/O Layer and
Client Callbacks</b> <br>
XXX To be written XXX</p>

<p style="margin-left:6%; margin-top: 1em"><b>Compression
Layer</b> <br>
XXX To be written XXX</p>

<p style="margin-left:6%; margin-top: 1em"><b>Format
Layer</b> <br>
XXX To be written XXX</p>

<p style="margin-left:6%; margin-top: 1em"><b>API Layer</b>
<br>
XXX To be written XXX</p>

<p style="margin-top: 1em"><b>WRITE_DISK
ARCHITECTURE</b></p>

<p style="margin-left:6%;">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.</p>

<p style="margin-top: 1em"><b>GENERAL SERVICES</b></p>

<p style="margin-left:6%;">The <b>archive_read</b>,
<b>archive_write</b>, and <b>archive_write_disk</b> objects
all contain an initial <b>archive</b> 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 <b>archive</b> 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.</p>

<p style="margin-top: 1em"><b>MISCELLANEOUS NOTES</b></p>

<p style="margin-left:6%;">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.</p>

<p style="margin-left:6%; margin-top: 1em">For example,
libarchive&rsquo;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&rsquo;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.</p>

<p style="margin-left:6%; margin-top: 1em">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.</p>

<p style="margin-top: 1em"><b>SEE ALSO</b></p>

<p style="margin-left:6%;">archive_entry(3),
archive_read(3), archive_write(3), archive_write_disk(3),
libarchive(3)</p>

<p style="margin-top: 1em"><b>HISTORY</b></p>

<p style="margin-left:6%;">The <b>libarchive</b> library
first appeared in FreeBSD&nbsp;5.3.</p>

<p style="margin-top: 1em"><b>AUTHORS</b></p>

<p style="margin-left:6%;">The <b>libarchive</b> library
was written by Tim Kientzle &lt;[email protected]&gt;.</p>

<p style="margin-left:6%; margin-top: 1em">BSD
January&nbsp;26, 2011 BSD</p>
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