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LIBARCHIVE_INTERNALS(3) DragonFly Library Functions Manual
libarchive_internals - description of libarchive internal interfaces
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.
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.
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.
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:
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
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.)
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
The read formats have a similar lifecycle to the decompression handlers:
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.)
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.
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.
On cleanup, the format should release all of its allocated
XXX to do XXX
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
XXX To be written XXX
XXX To be written XXX
XXX To be written XXX
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.
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.
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
archive_entry(3), archive_read(3), archive_write(3),
The libarchive library first appeared in FreeBSD 5.3.
The libarchive library was written by Tim Kientzle <firstname.lastname@example.org>.
DragonFly 5.9-DEVELOPMENT January 26, 2011 DragonFly 5.9-DEVELOPMENT