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+.TH LIBARCHIVE 3 "April 16, 2007" ""
+.SH NAME
+.ad l
+\fB\%libarchive_internals\fP
+\- description of libarchive internal interfaces
+.SH OVERVIEW
+.ad l
+The
+\fB\%libarchive\fP
+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.
+.SH GENERAL ARCHITECTURE
+.ad l
+Externally, libarchive exposes most operations through an
+opaque, object-style interface.
+The
+\fBarchive_entry\fP(1)
+objects store information about a single filesystem object.
+The rest of the library provides facilities to write
+\fBarchive_entry\fP(1)
+objects to archive files,
+read them from archive files,
+and write them to disk.
+(There are plans to add a facility to read
+\fBarchive_entry\fP(1)
+objects from disk as well.)
+.PP
+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.
+.PP
+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.
+.SH READ ARCHITECTURE
+.ad l
+From the outside, clients use the
+\fBarchive_read\fP(3)
+API to manipulate an
+\fB\%archive\fP
+object to read entries and bodies from an archive stream.
+Internally, the
+\fB\%archive\fP
+object is cast to an
+\fB\%archive_read\fP
+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
+\fBarchive_read_open_memory\fP(3),
+and
+\fBarchive_read_open_fd\fP(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
+\fB\%archive_entry\fP
+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.)
+.SS 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.
+.PP
+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.
+.PP
+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.
+.PP
+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
+\fBmmap\fP(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.
+.SS 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.
+.PP
+A subsequent call to the
+\fB\%consume\fP()
+function advances the read pointer.
+Note that data returned from a
+\fB\%read_ahead\fP()
+call is guaranteed to remain in place until
+the next call to
+\fB\%read_ahead\fP().
+Intervening calls to
+\fB\%consume\fP()
+should not cause the data to move.
+.PP
+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.
+.PP
+A decompression handler has a specific lifecycle:
+.RS 5
+.TP
+Registration/Configuration
+When the client invokes the public support function,
+the decompression handler invokes the internal
+\fB\%__archive_read_register_compression\fP()
+function to provide bid and initialization functions.
+This function returns
+\fBNULL\fP
+on error or else a pointer to a
+\fBstruct\fP decompressor_t.
+This structure contains a
+\fIvoid\fP * config
+slot that can be used for storing any customization information.
+.TP
+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
+\fIdecompressor\fP
+element of the
+\fBarchive_read\fP
+object.
+The bid function is otherwise stateless.
+In particular, it must not perform any I/O operations.
+.PP
+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.)
+.TP
+Initialize
+The winning bidder will have its init function called.
+This function should initialize the remaining slots of the
+\fIstruct\fP decompressor_t
+object pointed to by the
+\fIdecompressor\fP
+element of the
+\fIarchive_read\fP
+object.
+In particular, it should allocate any working data it needs
+in the
+\fIdata\fP
+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.
+.TP
+Satisfy I/O requests
+The format handler will invoke the
+\fIread_ahead\fP,
+\fIconsume\fP,
+and
+\fIskip\fP
+functions as needed.
+.TP
+Finish
+The finish method is called only once when the archive is closed.
+It should release anything stored in the
+\fIdata\fP
+and
+\fIconfig\fP
+slots of the
+\fIdecompressor\fP
+object.
+It should not invoke the client close callback.
+.RE
+.SS Format Layer
+The read formats have a similar lifecycle to the decompression handlers:
+.RS 5
+.TP
+Registration
+Allocate your private data and initialize your pointers.
+.TP
+Bid
+Formats bid by invoking the
+\fB\%read_ahead\fP()
+decompression method but not calling the
+\fB\%consume\fP()
+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.)
+.TP
+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.
+.TP
+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.
+.TP
+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
+\fB\%data_skip\fP()
+function.
+.TP
+Cleanup
+On cleanup, the format should release all of its allocated memory.
+.RE
+.SS API Layer
+XXX to do XXX
+.SH WRITE ARCHITECTURE
+.ad l
+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.
+.SS I/O Layer and Client Callbacks
+XXX To be written XXX
+.SS Compression Layer
+XXX To be written XXX
+.SS Format Layer
+XXX To be written XXX
+.SS API Layer
+XXX To be written XXX
+.SH WRITE_DISK ARCHITECTURE
+.ad l
+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.
+.SH GENERAL SERVICES
+.ad l
+The
+\fB\%archive_read\fP,
+\fB\%archive_write\fP,
+and
+\fB\%archive_write_disk\fP
+objects all contain an initial
+\fB\%archive\fP
+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
+\fB\%archive\fP
+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.
+.SH MISCELLANEOUS NOTES
+.ad l
+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.
+.PP
+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.
+.PP
+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.
+.SH SEE ALSO
+.ad l
+\fBarchive\fP(3),
+\fBarchive_entry\fP(3),
+\fBarchive_read\fP(3),
+\fBarchive_write\fP(3),
+\fBarchive_write_disk\fP(3)
+.SH HISTORY
+.ad l
+The
+\fB\%libarchive\fP
+library first appeared in
+FreeBSD 5.3.
+.SH AUTHORS
+.ad l
+-nosplit
+The
+\fB\%libarchive\fP
+library was written by
+Tim Kientzle \%<kientzle@acm.org.>
+.SH BUGS
+.ad l