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LIBARCHIVE_INTERNALS(3) Library Functions Manual LIBARCHIVE_INTERNALS(3) 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, ob- ject-style interface. The archive_entry(3) objects store information about a single filesystem object. The rest of the library provides fa- cilities 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. In- ternally, 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 compres- sion 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 com- pression 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 require- ment is that the skip not be larger. In particular, clients are al- lowed 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 read- ing 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. Decompression 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 de- compression API is a two stage peek/consume model. A read_ahead re- quest specifies a minimum read amount; the decompression layer must provide a pointer to at least that much data. If more data is immedi- ately 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 re- main 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 decom- pression 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 suitabil- ity 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 exam- ple, 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 cur- rent 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 han- dlers: 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 for- mats 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 up- per 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 straightfor- ward 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 off- set and size. This may require you to carefully track the lo- cation 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, exam- ine the return value to see how much data is available, and possibly trim that to the amount you can use. You should in- voke 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 mem- ory. 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 regis- tered 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 as- sume 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 loca- tion 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. Fortu- nately, such archives are very rare, and libarchive can read most ZIP archives, though it cannot always extract as much information as a ded- icated ZIP program. SEE ALSO archive_entry(3), archive_read(3), archive_write(3), archive_write_disk(3), libarchive(3) HISTORY The libarchive library first appeared in FreeBSD 5.3. AUTHORS The libarchive library was written by Tim Kientzle <kientzle@acm.org>. FreeBSD Ports 14.quarterly January 26, 2011 LIBARCHIVE_INTERNALS(3)
NAME | OVERVIEW | GENERAL ARCHITECTURE | READ ARCHITECTURE | WRITE ARCHITECTURE | WRITE_DISK ARCHITECTURE | GENERAL SERVICES | MISCELLANEOUS NOTES | SEE ALSO | HISTORY | AUTHORS
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