1LIBARCHIVE_INTERNALS(3) BSD Library Functions Manual LIBARCHIVE_INTERNALS(3)
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4 libarchive_internals — description of libarchive internal interfaces
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7 The libarchive library provides a flexible interface for reading and
8 writing streaming archive files such as tar and cpio. Internally, it
9 follows a modular layered design that should make it easy to add new ar‐
10 chive and compression formats.
11
13 Externally, libarchive exposes most operations through an opaque, object-
14 style interface. The archive_entry(3) objects store information about a
15 single filesystem object. The rest of the library provides facilities to
16 write archive_entry(3) objects to archive files, read them from archive
17 files, and write them to disk. (There are plans to add a facility to
18 read archive_entry(3) objects from disk as well.)
19 The read and write APIs each have four layers: a public API layer, a for‐
21 mat layer that understands the archive file format, a compression layer,
22 and an I/O layer. The I/O layer is completely exposed to clients who can
23 replace it entirely with their own functions.
24 In order to provide as much consistency as possible for clients, some
26 public functions are virtualized. Eventually, it should be possible for
27 clients to open an archive or disk writer, and then use a single set of
28 code to select and write entries, regardless of the target.
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31 From the outside, clients use the archive_read(3) API to manipulate an
32 archive object to read entries and bodies from an archive stream. Inter‐
33 nally, the archive object is cast to an archive_read object, which holds
34 all read-specific data. The API has four layers: The lowest layer is the
35 I/O layer. This layer can be overridden by clients, but most clients use
36 the packaged I/O callbacks provided, for example, by
37 archive_read_open_memory(3), and archive_read_open_fd(3). The compres‐
38 sion layer calls the I/O layer to read bytes and decompresses them for
39 the format layer. The format layer unpacks a stream of uncompressed
40 bytes and creates archive_entry objects from the incoming data. The API
41 layer tracks overall state (for example, it prevents clients from reading
42 data before reading a header) and invokes the format and compression
43 layer operations through registered function pointers. In particular,
44 the API layer drives the format-detection process: When opening the ar‐
45 chive, it reads an initial block of data and offers it to each registered
46 compression handler. The one with the highest bid is initialized with
47 the first block. Similarly, the format handlers are polled to see which
48 handler is the best for each archive. (Prior to 2.4.0, the format bid‐
49 ders were invoked for each entry, but this design hindered error recov‐
50 ery.)
51 I/O Layer and Client Callbacks
53 The read API goes to some lengths to be nice to clients. As a result,
54 there are few restrictions on the behavior of the client callbacks.
55 The client read callback is expected to provide a block of data on each
57 call. A zero-length return does indicate end of file, but otherwise
58 blocks may be as small as one byte or as large as the entire file. In
59 particular, blocks may be of different sizes.
60 The client skip callback returns the number of bytes actually skipped,
62 which may be much smaller than the skip requested. The only requirement
63 is that the skip not be larger. In particular, clients are allowed to
64 return zero for any skip that they don't want to handle. The skip call‐
65 back must never be invoked with a negative value.
66 Keep in mind that not all clients are reading from disk: clients reading
68 from networks may provide different-sized blocks on every request and
69 cannot skip at all; advanced clients may use mmap(2) to read the entire
70 file into memory at once and return the entire file to libarchive as a
71 single block; other clients may begin asynchronous I/O operations for the
72 next block on each request.
73 Decompresssion Layer
75 The decompression layer not only handles decompression, it also buffers
76 data so that the format handlers see a much nicer I/O model. The decom‐
77 pression API is a two stage peek/consume model. A read_ahead request
78 specifies a minimum read amount; the decompression layer must provide a
79 pointer to at least that much data. If more data is immediately avail‐
80 able, it should return more: the format layer handles bulk data reads by
81 asking for a minimum of one byte and then copying as much data as is
82 available.
83 A subsequent call to the consume() function advances the read pointer.
85 Note that data returned from a read_ahead() call is guaranteed to remain
86 in place until the next call to read_ahead(). Intervening calls to
87 consume() should not cause the data to move.
88 Skip requests must always be handled exactly. Decompression handlers
90 that cannot seek forward should not register a skip handler; the API
91 layer fills in a generic skip handler that reads and discards data.
92 A decompression handler has a specific lifecycle:
94 Registration/Configuration
95 When the client invokes the public support function, the decom‐
96 pression handler invokes the internal
97 __archive_read_register_compression() function to provide bid and
98 initialization functions. This function returns NULL on error or
99 else a pointer to a struct decompressor_t. This structure con‐
100 tains a void * config slot that can be used for storing any cus‐
101 tomization information.
102 Bid The bid function is invoked with a pointer and size of a block of
103 data. The decompressor can access its config data through the
104 decompressor element of the archive_read object. The bid func‐
105 tion is otherwise stateless. In particular, it must not perform
106 any I/O operations.
107 The value returned by the bid function indicates its suitability
109 for handling this data stream. A bid of zero will ensure that
110 this decompressor is never invoked. Return zero if magic number
111 checks fail. Otherwise, your initial implementation should
112 return the number of bits actually checked. For example, if you
113 verify two full bytes and three bits of another byte, bid 19.
114 Note that the initial block may be very short; be careful to only
115 inspect the data you are given. (The current decompressors
116 require two bytes for correct bidding.)
117 Initialize
118 The winning bidder will have its init function called. This
119 function should initialize the remaining slots of the struct
120 decompressor_t object pointed to by the decompressor element of
121 the archive_read object. In particular, it should allocate any
122 working data it needs in the data slot of that structure. The
123 init function is called with the block of data that was used for
124 tasting. At this point, the decompressor is responsible for all
125 I/O requests to the client callbacks. The decompressor is free
126 to read more data as and when necessary.
127 Satisfy I/O requests
128 The format handler will invoke the read_ahead, consume, and skip
129 functions as needed.
130 Finish The finish method is called only once when the archive is closed.
131 It should release anything stored in the data and config slots of
132 the decompressor object. It should not invoke the client close
133 callback.
134 Format Layer
136 The read formats have a similar lifecycle to the decompression handlers:
137 Registration
138 Allocate your private data and initialize your pointers.
139 Bid Formats bid by invoking the read_ahead() decompression method but
140 not calling the consume() method. This allows each bidder to
141 look ahead in the input stream. Bidders should not look further
142 ahead than necessary, as long look aheads put pressure on the
143 decompression layer to buffer lots of data. Most formats only
144 require a few hundred bytes of look ahead; look aheads of a few
145 kilobytes are reasonable. (The ISO9660 reader sometimes looks
146 ahead by 48k, which should be considered an upper limit.)
147 Read header
148 The header read is usually the most complex part of any format.
149 There are a few strategies worth mentioning: For formats such as
150 tar or cpio, reading and parsing the header is straightforward
151 since headers alternate with data. For formats that store all
152 header data at the beginning of the file, the first header read
153 request may have to read all headers into memory and store that
154 data, sorted by the location of the file data. Subsequent header
155 read requests will skip forward to the beginning of the file data
156 and return the corresponding header.
157 Read Data
158 The read data interface supports sparse files; this requires that
159 each call return a block of data specifying the file offset and
160 size. This may require you to carefully track the location so
161 that you can return accurate file offsets for each read. Remem‐
162 ber that the decompressor will return as much data as it has.
163 Generally, you will want to request one byte, examine the return
164 value to see how much data is available, and possibly trim that
165 to the amount you can use. You should invoke consume for each
166 block just before you return it.
167 Skip All Data
168 The skip data call should skip over all file data and trailing
169 padding. This is called automatically by the API layer just
170 before each header read. It is also called in response to the
171 client calling the public data_skip() function.
172 Cleanup
173 On cleanup, the format should release all of its allocated mem‐
174 ory.
175 API Layer
177 XXX to do XXX
178
180 The write API has a similar set of four layers: an API layer, a format
181 layer, a compression layer, and an I/O layer. The registration here is
182 much simpler because only one format and one compression can be regis‐
183 tered at a time.
184 I/O Layer and Client Callbacks
186 XXX To be written XXX
187 Compression Layer
189 XXX To be written XXX
190 Format Layer
192 XXX To be written XXX
193 API Layer
195 XXX To be written XXX
196
198 The write_disk API is intended to look just like the write API to
199 clients. Since it does not handle multiple formats or compression, it is
200 not layered internally.
201
203 The archive_read, archive_write, and archive_write_disk objects all con‐
204 tain an initial archive object which provides common support for a set of
205 standard services. (Recall that ANSI/ISO C90 guarantees that you can
206 cast freely between a pointer to a structure and a pointer to the first
207 element of that structure.) The archive object has a magic value that
208 indicates which API this object is associated with, slots for storing
209 error information, and function pointers for virtualized API functions.
210
212 Connecting existing archiving libraries into libarchive is generally
213 quite difficult. In particular, many existing libraries strongly assume
214 that you are reading from a file; they seek forwards and backwards as
215 necessary to locate various pieces of information. In contrast,
216 libarchive never seeks backwards in its input, which sometimes requires
217 very different approaches.
218 For example, libarchive's ISO9660 support operates very differently from
220 most ISO9660 readers. The libarchive support utilizes a work-queue
221 design that keeps a list of known entries sorted by their location in the
222 input. Whenever libarchive's ISO9660 implementation is asked for the
223 next header, checks this list to find the next item on the disk. Direc‐
224 tories are parsed when they are encountered and new items are added to
225 the list. This design relies heavily on the ISO9660 image being opti‐
226 mized so that directories always occur earlier on the disk than the files
227 they describe.
228 Depending on the specific format, such approaches may not be possible.
230 The ZIP format specification, for example, allows archivers to store key
231 information only at the end of the file. In theory, it is possible to
232 create ZIP archives that cannot be read without seeking. Fortunately,
233 such archives are very rare, and libarchive can read most ZIP archives,
234 though it cannot always extract as much information as a dedicated ZIP
235 program.
236
238 archive(3), archive_entry(3), archive_read(3), archive_write(3),
239 archive_write_disk(3)
240
242 The libarchive library first appeared in FreeBSD 5.3.
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245 The libarchive library was written by Tim Kientzle <kientzle@acm.org>.
246BSD January 26, 2011 BSD