README.md
1# Welcome to libarchive!
2
3The libarchive project develops a portable, efficient C library that
4can read and write streaming archives in a variety of formats. It
5also includes implementations of the common `tar`, `cpio`, and `zcat`
6command-line tools that use the libarchive library.
7
8## Questions? Issues?
9
10* http://www.libarchive.org is the home for ongoing
11 libarchive development, including documentation,
12 and links to the libarchive mailing lists.
13* To report an issue, use the issue tracker at
14 https://github.com/libarchive/libarchive/issues
15* To submit an enhancement to libarchive, please
16 submit a pull request via GitHub: https://github.com/libarchive/libarchive/pulls
17
18## Contents of the Distribution
19
20This distribution bundle includes the following major components:
21
22* **libarchive**: a library for reading and writing streaming archives
23* **tar**: the 'bsdtar' program is a full-featured 'tar' implementation built on libarchive
24* **cpio**: the 'bsdcpio' program is a different interface to essentially the same functionality
25* **cat**: the 'bsdcat' program is a simple replacement tool for zcat, bzcat, xzcat, and such
26* **examples**: Some small example programs that you may find useful.
27* **examples/minitar**: a compact sample demonstrating use of libarchive.
28* **contrib**: Various items sent to me by third parties; please contact the authors with any questions.
29
30The top-level directory contains the following information files:
31
32* **NEWS** - highlights of recent changes
33* **COPYING** - what you can do with this
34* **INSTALL** - installation instructions
35* **README** - this file
36* **CMakeLists.txt** - input for "cmake" build tool, see INSTALL
37* **configure** - configuration script, see INSTALL for details. If your copy of the source lacks a `configure` script, you can try to construct it by running the script in `build/autogen.sh` (or use `cmake`).
38
39The following files in the top-level directory are used by the 'configure' script:
40* `Makefile.am`, `aclocal.m4`, `configure.ac` - used to build this distribution, only needed by maintainers
41* `Makefile.in`, `config.h.in` - templates used by configure script
42
43## Documentation
44
45In addition to the informational articles and documentation
46in the online [libarchive Wiki](https://github.com/libarchive/libarchive/wiki),
47the distribution also includes a number of manual pages:
48
49 * bsdtar.1 explains the use of the bsdtar program
50 * bsdcpio.1 explains the use of the bsdcpio program
51 * bsdcat.1 explains the use of the bsdcat program
52 * libarchive.3 gives an overview of the library as a whole
53 * archive_read.3, archive_write.3, archive_write_disk.3, and
54 archive_read_disk.3 provide detailed calling sequences for the read
55 and write APIs
56 * archive_entry.3 details the "struct archive_entry" utility class
57 * archive_internals.3 provides some insight into libarchive's
58 internal structure and operation.
59 * libarchive-formats.5 documents the file formats supported by the library
60 * cpio.5, mtree.5, and tar.5 provide detailed information about these
61 popular archive formats, including hard-to-find details about
62 modern cpio and tar variants.
63
64The manual pages above are provided in the 'doc' directory in
65a number of different formats.
66
67You should also read the copious comments in `archive.h` and the
68source code for the sample programs for more details. Please let us
69know about any errors or omissions you find.
70
71## Supported Formats
72
73Currently, the library automatically detects and reads the following fomats:
74 * Old V7 tar archives
75 * POSIX ustar
76 * GNU tar format (including GNU long filenames, long link names, and sparse files)
77 * Solaris 9 extended tar format (including ACLs)
78 * POSIX pax interchange format
79 * POSIX octet-oriented cpio
80 * SVR4 ASCII cpio
81 * POSIX octet-oriented cpio
82 * Binary cpio (big-endian or little-endian)
83 * ISO9660 CD-ROM images (with optional Rockridge or Joliet extensions)
84 * ZIP archives (with uncompressed or "deflate" compressed entries, including support for encrypted Zip archives)
85 * GNU and BSD 'ar' archives
86 * 'mtree' format
87 * 7-Zip archives
88 * Microsoft CAB format
89 * LHA and LZH archives
90 * RAR archives (with some limitations due to RAR's proprietary status)
91 * XAR archives
92
93The library also detects and handles any of the following before evaluating the archive:
94 * uuencoded files
95 * files with RPM wrapper
96 * gzip compression
97 * bzip2 compression
98 * compress/LZW compression
99 * lzma, lzip, and xz compression
100 * lz4 compression
101 * lzop compression
102
103The library can create archives in any of the following formats:
104 * POSIX ustar
105 * POSIX pax interchange format
106 * "restricted" pax format, which will create ustar archives except for
107 entries that require pax extensions (for long filenames, ACLs, etc).
108 * Old GNU tar format
109 * Old V7 tar format
110 * POSIX octet-oriented cpio
111 * SVR4 "newc" cpio
112 * shar archives
113 * ZIP archives (with uncompressed or "deflate" compressed entries)
114 * GNU and BSD 'ar' archives
115 * 'mtree' format
116 * ISO9660 format
117 * 7-Zip archives
118 * XAR archives
119
120When creating archives, the result can be filtered with any of the following:
121 * uuencode
122 * gzip compression
123 * bzip2 compression
124 * compress/LZW compression
125 * lzma, lzip, and xz compression
126 * lz4 compression
127 * lzop compression
128
129## Notes about the Library Design
130
131The following notes address many of the most common
132questions we are asked about libarchive:
133
134* This is a heavily stream-oriented system. That means that
135 it is optimized to read or write the archive in a single
136 pass from beginning to end. For example, this allows
137 libarchive to process archives too large to store on disk
138 by processing them on-the-fly as they are read from or
139 written to a network or tape drive. This also makes
140 libarchive useful for tools that need to produce
141 archives on-the-fly (such as webservers that provide
142 archived contents of a users account).
143
144* In-place modification and random access to the contents
145 of an archive are not directly supported. For some formats,
146 this is not an issue: For example, tar.gz archives are not
147 designed for random access. In some other cases, libarchive
148 can re-open an archive and scan it from the beginning quickly
149 enough to provide the needed abilities even without true
150 random access. Of course, some applications do require true
151 random access; those applications should consider alternatives
152 to libarchive.
153
154* The library is designed to be extended with new compression and
155 archive formats. The only requirement is that the format be
156 readable or writable as a stream and that each archive entry be
157 independent. There are articles on the libarchive Wiki explaining
158 how to extend libarchive.
159
160* On read, compression and format are always detected automatically.
161
162* The same API is used for all formats; in particular, it's very
163 easy for software using libarchive to transparently handle
164 any of libarchive's archiving formats.
165
166* Libarchive's automatic support for decompression can be used
167 without archiving by explicitly selecting the "raw" and "empty"
168 formats.
169
170* I've attempted to minimize static link pollution. If you don't
171 explicitly invoke a particular feature (such as support for a
172 particular compression or format), it won't get pulled in to
173 statically-linked programs. In particular, if you don't explicitly
174 enable a particular compression or decompression support, you won't
175 need to link against the corresponding compression or decompression
176 libraries. This also reduces the size of statically-linked
177 binaries in environments where that matters.
178
179* The library is generally _thread safe_ depending on the platform:
180 it does not define any global variables of its own. However, some
181 platforms do not provide fully thread-safe versions of key C library
182 functions. On those platforms, libarchive will use the non-thread-safe
183 functions. Patches to improve this are of great interest to us.
184
185* In particular, libarchive's modules to read or write a directory
186 tree do use `chdir()` to optimize the directory traversals. This
187 can cause problems for programs that expect to do disk access from
188 multiple threads. Of course, those modules are completely
189 optional and you can use the rest of libarchive without them.
190
191* The library is _not_ thread aware, however. It does no locking
192 or thread management of any kind. If you create a libarchive
193 object and need to access it from multiple threads, you will
194 need to provide your own locking.
195
196* On read, the library accepts whatever blocks you hand it.
197 Your read callback is free to pass the library a byte at a time
198 or mmap the entire archive and give it to the library at once.
199 On write, the library always produces correctly-blocked output.
200
201* The object-style approach allows you to have multiple archive streams
202 open at once. bsdtar uses this in its "@archive" extension.
203
204* The archive itself is read/written using callback functions.
205 You can read an archive directly from an in-memory buffer or
206 write it to a socket, if you wish. There are some utility
207 functions to provide easy-to-use "open file," etc, capabilities.
208
209* The read/write APIs are designed to allow individual entries
210 to be read or written to any data source: You can create
211 a block of data in memory and add it to a tar archive without
212 first writing a temporary file. You can also read an entry from
213 an archive and write the data directly to a socket. If you want
214 to read/write entries to disk, there are convenience functions to
215 make this especially easy.
216
217* Note: The "pax interchange format" is a POSIX standard extended tar
218 format that should be used when the older _ustar_ format is not
219 appropriate. It has many advantages over other tar formats
220 (including the legacy GNU tar format) and is widely supported by
221 current tar implementations.
222
223