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* https://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* **unzip**: the 'bsdunzip' program is a simple replacement tool for Info-ZIP's unzip
27* **examples**: Some small example programs that you may find useful.
28* **examples/minitar**: a compact sample demonstrating use of libarchive.
29* **contrib**: Various items sent to me by third parties; please contact the authors with any questions.
30
31The top-level directory contains the following information files:
32
33* **NEWS** - highlights of recent changes
34* **COPYING** - what you can do with this
35* **INSTALL** - installation instructions
36* **README** - this file
37* **CMakeLists.txt** - input for "cmake" build tool, see INSTALL
38* **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`).
39
40The following files in the top-level directory are used by the 'configure' script:
41
42* `Makefile.am`, `aclocal.m4`, `configure.ac` - used to build this distribution, only needed by maintainers
43* `Makefile.in`, `config.h.in` - templates used by configure script
44
45## Documentation
46
47In addition to the informational articles and documentation
48in the online [libarchive Wiki](https://github.com/libarchive/libarchive/wiki),
49the distribution also includes a number of manual pages:
50
51 * bsdtar.1 explains the use of the bsdtar program
52 * bsdcpio.1 explains the use of the bsdcpio program
53 * bsdcat.1 explains the use of the bsdcat program
54 * libarchive.3 gives an overview of the library as a whole
55 * archive_read.3, archive_write.3, archive_write_disk.3, and
56 archive_read_disk.3 provide detailed calling sequences for the read
57 and write APIs
58 * archive_entry.3 details the "struct archive_entry" utility class
59 * archive_internals.3 provides some insight into libarchive's
60 internal structure and operation.
61 * libarchive-formats.5 documents the file formats supported by the library
62 * cpio.5, mtree.5, and tar.5 provide detailed information about these
63 popular archive formats, including hard-to-find details about
64 modern cpio and tar variants.
65
66The manual pages above are provided in the 'doc' directory in
67a number of different formats.
68
69You should also read the copious comments in `archive.h` and the
70source code for the sample programs for more details. Please let us
71know about any errors or omissions you find.
72
73## Supported Formats
74
75Currently, the library automatically detects and reads the following formats:
76
77 * Old V7 tar archives
78 * POSIX ustar
79 * GNU tar format (including GNU long filenames, long link names, and sparse files)
80 * Solaris 9 extended tar format (including ACLs)
81 * POSIX pax interchange format
82 * POSIX octet-oriented cpio
83 * SVR4 ASCII cpio
84 * Binary cpio (big-endian or little-endian)
85 * PWB binary cpio
86 * ISO9660 CD-ROM images (with optional Rockridge or Joliet extensions)
87 * ZIP archives (with uncompressed or "deflate" compressed entries, including support for encrypted Zip archives)
88 * ZIPX archives (with support for bzip2, zstd, ppmd8, lzma and xz compressed entries)
89 * GNU and BSD 'ar' archives
90 * 'mtree' format
91 * 7-Zip archives (including archives that use zstandard compression)
92 * Microsoft CAB format
93 * LHA and LZH archives
94 * RAR and RAR 5.0 archives (with some limitations due to RAR's proprietary status)
95 * WARC archives
96 * XAR archives
97
98The library also detects and handles any of the following before evaluating the archive:
99
100 * uuencoded files
101 * files with RPM wrapper
102 * gzip compression
103 * bzip2 compression
104 * compress/LZW compression
105 * lzma, lzip, and xz compression
106 * lz4 compression
107 * lzop compression
108 * zstandard compression
109
110The library can create archives in any of the following formats:
111
112 * POSIX ustar
113 * POSIX pax interchange format
114 * "restricted" pax format, which will create ustar archives except for
115 entries that require pax extensions (for long filenames, ACLs, etc).
116 * Old GNU tar format
117 * Old V7 tar format
118 * POSIX octet-oriented cpio
119 * SVR4 "newc" cpio
120 * Binary cpio (little-endian)
121 * PWB binary cpio
122 * shar archives
123 * ZIP archives (with uncompressed or "deflate" compressed entries)
124 * ZIPX archives (with bzip2, zstd, lzma or xz compressed entries)
125 * GNU and BSD 'ar' archives
126 * 'mtree' format
127 * ISO9660 format
128 * 7-Zip archives (including archives that use zstandard compression)
129 * WARC archives
130 * XAR archives
131
132When creating archives, the result can be filtered with any of the following:
133
134 * uuencode
135 * base64
136 * gzip compression
137 * bzip2 compression
138 * compress/LZW compression
139 * lzma, lzip, and xz compression
140 * lz4 compression
141 * lzop compression
142 * zstandard compression
143
144## Notes about the Library Design
145
146The following notes address many of the most common
147questions we are asked about libarchive:
148
149* This is a heavily stream-oriented system. That means that
150 it is optimized to read or write the archive in a single
151 pass from beginning to end. For example, this allows
152 libarchive to process archives too large to store on disk
153 by processing them on-the-fly as they are read from or
154 written to a network or tape drive. This also makes
155 libarchive useful for tools that need to produce
156 archives on-the-fly (such as webservers that provide
157 archived contents of a users account).
158
159* In-place modification and random access to the contents
160 of an archive are not directly supported. For some formats,
161 this is not an issue: For example, tar.gz archives are not
162 designed for random access. In some other cases, libarchive
163 can re-open an archive and scan it from the beginning quickly
164 enough to provide the needed abilities even without true
165 random access. Of course, some applications do require true
166 random access; those applications should consider alternatives
167 to libarchive.
168
169* The library is designed to be extended with new compression and
170 archive formats. The only requirement is that the format be
171 readable or writable as a stream and that each archive entry be
172 independent. There are articles on the libarchive Wiki explaining
173 how to extend libarchive.
174
175* On read, compression and format are always detected automatically.
176
177* The same API is used for all formats; it should be very
178 easy for software using libarchive to transparently handle
179 any of libarchive's archiving formats.
180
181* Libarchive's automatic support for decompression can be used
182 without archiving by explicitly selecting the "raw" and "empty"
183 formats.
184
185* I've attempted to minimize static link pollution. If you don't
186 explicitly invoke a particular feature (such as support for a
187 particular compression or format), it won't get pulled in to
188 statically-linked programs. In particular, if you don't explicitly
189 enable a particular compression or decompression support, you won't
190 need to link against the corresponding compression or decompression
191 libraries. This also reduces the size of statically-linked
192 binaries in environments where that matters.
193
194* The library is generally _thread safe_ depending on the platform:
195 it does not define any global variables of its own. However, some
196 platforms do not provide fully thread-safe versions of key C library
197 functions. On those platforms, libarchive will use the non-thread-safe
198 functions. Patches to improve this are of great interest to us.
199
200* The function `archive_write_disk_header()` is _not_ thread safe on
201 POSIX machines and could lead to security issue resulting in world
202 writeable directories. Thus it must be mutexed by the calling code.
203 This is due to calling `umask(oldumask = umask(0))`, which sets the
204 umask for the whole process to 0 for a short time frame.
205 In case other thread calls the same function in parallel, it might
206 get interrupted by it and cause the executable to use umask=0 for the
207 remaining execution.
208 This will then lead to implicitly created directories to have 777
209 permissions without sticky bit.
210
211* In particular, libarchive's modules to read or write a directory
212 tree do use `chdir()` to optimize the directory traversals. This
213 can cause problems for programs that expect to do disk access from
214 multiple threads. Of course, those modules are completely
215 optional and you can use the rest of libarchive without them.
216
217* The library is _not_ thread aware, however. It does no locking
218 or thread management of any kind. If you create a libarchive
219 object and need to access it from multiple threads, you will
220 need to provide your own locking.
221
222* On read, the library accepts whatever blocks you hand it.
223 Your read callback is free to pass the library a byte at a time
224 or mmap the entire archive and give it to the library at once.
225 On write, the library always produces correctly-blocked output.
226
227* The object-style approach allows you to have multiple archive streams
228 open at once. bsdtar uses this in its "@archive" extension.
229
230* The archive itself is read/written using callback functions.
231 You can read an archive directly from an in-memory buffer or
232 write it to a socket, if you wish. There are some utility
233 functions to provide easy-to-use "open file," etc, capabilities.
234
235* The read/write APIs are designed to allow individual entries
236 to be read or written to any data source: You can create
237 a block of data in memory and add it to a tar archive without
238 first writing a temporary file. You can also read an entry from
239 an archive and write the data directly to a socket. If you want
240 to read/write entries to disk, there are convenience functions to
241 make this especially easy.
242
243* Note: The "pax interchange format" is a POSIX standard extended tar
244 format that should be used when the older _ustar_ format is not
245 appropriate. It has many advantages over other tar formats
246 (including the legacy GNU tar format) and is widely supported by
247 current tar implementations.
248