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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 formats:
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  * Binary cpio (big-endian or little-endian)
82  * ISO9660 CD-ROM images (with optional Rockridge or Joliet extensions)
83  * ZIP archives (with uncompressed or "deflate" compressed entries, including support for encrypted Zip archives)
84  * ZIPX archives (with support for bzip2, ppmd8, lzma and xz compressed entries)
85  * GNU and BSD 'ar' archives
86  * 'mtree' format
87  * 7-Zip archives
88  * Microsoft CAB format
89  * LHA and LZH archives
90  * RAR and RAR 5.0 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  * zstandard compression
103
104The library can create archives in any of the following formats:
105  * POSIX ustar
106  * POSIX pax interchange format
107  * "restricted" pax format, which will create ustar archives except for
108    entries that require pax extensions (for long filenames, ACLs, etc).
109  * Old GNU tar format
110  * Old V7 tar format
111  * POSIX octet-oriented cpio
112  * SVR4 "newc" cpio
113  * shar archives
114  * ZIP archives (with uncompressed or "deflate" compressed entries)
115  * GNU and BSD 'ar' archives
116  * 'mtree' format
117  * ISO9660 format
118  * 7-Zip archives
119  * XAR archives
120
121When creating archives, the result can be filtered with any of the following:
122  * uuencode
123  * gzip compression
124  * bzip2 compression
125  * compress/LZW compression
126  * lzma, lzip, and xz compression
127  * lz4 compression
128  * lzop compression
129  * zstandard compression
130
131## Notes about the Library Design
132
133The following notes address many of the most common
134questions we are asked about libarchive:
135
136* This is a heavily stream-oriented system.  That means that
137  it is optimized to read or write the archive in a single
138  pass from beginning to end.  For example, this allows
139  libarchive to process archives too large to store on disk
140  by processing them on-the-fly as they are read from or
141  written to a network or tape drive.  This also makes
142  libarchive useful for tools that need to produce
143  archives on-the-fly (such as webservers that provide
144  archived contents of a users account).
145
146* In-place modification and random access to the contents
147  of an archive are not directly supported.  For some formats,
148  this is not an issue: For example, tar.gz archives are not
149  designed for random access.  In some other cases, libarchive
150  can re-open an archive and scan it from the beginning quickly
151  enough to provide the needed abilities even without true
152  random access.  Of course, some applications do require true
153  random access; those applications should consider alternatives
154  to libarchive.
155
156* The library is designed to be extended with new compression and
157  archive formats.  The only requirement is that the format be
158  readable or writable as a stream and that each archive entry be
159  independent.  There are articles on the libarchive Wiki explaining
160  how to extend libarchive.
161
162* On read, compression and format are always detected automatically.
163
164* The same API is used for all formats; it should be very
165  easy for software using libarchive to transparently handle
166  any of libarchive's archiving formats.
167
168* Libarchive's automatic support for decompression can be used
169  without archiving by explicitly selecting the "raw" and "empty"
170  formats.
171
172* I've attempted to minimize static link pollution.  If you don't
173  explicitly invoke a particular feature (such as support for a
174  particular compression or format), it won't get pulled in to
175  statically-linked programs.  In particular, if you don't explicitly
176  enable a particular compression or decompression support, you won't
177  need to link against the corresponding compression or decompression
178  libraries.  This also reduces the size of statically-linked
179  binaries in environments where that matters.
180
181* The library is generally _thread safe_ depending on the platform:
182  it does not define any global variables of its own.  However, some
183  platforms do not provide fully thread-safe versions of key C library
184  functions.  On those platforms, libarchive will use the non-thread-safe
185  functions.  Patches to improve this are of great interest to us.
186
187* In particular, libarchive's modules to read or write a directory
188  tree do use `chdir()` to optimize the directory traversals.  This
189  can cause problems for programs that expect to do disk access from
190  multiple threads.  Of course, those modules are completely
191  optional and you can use the rest of libarchive without them.
192
193* The library is _not_ thread aware, however.  It does no locking
194  or thread management of any kind.  If you create a libarchive
195  object and need to access it from multiple threads, you will
196  need to provide your own locking.
197
198* On read, the library accepts whatever blocks you hand it.
199  Your read callback is free to pass the library a byte at a time
200  or mmap the entire archive and give it to the library at once.
201  On write, the library always produces correctly-blocked output.
202
203* The object-style approach allows you to have multiple archive streams
204  open at once.  bsdtar uses this in its "@archive" extension.
205
206* The archive itself is read/written using callback functions.
207  You can read an archive directly from an in-memory buffer or
208  write it to a socket, if you wish.  There are some utility
209  functions to provide easy-to-use "open file," etc, capabilities.
210
211* The read/write APIs are designed to allow individual entries
212  to be read or written to any data source:  You can create
213  a block of data in memory and add it to a tar archive without
214  first writing a temporary file.  You can also read an entry from
215  an archive and write the data directly to a socket.  If you want
216  to read/write entries to disk, there are convenience functions to
217  make this especially easy.
218
219* Note: The "pax interchange format" is a POSIX standard extended tar
220  format that should be used when the older _ustar_ format is not
221  appropriate.  It has many advantages over other tar formats
222  (including the legacy GNU tar format) and is widely supported by
223  current tar implementations.
224
225