2710e4d1ef
What it is:
A library for reading and writing various streaming archive
formats, especially tar and cpio. Being a library, it should
be easy to incorporate into pkg_* tools, sysinstall, and any
other place that needs to read or write such archives.
Features:
* Full automatic detection of both compression and archive format.
* Extensible internal architecture to make it easy to add new formats.
* Support for "pax interchange format," a new POSIX-standard tar format
that eliminates essentially all of the restrictions of historic formats.
* BSD license
Thanks to: jkh for pushing me to start this work, gordon for
encouraging me to commit it, bde for answering endless style
questions, and many others for feedback and encouragement.
Status: Pretty good overall, though there are still a few rough edges and
the library could always use more testing. Feedback eagerly solicited.
91 lines
4.4 KiB
Plaintext
91 lines
4.4 KiB
Plaintext
$FreeBSD$
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libarchive: a library for reading and writing streaming archives
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This is all under a BSD license. Use, enjoy, but don't blame me if it breaks!
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As of February, 2004, the library proper is fairly complete and compiles
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cleanly on FreeBSD 5-CURRENT. The API should be stable now.
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Documentation:
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* libarchive(3) gives an overview of the library as a whole
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* archive_read(3) and archive_write(3) provide detailed calling
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sequences for the read and write APIs
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* archive_entry(3) details the "struct archive_entry" utility class
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* tar(5) documents the "tar" file formats supported by the library
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You should also read the copious comments in "archive.h" and the source
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code for the sample "bsdtar" program for more details. Please let me know
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about any errors or omissions you find. (In particular, I no doubt missed
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a few things when researching the tar(5) page.)
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Notes:
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* This is a heavily stream-oriented system. There is no direct
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support for in-place modification or random access and no intention
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of ever adding such support. Adding such support would require
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sacrificing a lot of other features, so don't bother asking.
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* The library is designed to be extended with new compression and
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archive formats. The only requirement is that the format be
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readable or writable as a stream and that each archive entry be
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independent. For example, zip archives can't be written as a
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stream because they require the compressed size of the data as part
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of the file header. Similarly, some file attributes for zip
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archives can't be extracted when streaming because those attributes
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are only stored in the end-of-archive central directory and thus
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aren't available when the corresponding entry is actually
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extracted.
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* Under certain circumstances, you can append entries to an archive
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by opening the file for reading, skimming to the end of the archive,
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noting the file location, then opening it for write with a custom write
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callback that seeks to the appropriate position before writing. Be
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sure to not enable any compression support if you do this!
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* Compression and blocking are handled implicitly and, as far as
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possible, transparently. All archive I/O is correctly blocked, even if
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it's compressed. On read, the compression format is detected
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automatically and the appropriate decompressor is invoked.
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* It should be easy to implement a system that reads one
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archive and writes entries to another archive, omitting
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or adding entries as appropriate along the way. This permits
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"re-writing" of archive streams in lieu of in-place modification.
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bsdtar has some code to demonstrate this.
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* The archive itself is read/written using callback functions.
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You can read an archive directly from an in-memory buffer or
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write it to a socket, if you wish. There are some utility
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functions to provide easy-to-use "open file," etc, capabilities.
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* The read/write APIs are designed to allow individual entries
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to be read or written to any data source: You can create
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a block of data in memory and add it to a tar archive without
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first writing a temporary file. You can also read an entry from
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an archive and write the data directly to a socket. If you want
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to read/write entries to disk, there are convenience functions to
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make this especially easy.
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* Read supports most common tar formats, including GNU tar,
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POSIX-compliant "ustar interchange format", and the
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shiny-and-improved POSIX "pax extended interchange format." The
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pax format, in particular, eliminates most of the traditional tar
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limitations in a standard way that is increasingly well supported.
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(GNU tar notably does not support "pax interchange format"; the
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GPL-licensed 'star' archiver does, however.) GNU format is only
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incompletely supported at this time; if you really need GNU-format
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sparse file support, volume headers, or GNU-format split archives,
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let me know.
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There's also support for a grab-bag of non-tar formats, including
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POSIX cpio and shar.
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* When writing tar formats, consider using "pax restricted" format
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by default. This avoids the pax extensions whenever it can, enabling
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them only on entries that cannot be correctly archived with ustar
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format. Thus, you get the broad compatibility of ustar with the
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safety of pax's support for very long filenames, etc.
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* Note: "pax interchange format" is really an extended tar format,
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despite what the name says.
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