libarchive —
functions for reading and writing streaming
archives
The
libarchive library provides a flexible
interface for reading and writing archives in various formats such as tar and
cpio.
libarchive also supports reading and
writing archives compressed using various compression filters such as gzip and
bzip2. The library is inherently stream-oriented; readers serially iterate
through the archive, writers serially add things to the archive. In
particular, note that there is currently no built-in support for random access
nor for in-place modification.
When reading an archive, the library automatically detects the format and the
compression. The library currently has read support for:
- old-style tar archives,
- most variants of the POSIX “ustar”
format,
- the POSIX “pax interchange” format,
- GNU-format tar archives,
- most common cpio archive formats,
- 7-Zip archives,
- ar archives (including GNU/SysV and BSD
extensions),
- Microsoft CAB archives,
- ISO9660 CD images (including RockRidge and Joliet
extensions),
- LHA archives,
- mtree file tree descriptions,
- RAR and most RAR5 archives,
- WARC archives,
- XAR archives,
- Zip archives.
The library automatically detects archives compressed with
compress(1),
bzip2(1),
grzip(1),
gzip(1),
lrzip(1),
lz4(1),
lzip(1),
lzop(1),
xz(1), or
zstd(1) and decompresses them transparently.
Decompression of some formats requires external decompressor utilities. It can
similarly detect and decode archives processed with
uuencode(1) or which have an
rpm(1) header.
When writing an archive, you can specify the compression to be used and the
format to use. The library can write
- POSIX-standard “ustar” archives,
- POSIX “pax interchange format”
archives,
- cpio archives,
- 7-Zip archives,
- ar archives,
- two different variants of shar archives,
- ISO9660 CD images,
- mtree file tree descriptions,
- XAR archives,
- Zip archive.
Pax interchange format is an extension of the tar archive format that eliminates
essentially all of the limitations of historic tar formats in a standard
fashion that is supported by POSIX-compliant
pax(1) implementations on many systems as well as
several newer implementations of
tar(1). Note
that the default write format will suppress the pax extended attributes for
most entries; explicitly requesting pax format will enable those attributes
for all entries.
The read and write APIs are accessed through the
archive_read_XXX() functions and the
archive_write_XXX() functions, respectively, and
either can be used independently of the other.
The rest of this manual page provides an overview of the library operation. More
detailed information can be found in the individual manual pages for each API
or utility function.
See
archive_read(3).
See
archive_write(3).
The
archive_write_disk(3) API allows you to write
archive_entry(3) objects to disk using the same
API used by
archive_write(3). The
archive_write_disk(3) API is used internally by
archive_read_extract(); using it directly can
provide greater control over how entries get written to disk. This API also
makes it possible to share code between archive-to-archive copy and
archive-to-disk extraction operations.
The
archive_read_disk(3) supports for populating
archive_entry(3) objects from information in the
filesystem. This includes the information accessible from the
stat(2) system call as well as ACLs, extended
attributes, and other metadata. The
archive_read_disk(3) API also supports iterating
over directory trees, which allows directories of files to be read using an
API compatible with the
archive_read(3) API.
Detailed descriptions of each function are provided by the corresponding manual
pages.
All of the functions utilize an opaque struct archive datatype that provides
access to the archive contents.
The struct archive_entry structure contains a complete description of a single
archive entry. It uses an opaque interface that is fully documented in
archive_entry(3).
Users familiar with historic formats should be aware that the newer variants
have eliminated most restrictions on the length of textual fields. Clients
should not assume that filenames, link names, user names, or group names are
limited in length. In particular, pax interchange format can easily
accommodate pathnames in arbitrary character sets that exceed
PATH_MAX.
Most functions return
ARCHIVE_OK (zero) on success,
non-zero on error. The return value indicates the general severity of the
error, ranging from
ARCHIVE_WARN, which indicates
a minor problem that should probably be reported to the user, to
ARCHIVE_FATAL, which indicates a serious problem
that will prevent any further operations on this archive. On error, the
archive_errno() function can be used to retrieve
a numeric error code (see
errno(2)). The
archive_error_string() returns a textual error
message suitable for display.
archive_read_new() and
archive_write_new() return pointers to an
allocated and initialized struct archive object.
archive_read_data() and
archive_write_data() return a count of the number
of bytes actually read or written. A value of zero indicates the end of the
data for this entry. A negative value indicates an error, in which case the
archive_errno() and
archive_error_string() functions can be used to
obtain more information.
There are character set conversions within the
archive_entry(3) functions that are impacted by
the currently-selected locale.
tar(1),
archive_entry(3),
archive_read(3),
archive_util(3),
archive_write(3),
tar(5)
The
libarchive library first appeared in
FreeBSD 5.3.
The
libarchive library was originally written by
Tim Kientzle
⟨
[email protected]⟩.
Some archive formats support information that is not supported by struct
archive_entry. Such information cannot be fully archived or restored using
this library. This includes, for example, comments, character sets, or the
arbitrary key/value pairs that can appear in pax interchange format archives.
Conversely, of course, not all of the information that can be stored in an
struct archive_entry is supported by all formats. For example, cpio formats do
not support nanosecond timestamps; old tar formats do not support large device
numbers.
The ISO9660 reader cannot yet read all ISO9660 images; it should learn how to
seek.
The AR writer requires the client program to use two passes, unlike all other
libarchive writers.