NAME
libarchive —
functions for reading and
writing streaming archives
OVERVIEW
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,
- ISO9660 CD images (including
RockRidge and Joliet extensions),
- Zip archives,
- ar archives (including
GNU/SysV and BSD extensions),
- Microsoft CAB archives,
- LHA archives,
- mtree file tree
descriptions,
- RAR archives,
- XAR archives.
The library automatically detects archives compressed with
gzip(1),
bzip2(1),
xz(1),
lzip(1), or
compress(1) and decompresses
them transparently. 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,
- POSIX octet-oriented cpio
archives,
- Zip archive,
- two different variants of
shar archives,
- ISO9660 CD images,
- 7-Zip archives,
- ar archives,
- mtree file tree
descriptions,
- XAR archives.
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.
READING AN ARCHIVE
See
archive_read(3).
WRITING AN ARCHIVE
See
archive_write(3).
WRITING ENTRIES TO DISK
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.
READING ENTRIES FROM DISK
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.
DESCRIPTION
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.
RETURN VALUES
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.
ENVIRONMENT
There are character set conversions within the
archive_entry(3)
functions that are impacted by the currently-selected locale.
SEE ALSO
tar(1),
archive_entry(3),
archive_read(3),
archive_util(3),
archive_write(3),
tar(5)
HISTORY
The
libarchive library first appeared in
FreeBSD 5.3.
AUTHORS
The
libarchive library was originally written by
Tim Kientzle ⟨kientzle@acm.org⟩.
BUGS
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.