Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
LIBARCHIVE(3)          FreeBSD Library Functions Manual          LIBARCHIVE(3)

     libarchive -- functions for reading and writing streaming archives

     Streaming Archive Library (libarchive, -larchive)

     The libarchive library provides a flexible interface for reading and
     writing streaming archive files such as tar and cpio.  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 no built-in support for random access nor for in-place modifica-

     When reading an archive, the library automatically detects the format and
     the compression.  The library currently has read support for:
     o   old-style tar
     o   most variants of the POSIX ``ustar'' format,
     o   the POSIX ``pax interchange'' format,
     o   GNU-format tar archives,
     o   POSIX octet-oriented cpio archives.
     The library automatically detects archives compressed with gzip(1),
     bzip2(1), or compress(1) and decompresses them transparently.

     When writing an archive, you can specify the compression to be used and
     the format to use.  The library can write
     o   POSIX-standard ``ustar'' archives,
     o   POSIX ``pax interchange format'' archives,
     o   POSIX octet-oriented cpio archives,
     o   two different variants of shar 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) implemen-
     tations 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() func-
     tions 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 opera-
     tion.  More detailed information can be found in the individual manual
     pages for each API or utility function.

     To read an archive, you must first obtain an initialized struct archive
     object from archive_read_new().  You can then modify this object for the
     desired operations with the various archive_read_set_XXX() and
     archive_read_support_XXX() functions.  In particular, you will need to
     invoke appropriate archive_read_support_XXX() functions to enable the
     corresponding compression and format support.  Note that these latter
     functions perform two distinct operations: they cause the corresponding
     support code to be linked into your program, and they enable the corre-
     sponding auto-detect code.  Unless you have specific constraints, you
     will generally want to invoke archive_read_support_compression_all() and
     archive_read_support_format_all() to enable auto-detect for all formats
     and compression types currently supported by the library.

     Once you have prepared the struct archive object, you call
     archive_read_open() to actually open the archive and prepare it for read-

     Each archive entry consists of a header followed by a certain amount of
     data.  You can obtain the next header with archive_read_next_header(),
     which returns a pointer to an struct archive_entry structure with infor-
     mation about the current archive element.  If the entry is a regular
     file, then the header will be followed by the file data.  You can use
     archive_read_data() (which works much like the read(2) system call) to
     read this data from the archive.  You may prefer to use the higher-level
     archive_read_data_skip(), which reads and discards the data for this
     entry, archive_read_data_to_buffer(), which reads the data into an in-
     memory buffer, archive_read_data_to_file(), which copies the data to the
     provided file descriptor, or archive_read_extract(), which recreates the
     specified entry on disk and copies data from the archive.  In particular,
     note that archive_read_extract() uses the struct archive_entry structure
     that you provide it, which may differ from the entry just read from the
     archive.  In particular, many applications will want to override the
     pathname, file permissions, or ownership.

     Once you have finished reading data from the archive, you should call
     archive_read_finish() to release all resources.  In particular,
     archive_read_finish() closes the archive and frees any memory that was
     allocated by the library.

     The archive_read(3) manual page provides more detailed calling informa-
     tion for this API.

     You use a similar process to write an archive.  The archive_write_new()
     function creates an archive object useful for writing, the various
     archive_write_set_XXX() functions are used to set parameters for writing
     the archive, and archive_write_open() completes the setup and opens the
     archive for writing.

     Individual archive entries are written in a three-step process: You first
     initialize a struct archive_entry structure with information about the
     new entry.  At a minimum, you should set the pathname of the entry and
     provide a struct stat with a valid st_mode field, which specifies the
     type of object and st_size field, which specifies the size of the data
     portion of the object.  The archive_write_header() function actually
     writes the header data to the archive.  You can then use
     archive_write_data() to write the actual data.

     After all entries have been written, use the archive_write_finish() func-
     tion to release all resources.

     The archive_write(3) manual page provides more detailed calling informa-
     tion for this API.

     Detailed descriptions of each function are provided by the corresponding
     manual pages.

     All of the functions utilize an opaque struct archive datatype that pro-
     vides 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 docu-
     mented in archive_entry(3).

     Users familiar with historic formats should be aware that the newer vari-
     ants 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 accomodate pathnames in arbitrary character sets that exceed

     Most functions return zero on success, non-zero on error.  The return
     value indicates the general severity of the error, ranging from
     ARCHIVE_WARNING, 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 mes-
     sage suitable for display.

     archive_read_new() and archive_write_new() return pointers to an allo-
     cated 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 written by Tim Kientzle <>.

     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 sup-
     port large device numbers.

FreeBSD 6.2                     January 8, 2005                    FreeBSD 6.2


Want to link to this manual page? Use this URL:

home | help