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.
326 lines
9.8 KiB
Groff
326 lines
9.8 KiB
Groff
.\" Copyright (c) 2003-2004 Tim Kientzle
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd October 1, 2003
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.Dt LIBARCHIVE 3
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.Os
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.Sh NAME
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.Nm libarchive
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.Nd functions for reading and writing streaming archives
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.Sh LIBRARY
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.Lb libarchive
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.Sh OVERVIEW
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The
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.Nm
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library provides a flexible interface for reading and writing
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streaming archive files such as tar and cpio.
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The library is inherently stream-oriented; readers serially iterate through
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the archive, writers serially add things to the archive.
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In particular, note that there is no built-in support for
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random access nor for in-place modification.
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.Pp
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When reading an archive, the library automatically detects the
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format and the compression.
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The library currently has read support for:
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.Bl -bullet -compact
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.It
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old-style tar
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.It
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most variants of the POSIX
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.Dq ustar
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format,
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.It
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the POSIX
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.Dq pax interchange
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format,
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.It
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GNU-format tar archives,
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.It
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POSIX octet-oriented cpio archives.
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.El
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The library automatically detects archives compressed with
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.Xr gzip 1
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or
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.Xr bzip2 1
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and decompresses them transparently.
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.Pp
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When writing an archive, you can specify the compression
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to be used and the format to use.
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The library can write
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.Bl -bullet -compact
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.It
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POSIX-standard
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.Dq ustar
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archives,
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.It
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POSIX
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.Dq pax interchange format
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archives,
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.It
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POSIX octet-oriented cpio archives.
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.El
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The default write format is the pax interchange
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format.
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Pax interchange format is an extension of the tar archive format that
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eliminates essentially all of the limitations of historic tar formats
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in a standard fashion that is supported
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by POSIX-compliant
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.Xr pax 1
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implementations on many systems as well as several newer implementations of
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.Xr tar 1 .
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.Pp
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The read and write APIs are accessed through the
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.Fn archive_read_XXX
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functions and the
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.Fn archive_write_XXX
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functions, respectively, and either can be used independently
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of the other.
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.Pp
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The rest of this manual page provides an overview of the library
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operation.
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More detailed information can be found in the individual manual
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pages for each API or utility function.
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.Sh READING AN ARCHIVE
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To read an archive, you must first obtain an initialized
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.Tn struct archive
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object from
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.Fn archive_read_new .
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You can then modify this object for the desired operations with the
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various
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.Fn archive_read_set_XXX
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and
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.Fn archive_read_support_XXX
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functions.
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In particular, you will need to invoke appropriate
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.Fn archive_read_support_XXX
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functions to enable the corresponding compression and format
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support.
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Note that these latter functions perform two distinct operations:
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they cause the corresponding support code to be linked into your
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program, and they enable the corresponding auto-detect code.
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Unless you have specific constraints, you will generally want
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to invoke
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.Fn archive_read_support_compression_all
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and
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.Fn archive_read_support_format_all
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to enable auto-detect for all formats and compression types
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currently supported by the library.
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.Pp
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Once you have prepared the
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.Tn struct archive
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object, you call
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.Fn archive_read_open
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to actually open the archive and prepare it for reading.
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.Pp
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Each archive entry consists of a header followed by a certain
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amount of data.
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You can obtain the next header with
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.Fn archive_read_next_header ,
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which returns a pointer to an
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.Tn struct archive_entry
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structure with information about the current archive element.
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If the entry is a regular file, then the header will be followed
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by the file data.
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You can use
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.Fn archive_read_data
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(which works much like the
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.Xr read 2
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system call)
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to read this data from the archive.
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You may prefer to use the higher-level
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.Fn archive_read_data_skip ,
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which reads and discards the data for this entry,
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.Fn archive_read_data_to_buffer ,
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which reads the data into an in-memory buffer,
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.Fn archive_read_data_to_file ,
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which copies the data to the provided file descriptor, or
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.Fn archive_read_extract ,
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which recreates the specified entry on disk and copies data
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from the archive.
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In particular, note that
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.Fn archive_read_extract
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uses the
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.Tn struct archive_entry
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structure that you provide it, which may differ from the
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entry just read from the archive.
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In particular, many applications will want to override the
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pathname, file permissions, or ownership.
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.Pp
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Once you have finished reading data from the archive, you
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should call
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.Fn archive_read_finish
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to release all resources.
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In particular,
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.Fn archive_read_finish
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closes the archive and frees any memory that was allocated by the library.
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.Pp
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The
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.Xr archive_read 3
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manual page provides more detailed calling information for this API.
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.Sh WRITING AN ARCHIVE
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You use a similar process to write an archive.
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The
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.Fn archive_write_new
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function creates an archive object useful for writing,
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the various
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.Fn archive_write_set_XXX
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functions are used to set parameters for writing the archive, and
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.Fn archive_write_open
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completes the setup and opens the archive for writing.
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.Pp
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Individual archive entries are written in a three-step
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process:
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You first initialize a
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.Tn struct archive_entry
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structure with information about the new entry.
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At a minimum, you should set the pathname of the
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entry and provide a
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.Va struct stat
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with a valid
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.Va st_mode
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field, which specifies the type of object and
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.Va st_size
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field, which specifies the size of the data portion of the object.
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The
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.Fn archive_write_header
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function actually writes the header data to the archive.
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You can then use
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.Fn archive_write_data
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to write the actual data.
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.Pp
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After all entries have been written, use the
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.Fn archive_write_finish
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function to release all resources.
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.Pp
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The
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.Xr archive_write 3
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manual page provides more detailed calling information for this API.
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.Sh DESCRIPTION
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Detailed descriptions of each function are provided by the
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corresponding manual pages.
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.Pp
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All of the functions utilize an opaque
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.Tn struct archive
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datatype that provides access to the archive contents.
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.Pp
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The
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.Tn struct archive_entry
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structure contains a complete description of a single archive
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entry.
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It uses an opaque interface that is fully documented in
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.Xr archive_entry 3 .
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.Pp
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Users familiar with historic formats should be aware that the newer
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variants have eliminated most restrictions on the length of textual fields.
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Clients should not assume that filenames, link names, user names, or
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group names are limited in length.
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In particular, pax interchange format can easily accomodate pathnames
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that exceed
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.Va PATH_MAX .
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.Sh RETURN VALUES
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Most functions return zero on success, non-zero on error.
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On error, the
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.Fn archive_errno
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function can be used to retrieve a numeric error code (see
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.Xr errno 2 ) .
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The
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.Fn archive_error_string
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returns a textual error message suitable for display.
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.Pp
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.Fn archive_read_new
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and
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.Fn archive_write_new
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return pointers to an allocated and initialized
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.Tn struct archive
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object.
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.Pp
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.Fn archive_read_next_header
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returns a pointer to an
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.Tn struct archive_entry
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structure or
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.Dv NULL .
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If
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.Dv NULL
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is returned, the value from
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.Fn archive_errno
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will be zero if the end of the archive was reached,
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-1 if there was a recoverable error reading the archive,
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or positive if there was a non-recoverable error reading the archive.
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If there was a recoverable error, the client should retry the
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operation.
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.Pp
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.Fn archive_read_data
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and
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.Fn archive_write_data
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return a count of the number of bytes actually read or written.
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A value of zero indicates the end of the data for this entry.
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A negative value indicates an error, in which case the
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.Fn archive_errno
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and
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.Fn archive_error_string
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functions can be used to obtain more information.
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.Sh ENVIRONMENT
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The library currently obeys no environment variables.
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.Sh SEE ALSO
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.Xr tar 1 ,
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.Xr archive_entry 3 ,
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.Xr archive_read 3 ,
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.Xr archive_util 3 ,
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.Xr archive_write 3 ,
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.Xr tar 5 .
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.Sh HISTORY
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The
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.Nm libarchive
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library first appeared in
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.Fx 5.3 .
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.Sh AUTHORS
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.An -nosplit
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The
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.Nm libarchive
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library was written by
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.An Tim Kientzle Aq kientzle@acm.org .
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.Sh BUGS
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Some archive formats support information that is not supported by
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.Tn struct archive_entry
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and cannot therefore be archived or restored using this library.
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This includes, for example, comments, character sets, sparse
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file information, or the arbitrary key/value pairs that can appear in
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pax interchange format archives.
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.Pp
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Conversely, of course, not all of the information that can be
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stored in an
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.Tn struct archive_entry
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is supported by all formats.
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For example, cpio formats do not support nanosecond timestamps;
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old tar formats do not support large device numbers.
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.Pp
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The library does not have write support for pre-POSIX tar archives.
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The support for GNU tar format is incomplete.
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.Pp
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The library should obey the current locale and convert
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UTF8 filenames stored by pax interchange format to and from the
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currently-active character coding.
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