freebsd-dev/lib/libarchive/libarchive.3

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.Dd July 17, 2010
.Dt LIBARCHIVE 3
.Os
.Sh NAME
.Nm libarchive
.Nd functions for reading and writing streaming archives
.Sh LIBRARY
.Lb libarchive
.Sh OVERVIEW
The
.Nm
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 modification.
.Pp
When reading an archive, the library automatically detects the
format and the compression.
The library currently has read support for:
.Bl -bullet -compact
.It
old-style tar archives,
.It
most variants of the POSIX
.Dq ustar
format,
.It
the POSIX
.Dq pax interchange
format,
.It
GNU-format tar archives,
.It
most common cpio archive formats,
.It
ISO9660 CD images (with or without RockRidge extensions),
.It
Zip archives.
.El
The library automatically detects archives compressed with
.Xr gzip 1 ,
.Xr bzip2 1 ,
or
.Xr compress 1
and decompresses them transparently.
.Pp
When writing an archive, you can specify the compression
to be used and the format to use.
The library can write
.Bl -bullet -compact
.It
POSIX-standard
.Dq ustar
archives,
.It
POSIX
.Dq pax interchange format
archives,
.It
POSIX octet-oriented cpio archives,
.It
two different variants of shar archives.
.El
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
.Xr pax 1
implementations on many systems as well as several newer implementations of
.Xr 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.
.Pp
The read and write APIs are accessed through the
.Fn archive_read_XXX
functions and the
.Fn archive_write_XXX
functions, respectively, and either can be used independently
of the other.
.Pp
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.
.Sh READING AN ARCHIVE
To read an archive, you must first obtain an initialized
.Tn struct archive
object from
.Fn archive_read_new .
You can then modify this object for the desired operations with the
various
.Fn archive_read_set_XXX
and
.Fn archive_read_support_XXX
functions.
In particular, you will need to invoke appropriate
.Fn 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 corresponding auto-detect code.
Unless you have specific constraints, you will generally want
to invoke
.Fn archive_read_support_compression_all
and
.Fn archive_read_support_format_all
to enable auto-detect for all formats and compression types
currently supported by the library.
.Pp
Once you have prepared the
.Tn struct archive
object, you call
.Fn archive_read_open
to actually open the archive and prepare it for reading.
There are several variants of this function;
the most basic expects you to provide pointers to several
functions that can provide blocks of bytes from the archive.
There are convenience forms that allow you to
specify a filename, file descriptor,
.Ft "FILE *"
object, or a block of memory from which to read the archive data.
Note that the core library makes no assumptions about the
size of the blocks read;
callback functions are free to read whatever block size is
most appropriate for the medium.
.Pp
Each archive entry consists of a header followed by a certain
amount of data.
You can obtain the next header with
.Fn archive_read_next_header ,
which returns a pointer to a
.Tn struct archive_entry
structure with information 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
.Fn archive_read_data
(which works much like the
.Xr read 2
system call)
to read this data from the archive.
You may prefer to use the higher-level
.Fn archive_read_data_skip ,
which reads and discards the data for this entry,
.Fn archive_read_data_to_buffer ,
which reads the data into an in-memory buffer,
.Fn archive_read_data_to_file ,
which copies the data to the provided file descriptor, or
.Fn archive_read_extract ,
which recreates the specified entry on disk and copies data
from the archive.
In particular, note that
.Fn archive_read_extract
uses the
.Tn 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.
.Pp
Once you have finished reading data from the archive, you
should call
.Fn archive_read_close
to close the archive, then call
.Fn archive_read_finish
to release all resources, including all memory allocated by the library.
.Pp
The
.Xr archive_read 3
manual page provides more detailed calling information for this API.
.Sh WRITING AN ARCHIVE
You use a similar process to write an archive.
The
.Fn archive_write_new
function creates an archive object useful for writing,
the various
.Fn archive_write_set_XXX
functions are used to set parameters for writing the archive, and
.Fn archive_write_open
completes the setup and opens the archive for writing.
.Pp
Individual archive entries are written in a three-step
process:
You first initialize a
.Tn struct archive_entry
structure with information about the new entry.
At a minimum, you should set the pathname of the
entry and provide a
.Va struct stat
with a valid
.Va st_mode
field, which specifies the type of object and
.Va st_size
field, which specifies the size of the data portion of the object.
The
.Fn archive_write_header
function actually writes the header data to the archive.
You can then use
.Fn archive_write_data
to write the actual data.
.Pp
After all entries have been written, use the
.Fn archive_write_finish
function to release all resources.
.Pp
The
.Xr archive_write 3
manual page provides more detailed calling information for this API.
.Sh DESCRIPTION
Detailed descriptions of each function are provided by the
corresponding manual pages.
.Pp
All of the functions utilize an opaque
.Tn struct archive
datatype that provides access to the archive contents.
.Pp
The
.Tn struct archive_entry
structure contains a complete description of a single archive
entry.
It uses an opaque interface that is fully documented in
.Xr archive_entry 3 .
.Pp
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
.Va PATH_MAX .
.Sh RETURN VALUES
Most functions return zero on success, non-zero on error.
The return value indicates the general severity of the error, ranging
from
.Cm ARCHIVE_WARN ,
which indicates a minor problem that should probably be reported
to the user, to
.Cm ARCHIVE_FATAL ,
which indicates a serious problem that will prevent any further
operations on this archive.
On error, the
.Fn archive_errno
function can be used to retrieve a numeric error code (see
.Xr errno 2 ) .
The
.Fn archive_error_string
returns a textual error message suitable for display.
.Pp
.Fn archive_read_new
and
.Fn archive_write_new
return pointers to an allocated and initialized
.Tn struct archive
object.
.Pp
.Fn archive_read_data
and
.Fn 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
.Fn archive_errno
and
.Fn archive_error_string
functions can be used to obtain more information.
.Sh ENVIRONMENT
There are character set conversions within the
.Xr archive_entry 3
functions that are impacted by the currently-selected locale.
.Sh SEE ALSO
.Xr tar 1 ,
.Xr archive_entry 3 ,
.Xr archive_read 3 ,
.Xr archive_util 3 ,
.Xr archive_write 3 ,
.Xr tar 5
.Sh HISTORY
The
.Nm libarchive
library first appeared in
.Fx 5.3 .
.Sh AUTHORS
.An -nosplit
The
.Nm libarchive
library was written by
.An Tim Kientzle Aq kientzle@acm.org .
.Sh BUGS
Some archive formats support information that is not supported by
.Tn 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.
.Pp
Conversely, of course, not all of the information that can be
stored in a
.Tn 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.