path: root/doc/libextractor.texi

\input texinfo                  @c -*- Texinfo -*-
@c % The structure of this document is based on the
@c % Texinfo manual from libgcrypt by Werner Koch and 
@c % and Moritz Schulte.
@c %**start of header
@setfilename libextractor.info
@include version.texi
@settitle The GNU libextractor Reference Manual
@c Unify all the indices into concept index.
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@copying
This manual is for GNU libextractor
(version @value{VERSION}, @value{UPDATED}), a library for metadata
extraction.

Copyright @copyright{} 2007, 2010, 2012 Christian Grothoff

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts.  A copy of the license is included in the section entitled ``GNU
Free Documentation License''.
@end quotation
@end copying

@dircategory Software libraries
@direntry
* Libextractor: (libextractor).    Metadata extraction library.
@end direntry



@c
@c Titlepage
@c
@titlepage
@title The GNU libextractor Reference Manual
@subtitle Version @value{VERSION}
@subtitle @value{UPDATED}
@author Christian Grothoff (@email{christian@@grothoff.org})

@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@summarycontents
@contents


@ifnottex
@node Top
@top The GNU libextractor Reference Manual
@insertcopying
@end ifnottex

@menu
* Introduction::                 What is GNU libextractor.
* Preparation::                  What you should do before using the library.
* Generalities::                 General library functions and data types.
* Extracting meta data::         How to use GNU libextractor to obtain meta data.
* Language bindings::            How to use GNU libextractor from languages other than C.
* Utility functions::            Utility functions of GNU libextractor.
* Existing Plugins::             What plugins are available.
* Writing new Plugins::          How to write new plugins for GNU libextractor.
* Internal utility functions::   Utility functions of GNU libextractor for writing plugins.
* Reporting bugs::               How to report bugs or request new features.

Appendices

* GNU Free Documentation License::  Copying this manual.

Indices

* Index::                       Index
@c * Function and Data Index::     Index of functions, variables and data types.
@c * Type Index::                  Index of data types.

@end menu



@c **********************************************************
@c *******************  Introduction  ***********************
@c **********************************************************
@node Introduction
@chapter Introduction

@cindex error handling
GNU libextractor is GNU's library for extracting meta data from
files.  Meta data includes format information (such as mime type,
image dimensions, color depth, recording frequency), content
descriptions (such as document title or document description) and
copyright information (such as license, author and contributors).
Meta data extraction is an inherently uncertain business --- a parse
error can be a corrupt file, an incompatibility in the file format
version, an entirely different file format or a bug in the parser.  As
a result of this uncertainty, GNU libextractor deliberately
avoids to ever report any errors.  Unexpected file contents simply
result in less or possibly no meta data being extracted.  

@cindex plugin
GNU libextractor uses plugins to handle various file formats.
Technically a plugin can support multiple file formats; however, most
plugins only support one particular format.  By default,
GNU libextractor will use all plugins that are available and found
in the plugin installation directory.  Applications can
request the use of only specific plugins or the exclusion of
certain plugins.

GNU libextractor is distributed with the @command{extract} 
command@footnote{Some distributions ship @command{extract} in a
seperate package.} which is a command-line tool for extracting
meta data.  @command{extract} is given a list of filenames and 
prints the resulting meta data to the console.  The @command{extract}
source code also serves as an advanced example for how to use
GNU libextractor.  

This manual focuses on providing documentation for writing software
with GNU libextractor.  The only relevant parts for end-users
are the chapter on compiling and installing GNU libextractor
(@xref{Preparation}.).  Also, the chapter on existing plugins maybe of
interest (@xref{Existing Plugins}.).  Additional documentation for
end-users can be find in the man page on @command{extract} (using
@verb{|man extract|}).

@cindex license
GNU libextractor is licensed under the GNU General Public License,
specifically, since version 0.7, GNU libextractor is licensed under GPLv3
@emph{or any later version}.

@node Preparation
@chapter Preparation

This chapter first describes the general build instructions that
should apply to all systems.  Specific instructions for known problems
for particular platforms are then described in individual sections
afterwards.

Compiling GNU libextractor follows the standard GNU autotools build process
using @command{configure} and @command{make}.  For details on the GNU
autotools build process, read the @file{INSTALL} file and query
@verb{|./configure --help|} for additional options.  

GNU libextractor has various dependencies, most of which are optional. 
Instead of specifying the names of the software packages, we
will give the list in terms of the names of the respective
Debian (wheezy) packages that should be installed.

You absolutely need:

@itemize @bullet
@item
libtool
@item
gcc
@item
make
@item
g++ 
@item
libltdl7-dev
@end itemize

Recommended dependencies are:
@itemize @bullet
@item
zlib1g-dev
@item
libbz2-dev
@item
libgif-dev
@item
libvorbis-dev
@item
libflac-dev
@item
libmpeg2-4-dev
@item
librpm-dev
@item
libgtk2.0-dev or libgtk3.0-dev
@item
libgsf-1-dev
@item
libqt4-dev
@item
libpoppler-dev
@item
libexiv2-dev
@item
libavformat-dev
@item
libswscale-dev
@item
libgstreamer1.0-dev
@end itemize

For Subversion access and compilation one also needs:
@itemize @bullet
@item
subversion
@item
autoconf
@item
automake
@end itemize

Please notify us if we missed some dependencies (note that the list is
supposed to only list direct dependencies, not transitive
dependencies).

Once you have compiled and installed GNU libextractor, you should have a file
@file{extractor.h} installed in your @file{include/} directory.  This
file should be the starting point for your C and C++ development with
GNU libextractor.  The build process also installs the @file{extract} binary and
man pages for @file{extract} and GNU libextractor.  The @file{extract} man page
documents the @file{extract} tool.  The GNU libextractor man page gives a brief
summary of the C API for GNU libextractor.

@cindex packageing
@cindex directory structure
@cindex plugin
@cindex environment variables
@vindex LIBEXTRACTOR_PREFIX
When you install GNU libextractor, various plugins will be
installed in the @file{lib/libextractor/} directory.  The main library
will be installed as @file{lib/libextractor.so}.  Note that
GNU libextractor will attempt to find the plugins relative to the
path of the main library.  Consequently, a package manager can move
the library and its plugins to a different location later --- as long
as the relative path between the main library and the plugins is
preserved.  As a method of last resort, the user can specify an
environment variable @verb{|LIBEXTRACTOR_PREFIX|}.  If
GNU libextractor cannot locate a plugin, it will look in
@verb{|LIBEXTRACTOR_PREFIX/lib/libextractor/|}.


@section Installation on GNU/Linux

Should work using the standard instructions without problems.


@section Installation on FreeBSD

Should work using the standard instructions without problems.


@section Installation on OpenBSD

OpenBSD 3.8 also doesn't have CODESET in @file{langinfo.h}.  CODESET
is used in GNU libextractor in about three places.  This causes problems
during compilation.


@section Installation on NetBSD

No reports so far.


@section Installation using MinGW

Linking -lstdc++ with the provided libtool fails on Cygwin, this
is a problem with libtool, there is unfortunately no flag to tell
libtool how to do its job on Cygwin and it seems that it cannot be the
default to set the library check to 'pass_all'.  Patching libtool may
help.

Note: this is a rather dated report and may no longer apply.


@section Installation on OS X

libextractor has two installation methods on Mac OS X: it can be
installed as a Mac OS X framework or with the standard
@command{./configure; make; make install} shell commands. The
framework package is self-contained, but currently omits some of the
extractor plugins that can be compiled in if libextractor is installed
with @command{./configure; make; make install} (provided that the
required dependencies exist.)

@subsection Installing and uninstalling the framework

The binary framework is distributed as a disk image (@file{Extractor-x.x.xx.dmg}).
Installation is done by opening the disk image and clicking @file{Extractor.pkg}
inside it. The Mac OS X installer application will then run. The framework
is installed to the root volume's @file{/Library/Frameworks} folder and installing
will require admin privileges.

The framework can be uninstalled by dragging @*
@file{/Library/Frameworks/Extractor.framework} to the @file{Trash}.


@subsection Using the framework

In the framework, the @command{extract} command line tool can be found at @*
@file{/Library/Frameworks/Extractor.framework/Versions/Current/bin/extract}

The framework can be used in software projects as a framework or as a dynamic
library. 

When using the framework as a dynamic library in projects using autotools,
one would most likely want to add  @*
"-I/Library/Frameworks/Extractor.framework/Versions/Current/include"
to CPPFLAGS and @*
"-L/Library/Frameworks/Extractor.framework/Versions/Current/lib"
to LDFLAGS.


@subsection Example for using the framework

@example
@verbatim
// hello.c
#include <Extractor/extractor.h>

int
main (int argc, char **argv)
{
  struct EXTRACTOR_PluginList *el;
  el = EXTRACTOR_plugin_load_defaults (EXTRACTOR_OPTION_DEFAULT_POLICY);
  // ...
  EXTRACTOR_plugin_remove_all (el);
  return 0;
}
@end verbatim
@end example

You can then compile the example using

@verbatim
$ gcc -o hello hello.c -framework Extractor
@end verbatim

@subsection Example for using the dynamic library

@example
@verbatim
// hello.c
#include <extractor.h>
int main()
{
  struct EXTRACTOR_PluginList *el;
  el = EXTRACTOR_plugin_load_defaults (EXTRACTOR_OPTION_DEFAULT_POLICY);
  // ...
  EXTRACTOR_plugin_remove_all (el);
  return 0;
}
@end verbatim
@end example

You can then compile the example using

@verbatim
$ gcc -I/Library/Frameworks/Extractor.framework/Versions/Current/include \
  -o hello hello.c \
  -L/Library/Frameworks/Extractor.framework/Versions/Current/lib \
  -lextractor
@end verbatim

Notice the difference in the @code{#include} line.






@section Note to package maintainers

The suggested way to package GNU libextractor is to split it into
roughly the following binary packages:

@itemize @bullet
@item
libextractor (main library only, only hard dependency for other packages depending on GNU libextractor)
@item
extract (command-line tool and man page extract.1)
@item
libextractor-dev (extractor.h header and man page libextractor.3)
@item
libextractor-doc (this manual)
@item
libextractor-plugins (plugins without external dependencies; recommended but not required by extract and libextractor package)
@item
libextractor-plugin-XXX (plugin with dependency on libXXX, for example for XXX=mpeg this would be @file{libextractor_mpeg.so})
@item
libextractor-plugins-all (meta package that requires all plugins except experimental plugins)
@end itemize

This would enable minimal installations (i.e. for embedded systems) to
not include any plugins, as well as moderate-size installations (that
do not trigger GTK and X11) for systems that have limited resources.
Right now, the MP4 plugin is experimental and does nothing and should
thus never be included at all.  The gstreamer plugin is experimental
but largely works with the correct version of gstreamer and can thus
be packaged (especially if the dependency is available on the target
system) but should probably not be part of libextractor-plugins-all.


@node Generalities
@chapter Generalities

@section Introduction to the ``extract'' command

The @command{extract} command takes a list of file names as arguments,
extracts meta data from each of those files and prints the result to
the console.  By default, @command{extract} will use all available
plugins and print all (non-binary) meta data that is found.

The set of plugins used by @command{extract} can be controlled using
the ``-l'' and ``-n'' options.  Use ``-n'' to not load all of the
default plugins.  Use ``-l NAME'' to specifically load a certain
plugin.  For example, specify ``-n -l mime'' to only use the MIME
plugin.

Using the ``-p'' option the output of @command{extract} can be limited
to only certain keyword types.  Similarly, using the ``-x'' option,
certain keyword types can be excluded.  A list of all known keyword
types can be obtained using the ``-L'' option.

The output format of @command{extract} can be influenced with the
``-V'' (more verbose, lists filenames), ``-g'' (grep-friendly, all
meta data on a single line per file) and ``-b'' (bibTeX style)
options.

@section Common usage examples for ``extract''

@example
$ extract test/test.jpg
comment - (C) 2001 by Christian Grothoff, using gimp 1.2 1
mimetype - image/jpeg

$ extract -V -x comment test/test.jpg
Keywords for file test/test.jpg:
mimetype - image/jpeg

$ extract -p comment test/test.jpg
comment - (C) 2001 by Christian Grothoff, using gimp 1.2 1

$ extract -nV -l png.so -p comment test/test.jpg test/test.png
Keywords for file test/test.jpg:
Keywords for file test/test.png:
comment - Testing keyword extraction
@end example


@section Introduction to the libextractor library

Each public symbol exported by GNU libextractor has the prefix
@verb{|EXTRACTOR_|}.  All-caps names are used for constants.  For the
impatient, the minimal C code for using GNU libextractor (on the
executing binary itself) looks like this:

@verbatim
#include <extractor.h>

int 
main (int argc, char ** argv) 
{
  struct EXTRACTOR_PluginList *plugins
    = EXTRACTOR_plugin_add_defaults (EXTRACTOR_OPTION_DEFAULT_POLICY);
  EXTRACTOR_extract (plugins, argv[1],
                     NULL, 0, 
                     &EXTRACTOR_meta_data_print, stdout);
  EXTRACTOR_plugin_remove_all (plugins);
  return 0;
}
@end verbatim

The minimal API illustrated by this example is actually sufficient for
many applications.  The full external C API of GNU libextractor is described
in chapter @xref{Extracting meta data}.  Bindings for other languages
are described in chapter @xref{Language bindings}.  The API for
writing new plugins is described in chapter @xref{Writing new Plugins}.

@node Extracting meta data
@chapter Extracting meta data

In order to extract meta data with GNU libextractor you first need to
load the respective plugins and then call the extraction API
with the plugins and the data to process.  This section
documents how to load and unload plugins, the various types
and formats in which meta data is returned to the application
and finally the extraction API itself.

@menu
* Plugin management::   How to load and unload plugins
* Meta types::          About meta types
* Meta formats::        About meta formats
* Extracting::          How to use the extraction API
@end menu


@node Plugin management
@section Plugin management

@cindex reentrant
@cindex concurrency
@cindex threads
@cindex thread-safety
@tindex enum EXTRACTOR_Options

Using GNU libextractor from a multi-threaded parent process requires some
care.  The problem is that on most platforms GNU libextractor starts
sub-processes for the actual extraction work.  This is useful to
isolate the parent process from potential bugs; however, it can cause
problems if the parent process is multi-threaded.  The issue is that
at the time of the fork, another thread of the application may hold a
lock (i.e. in gettext or libc).  That lock would then never be
released in the child process (as the other thread is not present in
the child process).  As a result, the child process would then
deadlock on trying to acquire the lock and never terminate.  This has
actually been observed with a lock in GNU gettext that is triggered by
the plugin startup code when it interacts with libltdl.

The problem can be solved by loading the plugins using the
@code{EXTRACTOR_OPTION_IN_PROCESS} option, which will run GNU libextractor
in-process and thus avoid the locking issue.  In this case, all of the
functions for loading and unloading plugins, including
@verb{|EXTRACTOR_plugin_add_defaults|} and
@verb{|EXTRACTOR_plugin_remove_all|}, are thread-safe and reentrant.
However, using the same plugin list from multiple threads at the same
time is not safe.  

All plugin code is expected required to be reentrant and state-less,
but due to the extensive use of 3rd party libraries this cannot
be guaranteed.


@deftp {C Struct} EXTRACTOR_PluginList
@tindex struct EXTRACTOR_PluginList

A plugin list represents a set of GNU libextractor plugins.  Most of
the GNU libextractor API is concerned with either constructing a
plugin list or using it to extract meta data.  The internal representation
of the plugin list is of no concern to users or plugin developers.
@end deftp


@deftypefun void EXTRACTOR_plugin_remove_all (struct EXTRACTOR_PluginList *plugins)
@findex EXTRACTOR_plugin_remove_all

Unload all of the plugins in the given list.
@end deftypefun

@deftypefun {struct EXTRACTOR_PluginList *} EXTRACTOR_plugin_remove (struct EXTRACTOR_PluginList *plugins, const char*name)
@findex EXTRACTOR_plugin_remove

Unloads a particular plugin.  The given name should be the short name of the plugin, for example ``mime'' for the mime-type extractor or ``mpeg'' for the MPEG extractor.
@end deftypefun

@deftypefun {struct EXTRACTOR_PluginList *} EXTRACTOR_plugin_add (struct EXTRACTOR_PluginList *plugins, const char* name,const char* options, enum EXTRACTOR_Options flags)
@findex EXTRACTOR_plugin_add

Loads a particular plugin.  The plugin is added to the existing list, which can be @code{NULL}.  The second argument specifies the name of the plugin (i.e. ``ogg'').  The third argument can be @code{NULL} and specifies plugin-specific options.  Finally, the last argument specifies if the plugin should be executed out-of-process (@code{EXTRACTOR_OPTION_DEFAULT_POLICY}) or not.
@end deftypefun

@deftypefun {struct EXTRACTOR_PluginList *} EXTRACTOR_plugin_add_config (struct EXTRACTOR_PluginList *plugins, const char* config, enum EXTRACTOR_Options flags)
@findex EXTRACTOR_plugin_add_config

Loads and unloads plugins based on a configuration string, modifying the existing list, which can be @code{NULL}.  The string has the format ``[-]NAME(OPTIONS)@{:[-]NAME(OPTIONS)@}*''.  Prefixing the plugin name with a ``-'' means that the plugin should be unloaded.
@end deftypefun

@deftypefun {struct EXTRACTOR_PluginList *} EXTRACTOR_plugin_add_defaults (enum EXTRACTOR_Options flags)
@findex EXTRACTOR_plugin_add_defaults

Loads all of the plugins in the plugin directory.  This function is what most GNU libextractor applications should use to setup the plugins.
@end deftypefun



@node Meta types
@section Meta types


@tindex enum EXTRACTOR_MetaType
@findex EXTRACTOR_metatype_get_max

@verb{|enum EXTRACTOR_MetaType|} is a C enum which defines a list of over 100 different types of meta data.  The total number can differ between different GNU libextractor releases; the maximum value for the current release can be obtained using the @verb{|EXTRACTOR_metatype_get_max|} function.  All values in this enumeration are of the form @verb{|EXTRACTOR_METATYPE_XXX|}.

@deftypefun {const char *} EXTRACTOR_metatype_to_string (enum EXTRACTOR_MetaType type)
@findex EXTRACTOR_metatype_to_string
@cindex gettext
@cindex internationalization

The function @verb{|EXTRACTOR_metatype_to_string|} can be used to obtain a short English string @samp{s} describing the meta data type.  The string can be translated into other languages using GNU gettext with the domain set to GNU libextractor (@verb{|dgettext("libextractor", s)|}).  
@end deftypefun

@deftypefun {const char *} EXTRACTOR_metatype_to_description (enum EXTRACTOR_MetaType type)
@findex EXTRACTOR_metatype_to_description
@cindex gettext
@cindex internationalization

The function @verb{|EXTRACTOR_metatype_to_description|} can be used to obtain a longer English string @samp{s} describing the meta data type.  The description may be empty if the short description returned by @code{EXTRACTOR_metatype_to_string} is already comprehensive.  The string can be translated into other languages using GNU gettext with the domain set to GNU libextractor (@verb{|dgettext("libextractor", s)|}).  
@end deftypefun



@node Meta formats
@section Meta formats

@tindex enum EXTRACTOR_MetaFormat

@verb{|enum EXTRACTOR_MetaFormat|} is a C enum which defines on a high level how the extracted meta data is represented.  Currently, the library uses three formats: UTF-8 strings, C strings and binary data.  A fourth value, @code{EXTRACTOR_METAFORMAT_UNKNOWN} is defined but not used.  UTF-8 strings are 0-terminated strings that have been converted to UTF-8.  The format code is @code{EXTRACTOR_METAFORMAT_UTF8}. Ideally, most text meta data will be of this format.  Some file formats fail to specify the encoding used for the text.  In this case, the text cannot be converted to UTF-8.  However, the meta data is still known to be 0-terminated and presumably human-readable.  In this case, the format code used is @code{EXTRACTOR_METAFORMAT_C_STRING}; however, this should not be understood to mean that the encoding is the same as that used by the C compiler.  Finally, for binary data (mostly images), the format @code{EXTRACTOR_METAFORMAT_BINARY} is used.

Naturally this is not a precise description of the meta format. Plugins can provide a more precise description (if known) by providing the respective mime type of the meta data.  For example, binary image meta data could be also tagged as ``image/png'' and normal text would typically be tagged as ``text/plain''.  



@node Extracting
@section Extracting

@deftypefn {Function Pointer} int (*EXTRACTOR_MetaDataProcessor)(void *cls, const char *plugin_name, enum EXTRACTOR_MetaType type, enum EXTRACTOR_MetaFormat format, const char *data_mime_type, const char *data, size_t data_len)
@tindex EXTRACTOR_MetaDataProcessor

Type of a function that libextractor calls for each meta data item found.

@table @var

@item cls 
closure (user-defined)

@item plugin_name 
name of the plugin that produced this value; special values can be used (i.e. '<zlib>' for zlib being used in the main libextractor library and yielding meta data);

@item type 
libextractor-type describing the meta data;

@item format basic 
format information about data

@item data_mime_type 
mime-type of data (not of the original file); can be @code{NULL} (if mime-type is not known);

@item data 
actual meta-data found

@item data_len 
number of bytes in data

@end table

Return 0 to continue extracting, 1 to abort.
@end deftypefn



@deftypefun void EXTRACTOR_extract (struct EXTRACTOR_PluginList *plugins, const char *filename, const void *data, size_t size, EXTRACTOR_MetaDataProcessor proc, void *proc_cls)
@findex EXTRACTOR_extract
@cindex reentrant
@cindex concurrency
@cindex threads
@cindex thread-safety

This is the main function for extracting keywords with GNU libextractor.  The first argument is a plugin list which specifies the set of plugins that should be used for extracting meta data.  The @samp{filename} argument is optional and can be used to specify the name of a file to process.  If @samp{filename} is @code{NULL}, then the @samp{data} argument must point to the in-memory data to extract meta data from.  If @samp{filename} is non-@code{NULL}, @samp{data} can be @code{NULL}.  If @samp{data} is non-null, then @samp{size} is the size of @samp{data} in bytes.  Otherwise @samp{size} should be zero.  For each meta data item found, GNU libextractor will call the @samp{proc} function, passing @samp{proc_cls} as the first argument to @samp{proc}.  The other arguments to @samp{proc} depend on the specific meta data found.  

@cindex SIGBUS
@cindex bus error
Meta data extraction should never really fail --- at worst, GNU libextractor should not call @samp{proc} with any meta data. By design, GNU libextractor should never crash or leak memory, even given corrupt files as input.  Note however, that running GNU libextractor on a corrupt file system (or incorrectly @verb{|mmap|}ed files) can result in the operating system sending a SIGBUS (bus error) to the process.  While GNU libextractor runs plugins out-of-process, it first maps the file into memory and then attempts to decompress it.  During decompression it is possible to encounter a SIGBUS.   GNU libextractor will @emph{not} attempt to catch this signal and your application is likely to crash.  Note again that this should only happen if the file @emph{system} is corrupt (not if individual files are corrupt).  If this is not acceptable, you might want to consider running GNU libextractor itself also out-of-process (as done, for example, by @url{http://grothoff.org/christian/doodle/,doodle}).

@end deftypefun


@node Language bindings
@chapter Language bindings
@cindex Java
@cindex Mono
@cindex Perl
@cindex Python
@cindex PHP
@cindex Ruby

GNU libextractor works immediately with C and C++ code. Bindings for Java, Mono, Ruby, Perl, PHP and Python are available for download from the main GNU libextractor website.  Documentation for these bindings (if available) is part of the downloads for the respective binding.  In all cases, a full installation of the C library is required before the binding can be installed.

@section Java

Compiling the GNU libextractor Java binding follows the usual process of
running @command{configure} and @command{make}.  The result will be a
shared C library @file{libextractor_java.so} with the native code and
a JAR file (installed to @file{$PREFIX/share/java/libextractor.java}).

A minimal example for using GNU libextractor's Java binding would look
like this:
@verbatim
import org.gnu.libextractor.*;
import java.util.ArrayList;

public static void main(String[] args) {
  Extractor ex = Extractor.getDefault();
  for (int i=0;i<args.length;i++) {
    ArrayList keywords = ex.extract(args[i]);
    System.out.println("Keywords for " + args[i] + ":");
    for (int j=0;j<keywords.size();j++)
      System.out.println(keywords.get(j));
  }
}
@end verbatim

The GNU libextractor library and the @file{libextractor_java.so} JNI binding
have to be in the library search path for this to work.  Furthermore, the
@file{libextractor.jar} file should be on the classpath.  

Note that the API does not use Java 5 style generics in order to work
with older versions of Java.

@section Mono

his binding is undocumented at this point.

@section Perl

This binding is undocumented at this point.

@section Python

This binding is undocumented at this point.

@section PHP

This binding is undocumented at this point.

@section Ruby

This binding is undocumented at this point.



@node Utility functions
@chapter Utility functions

@cindex reentrant
@cindex concurrency
@cindex threads
@cindex thread-safety
This chapter describes various utility functions for GNU libextractor usage. All of the functions are reentrant.

@menu
* Utility Constants::
* Meta data printing::
@end menu

@node Utility Constants
@section Utility Constants

@findex EXTRACTOR_VERSION
The constant @verb{|EXTRACTOR_VERSION|} is a hexadecimal
representation of the version number of the installed libextractor
header.  The hexadecimal format is 0xAABBCCDD where AA is the major
version (so far always 0), BB is the minor version, CC is the revision
and DD the patch number.  For example, for version 0.5.18, we would
have AA=0, BB=5, CC=18 and DD=0.  Minor releases such as 0.5.18a or
significant changes in unreleased versions would be marked with DD=1
or higher.


@node Meta data printing
@section Meta data printing


@findex EXTRACTOR_meta_data_print
The @verb{|EXTRACTOR_meta_data_print|} is a simple function which prints the meta data found with libextractor to a file.  The function is mostly useful for debugging and as an example for how to manipulate the keyword list and can be passed as the @samp{proc} argument to @code{EXTRACTOR_extract}.  The file to print to should be passed as @samp{proc_cls} (which must be of type @code{FILE *}), for example @code{stdout}.



@node Existing Plugins
@chapter Existing Plugins

@itemize @bullet
@item
ARCHIVE (using libarchive)
@item
DVI
@item
EXIV2 (using libexiv2, 0.23 or later preferred)
@item 
FLAC (using libFLAC)
@item
GIF (using libgif)
@item
GSTREAMER (using libgstreamer v1.0 or later)
@item
HTML (using libtidy)
@item
IT 
@item
JPEG (using libjpeg v8 or later)
@item
MAN
@item
MIDI (using libsmf)
@item
MIME (using libmagic)
@item
MPEG (using libmpeg2)
@item
NSF
@item
NSFE
@item
ODF
@item
OLE2 (with libgsf)
@item
OGG (with libogg)
@item
PNG
@item
PS
@item
RIFF
@item
RPM (using librpm)
@item
S3M
@item
SID
@item
ThumbnailFFMPEG (using libavformat and related libav-libraries, including libswscale)
@item
ThumbnailGtk (using libgtk)
@item
TIFF (with libtiff, tested with v4)
@item
WAV
@item
XM
@item
ZIP
@end itemize

@file{gzip} and @file{bzip2} compressed versions of these formats are 
also supported (as well as meta data embedded by @file{gzip} itself)
if zlib or libbz2 are available.

@node Writing new Plugins
@chapter Writing new Plugins

Writing a new plugin for libextractor usually requires writing of or
interfacing with an actual parser for a specific format.  How this is
can be accomplished depends on the format and cannot be specified in
general.  However, care should be taken for the code to be reentrant
and highly fault-tolerant, especially with respect to malformed
inputs.

Plugins should start by verifying that the header of the data matches
the specific format and immediately return if that is not the case.
Even if the header matches the expected file format, plugins must not
assume that the remainder of the file is well formed.

The plugin library must be called libextractor_XXX.so, where XXX 
denotes the file format of the plugin. The library must export a 
method @verb{|libextractor_XXX_extract_method|}, with the following 
signature:
@verbatim
void
EXTRACTOR_XXX_extract_method (struct EXTRACTOR_ExtractContext *ec);
@end verbatim

@samp{ec} contains various information the plugin may need for its
execution.  Most importantly, it contains functions for reading
(``read'') and seeking (``seek'') the input data and for returning
extracted data (``proc'').  The ``config'' member can contain
additional configuration options.  ``proc'' should be called on
each meta data item found.  If ``proc'' returns non-zero,
processing should be aborted (if possible).

In order to test new plugins, the @file{extract} command can be run
with the options ``-ni'' and ``-l XXX'' .  This will run the plugin
in-process (making it easier to debug) and without any of the other
plugins.


@section Example for a minimal extract method

The following example shows how a plugin can return the mime type of
a file.
@example
@verbatim
void
EXTRACTOR_mymime_extract (struct EXTRACTOR_ExtractContext *ec)
{
  void *data;
  ssize_t data_size,

  if (-1 == (data_size = ec->read (ec->cls, &data, 4)))
    return; /* read error */
  if (data_size < 4)
    return; /* file too small */
  if (0 != memcmp (data, "\177ELF", 4))
    return; /* not ELF */
  if (0 != ec->proc (ec->cls, 
                     "mymime",
                     EXTRACTOR_METATYPE_MIMETYPE,
                     EXTRACTOR_METAFORMAT_UTF8,
                     "text/plain",
                     "application/x-executable",
                     1 + strlen("application/x-executable")))
    return;
  /* more calls to 'proc' here as needed */
}
@end verbatim
@end example


@node Internal utility functions
@chapter Internal utility functions

Some plugins link against the @code{libextractor_common} library which
provides common abstractions needed by many plugins.  This section
documents this internal API for plugin developers.  Note that the headers
for this library are (intentionally) not installed: we do not consider
this API stable and it should hence only be used by plugins that are 
build and shipped with GNU libextractor.  Third-party plugins should
not use it.

@file{convert_numeric.h} defines various conversion functions for
numbers (in particular, byte-order conversion for floating point
numbers).  

@file{unzip.h} defines an API for accessing compressed files.

@file{pack.h} provides an interpreter for unpacking structs of integer
numbers from streams and converting from big or little endian to host
byte order at the same time.

@file{convert.h} provides a function for character set conversion described
below.

@deftypefun {char *} EXTRACTOR_common_convert_to_utf8 (const char *input, size_t len, const char *charset)
@cindex UTF-8
@cindex character set
@findex EXTRACTOR_common_convert_to_utf8
Various GNU libextractor plugins make use of the internal
@file{convert.h} header which defines a function

@verb{|EXTRACTOR_common_convert_to_utf8|} which can be used to easily convert text from
any character set to UTF-8.  This conversion is important since the
linked list of keywords that is returned by GNU libextractor is
expected to contain only UTF-8 strings.  Naturally, proper conversion
may not always be possible since some file formats fail to specify the
character set.  In that case, it is often better to not convert at
all.

The arguments to @verb{|EXTRACTOR_common_convert_to_utf8|} are the input string (which
does @emph{not} have to be zero-terminated), the length of the input
string, and the character set (which @emph{must} be zero-terminated).
Which character sets are supported depends on the platform, a list can
generally be obtained using the @command{iconv -l} command.  The
return value from @verb{|EXTRACTOR_common_convert_to_utf8|} is a zero-terminated string
in UTF-8 format.  The responsibility to free the string is with the
caller, so storing the string in the keyword list is acceptable.
@end deftypefun





@node Reporting bugs
@chapter Reporting bugs

@cindex bug
GNU libextractor uses the @url{https://gnunet.org/bugs/,Mantis bugtracking
system}.  If possible, please report bugs there.  You can also e-mail
the GNU libextractor mailinglist at @url{libextractor@@gnu.org}.



@c **********************************************************
@c *******************  Appendices  *************************
@c **********************************************************

@node GNU Free Documentation License
@appendix GNU Free Documentation License

@include fdl-1.3.texi


@node Index
@unnumbered Index

@printindex cp

@c @node Function and Data Index
@c @unnumbered Function and Data Index
@c @printindex fn

@c @node Type Index
@c @unnumbered Type Index
@c @printindex tp

@bye