LJ Archive

Reading File Metadata with extract and libextractor

Christian Grothoff

Issue #134, June 2005

Don't just guess about a file's characteristics in a search. Use specific extractor plugins to build an accurate database of files.

Modern file formats have provisions to annotate the contents of the file with descriptive information. This development is driven by the need to find a better way to organize data than merely by using filenames. The problem with such metadata is it is not stored in a standardized manner across different file formats. This makes it difficult for format-agnostic tools, such as file managers or file-sharing applications, to make use of the information. It also results in a plethora of format-specific tools used to extract the metadata, such as AVInfo, id3edit, jpeginfo and Vocoditor.

In this article, the libextractor library and the extract tool are introduced. The goal of the libextractor Project is to provide a uniform interface for obtaining metadata from different file formats. libextractor currently is used by evidence, the file manager for the forthcoming version of Enlightenment, as well as for GNUnet, an anonymous, censorship-resistant peer-to-peer file-sharing system. The extract tool is a command-line interface to the library. libextractor is licensed under the GNU General Public License.

libextractor shares some similarities with the popular file tool, which uses the first bytes in a file to guess the MIME type. libextractor differs from file in that it tries to obtain much more information than the MIME type. Depending on the file format, libextractor can obtain additional information, including the name of the software used to create the file, the author, descriptions, album titles, image dimensions or the duration of a movie.

libextractor achieves this information by using specific parser code for many popular formats. The list currently includes MP3, Ogg, Real Media, MPEG, RIFF (avi), GIF, JPEG, PNG, TIFF, HTML, PDF, PostScript, Zip, OpenOffice.org, StarOffice, Microsoft Office, tar, DVI, man, Deb, elf, RPM, asf, as well as generic methods such as MIME-type detection. Many other formats exist, and among the more popular formats only a few proprietary formats are not supported.

Integrating support for new formats is easy, because libextractor uses plugins to gather data. libextractor plugins are shared libraries that typically provide code to parse one particular format. At the end of this article, we demonstrate how to integrate support for new formats into the library. libextractor gathers the metadata obtained from various plugins and provides clients with a list of pairs, consisting of a classification and a character sequence. The classification is used to organize the metadata into categories such as title, creator, subject and description.

Installing libextractor and Using extract

The simplest way to install libextractor is to use one of the binary packages available for many distributions. Under Debian, the extract tool is in a separate package, extract. Headers required to compile other applications against libextractor are contained in libextractor0-devel. If you want to compile libextractor from source, you need an unusual amount of memory: 256MB of system memory is roughly the minimum, as GCC uses about 200MB to compile one of the plugins. Otherwise, compiling by hand follows the usual sequence of steps, as shown in Listing 1.

After installing libextractor, the extract tool can be used to obtain metadata from documents. By default, the extract tool uses a canonical set of plugins, which consists of all file-format-specific plugins supported by the current version of libextractor, together with the mime-type detection plugin. Example output for the Linux Journal Web site is shown in Listing 2.

If you are a user of BibTeX, the option -b is likely to come in handy to create BibTeX entries automatically from documents that have been equipped properly with metadata, as shown in Listing 3.

Another interesting option is -B LANG. This option loads one of the language-specific but format-agnostic plugins. These plugins attempt to find plain text in a document by matching strings in the document against a dictionary. If the need for 200MB of memory to compile libextractor seems mysterious, the answer lies in these plugins. In order to perform a fast dictionary search, a bloomfilter is created that allows fast probabilistic matching; GCC finds the resulting data structure a bit hard to swallow.

The option -B is useful for formats that currently are undocumented or unsupported. The printable plugins typically print the entire text of the document in order. Listing 4 shows the output of extract run on a Microsoft Word document.

This is a rather precise description of the text for a German speaker. The supported languages at the moment are Danish (da), German (de), English (en), Spanish (es), Italian (it) and Norwegian (no). Supporting other languages merely is a question of adding free dictionaries in an appropriate character set. Further options are described in the extract man page; see man 1 extract.

Using libextractor in Your Projects

Listing 5 shows the code of a minimalistic program that uses libextractor. Compiling minimal.c requires passing the option -lextractor to GCC. The EXTRACTOR_KeywordList is a simple linked list containing a keyword and a keyword type. For details and additional functions for loading plugins and manipulating the keyword list, see the libextractor man page, man 3 libextractor. Java programmers should know that a Java class that uses JNI to communicate with libextractor also is available.

Writing Plugins

The most complicated thing about writing a new plugin for libextractor is writing the actual parser for a specific format. Nevertheless, the basic pattern is always the same. The plugin library must be called libextractor_XXX.so, where XXX denotes the file format of the plugin. The library must export a method libextractor_XXX_extract, with the following signature shown in Listing 6.

The argument filename specifies the name of the file being processed. data is a pointer to the typically mmapped contents of the file, and size is the file size. Most plugins do not make use of the filename and simply parse data directly, starting by verifying that the header of the data matches the specific format.

prev is the list of keywords extracted so far by other plugins for the file. The function is expected to return an updated list of keywords. If the format does not match the expectations of the plugin, prev is returned. Most plugins use a function such as addKeyword (Listing 7) to extend the list.

A typical use of addKeyword is to add the MIME type once the file format has been established. For example, the JPEG-extractor (Listing 8) checks the first bytes of the JPEG header and then either aborts or claims the file to be a JPEG. The strdup in the code is important, because the string will be deallocated later, typically in EXTRACTOR_freeKeywords(). A list of supported keyword classifications, in the example EXTRACTOR_MIMETYPE can be found in the extractor.h header file.


libextractor is a simple extensible C library for obtaining metadata from documents. Its plugin architecture and broad support for formats set it apart from format-specific tools. The design is limited by the fact that libextractor cannot be used to update metadata, which more specialized tools typically support.

Resources for this article: /article/8207.

Christian Grothoff graduated from the University of Wuppertal in 2000 with a degree in mathematics. He currently is a PhD student in computer science at Purdue University, studying static program analysis and secure peer-to-peer networking. A Linux user since 1995, he has contributed to various free software projects and now is the maintainer of GNUnet and a member of the core team for libextractor. His home page can be found at grothoff.org/christian.

LJ Archive