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Reading File Metadata with extract and libextractor

Issue 134

From Issue #134
June 2005

Apr 28, 2005  By Christian Grothoff
 in
Don't just guess about a file's characteristics in a search. Use specific extractor plugins to build an accurate database of files.

Listing 8. jpegextractor.c adds the MIME type to the list after parsing the file header.



if ( (data[0] != 0xFF) || (data[1] != 0xD8) )
  return prev; /* not a JPEG */
addKeyword(&prev,
           strdup("image/jpeg"),
           EXTRACTOR_MIMETYPE);
/* ... more parsing code here ... */
return prev;
Conclusion

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.

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Test this tool online

Anonymous's picture
Submitted by Anonymous (not verified) on Sat, 12/19/2009 - 08:52.

Online metadata reader is using libextractor. Might be handy if someone wants to test the results first without installing libextractor.

Can this extract index

Anonymous's picture
Submitted by Anonymous (not verified) on Thu, 12/17/2009 - 23:07.

Can this extract index information of PDF files?

Extracting titles from word documents on linux

Anonymous's picture
Submitted by Anonymous (not verified) on Thu, 09/22/2005 - 08:51.

WORD DOCS ARE SUPPORTED

A quick scan of the examples on this page initally made it seem to me as if the extract program does not support Microsoft Word Documents.

Closer inspection reveals that extracting metadata from Office documents is supported.


[foo@localhost ~]$ extract foo.doc
mimetype - application/vnd.ms-files
os - Win32
organization - Foo Publishing
page count - 1
modification date - Tue Sep 6 16:10:00 2005
software - Microsoft Office Word
version - 3
format - ABC123
keywords - SCADA, Cryptographic Protection, Communications
author - ABC123 Task Group
subject - Cryptographic Protection of SCADA Communications
title - ABC123 Draft 3
[foo@localhost ~]$

Missing strdup()?

Aron Stansvik's picture
Submitted by Aron Stansvik (not verified) on Mon, 08/29/2005 - 04:47.

"The strdup in the code is important, because the string will be deallocated later, typically in EXTRACTOR_freeKeywords()."

If that strdup() is so important, then where is it? ;)

strdup necessary

Mike W's picture
Submitted by Mike W (not verified) on Tue, 10/11/2005 - 04:53.

The strdup() referred to is in Listing 8 !!
R-E-A-D M-O-R-E C-A-R-E-F-U-L-L-Y !

Right there?

Christian_Grothoff's picture
Submitted by Christian_Grothoff (not verified) on Tue, 09/27/2005 - 12:59.

The strdup can either be in addKeyword or, as in the article, before the call to addKeyword:

addKeyword(&prev,
strdup("image/jpeg"),
EXTRACTOR_MIMETYPE);

I'm also not aware of any strdup's missing (at the moment) in the actual source, so I'm not sure what your comment refers to. :-)

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