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<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE book [
<!ENTITY ldquo "&#x201C;">
<!ENTITY rdquo "&#x201D;">
<!ENTITY mdash "&#x2014;">
<!ENTITY ndash "&#x2013;">
<!ENTITY nbsp "&#xA0;">
<!ENTITY swversion "4.1.0">
<!ENTITY lastreleased "April 14, 2013">
]>
<book>
<bookinfo>
<title>QPDF Manual</title>
<subtitle>For QPDF Version &swversion;, &lastreleased;</subtitle>
<author>
<firstname>Jay</firstname><surname>Berkenbilt</surname>
</author>
<copyright>
<year>2005&ndash;2013</year>
<holder>Jay Berkenbilt</holder>
</copyright>
</bookinfo>
<preface id="acknowledgments">
<title>General Information</title>
<para>
QPDF is a program that does structural, content-preserving
transformations on PDF files. QPDF's website is located at <ulink
url="http://qpdf.sourceforge.net/">http://qpdf.sourceforge.net/</ulink>.
QPDF's source code is hosted on github at <ulink
url="https://github.com/qpdf/qpdf">https://github.com/qpdf/qpdf</ulink>.
</para>
<para>
QPDF has been released under the terms of <ulink
url="http://www.opensource.org/licenses/artistic-license-2.0.php">Version
2.0 of the Artistic License</ulink>, a copy of which appears in the
file <filename>Artistic-2.0</filename> in the source distribution.
</para>
<para>
QPDF was originally created in 2001 and modified periodically
between 2001 and 2005 during my employment at <ulink
url="http://www.apexcovantage.com">Apex CoVantage</ulink>. Upon my
departure from Apex, the company graciously allowed me to take
ownership of the software and continue maintaining as an open
source project, a decision for which I am very grateful. I have
made considerable enhancements to it since that time. I feel
fortunate to have worked for people who would make such a decision.
This work would not have been possible without their support.
</para>
</preface>
<chapter id="ref.overview">
<title>What is QPDF?</title>
<para>
QPDF is a program that does structural, content-preserving
transformations on PDF files. It could have been called something
like <emphasis>pdf-to-pdf</emphasis>. It also provides many useful
capabilities to developers of PDF-producing software or for people
who just want to look at the innards of a PDF file to learn more
about how they work.
</para>
<para>
With QPDF, it is possible to copy objects from one PDF file into
another and to manipulate the list of pages in a PDF file. This
makes it possible to merge and split PDF files. The QPDF library
also makes it possible for you to create PDF files from scratch.
In this mode, you are responsible for supplying all the contents of
the file, while the QPDF library takes care off all the syntactical
representation of the objects, creation of cross references tables
and, if you use them, object streams, encryption, linearization,
and other syntactic details. You are still responsible for
generating PDF content on your own.
</para>
<para>
QPDF has been designed with very few external dependencies, and it
is intentionally very lightweight. QPDF is
<emphasis>not</emphasis> a PDF content creation library, a PDF
viewer, or a program capable of converting PDF into other formats.
In particular, QPDF knows nothing about the semantics of PDF
content streams. If you are looking for something that can do
that, you should look elsewhere. However, once you have a valid
PDF file, QPDF can be used to transform that file in ways perhaps
your original PDF creation can't handle. For example, many
programs generate simple PDF files but can't password-protect them,
web-optimize them, or perform other transformations of that type.
</para>
</chapter>
<chapter id="ref.installing">
<title>Building and Installing QPDF</title>
<para>
This chapter describes how to build and install qpdf. Please see
also the <filename>README</filename> and
<filename>INSTALL</filename> files in the source distribution.
</para>
<sect1 id="ref.prerequisites">
<title>System Requirements</title>
<para>
The qpdf package has relatively few external dependencies. In
order to build qpdf, the following packages are required:
<itemizedlist>
<listitem>
<para>
zlib: <ulink url="http://www.zlib.net/">http://www.zlib.net/</ulink>
</para>
</listitem>
<listitem>
<para>
pcre: <ulink url="http://www.pcre.org/">http://www.pcre.org/</ulink>
</para>
</listitem>
<listitem>
<para>
gnu make 3.81 or newer: <ulink url="http://www.gnu.org/software/make">http://www.gnu.org/software/make</ulink>
</para>
</listitem>
<listitem>
<para>
perl version 5.8 or newer:
<ulink url="http://www.perl.org/">http://www.perl.org/</ulink>;
required for <command>fix-qdf</command> and the test suite.
</para>
</listitem>
<listitem>
<para>
GNU diffutils (any version): <ulink
url="http://www.gnu.org/software/diffutils/">http://www.gnu.org/software/diffutils/</ulink>
is required to run the test suite. Note that this is the
version of diff present on virtually all GNU/Linux systems.
This is required because the test suite uses <command>diff
-u</command>.
</para>
</listitem>
<listitem>
<para>
A C++ compiler that works well with STL and has the <type>long
long</type> type. Most modern C++ compilers should fit the
bill fine. QPDF is tested with gcc and Microsoft Visual C++.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Part of qpdf's test suite does comparisons of the contents PDF
files by converting them images and comparing the images. The
image comparison tests are disabled by default. Those tests are
not required for determining correctness of a qpdf build if you
have not modified the code since the test suite also contains
expected output files that are compared literally. The image
comparison tests provide an extra check to make sure that any
content transformations don't break the rendering of pages.
Transformations that affect the content streams themselves are off
by default and are only provided to help developers look into the
contents of PDF files. If you are making deep changes to the
library that cause changes in the contents of the files that qpdf
generates, then you should enable the image comparison tests.
Enable them by running <command>configure</command> with the
<option>--enable-test-compare-images</option> flag. If you enable
this, the following additional requirements are required by the
test suite. Note that in no case are these items required to use
qpdf.
<itemizedlist>
<listitem>
<para>
libtiff: <ulink url="http://www.remotesensing.org/libtiff/">http://www.remotesensing.org/libtiff/</ulink>
</para>
</listitem>
<listitem>
<para>
GhostScript version 8.60 or newer: <ulink
url="http://www.ghostscript.com">http://www.ghostscript.com</ulink>
</para>
</listitem>
</itemizedlist>
If you do not enable this, then you do not need to have tiff and
ghostscript.
</para>
<para>
If Adobe Reader is installed as <command>acroread</command>, some
additional test cases will be enabled. These test cases simply
verify that Adobe Reader can open the files that qpdf creates.
They require version 8.0 or newer to pass. However, in order to
avoid having qpdf depend on non-free (as in liberty) software, the
test suite will still pass without Adobe reader, and the test
suite still exercises the full functionality of the software.
</para>
<para>
Pre-built documentation is distributed with qpdf, so you should
generally not need to rebuild the documentation. In order to
build the documentation from its docbook sources, you need the
docbook XML style sheets (<ulink
url="http://downloads.sourceforge.net/docbook/">http://downloads.sourceforge.net/docbook/</ulink>).
To build the PDF version of the documentation, you need Apache fop
(<ulink
url="http://xml.apache.org/fop/">http://xml.apache.org/fop/</ulink>)
version 0.94 or higher.
</para>
</sect1>
<sect1 id="ref.building">
<title>Build Instructions</title>
<para>
Building qpdf on UNIX is generally just a matter of running
<programlisting>./configure
make
</programlisting>
You can also run <command>make check</command> to run the test
suite and <command>make install</command> to install. Please run
<command>./configure --help</command> for options on what can be
configured. You can also set the value of
<varname>DESTDIR</varname> during installation to install to a
temporary location, as is common with many open source packages.
Please see also the <filename>README</filename> and
<filename>INSTALL</filename> files in the source distribution.
</para>
<para>
Building on Windows is a little bit more complicated. For
details, please see <filename>README-windows.txt</filename> in the
source distribution. You can also download a binary distribution
for Windows. There is a port of qpdf to Visual C++ version 6 in
the <filename>contrib</filename> area generously contributed by
Jian Ma. This is also discussed in more detail in
<filename>README-windows.txt</filename>.
</para>
<para>
There are some other things you can do with the build. Although
qpdf uses <application>autoconf</application>, it does not use
<application>automake</application> but instead uses a
hand-crafted non-recursive Makefile that requires gnu make. If
you're really interested, please read the comments in the
top-level <filename>Makefile</filename>.
</para>
</sect1>
</chapter>
<chapter id="ref.using">
<title>Running QPDF</title>
<para>
This chapter describes how to run the qpdf program from the command
line.
</para>
<sect1 id="ref.invocation">
<title>Basic Invocation</title>
<para>
When running qpdf, the basic invocation is as follows:
<programlisting><command>qpdf</command><option> [ <replaceable>options</replaceable> ] <replaceable>infilename</replaceable> [ <replaceable>outfilename</replaceable> ]</option>
</programlisting>
This converts PDF file <option>infilename</option> to PDF file
<option>outfilename</option>. The output file is functionally
identical to the input file but may have been structurally
reorganized. Also, orphaned objects will be removed from the
file. Many transformations are available as controlled by the
options below. In place of <option>infilename</option>, the
parameter <option>--empty</option> may be specified. This causes
qpdf to use a dummy input file that contains zero pages. The only
normal use case for using <option>--empty</option> would be if you
were going to add pages from another source, as discussed in <xref
linkend="ref.page-selection"/>.
</para>
<para>
<option>outfilename</option> does not have to be seekable, even
when generating linearized files. Specifying
&ldquo;<option>-</option>&rdquo; as <option>outfilename</option>
means to write to standard output. However, you can't specify the
same file as both the input and the output because qpdf reads data
from the input file as it writes to the output file.
</para>
<para>
Most options require an output file, but some testing or
inspection commands do not. These are specifically noted.
</para>
</sect1>
<sect1 id="ref.basic-options">
<title>Basic Options</title>
<para>
The following options are the most common ones and perform
commonly needed transformations.
<variablelist>
<varlistentry>
<term><option>--password=password</option></term>
<listitem>
<para>
Specifies a password for accessing encrypted files.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--linearize</option></term>
<listitem>
<para>
Causes generation of a linearized (web-optimized) output file.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--copy-encryption=file</option></term>
<listitem>
<para>
Encrypt the file using the same encryption parameters,
including user and owner password, as the specified file. Use
<option>--encrypt-file-password</option> to specify a password
if one is needed to open this file. Note that copying the
encryption parameters from a file also copies the first half
of <literal>/ID</literal> from the file since this is part of
the encryption parameters.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--encrypt-file-password=password</option></term>
<listitem>
<para>
If the file specified with <option>--copy-encryption</option>
requires a password, specify the password using this option.
Note that only one of the user or owner password is required.
Both passwords will be preserved since QPDF does not
distinguish between the two passwords. It is possible to
preserve encryption parameters, including the owner password,
from a file even if you don't know the file's owner password.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--encrypt options --</option></term>
<listitem>
<para>
Causes generation an encrypted output file. Please see <xref
linkend="ref.encryption-options"/> for details on how to
specify encryption parameters.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--decrypt</option></term>
<listitem>
<para>
Removes any encryption on the file. A password must be
supplied if the file is password protected.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--pages options --</option></term>
<listitem>
<para>
Select specific pages from one or more input files. See <xref
linkend="ref.page-selection"/> for details on how to do page
selection (splitting and merging).
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
Password-protected files may be opened by specifying a password.
By default, qpdf will preserve any encryption data associated with
a file. If <option>--decrypt</option> is specified, qpdf will
attempt to remove any encryption information. If
<option>--encrypt</option> is specified, qpdf will replace the
document's encryption parameters with whatever is specified.
</para>
<para>
Note that qpdf does not obey encryption restrictions already
imposed on the file. Doing so would be meaningless since qpdf can
be used to remove encryption from the file entirely. This
functionality is not intended to be used for bypassing copyright
restrictions or other restrictions placed on files by their
producers.
</para>
<para>
In all cases where qpdf allows specification of a password, care
must be taken if the password contains characters that fall
outside of the 7-bit US-ASCII character range to ensure that the
exact correct byte sequence is provided. It is possible that a
future version of qpdf may handle this more gracefully. For
example, if a password was encrypted using a password that was
encoded in ISO-8859-1 and your terminal is configured to use
UTF-8, the password you supply may not work properly. There are
various approaches to handling this. For example, if you are
using Linux and have the iconv executable (part of the ICU
package) installed, you could pass <option>--password=`echo
<replaceable>password</replaceable> | iconv -t
iso-8859-1`</option> to qpdf where
<replaceable>password</replaceable> is a password specified in
your terminal's locale. A detailed discussion of this is out of
scope for this manual, but just be aware of this issue if you have
trouble with a password that contains 8-bit characters.
</para>
</sect1>
<sect1 id="ref.encryption-options">
<title>Encryption Options</title>
<para>
To change the encryption parameters of a file, use the --encrypt
flag. The syntax is
<programlisting><option>--encrypt <replaceable>user-password</replaceable> <replaceable>owner-password</replaceable> <replaceable>key-length</replaceable> [ <replaceable>restrictions</replaceable> ] --</option>
</programlisting>
Note that &ldquo;<option>--</option>&rdquo; terminates parsing of
encryption flags and must be present even if no restrictions are
present.
</para>
<para>
Either or both of the user password and the owner password may be
empty strings.
</para>
<para>
The value for
<option><replaceable>key-length</replaceable></option> may be 40,
128, or 256. The restriction flags are dependent upon key length.
When no additional restrictions are given, the default is to be
fully permissive.
</para>
<para>
If <option><replaceable>key-length</replaceable></option> is 40,
the following restriction options are available:
<variablelist>
<varlistentry>
<term><option>--print=[yn]</option></term>
<listitem>
<para>
Determines whether or not to allow printing.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--modify=[yn]</option></term>
<listitem>
<para>
Determines whether or not to allow document modification.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--extract=[yn]</option></term>
<listitem>
<para>
Determines whether or not to allow text/image extraction.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--annotate=[yn]</option></term>
<listitem>
<para>
Determines whether or not to allow comments and form fill-in
and signing.
</para>
</listitem>
</varlistentry>
</variablelist>
If <option><replaceable>key-length</replaceable></option> is 128,
the following restriction options are available:
<variablelist>
<varlistentry>
<term><option>--accessibility=[yn]</option></term>
<listitem>
<para>
Determines whether or not to allow accessibility to visually
impaired.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--extract=[yn]</option></term>
<listitem>
<para>
Determines whether or not to allow text/graphic extraction.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--print=<replaceable>print-opt</replaceable></option></term>
<listitem>
<para>
Controls printing access.
<option><replaceable>print-opt</replaceable></option> may be
one of the following:
<itemizedlist>
<listitem>
<para>
<option>full</option>: allow full printing
</para>
</listitem>
<listitem>
<para>
<option>low</option>: allow low-resolution printing only
</para>
</listitem>
<listitem>
<para>
<option>none</option>: disallow printing
</para>
</listitem>
</itemizedlist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--modify=<replaceable>modify-opt</replaceable></option></term>
<listitem>
<para>
Controls modify access.
<option><replaceable>modify-opt</replaceable></option> may be
one of the following, each of which implies all the options
that follow it:
<itemizedlist>
<listitem>
<para>
<option>all</option>: allow full document modification
</para>
</listitem>
<listitem>
<para>
<option>annotate</option>: allow comment authoring and form operations
</para>
</listitem>
<listitem>
<para>
<option>form</option>: allow form field fill-in and signing
</para>
</listitem>
<listitem>
<para>
<option>assembly</option>: allow document assembly only
</para>
</listitem>
<listitem>
<para>
<option>none</option>: allow no modifications
</para>
</listitem>
</itemizedlist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--cleartext-metadata</option></term>
<listitem>
<para>
If specified, any metadata stream in the document will be left
unencrypted even if the rest of the document is encrypted.
This also forces the PDF version to be at least 1.5.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--use-aes=[yn]</option></term>
<listitem>
<para>
If <option>--use-aes=y</option> is specified, AES encryption
will be used instead of RC4 encryption. This forces the PDF
version to be at least 1.6.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--force-V4</option></term>
<listitem>
<para>
Use of this option forces the <literal>/V</literal> and
<literal>/R</literal> parameters in the document's encryption
dictionary to be set to the value <literal>4</literal>. As
qpdf will automatically do this when required, there is no
reason to ever use this option. It exists primarily for use
in testing qpdf itself. This option also forces the PDF
version to be at least 1.5.
</para>
</listitem>
</varlistentry>
</variablelist>
If <option><replaceable>key-length</replaceable></option> is 256,
the minimum PDF version is 1.7 with extension level 8, and the
AES-based encryption format used is the PDF 2.0 encryption method
supported by Acrobat X. the same options are available as with
128 bits with the following exceptions:
<variablelist>
<varlistentry>
<term><option>--use-aes</option></term>
<listitem>
<para>
This option is not available with 256-bit keys. AES is always
used with 256-bit encryption keys.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--force-V4</option></term>
<listitem>
<para>
This option is not available with 256 keys.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--force-R5</option></term>
<listitem>
<para>
If specified, qpdf sets the minimum version to 1.7 at
extension level 3 and writes the deprecated encryption format
used by Acrobat version IX. This option should not be used in
practice to generate PDF files that will be in general use,
but it can be useful to generate files if you are trying to
test proper support in another application for PDF files
encrypted in this way.
</para>
</listitem>
</varlistentry>
</variablelist>
The default for each permission option is to be fully permissive.
</para>
</sect1>
<sect1 id="ref.page-selection">
<title>Page Selection Options</title>
<para>
Starting with qpdf 3.0, it is possible to split and merge PDF
files by selecting pages from one or more input files. Whatever
file is given as the primary input file is used as the starting
point, but its pages are replaced with pages as specified.
<programlisting><option>--pages <replaceable>input-file</replaceable> [ <replaceable>--password=password</replaceable> ] <replaceable>page-range</replaceable> [ ... ] --</option>
</programlisting>
Multiple input files may be specified. Each one is given as the
name of the input file, an optional password (if required to open
the file), and the range of pages. Note that
&ldquo;<option>--</option>&rdquo; terminates parsing of page
selection flags.
</para>
<para>
For each file that pages should be taken from, specify the file, a
password needed to open the file (if any), and a page range. The
password needs to be given only once per file. If any of the
input files are the same as the primary input file or the file
used to copy encryption parameters (if specified), you do not need
to repeat the password here. The same file can be repeated
multiple times. If a file that is repeated has a password, the
password only has to be given the first time. All non-page data
(info, outlines, page numbers, etc.) are taken from the primary
input file. To discard these, use <option>--empty</option> as the
primary input.
</para>
<para>
It is not presently possible to specify the same page from the
same file directly more than once, but you can make this work by
specifying two different paths to the same file (such as by
putting <filename>./</filename> somewhere in the path). This can
also be used if you want to repeat a page from one of the input
files in the output file. This may be made more convenient in a
future version of qpdf if there is enough demand for this feature.
</para>
<para>
The page range is a set of numbers separated by commas, ranges of
numbers separated dashes, or combinations of those. The character
&ldquo;z&rdquo; represents the last page. Pages can appear in any
order. Ranges can appear with a high number followed by a low
number, which causes the pages to appear in reverse. Repeating a
number will cause an error, but you can use the workaround
discussed above should you really want to include the same page
twice.
</para>
<para>
Example page ranges:
<itemizedlist>
<listitem>
<para>
<literal>1,3,5-9,15-12</literal>: pages 1, 2, 3, 5, 6, 7, 8,
9, 15, 14, 13, and 12.
</para>
</listitem>
<listitem>
<para>
<literal>z-1</literal>: all pages in the document in reverse
</para>
</listitem>
</itemizedlist>
</para>
<para>
Note that qpdf doesn't presently do anything special about other
constructs in a PDF file that may know about pages, so semantics
of splitting and merging vary across features. For example, the
document's outlines (bookmarks) point to actual page objects, so
if you select some pages and not others, bookmarks that point to
pages that are in the output file will work, and remaining
bookmarks will not work. On the other hand, page labels (page
numbers specified in the file) are just sequential, so page labels
will be messed up in the output file. A future version of
<command>qpdf</command> may do a better job at handling these
issues. (Note that the qpdf library already contains all of the
APIs required in order to implement this in your own application
if you need it.) In the mean time, you can always use
<option>--empty</option> as the primary input file to avoid
copying all of that from the first file. For example, to take
pages 1 through 5 from a <filename>infile.pdf</filename> while
preserving all metadata associated with that file, you could use
<programlisting><command>qpdf</command> <option>infile.pdf --pages infile.pdf 1-5 -- outfile.pdf</option>
</programlisting>
If you wanted pages 1 through 5 from
<filename>infile.pdf</filename> but you wanted the rest of the
metadata to be dropped, you could instead run
<programlisting><command>qpdf</command> <option>--empty --pages infile.pdf 1-5 -- outfile.pdf</option>
</programlisting>
If you wanted to take pages 1&ndash;5 from
<filename>file1.pdf</filename> and pages 11&ndash;15 from
<filename>file2.pdf</filename> in reverse, you would run
<programlisting><command>qpdf</command> <option>file1.pdf --pages file1.pdf 1-5 file2.pdf 15-11 -- outfile.pdf</option>
</programlisting>
If, for some reason, you wanted to take the first page of an
encrypted file called <filename>encrypted.pdf</filename> with
password <literal>pass</literal> and repeat it twice in an output
file, and if you wanted to drop metadata (like page numbers and
outlines) but preserve encryption, you would use
<programlisting><command>qpdf</command> <option>--empty --copy-encryption=encrypted.pdf --encryption-file-password=pass
--pages encrypted.pdf --password=pass 1 ./encrypted.pdf --password=pass 1 --
outfile.pdf</option>
</programlisting>
Note that we had to specify the password all three times because
giving a password as <option>--encryption-file-password</option>
doesn't count for page selection, and as far as qpdf is concerned,
<filename>encrypted.pdf</filename> and
<filename>./encrypted.pdf</filename> are separated files. These
are all corner cases that most users should hopefully never have
to be bothered with.
</para>
</sect1>
<sect1 id="ref.advanced-transformation">
<title>Advanced Transformation Options</title>
<para>
These transformation options control fine points of how qpdf
creates the output file. Mostly these are of use only to people
who are very familiar with the PDF file format or who are PDF
developers. The following options are available:
<variablelist>
<varlistentry>
<term><option>--stream-data=<replaceable>option</replaceable></option></term>
<listitem>
<para>
Controls transformation of stream data. The value of
<option><replaceable>option</replaceable></option> may be one
of the following:
<itemizedlist>
<listitem>
<para>
<option>compress</option>: recompress stream data when
possible (default)
</para>
</listitem>
<listitem>
<para>
<option>preserve</option>: leave all stream data as is
</para>
</listitem>
<listitem>
<para>
<option>uncompress</option>: uncompress stream data when
possible
</para>
</listitem>
</itemizedlist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--normalize-content=[yn]</option></term>
<listitem>
<para>
Enables or disables normalization of content streams.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--suppress-recovery</option></term>
<listitem>
<para>
Prevents qpdf from attempting to recover damaged files.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--object-streams=<replaceable>mode</replaceable></option></term>
<listitem>
<para>
Controls handing of object streams. The value of
<option><replaceable>mode</replaceable></option> may be one of
the following:
<itemizedlist>
<listitem>
<para>
<option>preserve</option>: preserve original object streams
(default)
</para>
</listitem>
<listitem>
<para>
<option>disable</option>: don't write any object streams
</para>
</listitem>
<listitem>
<para>
<option>generate</option>: use object streams wherever
possible
</para>
</listitem>
</itemizedlist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--ignore-xref-streams</option></term>
<listitem>
<para>
Tells qpdf to ignore any cross-reference streams.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--qdf</option></term>
<listitem>
<para>
Turns on QDF mode. For additional information on QDF, please
see <xref linkend="ref.qdf"/>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--min-version=<replaceable>version</replaceable></option></term>
<listitem>
<para>
Forces the PDF version of the output file to be at least
<replaceable>version</replaceable>. In other words, if the
input file has a lower version than the specified version, the
specified version will be used. If the input file has a
higher version, the input file's original version will be
used. It is seldom necessary to use this option since qpdf
will automatically increase the version as needed when adding
features that require newer PDF readers.
</para>
<para>
The version number may be expressed in the form
<replaceable>major.minor.extension-level</replaceable>, in
which case the version is interpreted as
<replaceable>major.minor</replaceable> at extension level
<replaceable>extension-level</replaceable>. For example,
version <literal>1.7.8</literal> represents version 1.7 at
extension level 8. Note that minimal syntax checking is done
on the command line.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--force-version=<replaceable>version</replaceable></option></term>
<listitem>
<para>
This option forces the PDF version to be the exact version
specified <emphasis>even when the file may have content that
is not supported in that version</emphasis>. The version
number is interpreted in the same way as with
<option>--min-version</option> so that extension levels can be
set. In some cases, forcing the output file's PDF version to
be lower than that of the input file will cause qpdf to
disable certain features of the document. Specifically,
256-bit keys are disabled if the version is less than 1.7 with
extension level 8 (except R5 is disabled if less than 1.7 with
extension level 3), AES encryption is disabled if the version
is less than 1.6, cleartext metadata and object streams are
disabled if less than 1.5, 128-bit encryption keys are
disabled if less than 1.4, and all encryption is disabled if
less than 1.3. Even with these precautions, qpdf won't be
able to do things like eliminate use of newer image
compression schemes, transparency groups, or other features
that may have been added in more recent versions of PDF.
</para>
<para>
As a general rule, with the exception of big structural things
like the use of object streams or AES encryption, PDF viewers
are supposed to ignore features in files that they don't
support from newer versions. This means that forcing the
version to a lower version may make it possible to open your
PDF file with an older version, though bear in mind that some
of the original document's functionality may be lost.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
By default, when a stream is encoded using non-lossy filters that
qpdf understands and is not already compressed using a good
compression scheme, qpdf will uncompress and recompress streams.
Assuming proper filter implements, this is safe and generally
results in smaller files. This behavior may also be explicitly
requested with <option>--stream-data=compress</option>.
</para>
<para>
When <option>--stream-data=preserve</option> is specified, qpdf
will never attempt to change the filtering of any stream data.
</para>
<para>
When <option>--stream-data=uncompress</option> is specified, qpdf
will attempt to remove any non-lossy filters that it supports.
This includes <literal>/FlateDecode</literal>,
<literal>/LZWDecode</literal>, <literal>/ASCII85Decode</literal>,
and <literal>/ASCIIHexDecode</literal>. This can be very useful
for inspecting the contents of various streams.
</para>
<para>
When <option>--normalize-content=y</option> is specified, qpdf
will attempt to normalize whitespace and newlines in page content
streams. This is generally safe but could, in some cases, cause
damage to the content streams. This option is intended for people
who wish to study PDF content streams or to debug PDF content.
You should not use this for &ldquo;production&rdquo; PDF files.
</para>
<para>
Ordinarily, qpdf will attempt to recover from certain types of
errors in PDF files. These include errors in the cross-reference
table, certain types of object numbering errors, and certain types
of stream length errors. Sometimes, qpdf may think it has
recovered but may not have actually recovered, so care should be
taken when using this option as some data loss is possible. The
<option>--suppress-recovery</option> option will prevent qpdf from
attempting recovery. In this case, it will fail on the first
error that it encounters.
</para>
<para>
Object streams, also known as compressed objects, were introduced
into the PDF specification at version 1.5, corresponding to
Acrobat 6. Some older PDF viewers may not support files with
object streams. qpdf can be used to transform files with object
streams to files without object streams or vice versa. As
mentioned above, there are three object stream modes:
<option>preserve</option>, <option>disable</option>, and
<option>generate</option>.
</para>
<para>
In <option>preserve</option> mode, the relationship to objects and
the streams that contain them is preserved from the original file.
In <option>disable</option> mode, all objects are written as
regular, uncompressed objects. The resulting file should be
readable by older PDF viewers. (Of course, the content of the
files may include features not supported by older viewers, but at
least the structure will be supported.) In
<option>generate</option> mode, qpdf will create its own object
streams. This will usually result in more compact PDF files,
though they may not be readable by older viewers. In this mode,
qpdf will also make sure the PDF version number in the header is
at least 1.5.
</para>
<para>
Ordinarily, qpdf reads cross-reference streams when they are
present in a PDF file. If <option>--ignore-xref-streams</option>
is specified, qpdf will ignore any cross-reference streams for
hybrid PDF files. The purpose of hybrid files is to make some
content available to viewers that are not aware of cross-reference
streams. It is almost never desirable to ignore them. The only
time when you might want to use this feature is if you are testing
creation of hybrid PDF files and wish to see how a PDF consumer
that doesn't understand object and cross-reference streams would
interpret such a file.
</para>
<para>
The <option>--qdf</option> flag turns on QDF mode, which changes
some of the defaults described above. Specifically, in QDF mode,
by default, stream data is uncompressed, content streams are
normalized, and encryption is removed. These defaults can still
be overridden by specifying the appropriate options as described
above. Additionally, in QDF mode, stream lengths are stored as
indirect objects, objects are laid out in a less efficient but
more readable fashion, and the documents are interspersed with
comments that make it easier for the user to find things and also
make it possible for <command>fix-qdf</command> to work properly.
QDF mode is intended for people, mostly developers, who wish to
inspect or modify PDF files in a text editor. For details, please
see <xref linkend="ref.qdf"/>.
</para>
</sect1>
<sect1 id="ref.testing-options">
<title>Testing, Inspection, and Debugging Options</title>
<para>
These options can be useful for digging into PDF files or for use
in automated test suites for software that uses the qpdf library.
When any of the options in this section are specified, no output
file should be given. The following options are available:
<variablelist>
<varlistentry>
<term><option>--static-id</option></term>
<listitem>
<para>
Causes generation of a fixed value for /ID. This is intended
for testing only. Never use it for production files.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--static-aes-iv</option></term>
<listitem>
<para>
Causes use of a static initialization vector for AES-CBC.
This is intended for testing only so that output files can be
reproducible. Never use it for production files. This option
in particular is not secure since it significantly weakens the
encryption.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--no-original-object-ids</option></term>
<listitem>
<para>
Suppresses inclusion of original object ID comments in QDF
files. This can be useful when generating QDF files for test
purposes, particularly when comparing them to determine
whether two PDF files have identical content.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-show-encryption</option></term>
<listitem>
<para>
Shows document encryption parameters. Also shows the
document's user password if the owner password is given.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-check-linearization</option></term>
<listitem>
<para>
Checks file integrity and linearization status.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-show-linearization</option></term>
<listitem>
<para>
Checks and displays all data in the linearization hint tables.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-show-xref</option></term>
<listitem>
<para>
Shows the contents of the cross-reference table in a
human-readable form. This is especially useful for files with
cross-reference streams which are stored in a binary format.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-show-object=obj[,gen]</option></term>
<listitem>
<para>
Show the contents of the given object. This is especially
useful for inspecting objects that are inside of object
streams (also known as &ldquo;compressed objects&rdquo;).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-raw-stream-data</option></term>
<listitem>
<para>
When used along with the <option>--show-object</option>
option, if the object is a stream, shows the raw stream data
instead of object's contents.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-filtered-stream-data</option></term>
<listitem>
<para>
When used along with the <option>--show-object</option>
option, if the object is a stream, shows the filtered stream
data instead of object's contents. If the stream is filtered
using filters that qpdf does not support, an error will be
issued.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-show-pages</option></term>
<listitem>
<para>
Shows the object and generation number for each page
dictionary object and for each content stream associated with
the page. Having this information makes it more convenient to
inspect objects from a particular page.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-with-images</option></term>
<listitem>
<para>
When used along with <option>--show-pages</option>, also shows
the object and generation numbers for the image objects on
each page. (At present, information about images in shared
resource dictionaries are not output by this command. This is
discussed in a comment in the source code.)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-check</option></term>
<listitem>
<para>
Checks file structure and well as encryption, linearization,
and encoding of stream data. A file for which
<option>--check</option> reports no errors may still have
errors in stream data content but should otherwise be
structurally sound. If <option>--check</option> any errors,
qpdf will exit with a status of 2. There are some recoverable
conditions that <option>--check</option> detects. These are
issued as warnings instead of errors. If qpdf finds no errors
but finds warnings, it will exit with a status of 3 (as of
version&nbsp;2.0.4).
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
The <option>--raw-stream-data</option> and
<option>--filtered-stream-data</option> options are ignored unless
<option>--show-object</option> is given. Either of these options
will cause the stream data to be written to standard output. In
order to avoid commingling of stream data with other output, it is
recommend that these objects not be combined with other
test/inspection options.
</para>
<para>
If <option>--filtered-stream-data</option> is given and
<option>--normalize-content=y</option> is also given, qpdf will
attempt to normalize the stream data as if it is a page content
stream. This attempt will be made even if it is not a page
content stream, in which case it will produce unusable results.
</para>
</sect1>
</chapter>
<chapter id="ref.qdf">
<title>QDF Mode</title>
<para>
In QDF mode, qpdf creates PDF files in what we call <firstterm>QDF
form</firstterm>. A PDF file in QDF form, sometimes called a QDF
file, is a completely valid PDF file that has
<literal>%QDF-1.0</literal> as its third line (after the pdf header
and binary characters) and has certain other characteristics. The
purpose of QDF form is to make it possible to edit PDF files, with
some restrictions, in an ordinary text editor. This can be very
useful for experimenting with different PDF constructs or for
making one-off edits to PDF files (though there are other reasons
why this may not always work).
</para>
<para>
It is ordinarily very difficult to edit PDF files in a text editor
for two reasons: most meaningful data in PDF files is compressed,
and PDF files are full of offset and length information that makes
it hard to add or remove data. A QDF file is organized in a manner
such that, if edits are kept within certain constraints, the
<command>fix-qdf</command> program, distributed with qpdf, is able
to restore edited files to a correct state. The
<command>fix-qdf</command> program takes no command-line
arguments. It reads a possibly edited QDF file from standard input
and writes a repaired file to standard output.
</para>
<para>
The following attributes characterize a QDF file:
<itemizedlist>
<listitem>
<para>
All objects appear in numerical order in the PDF file, including
when objects appear in object streams.
</para>
</listitem>
<listitem>
<para>
Objects are printed in an easy-to-read format, and all line
endings are normalized to UNIX line endings.
</para>
</listitem>
<listitem>
<para>
Unless specifically overridden, streams appear uncompressed
(when qpdf supports the filters and they are compressed with a
non-lossy compression scheme), and most content streams are
normalized (line endings are converted to just a UNIX-style
linefeeds).
</para>
</listitem>
<listitem>
<para>
All streams lengths are represented as indirect objects, and the
stream length object is always the next object after the stream.
If the stream data does not end with a newline, an extra newline
is inserted, and a special comment appears after the stream
indicating that this has been done.
</para>
</listitem>
<listitem>
<para>
If the PDF file contains object streams, if object stream
<emphasis>n</emphasis> contains <emphasis>k</emphasis> objects,
those objects are numbered from <emphasis>n+1</emphasis> through
<emphasis>n+k</emphasis>, and the object number/offset pairs
appear on a separate line for each object. Additionally, each
object in the object stream is preceded by a comment indicating
its object number and index. This makes it very easy to find
objects in object streams.
</para>
</listitem>
<listitem>
<para>
All beginnings of objects, <literal>stream</literal> tokens,
<literal>endstream</literal> tokens, and
<literal>endobj</literal> tokens appear on lines by themselves.
A blank line follows every <literal>endobj</literal> token.
</para>
</listitem>
<listitem>
<para>
If there is a cross-reference stream, it is unfiltered.
</para>
</listitem>
<listitem>
<para>
Page dictionaries and page content streams are marked with
special comments that make them easy to find.
</para>
</listitem>
<listitem>
<para>
Comments precede each object indicating the object number of the
corresponding object in the original file.
</para>
</listitem>
</itemizedlist>
</para>
<para>
When editing a QDF file, any edits can be made as long as the above
constraints are maintained. This means that you can freely edit a
page's content without worrying about messing up the QDF file. It
is also possible to add new objects so long as those objects are
added after the last object in the file or subsequent objects are
renumbered. If a QDF file has object streams in it, you can always
add the new objects before the xref stream and then change the
number of the xref stream, since nothing generally ever references
it by number.
</para>
<para>
It is not generally practical to remove objects from QDF files
without messing up object numbering, but if you remove all
references to an object, you can run qpdf on the file (after
running <command>fix-qdf</command>), and qpdf will omit the
now-orphaned object.
</para>
<para>
When <command>fix-qdf</command> is run, it goes through the file
and recomputes the following parts of the file:
<itemizedlist>
<listitem>
<para>
the <literal>/N</literal>, <literal>/W</literal>, and
<literal>/First</literal> keys of all object stream dictionaries
</para>
</listitem>
<listitem>
<para>
the pairs of numbers representing object numbers and offsets of
objects in object streams
</para>
</listitem>
<listitem>
<para>
all stream lengths
</para>
</listitem>
<listitem>
<para>
the cross-reference table or cross-reference stream
</para>
</listitem>
<listitem>
<para>
the offset to the cross-reference table or cross-reference
stream following the <literal>startxref</literal> token
</para>
</listitem>
</itemizedlist>
</para>
</chapter>
<chapter id="ref.using-library">
<title>Using the QPDF Library</title>
<para>
The source tree for the qpdf package has an
<filename>examples</filename> directory that contains a few
example programs. The <filename>qpdf/qpdf.cc</filename> source
file also serves as a useful example since it exercises almost all
of the qpdf library's public interface. The best source of
documentation on the library itself is reading comments in
<filename>include/qpdf/QPDF.hh</filename>,
<filename>include/qpdf/QDFWriter.hh</filename>, and
<filename>include/qpdf/QPDFObjectHandle.hh</filename>.
</para>
<para>
All header files are installed in the <filename>include/qpdf</filename> directory. It
is recommend that you use <literal>#include
&lt;qpdf/QPDF.hh&gt;</literal> rather than adding
<filename>include/qpdf</filename> to your include path.
</para>
<para>
When linking against the qpdf static library, you may also need to
specify <literal>-lpcre -lz</literal> on your link command. If
your system understands how to read libtool
<filename>.la</filename> files, this may not be necessary.
</para>
<para>
The qpdf library is safe to use in a multithreaded program, but no
individual <type>QPDF</type> object instance (including
<type>QPDF</type>, <type>QPDFObjectHandle</type>, or
<type>QPDFWriter</type>) can be used in more than one thread at a
time. Multiple threads may simultaneously work with different
instances of these and all other QPDF objects.
</para>
</chapter>
<chapter id="ref.design">
<title>Design and Library Notes</title>
<sect1 id="ref.design.intro">
<title>Introduction</title>
<para>
This section was written prior to the implementation of the qpdf
package and was subsequently modified to reflect the
implementation. In some cases, for purposes of explanation, it
may differ slightly from the actual implementation. As always,
the source code and test suite are authoritative. Even if there
are some errors, this document should serve as a road map to
understanding how this code works.
</para>
<para>
In general, one should adhere strictly to a specification when
writing but be liberal in reading. This way, the product of our
software will be accepted by the widest range of other programs,
and we will accept the widest range of input files. This library
attempts to conform to that philosophy whenever possible but also
aims to provide strict checking for people who want to validate
PDF files. If you don't want to see warnings and are trying to
write something that is tolerant, you can call
<literal>setSuppressWarnings(true)</literal>. If you want to fail
on the first error, you can call
<literal>setAttemptRecovery(false)</literal>. The default
behavior is to generating warnings for recoverable problems. Note
that recovery will not always produce the desired results even if
it is able to get through the file. Unlike most other PDF files
that produce generic warnings such as &ldquo;This file is
damaged,&rdquo;, qpdf generally issues a detailed error message
that would be most useful to a PDF developer. This is by design
as there seems to be a shortage of PDF validation tools out
there. (This was, in fact, one of the major motivations behind
the initial creation of qpdf.)
</para>
</sect1>
<sect1 id="ref.design-goals">
<title>Design Goals</title>
<para>
The QPDF package includes support for reading and rewriting PDF
files. It aims to hide from the user details involving object
locations, modified (appended) PDF files, the
directness/indirectness of objects, and stream filters including
encryption. It does not aim to hide knowledge of the object
hierarchy or content stream contents. Put another way, a user of
the qpdf library is expected to have knowledge about how PDF files
work, but is not expected to have to keep track of bookkeeping
details such as file positions.
</para>
<para>
A user of the library never has to care whether an object is
direct or indirect. All access to objects deals with this
transparently. All memory management details are also handled by
the library.
</para>
<para>
The <classname>PointerHolder</classname> object is used internally
by the library to deal with memory management. This is basically
a smart pointer object very similar in spirit to the Boost
library's <classname>shared_ptr</classname> object, but predating
it by several years. This library also makes use of a technique
for giving fine-grained access to methods in one class to other
classes by using public subclasses with friends and only private
members that in turn call private methods of the containing class.
See <classname>QPDFObjectHandle::Factory</classname> as an
example.
</para>
<para>
The top-level qpdf class is <classname>QPDF</classname>. A
<classname>QPDF</classname> object represents a PDF file. The
library provides methods for both accessing and mutating PDF
files.
</para>
<para>
<classname>QPDFObject</classname> is the basic PDF Object class.
It is an abstract base class from which are derived classes for
each type of PDF object. Clients do not interact with Objects
directly but instead interact with
<classname>QPDFObjectHandle</classname>.
</para>
<para>
<classname>QPDFObjectHandle</classname> contains
<classname>PointerHolder&lt;QPDFObject&gt;</classname> and
includes accessor methods that are type-safe proxies to the
methods of the derived object classes as well as methods for
querying object types. They can be passed around by value,
copied, stored in containers, etc. with very low overhead.
Instances of <classname>QPDFObjectHandle</classname> always
contain a reference back to the <classname>QPDF</classname> object
from which they were created. A
<classname>QPDFObjectHandle</classname> may be direct or indirect.
If indirect, the <classname>QPDFObject</classname> the
<classname>PointerHolder</classname> initially points to is a null
pointer. In this case, the first attempt to access the underlying
<classname>QPDFObject</classname> will result in the
<classname>QPDFObject</classname> being resolved via a call to the
referenced <classname>QPDF</classname> instance. This makes it
essentially impossible to make coding errors in which certain
things will work for some PDF files and not for others based on
which objects are direct and which objects are indirect.
</para>
<para>
Instances of <classname>QPDFObjectHandle</classname> can be
directly created and modified using static factory methods in the
<classname>QPDFObjectHandle</classname> class. There are factory
methods for each type of object as well as a convenience method
<function>QPDFObjectHandle::parse</function> that creates an
object from a string representation of the object. Existing
instances of <classname>QPDFObjectHandle</classname> can also be
modified in several ways. See comments in
<filename>QPDFObjectHandle.hh</filename> for details.
</para>
<para>
When the <classname>QPDF</classname> class creates a new object,
it dynamically allocates the appropriate type of
<classname>QPDFObject</classname> and immediately hands the
pointer to an instance of <classname>QPDFObjectHandle</classname>.
The parser reads a token from the current file position. If the
token is a not either a dictionary or array opener, an object is
immediately constructed from the single token and the parser
returns. Otherwise, the parser is invoked recursively in a
special mode in which it accumulates objects until it finds a
balancing closer. During this process, the
&ldquo;<literal>R</literal>&rdquo; keyword is recognized and an
indirect <classname>QPDFObjectHandle</classname> may be
constructed.
</para>
<para>
The <function>QPDF::resolve()</function> method, which is used to
resolve an indirect object, may be invoked from the
<classname>QPDFObjectHandle</classname> class. It first checks a
cache to see whether this object has already been read. If not,
it reads the object from the PDF file and caches it. It the
returns the resulting <classname>QPDFObjectHandle</classname>.
The calling object handle then replaces its
<classname>PointerHolder&lt;QDFObject&gt;</classname> with the one
from the newly returned <classname>QPDFObjectHandle</classname>.
In this way, only a single copy of any direct object need exist
and clients can access objects transparently without knowing
caring whether they are direct or indirect objects. Additionally,
no object is ever read from the file more than once. That means
that only the portions of the PDF file that are actually needed
are ever read from the input file, thus allowing the qpdf package
to take advantage of this important design goal of PDF files.
</para>
<para>
If the requested object is inside of an object stream, the object
stream itself is first read into memory. Then the tokenizer reads
objects from the memory stream based on the offset information
stored in the stream. Those individual objects are cached, after
which the temporary buffer holding the object stream contents are
discarded. In this way, the first time an object in an object
stream is requested, all objects in the stream are cached.
</para>
<para>
An instance of <classname>QPDF</classname> is constructed by using
the class's default constructor. If desired, the
<classname>QPDF</classname> object may be configured with various
methods that change its default behavior. Then the
<function>QPDF::processFile()</function> method is passed the name
of a PDF file, which permanently associates the file with that
QPDF object. A password may also be given for access to
password-protected files. QPDF does not enforce encryption
parameters and will treat user and owner passwords equivalently.
Either password may be used to access an encrypted file.
<footnote>
<para>
As pointed out earlier, the intention is not for qpdf to be used
to bypass security on files. but as any open source PDF consumer
may be easily modified to bypass basic PDF document security,
and qpdf offers may transformations that can do this as well,
there seems to be little point in the added complexity of
conditionally enforcing document security.
</para>
</footnote>
<classname>QPDF</classname> will allow recovery of a user password
given an owner password. The input PDF file must be seekable.
(Output files written by <classname>QPDFWriter</classname> need
not be seekable, even when creating linearized files.) During
construction, <classname>QPDF</classname> validates the PDF file's
header, and then reads the cross reference tables and trailer
dictionaries. The <classname>QPDF</classname> class keeps only
the first trailer dictionary though it does read all of them so it
can check the <literal>/Prev</literal> key.
<classname>QPDF</classname> class users may request the root
object and the trailer dictionary specifically. The cross
reference table is kept private. Objects may then be requested by
number of by walking the object tree.
</para>
<para>
When a PDF file has a cross-reference stream instead of a
cross-reference table and trailer, requesting the document's
trailer dictionary returns the stream dictionary from the
cross-reference stream instead.
</para>
<para>
There are some convenience routines for very common operations
such as walking the page tree and returning a vector of all page
objects. For full details, please see the header file
<filename>QPDF.hh</filename>.
</para>
<para>
The following example should clarify how
<classname>QPDF</classname> processes a simple file.
<itemizedlist>
<listitem>
<para>
Client constructs <classname>QPDF</classname>
<varname>pdf</varname> and calls
<function>pdf.processFile("a.pdf");</function>.
</para>
</listitem>
<listitem>
<para>
The <classname>QPDF</classname> class checks the beginning of
<filename>a.pdf</filename> for
<literal>%!PDF-1.[0-9]+</literal>. It then reads the cross
reference table mentioned at the end of the file, ensuring that
it is looking before the last <literal>%%EOF</literal>. After
getting to <literal>trailer</literal> keyword, it invokes the
parser.
</para>
</listitem>
<listitem>
<para>
The parser sees &ldquo;<literal>&lt;&lt;</literal>&rdquo;, so
it calls itself recursively in dictionary creation mode.
</para>
</listitem>
<listitem>
<para>
In dictionary creation mode, the parser keeps accumulating
objects until it encounters
&ldquo;<literal>&gt;&gt;</literal>&rdquo;. Each object that is
read is pushed onto a stack. If
&ldquo;<literal>R</literal>&rdquo; is read, the last two
objects on the stack are inspected. If they are integers, they
are popped off the stack and their values are used to construct
an indirect object handle which is then pushed onto the stack.
When &ldquo;<literal>&gt;&gt;</literal>&rdquo; is finally read,
the stack is converted into a
<classname>QPDF_Dictionary</classname> which is placed in a
<classname>QPDFObjectHandle</classname> and returned.
</para>
</listitem>
<listitem>
<para>
The resulting dictionary is saved as the trailer dictionary.
</para>
</listitem>
<listitem>
<para>
The <literal>/Prev</literal> key is searched. If present,
<classname>QPDF</classname> seeks to that point and repeats
except that the new trailer dictionary is not saved. If
<literal>/Prev</literal> is not present, the initial parsing
process is complete.
</para>
<para>
If there is an encryption dictionary, the document's encryption
parameters are initialized.
</para>
</listitem>
<listitem>
<para>
The client requests root object. The
<classname>QPDF</classname> class gets the value of root key
from trailer dictionary and returns it. It is an unresolved
indirect <classname>QPDFObjectHandle</classname>.
</para>
</listitem>
<listitem>
<para>
The client requests the <literal>/Pages</literal> key from root
<classname>QPDFObjectHandle</classname>. The
<classname>QPDFObjectHandle</classname> notices that it is
indirect so it asks <classname>QPDF</classname> to resolve it.
<classname>QPDF</classname> looks in the object cache for an
object with the root dictionary's object ID and generation
number. Upon not seeing it, it checks the cross reference
table, gets the offset, and reads the object present at that
offset. It stores the result in the object cache and returns
the cached result. The calling
<classname>QPDFObjectHandle</classname> replaces its object
pointer with the one from the resolved
<classname>QPDFObjectHandle</classname>, verifies that it a
valid dictionary object, and returns the (unresolved indirect)
<classname>QPDFObject</classname> handle to the top of the
Pages hierarchy.
</para>
<para>
As the client continues to request objects, the same process is
followed for each new requested object.
</para>
</listitem>
</itemizedlist>
</para>
</sect1>
<sect1 id="ref.casting">
<title>Casting Policy</title>
<para>
This section describes the casting policy followed by qpdf's
implementation. This is no concern to qpdf's end users and
largely of no concern to people writing code that uses qpdf, but
it could be of interest to people who are porting qpdf to a new
platform or who are making modifications to the code.
</para>
<para>
The C++ code in qpdf is free of old-style casts except where
unavoidable (e.g. where the old-style cast is in a macro provided
by a third-party header file). When there is a need for a cast,
it is handled, in order of preference, by rewriting the code to
avoid the need for a cast, calling
<function>const_cast</function>, calling
<function>static_cast</function>, calling
<function>reinterpret_cast</function>, or calling some combination
of the above. As a last resort, a compiler-specific
<literal>#pragma</literal> may be used to suppress a warning that
we don't want to fix. Examples may include suppressing warnings
about the use of old-style casts in code that is shared between C
and C++ code.
</para>
<para>
The casting policy explicitly prohibits casting between integer
sizes for no purpose other than to quiet a compiler warning when
there is no reasonable chance of a problem resulting. The reason
for this exclusion is that the practice of adding these additional
casts precludes future use of additional compiler warnings as a
tool for making future improvements to this aspect of the code,
and it also damages the readability of the code.
</para>
<para>
There are a few significant areas where casting is common in the
qpdf sources or where casting would be required to quiet higher
levels of compiler warnings but is omitted at present:
<itemizedlist>
<listitem>
<para>
<type>char</type> vs. <type>unsigned char</type>. For
historical reasons, there are a lot of places in qpdf's
internals that deal with <type>unsigned char</type>, which
means that a lot of casting is required to interoperate with
standard library calls and <type>std::string</type>. In
retrospect, qpdf should have probably used regular (signed)
<type>char</type> and <type>char*</type> everywhere and just
cast to <type>unsigned char</type> when needed, but it's too
late to make that change now. There are
<function>reinterpret_cast</function> calls to go between
<type>char*</type> and <type>unsigned char*</type>, and there
are <function>static_cast</function> calls to go between
<type>char</type> and <type>unsigned char</type>. These should
always be safe.
</para>
</listitem>
<listitem>
<para>
Non-const <type>unsigned char*</type> used in the
<type>Pipeline</type> interface. The pipeline interface has a
<function>write</function> call that uses <type>unsigned
char*</type> without a <type>const</type> qualifier. The main
reason for this is to support pipelines that make calls to
third-party libraries, such as zlib, that don't include
<type>const</type> in their interfaces. Unfortunately, there
are many places in the code where it is desirable to have
<type>const char*</type> with pipelines. None of the pipeline
implementations in qpdf currently modify the data passed to
write, and doing so would be counter to the intent of
<type>Pipeline</type>, but there is nothing in the code to
prevent this from being done. There are places in the code
where <function>const_cast</function> is used to remove the
const-ness of pointers going into <type>Pipeline</type>s. This
could theoretically be unsafe, but there is adequate testing to
assert that it is safe and will remain safe in qpdf's code.
</para>
</listitem>
<listitem>
<para>
<type>size_t</type> vs. <type>qpdf_offset_t</type>. This is
pretty much unavoidable since sizes are unsigned types and
offsets are signed types. Whenever it is necessary to seek by
an amount given by a <type>size_t</type>, it becomes necessary
to mix and match between <type>size_t</type> and
<type>qpdf_offset_t</type>. Additionally, qpdf sometimes
treats memory buffers like files (as with
<type>BufferInputSource</type>, and those seek interfaces have
to be consistent with file-based input sources. Neither gcc
nor MSVC give warnings for this case by default, but both have
warning flags that can enable this. (MSVC:
<option>/W14267</option> or <option>/W3</option>, which also
enables some additional warnings that we ignore; gcc:
<option>-Wconversion -Wsign-conversion</option>). This could
matter for files whose sizes are larger than
2<superscript>63</superscript> bytes, but it is reasonable to
expect that a world where such files are common would also have
larger <type>size_t</type> and <type>qpdf_offset_t</type> types
in it. On most 64-bit systems at the time of this writing (the
release of version 4.1.0 of qpdf), both <type>size_t</type> and
<type>qpdf_offset_t</type> are 64-bit integer types, while on
many current 32-bit systems, <type>size_t</type> is a 32-bit
type while <type>qpdf_offset_t</type> is a 64-bit type. I am
not aware of any cases where 32-bit systems that have
<type>size_t</type> smaller than <type>qpdf_offset_t</type>
could run into problems. Although I can't conclusively rule
out the possibility of such problems existing, I suspect any
cases would be pretty contrived. In the event that someone
should produce a file that qpdf can't handle because of what is
suspected to be issues involving the handling of
<type>size_t</type> vs. <type>qpdf_offset_t</type> (such files
may behave properly on 64-bit systems but not on 32-bit systems
because they have very large embedded files or streams, for
example), the above mentioned warning flags could be enabled
and all those implicit conversions could be carefully
scrutinized. (I have already gone through that exercise once
in adding support for files larger than 4&nbsp;GB in size.) I
continue to be committed to supporting large files on 32-bit
systems, but I would not go to any lengths to support corner
cases involving large embedded files or large streams that work
on 64-bit systems but not on 32-bit systems because of
<type>size_t</type> being too small. It is reasonable to
assume that anyone working with such files would be using a
64-bit system anyway since many 32-bit applications would have
similar difficulties.
</para>
</listitem>
<listitem>
<para>
<type>size_t</type> vs. <type>int</type> or <type>long</type>.
There are some cases where <type>size_t</type> and
<type>int</type> or <type>long</type> or <type>size_t</type>
and <type>unsigned int</type> or <type>unsigned long</type> are
used interchangeably. These cases occur when working with very
small amounts of memory, such as with the bit readers (where
we're working with just a few bytes at a time), some cases of
<function>strlen</function>, and a few other cases. I have
scrutinized all of these cases and determined them to be safe,
but there is no mechanism in the code to ensure that new unsafe
conversions between <type>int</type> and <type>size_t</type>
aren't introduced short of good testing and strong awareness of
the issues. Again, if any such bugs are suspected in the
future, enabling the additional warning flags and scrutinizing
the warnings would be in order.
</para>
</listitem>
</itemizedlist>
</para>
<para>
To be clear, I believe qpdf to be well-behaved with respect to
sizes and offsets, and qpdf's test suite includes actual
generation and full processing of files larger than 4&nbsp;GB in
size. The issues raised here are largely academic and should not
in any way be interpreted to mean that qpdf has practical problems
involving sloppiness with integer types. I also believe that
appropriate measures have been taken in the code to avoid problems
with signed vs. unsigned integers from resulting in memory
overwrites or other issues with potential security implications,
though there are never any absolute guarantees.
</para>
</sect1>
<sect1 id="ref.encryption">
<title>Encryption</title>
<para>
Encryption is supported transparently by qpdf. When opening a PDF
file, if an encryption dictionary exists, the
<classname>QPDF</classname> object processes this dictionary using
the password (if any) provided. The primary decryption key is
computed and cached. No further access is made to the encryption
dictionary after that time. When an object is read from a file,
the object ID and generation of the object in which it is
contained is always known. Using this information along with the
stored encryption key, all stream and string objects are
transparently decrypted. Raw encrypted objects are never stored
in memory. This way, nothing in the library ever has to know or
care whether it is reading an encrypted file.
</para>
<para>
An interface is also provided for writing encrypted streams and
strings given an encryption key. This is used by
<classname>QPDFWriter</classname> when it rewrites encrypted
files.
</para>
<para>
When copying encrypted files, unless otherwise directed, qpdf will
preserve any encryption in force in the original file. qpdf can
do this with either the user or the owner password. There is no
difference in capability based on which password is used. When 40
or 128 bit encryption keys are used, the user password can be
recovered with the owner password. With 256 keys, the user and
owner passwords are used independently to encrypt the actual
encryption key, so while either can be used, the owner password
can no longer be used to recover the user password.
</para>
<para>
Starting with version 4.0.0, qpdf can read files that are not
encrypted but that contain encrypted attachments, but it cannot
write such files. qpdf also requires the password to be specified
in order to open the file, not just to extract attachments, since
once the file is open, all decryption is handled transparently.
When copying files like this while preserving encryption, qpdf
will apply the file's encryption to everything in the file, not
just to the attachments. When decrypting the file, qpdf will
decrypt the attachments. In general, when copying PDF files with
multiple encryption formats, qpdf will choose the newest format.
The only exception to this is that clear-text metadata will be
preserved as clear-text if it is that way in the original file.
</para>
</sect1>
<sect1 id="ref.adding-and-remove-pages">
<title>Adding and Removing Pages</title>
<para>
While qpdf's API has supported adding and modifying objects for
some time, version 3.0 introduces specific methods for adding and
removing pages. These are largely convenience routines that
handle two tricky issues: pushing inheritable resources from the
<literal>/Pages</literal> tree down to individual pages and
manipulation of the <literal>/Pages</literal> tree itself. For
details, see <function>addPage</function> and surrounding methods
in <filename>QPDF.hh</filename>.
</para>
</sect1>
<sect1 id="ref.reserved-objects">
<title>Reserving Object Numbers</title>
<para>
Version 3.0 of qpdf introduced the concept of reserved objects.
These are seldom needed for ordinary operations, but there are
cases in which you may want to add a series of indirect objects
with references to each other to a <classname>QPDF</classname>
object. This causes a problem because you can't determine the
object ID that a new indirect object will have until you add it to
the <classname>QPDF</classname> object with
<function>QPDF::makeIndirectObject</function>. The only way to
add two mutually referential objects to a
<classname>QPDF</classname> object prior to version 3.0 would be
to add the new objects first and then make them refer to each
other after adding them. Now it is possible to create a
<firstterm>reserved object</firstterm> using
<function>QPDFObjectHandle::newReserved</function>. This is an
indirect object that stays &ldquo;unresolved&rdquo; even if it is
queried for its type. So now, if you want to create a set of
mutually referential objects, you can create reservations for each
one of them and use those reservations to construct the
references. When finished, you can call
<function>QPDF::replaceReserved</function> to replace the reserved
objects with the real ones. This functionality will never be
needed by most applications, but it is used internally by QPDF
when copying objects from other PDF files, as discussed in <xref
linkend="ref.foreign-objects"/>. For an example of how to use
reserved objects, search for <function>newReserved</function> in
<filename>test_driver.cc</filename> in qpdf's sources.
</para>
</sect1>
<sect1 id="ref.foreign-objects">
<title>Copying Objects From Other PDF Files</title>
<para>
Version 3.0 of qpdf introduced the ability to copy objects into a
<classname>QPDF</classname> object from a different
<classname>QPDF</classname> object, which we refer to as
<firstterm>foreign objects</firstterm>. This allows arbitrary
merging of PDF files. The <command>qpdf</command> command-line
tool provides limited support for basic page selection, including
merging in pages from other files, but the library's API makes it
possible to implement arbitrarily complex merging operations. The
main method for copying foreign objects is
<function>QPDF::copyForeignObject</function>. This takes an
indirect object from another <classname>QPDF</classname> and
copies it recursively into this object while preserving all object
structure, including circular references. This means you can add
a direct object that you create from scratch to a
<classname>QPDF</classname> object with
<function>QPDF::makeIndirectObject</function>, and you can add an
indirect object from another file with
<function>QPDF::copyForeignObject</function>. The fact that
<function>QPDF::makeIndirectObject</function> does not
automatically detect a foreign object and copy it is an explicit
design decision. Copying a foreign object seems like a
sufficiently significant thing to do that it should be done
explicitly.
</para>
<para>
The other way to copy foreign objects is by passing a page from
one <classname>QPDF</classname> to another by calling
<function>QPDF::addPage</function>. In contrast to
<function>QPDF::makeIndirectObject</function>, this method
automatically distinguishes between indirect objects in the
current file, foreign objects, and direct objects.
</para>
</sect1>
<sect1 id="ref.rewriting">
<title>Writing PDF Files</title>
<para>
The qpdf library supports file writing of
<classname>QPDF</classname> objects to PDF files through the
<classname>QPDFWriter</classname> class. The
<classname>QPDFWriter</classname> class has two writing modes: one
for non-linearized files, and one for linearized files. See <xref
linkend="ref.linearization"/> for a description of linearization
is implemented. This section describes how we write
non-linearized files including the creation of QDF files (see
<xref linkend="ref.qdf"/>.
</para>
<para>
This outline was written prior to implementation and is not
exactly accurate, but it provides a correct &ldquo;notional&rdquo;
idea of how writing works. Look at the code in
<classname>QPDFWriter</classname> for exact details.
<itemizedlist>
<listitem>
<para>
Initialize state:
<itemizedlist>
<listitem>
<para>
next object number = 1
</para>
</listitem>
<listitem>
<para>
object queue = empty
</para>
</listitem>
<listitem>
<para>
renumber table: old object id/generation to new id/0 = empty
</para>
</listitem>
<listitem>
<para>
xref table: new id -> offset = empty
</para>
</listitem>
</itemizedlist>
</para>
</listitem>
<listitem>
<para>
Create a QPDF object from a file.
</para>
</listitem>
<listitem>
<para>
Write header for new PDF file.
</para>
</listitem>
<listitem>
<para>
Request the trailer dictionary.
</para>
</listitem>
<listitem>
<para>
For each value that is an indirect object, grab the next object
number (via an operation that returns and increments the
number). Map object to new number in renumber table. Push
object onto queue.
</para>
</listitem>
<listitem>
<para>
While there are more objects on the queue:
<itemizedlist>
<listitem>
<para>
Pop queue.
</para>
</listitem>
<listitem>
<para>
Look up object's new number <emphasis>n</emphasis> in the
renumbering table.
</para>
</listitem>
<listitem>
<para>
Store current offset into xref table.
</para>
</listitem>
<listitem>
<para>
Write <literal><replaceable>n</replaceable> 0 obj</literal>.
</para>
</listitem>
<listitem>
<para>
If object is null, whether direct or indirect, write out
null, thus eliminating unresolvable indirect object
references.
</para>
</listitem>
<listitem>
<para>
If the object is a stream stream, write stream contents,
piped through any filters as required, to a memory buffer.
Use this buffer to determine the stream length.
</para>
</listitem>
<listitem>
<para>
If object is not a stream, array, or dictionary, write out
its contents.
</para>
</listitem>
<listitem>
<para>
If object is an array or dictionary (including stream),
traverse its elements (for array) or values (for
dictionaries), handling recursive dictionaries and arrays,
looking for indirect objects. When an indirect object is
found, if it is not resolvable, ignore. (This case is
handled when writing it out.) Otherwise, look it up in the
renumbering table. If not found, grab the next available
object number, assign to the referenced object in the
renumbering table, and push the referenced object onto the
queue. As a special case, when writing out a stream
dictionary, replace length, filters, and decode parameters
as required.
</para>
<para>
Write out dictionary or array, replacing any unresolvable
indirect object references with null (pdf spec says
reference to non-existent object is legal and resolves to
null) and any resolvable ones with references to the
renumbered objects.
</para>
</listitem>
<listitem>
<para>
If the object is a stream, write
<literal>stream\n</literal>, the stream contents (from the
memory buffer), and <literal>\nendstream\n</literal>.
</para>
</listitem>
<listitem>
<para>
When done, write <literal>endobj</literal>.
</para>
</listitem>
</itemizedlist>
</para>
</listitem>
</itemizedlist>
</para>
<para>
Once we have finished the queue, all referenced objects will have
been written out and all deleted objects or unreferenced objects
will have been skipped. The new cross-reference table will
contain an offset for every new object number from 1 up to the
number of objects written. This can be used to write out a new
xref table. Finally we can write out the trailer dictionary with
appropriately computed /ID (see spec, 8.3, File Identifiers), the
cross reference table offset, and <literal>%%EOF</literal>.
</para>
</sect1>
<sect1 id="ref.filtered-streams">
<title>Filtered Streams</title>
<para>
Support for streams is implemented through the
<classname>Pipeline</classname> interface which was designed for
this package.
</para>
<para>
When reading streams, create a series of
<classname>Pipeline</classname> objects. The
<classname>Pipeline</classname> abstract base requires
implementation <function>write()</function> and
<function>finish()</function> and provides an implementation of
<function>getNext()</function>. Each pipeline object, upon
receiving data, does whatever it is going to do and then writes
the data (possibly modified) to its successor. Alternatively, a
pipeline may be an end-of-the-line pipeline that does something
like store its output to a file or a memory buffer ignoring a
successor. For additional details, look at
<filename>Pipeline.hh</filename>.
</para>
<para>
<classname>QPDF</classname> can read raw or filtered streams.
When reading a filtered stream, the <classname>QPDF</classname>
class creates a <classname>Pipeline</classname> object for one of
each appropriate filter object and chains them together. The last
filter should write to whatever type of output is required. The
<classname>QPDF</classname> class has an interface to write raw or
filtered stream contents to a given pipeline.
</para>
</sect1>
</chapter>
<chapter id="ref.linearization">
<title>Linearization</title>
<para>
This chapter describes how <classname>QPDF</classname> and
<classname>QPDFWriter</classname> implement creation and processing
of linearized PDFS.
</para>
<sect1 id="ref.linearization-strategy">
<title>Basic Strategy for Linearization</title>
<para>
To avoid the incestuous problem of having the qpdf library
validate its own linearized files, we have a special linearized
file checking mode which can be invoked via <command>qpdf
--check-linearization</command> (or <command>qpdf
--check</command>). This mode reads the linearization parameter
dictionary and the hint streams and validates that object
ordering, parameters, and hint stream contents are correct. The
validation code was first tested against linearized files created
by external tools (Acrobat and pdlin) and then used to validate
files created by <classname>QPDFWriter</classname> itself.
</para>
</sect1>
<sect1 id="ref.linearized.preparation">
<title>Preparing For Linearization</title>
<para>
Before creating a linearized PDF file from any other PDF file, the
PDF file must be altered such that all page attributes are
propagated down to the page level (and not inherited from parents
in the <literal>/Pages</literal> tree). We also have to know
which objects refer to which other objects, being concerned with
page boundaries and a few other cases. We refer to this part of
preparing the PDF file as <firstterm>optimization</firstterm>,
discussed in <xref linkend="ref.optimization"/>. Note the, in
this context, the term <firstterm>optimization</firstterm> is a
qpdf term, and the term <firstterm>linearization</firstterm> is a
term from the PDF specification. Do not be confused by the fact
that many applications refer to linearization as optimization or
web optimization.
</para>
<para>
When creating linearized PDF files from optimized PDF files, there
are really only a few issues that need to be dealt with:
<itemizedlist>
<listitem>
<para>
Creation of hints tables
</para>
</listitem>
<listitem>
<para>
Placing objects in the correct order
</para>
</listitem>
<listitem>
<para>
Filling in offsets and byte sizes
</para>
</listitem>
</itemizedlist>
</para>
</sect1>
<sect1 id="ref.optimization">
<title>Optimization</title>
<para>
In order to perform various operations such as linearization and
splitting files into pages, it is necessary to know which objects
are referenced by which pages, page thumbnails, and root and
trailer dictionary keys. It is also necessary to ensure that all
page-level attributes appear directly at the page level and are
not inherited from parents in the pages tree.
</para>
<para>
We refer to the process of enforcing these constraints as
<firstterm>optimization</firstterm>. As mentioned above, note
that some applications refer to linearization as optimization.
Although this optimization was initially motivated by the need to
create linearized files, we are using these terms separately.
</para>
<para>
PDF file optimization is implemented in the
<filename>QPDF_optimization.cc</filename> source file. That file
is richly commented and serves as the primary reference for the
optimization process.
</para>
<para>
After optimization has been completed, the private member
variables <varname>obj_user_to_objects</varname> and
<varname>object_to_obj_users</varname> in
<classname>QPDF</classname> have been populated. Any object that
has more than one value in the
<varname>object_to_obj_users</varname> table is shared. Any
object that has exactly one value in the
<varname>object_to_obj_users</varname> table is private. To find
all the private objects in a page or a trailer or root dictionary
key, one merely has make this determination for each element in
the <varname>obj_user_to_objects</varname> table for the given
page or key.
</para>
<para>
Note that pages and thumbnails have different object user types,
so the above test on a page will not include objects referenced by
the page's thumbnail dictionary and nothing else.
</para>
</sect1>
<sect1 id="ref.linearization.writing">
<title>Writing Linearized Files</title>
<para>
We will create files with only primary hint streams. We will
never write overflow hint streams. (As of PDF version 1.4,
Acrobat doesn't either, and they are never necessary.) The hint
streams contain offset information to objects that point to where
they would be if the hint stream were not present. This means
that we have to calculate all object positions before we can
generate and write the hint table. This means that we have to
generate the file in two passes. To make this reliable,
<classname>QPDFWriter</classname> in linearization mode invokes
exactly the same code twice to write the file to a pipeline.
</para>
<para>
In the first pass, the target pipeline is a count pipeline chained
to a discard pipeline. The count pipeline simply passes its data
through to the next pipeline in the chain but can return the
number of bytes passed through it at any intermediate point. The
discard pipeline is an end of line pipeline that just throws its
data away. The hint stream is not written and dummy values with
adequate padding are stored in the first cross reference table,
linearization parameter dictionary, and /Prev key of the first
trailer dictionary. All the offset, length, object renumbering
information, and anything else we need for the second pass is
stored.
</para>
<para>
At the end of the first pass, this information is passed to the
<classname>QPDF</classname> class which constructs a compressed
hint stream in a memory buffer and returns it.
<classname>QPDFWriter</classname> uses this information to write a
complete hint stream object into a memory buffer. At this point,
the length of the hint stream is known.
</para>
<para>
In the second pass, the end of the pipeline chain is a regular
file instead of a discard pipeline, and we have known values for
all the offsets and lengths that we didn't have in the first pass.
We have to adjust offsets that appear after the start of the hint
stream by the length of the hint stream, which is known. Anything
that is of variable length is padded, with the padding code
surrounding any writing code that differs in the two passes. This
ensures that changes to the way things are represented never
results in offsets that were gathered during the first pass
becoming incorrect for the second pass.
</para>
<para>
Using this strategy, we can write linearized files to a
non-seekable output stream with only a single pass to disk or
wherever the output is going.
</para>
</sect1>
<sect1 id="ref.linearization-data">
<title>Calculating Linearization Data</title>
<para>
Once a file is optimized, we have information about which objects
access which other objects. We can then process these tables to
decide which part (as described in &ldquo;Linearized PDF Document
Structure&rdquo; in the PDF specification) each object is
contained within. This tells us the exact order in which objects
are written. The <classname>QPDFWriter</classname> class asks for
this information and enqueues objects for writing in the proper
order. It also turns on a check that causes an exception to be
thrown if an object is encountered that has not already been
queued. (This could happen only if there were a bug in the
traversal code used to calculate the linearization data.)
</para>
</sect1>
<sect1 id="ref.linearization-issues">
<title>Known Issues with Linearization</title>
<para>
There are a handful of known issues with this linearization code.
These issues do not appear to impact the behavior of linearized
files which still work as intended: it is possible for a web
browser to begin to display them before they are fully
downloaded. In fact, it seems that various other programs that
create linearized files have many of these same issues. These
items make reference to terminology used in the linearization
appendix of the PDF specification.
<itemizedlist>
<listitem>
<para>
Thread Dictionary information keys appear in part 4 with the
rest of Threads instead of in part 9. Objects in part 9 are
not grouped together functionally.
</para>
</listitem>
<listitem>
<para>
We are not calculating numerators for shared object positions
within content streams or interleaving them within content
streams.
</para>
</listitem>
<listitem>
<para>
We generate only page offset, shared object, and outline hint
tables. It would be relatively easy to add some additional
tables. We gather most of the information needed to create
thumbnail hint tables. There are comments in the code about
this.
</para>
</listitem>
</itemizedlist>
</para>
</sect1>
<sect1 id="ref.linearization-debugging">
<title>Debugging Note</title>
<para>
The <command>qpdf --show-linearization</command> command can show
the complete contents of linearization hint streams. To look at
the raw data, you can extract the filtered contents of the
linearization hint tables using <command>qpdf --show-object=n
--filtered-stream-data</command>. Then, to convert this into a
bit stream (since linearization tables are bit streams written
without regard to byte boundaries), you can pipe the resulting
data through the following perl code:
<programlisting>use bytes;
binmode STDIN;
undef $/;
my $a = &lt;STDIN&gt;;
my @ch = split(//, $a);
map { printf("%08b", ord($_)) } @ch;
print "\n";
</programlisting>
</para>
</sect1>
</chapter>
<chapter id="ref.object-and-xref-streams">
<title>Object and Cross-Reference Streams</title>
<para>
This chapter provides information about the implementation of
object stream and cross-reference stream support in qpdf.
</para>
<sect1 id="ref.object-streams">
<title>Object Streams</title>
<para>
Object streams can contain any regular object except the
following:
<itemizedlist>
<listitem>
<para>
stream objects
</para>
</listitem>
<listitem>
<para>
objects with generation &gt; 0
</para>
</listitem>
<listitem>
<para>
the encryption dictionary
</para>
</listitem>
<listitem>
<para>
objects containing the /Length of another stream
</para>
</listitem>
</itemizedlist>
In addition, Adobe reader (at least as of version 8.0.0) appears
to not be able to handle having the document catalog appear in an
object stream if the file is encrypted, though this is not
specifically disallowed by the specification.
</para>
<para>
There are additional restrictions for linearized files. See <xref
linkend="ref.object-streams-linearization"/>for details.
</para>
<para>
The PDF specification refers to objects in object streams as
&ldquo;compressed objects&rdquo; regardless of whether the object
stream is compressed.
</para>
<para>
The generation number of every object in an object stream must be
zero. It is possible to delete and replace an object in an object
stream with a regular object.
</para>
<para>
The object stream dictionary has the following keys:
<itemizedlist>
<listitem>
<para>
<literal>/N</literal>: number of objects
</para>
</listitem>
<listitem>
<para>
<literal>/First</literal>: byte offset of first object
</para>
</listitem>
<listitem>
<para>
<literal>/Extends</literal>: indirect reference to stream that
this extends
</para>
</listitem>
</itemizedlist>
</para>
<para>
Stream collections are formed with <literal>/Extends</literal>.
They must form a directed acyclic graph. These can be used for
semantic information and are not meaningful to the PDF document's
syntactic structure. Although qpdf preserves stream collections,
it never generates them and doesn't make use of this information
in any way.
</para>
<para>
The specification recommends limiting the number of objects in
object stream for efficiency in reading and decoding. Acrobat 6
uses no more than 100 objects per object stream for linearized
files and no more 200 objects per stream for non-linearized files.
<classname>QPDFWriter</classname>, in object stream generation
mode, never puts more than 100 objects in an object stream.
</para>
<para>
Object stream contents consists of <emphasis>N</emphasis> pairs of
integers, each of which is the object number and the byte offset
of the object relative to the first object in the stream, followed
by the objects themselves, concatenated.
</para>
</sect1>
<sect1 id="ref.xref-streams">
<title>Cross-Reference Streams</title>
<para>
For non-hybrid files, the value following
<literal>startxref</literal> is the byte offset to the xref stream
rather than the word <literal>xref</literal>.
</para>
<para>
For hybrid files (files containing both xref tables and
cross-reference streams), the xref table's trailer dictionary
contains the key <literal>/XRefStm</literal> whose value is the
byte offset to a cross-reference stream that supplements the xref
table. A PDF 1.5-compliant application should read the xref table
first. Then it should replace any object that it has already seen
with any defined in the xref stream. Then it should follow any
<literal>/Prev</literal> pointer in the original xref table's
trailer dictionary. The specification is not clear about what
should be done, if anything, with a <literal>/Prev</literal>
pointer in the xref stream referenced by an xref table. The
<classname>QPDF</classname> class ignores it, which is probably
reasonable since, if this case were to appear for any sensible PDF
file, the previous xref table would probably have a corresponding
<literal>/XRefStm</literal> pointer of its own. For example, if a
hybrid file were appended, the appended section would have its own
xref table and <literal>/XRefStm</literal>. The appended xref
table would point to the previous xref table which would point the
<literal>/XRefStm</literal>, meaning that the new
<literal>/XRefStm</literal> doesn't have to point to it.
</para>
<para>
Since xref streams must be read very early, they may not be
encrypted, and the may not contain indirect objects for keys
required to read them, which are these:
<itemizedlist>
<listitem>
<para>
<literal>/Type</literal>: value <literal>/XRef</literal>
</para>
</listitem>
<listitem>
<para>
<literal>/Size</literal>: value <emphasis>n+1</emphasis>: where
<emphasis>n</emphasis> is highest object number (same as
<literal>/Size</literal> in the trailer dictionary)
</para>
</listitem>
<listitem>
<para>
<literal>/Index</literal> (optional): value
<literal>[<replaceable>n count</replaceable> ...]</literal>
used to determine which objects' information is stored in this
stream. The default is <literal>[0 /Size]</literal>.
</para>
</listitem>
<listitem>
<para>
<literal>/Prev</literal>: value
<replaceable>offset</replaceable>: byte offset of previous xref
stream (same as <literal>/Prev</literal> in the trailer
dictionary)
</para>
</listitem>
<listitem>
<para>
<literal>/W [...]</literal>: sizes of each field in the xref
table
</para>
</listitem>
</itemizedlist>
</para>
<para>
The other fields in the xref stream, which may be indirect if
desired, are the union of those from the xref table's trailer
dictionary.
</para>
<sect2 id="ref.xref-stream-data">
<title>Cross-Reference Stream Data</title>
<para>
The stream data is binary and encoded in big-endian byte order.
Entries are concatenated, and each entry has a length equal to
the total of the entries in <literal>/W</literal> above. Each
entry consists of one or more fields, the first of which is the
type of the field. The number of bytes for each field is given
by <literal>/W</literal> above. A 0 in <literal>/W</literal>
indicates that the field is omitted and has the default value.
The default value for the field type is
&ldquo;<literal>1</literal>&rdquo;. All other default values are
&ldquo;<literal>0</literal>&rdquo;.
</para>
<para>
PDF 1.5 has three field types:
<itemizedlist>
<listitem>
<para>
0: for free objects. Format: <literal>0 obj
next-generation</literal>, same as the free table in a
traditional cross-reference table
</para>
</listitem>
<listitem>
<para>
1: regular non-compressed object. Format: <literal>1 offset
generation</literal>
</para>
</listitem>
<listitem>
<para>
2: for objects in object streams. Format: <literal>2
object-stream-number index</literal>, the number of object
stream containing the object and the index within the object
stream of the object.
</para>
</listitem>
</itemizedlist>
</para>
<para>
It seems standard to have the first entry in the table be
<literal>0 0 0</literal> instead of <literal>0 0 ffff</literal>
if there are no deleted objects.
</para>
</sect2>
</sect1>
<sect1 id="ref.object-streams-linearization">
<title>Implications for Linearized Files</title>
<para>
For linearized files, the linearization dictionary, document
catalog, and page objects may not be contained in object streams.
</para>
<para>
Objects stored within object streams are given the highest range
of object numbers within the main and first-page cross-reference
sections.
</para>
<para>
It is okay to use cross-reference streams in place of regular xref
tables. There are on special considerations.
</para>
<para>
Hint data refers to object streams themselves, not the objects in
the streams. Shared object references should also be made to the
object streams. There are no reference in any hint tables to the
object numbers of compressed objects (objects within object
streams).
</para>
<para>
When numbering objects, all shared objects within both the first
and second halves of the linearized files must be numbered
consecutively after all normal uncompressed objects in that half.
</para>
</sect1>
<sect1 id="ref.object-stream-implementation">
<title>Implementation Notes</title>
<para>
There are three modes for writing object streams:
<option>disable</option>, <option>preserve</option>, and
<option>generate</option>. In disable mode, we do not generate
any object streams, and we also generate an xref table rather than
xref streams. This can be used to generate PDF files that are
viewable with older readers. In preserve mode, we write object
streams such that written object streams contain the same objects
and <literal>/Extends</literal> relationships as in the original
file. This is equal to disable if the file has no object streams.
In generate, we create object streams ourselves by grouping
objects that are allowed in object streams together in sets of no
more than 100 objects. We also ensure that the PDF version is at
least 1.5 in generate mode, but we preserve the version header in
the other modes. The default is <option>preserve</option>.
</para>
<para>
We do not support creation of hybrid files. When we write files,
even in preserve mode, we will lose any xref tables and merge any
appended sections.
</para>
</sect1>
</chapter>
<appendix id="ref.release-notes">
<title>Release Notes</title>
<para>
For a detailed list of changes, please see the file
<filename>ChangeLog</filename> in the source distribution.
</para>
<variablelist>
<varlistentry>
<term>4.1.0: April 14, 2013</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Note to people including qpdf in distributions: the
<filename>.la</filename> files generated by libtool are now
installed by qpdf's <command>make install</command> target.
Before, they were not installed. This means that if your
distribution does not want to include <filename>.la</filename>
files, you must remove them as part of your packaging process.
</para>
</listitem>
<listitem>
<para>
Major enhancement: API enhancements have been made to support
parsing of content streams. This enhancement includes the
following changes:
<itemizedlist>
<listitem>
<para>
<function>QPDFObjectHandle::parseContentStream</function>
method parses objects in a content stream and calls
handlers in a callback class. The example
<filename>examples/pdf-parse-content.cc</filename>
illustrates how this may be used.
</para>
</listitem>
<listitem>
<para>
<type>QPDFObjectHandle</type> can now represent operators
and inline images, object types that may only appear in
content streams.
</para>
</listitem>
<listitem>
<para>
Method <function>QPDFObjectHandle::getTypeCode()</function>
returns an enumerated type value representing the
underlying object type. Method
<function>QPDFObjectHandle::getTypeName()</function>
returns a text string describing the name of the type of a
<type>QPDFObjectHandle</type> object. These methods can be
used for more efficient parsing and debugging/diagnostic
messages.
</para>
</listitem>
</itemizedlist>
</para>
</listitem>
<listitem>
<para>
<command>qpdf --check</command> now parses all pages' content
streams in addition to doing other checks. While there are
still many types of errors that cannot be detected, syntactic
errors in content streams will now be reported.
</para>
</listitem>
<listitem>
<para>
Minor compilation enhancements have been made to facilitate
easier for support for a broader range of compilers and
compiler versions.
<itemizedlist>
<listitem>
<para>
Warning flags have been moved into a separate variable in
<filename>autoconf.mk</filename>
</para>
</listitem>
<listitem>
<para>
The configure flag <option>--enable-werror</option> work
for Microsoft compilers
</para>
</listitem>
<listitem>
<para>
All MSVC CRT security warnings have been resolved.
</para>
</listitem>
<listitem>
<para>
All C-style casts in C++ Code have been replaced by C++
casts, and many casts that had been included to suppress
higher warning levels for some compilers have been removed,
primarily for clarity. Places where integer type coercion
occurs have been scrutinized. A new casting policy has
been documented in the manual. This is of concern mainly
to people porting qpdf to new platforms or compilers. It
is not visible to programmers writing code that uses the
library
</para>
</listitem>
<listitem>
<para>
Some internal limits have been removed in code that
converts numbers to strings. This is largely invisible to
users, but it does trigger a bug in some older versions of
mingw-w64's C++ library. See
<filename>README-windows.txt</filename> in the source
distribution if you think this may affect you. The copy of
the DLL distributed with qpdf's binary distribution is not
affected by this problem.
</para>
</listitem>
</itemizedlist>
</para>
</listitem>
<listitem>
<para>
The RPM spec file previously included with qpdf has been
removed. This is because virtually all Linux distributions
include qpdf now that it is a dependency of CUPS filters.
</para>
</listitem>
<listitem>
<para>
A few bug fixes are included:
<itemizedlist>
<listitem>
<para>
Overridden compressed objects are properly handled.
Before, there were certain constructs that could cause qpdf
to see old versions of some objects. The most usual
manifestation of this was loss of filled in form values for
certain files.
</para>
</listitem>
<listitem>
<para>
Installation no longer uses GNU/Linux-specific versions of
some commands, so <command>make install</command> works on
Solaris with native tools.
</para>
</listitem>
<listitem>
<para>
The 64-bit mingw Windows binary package no longer includes
a 32-bit DLL.
</para>
</listitem>
</itemizedlist>
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>4.0.1: January 17, 2013</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Fix detection of binary attachments in test suite to avoid
false test failures on some platforms.
</para>
</listitem>
<listitem>
<para>
Add clarifying comment in <filename>QPDF.hh</filename> to
methods that return the user password explaining that it is no
longer possible with newer encryption formats to recover the
user password knowing the owner password. In earlier
encryption formats, the user password was encrypted in the
file using the owner password. In newer encryption formats, a
separate encryption key is used on the file, and that key is
independently encrypted using both the user password and the
owner password.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>4.0.0: December 31, 2012</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Major enhancement: support has been added for newer encryption
schemes supported by version X of Adobe Acrobat. This
includes use of 127-character passwords, 256-bit encryption
keys, and the encryption scheme specified in ISO 32000-2, the
PDF 2.0 specification. This scheme can be chosen from the
command line by specifying use of 256-bit keys. qpdf also
supports the deprecated encryption method used by Acrobat IX.
This encryption style has known security weaknesses and should
not be used in practice. However, such files exist &ldquo;in
the wild,&rdquo; so support for this scheme is still useful.
New methods
<function>QPDFWriter::setR6EncryptionParameters</function>
(for the PDF 2.0 scheme) and
<function>QPDFWriter::setR5EncryptionParameters</function>
(for the deprecated scheme) have been added to enable these
new encryption schemes. Corresponding functions have been
added to the C API as well.
</para>
</listitem>
<listitem>
<para>
Full support for Adobe extension levels in PDF version
information. Starting with PDF version 1.7, corresponding to
ISO 32000, Adobe adds new functionality by increasing the
extension level rather than increasing the version. This
support includes addition of the
<function>QPDF::getExtensionLevel</function> method for
retrieving the document's extension level, addition of
versions of
<function>QPDFWriter::setMinimumPDFVersion</function> and
<function>QPDFWriter::forcePDFVersion</function> that accept
an extension level, and extended syntax for specifying forced
and minimum versions on the command line as described in <xref
linkend="ref.advanced-transformation"/>. Corresponding
functions have been added to the C API as well.
</para>
</listitem>
<listitem>
<para>
Minor fixes to prevent qpdf from referencing objects in the
file that are not referenced in the file's overall structure.
Most files don't have any such objects, but some files have
contain unreferenced objects with errors, so these fixes
prevent qpdf from needlessly rejecting or complaining about
such objects.
</para>
</listitem>
<listitem>
<para>
Add new generalized methods for reading and writing files
from/to programmer-defined sources. The method
<function>QPDF::processInputSource</function> allows the
programmer to use any input source for the input file, and
<function>QPDFWriter::setOutputPipeline</function> allows the
programmer to write the output file through any pipeline.
These methods would make it possible to perform any number of
specialized operations, such as accessing external storage
systems, creating bindings for qpdf in other programming
languages that have their own I/O systems, etc.
</para>
</listitem>
<listitem>
<para>
Add new method <function>QPDF::getEncryptionKey</function> for
retrieving the underlying encryption key used in the file.
</para>
</listitem>
<listitem>
<para>
This release includes a small handful of non-compatible API
changes. While effort is made to avoid such changes, all the
non-compatible API changes in this version were to parts of
the API that would likely never be used outside the library
itself. In all cases, the altered methods or structures were
parts of the <classname>QPDF</classname> that were public to
enable them to be called from either
<classname>QPDFWriter</classname> or were part of validation
code that was over-zealous in reporting problems in parts of
the file that would not ordinarily be referenced. In no case
did any of the removed methods do anything worse that falsely
report error conditions in files that were broken in ways that
didn't matter. The following public parts of the
<classname>QPDF</classname> class were changed in a
non-compatible way:
<itemizedlist>
<listitem>
<para>
Updated nested <classname>QPDF::EncryptionData</classname>
class to add fields needed by the newer encryption formats,
member variables changed to private so that future changes
will not require breaking backward compatibility.
</para>
</listitem>
<listitem>
<para>
Added additional parameters to
<function>compute_data_key</function>, which is used by
<classname>QPDFWriter</classname> to compute the encryption
key used to encrypt a specific object.
</para>
</listitem>
<listitem>
<para>
Removed the method
<function>flattenScalarReferences</function>. This method
was previously used prior to writing a new PDF file, but it
has the undesired side effect of causing qpdf to read
objects in the file that were not referenced. Some
otherwise files have unreferenced objects with errors in
them, so this could cause qpdf to reject files that would
be accepted by virtually all other PDF readers. In fact,
qpdf relied on only a very small part of what
flattenScalarReferences did, so only this part has been
preserved, and it is now done directly inside
<classname>QPDFWriter</classname>.
</para>
</listitem>
<listitem>
<para>
Removed the method <function>decodeStreams</function>.
This method was used by the <option>--check</option> option
of the <command>qpdf</command> command-line tool to force
all streams in the file to be decoded, but it also suffered
from the problem of opening otherwise unreferenced streams
and thus could report false positive. The
<option>--check</option> option now causes qpdf to go
through all the motions of writing a new file based on the
original one, so it will always reference and check exactly
those parts of a file that any ordinary viewer would check.
</para>
</listitem>
<listitem>
<para>
Removed the method
<function>trimTrailerForWrite</function>. This method was
used by <classname>QPDFWriter</classname> to modify the
original QPDF object by removing fields from the trailer
dictionary that wouldn't apply to the newly written file.
This functionality, though generally harmless, was a poor
implementation and has been replaced by having QPDFWriter
filter these out when copying the trailer rather than
modifying the original QPDF object. (Note that qpdf never
modifies the original file itself.)
</para>
</listitem>
</itemizedlist>
</para>
</listitem>
<listitem>
<para>
Allow the PDF header to appear anywhere in the first 1024
bytes of the file. This is consistent with what other readers
do.
</para>
</listitem>
<listitem>
<para>
Fix the <command>pkg-config</command> files to list zlib and
pcre in <function>Requires.private</function> to better
support static linking using <command>pkg-config</command>.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>3.0.2: September 6, 2012</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Bug fix: <function>QPDFWriter::setOutputMemory</function> did
not work when not used with
<function>QPDFWriter::setStaticID</function>, which made it
pretty much useless. This has been fixed.
</para>
</listitem>
<listitem>
<para>
New API call
<function>QPDFWriter::setExtraHeaderText</function> inserts
additional text near the header of the PDF file. The intended
use case is to insert comments that may be consumed by a
downstream application, though other use cases may exist.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>3.0.1: August 11, 2012</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Version 3.0.0 included addition of files for
<command>pkg-config</command>, but this was not mentioned in
the release notes. The release notes for 3.0.0 were updated
to mention this.
</para>
</listitem>
<listitem>
<para>
Bug fix: if an object stream ended with a scalar object not
followed by space, qpdf would incorrectly report that it
encountered a premature EOF. This bug has been in qpdf since
version&nbsp;2.0.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>3.0.0: August 2, 2012</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Acknowledgment: I would like to express gratitude for the
contributions of Tobias Hoffmann toward the release of qpdf
version 3.0. He is responsible for most of the implementation
and design of the new API for manipulating pages, and
contributed code and ideas for many of the improvements made
in version 3.0. Without his work, this release would
certainly not have happened as soon as it did, if at all.
</para>
</listitem>
<listitem>
<para>
<emphasis>Non-compatible API change:</emphasis> The version of
<function>QPDFObjectHandle::replaceStreamData</function> that
uses a <classname>StreamDataProvider</classname> no longer
requires (or accepts) a <varname>length</varname> parameter.
See <xref linkend="ref.upgrading-to-3.0"/> for an explanation.
While care is taken to avoid non-compatible API changes in
general, an exception was made this time because the new
interface offers an opportunity to significantly simplify
calling code.
</para>
</listitem>
<listitem>
<para>
Support has been added for large files. The test suite
verifies support for files larger than 4 gigabytes, and manual
testing has verified support for files larger than 10
gigabytes. Large file support is available for both 32-bit
and 64-bit platforms as long as the compiler and underlying
platforms support it.
</para>
</listitem>
<listitem>
<para>
Support for page selection (splitting and merging PDF files)
has been added to the <command>qpdf</command> command-line
tool. See <xref linkend="ref.page-selection"/>.
</para>
</listitem>
<listitem>
<para>
Options have been added to the <command>qpdf</command>
command-line tool for copying encryption parameters from
another file. See <xref linkend="ref.basic-options"/>.
</para>
</listitem>
<listitem>
<para>
New methods have been added to the <classname>QPDF</classname>
object for adding and removing pages. See <xref
linkend="ref.adding-and-remove-pages"/>.
</para>
</listitem>
<listitem>
<para>
New methods have been added to the <classname>QPDF</classname>
object for copying objects from other PDF files. See <xref
linkend="ref.foreign-objects"/>
</para>
</listitem>
<listitem>
<para>
A new method <function>QPDFObjectHandle::parse</function> has
been added for constructing
<classname>QPDFObjectHandle</classname> objects from a string
description.
</para>
</listitem>
<listitem>
<para>
Methods have been added to <classname>QPDFWriter</classname>
to allow writing to an already open stdio <type>FILE*</type>
addition to writing to standard output or a named file.
Methods have been added to <classname>QPDF</classname> to be
able to process a file from an already open stdio
<type>FILE*</type>. This makes it possible to read and write
PDF from secure temporary files that have been unlinked prior
to being fully read or written.
</para>
</listitem>
<listitem>
<para>
The <function>QPDF::emptyPDF</function> can be used to allow
creation of PDF files from scratch. The example
<filename>examples/pdf-create.cc</filename> illustrates how it
can be used.
</para>
</listitem>
<listitem>
<para>
Several methods to take
<classname>PointerHolder&lt;Buffer&gt;</classname> can now
also accept <type>std::string</type> arguments.
</para>
</listitem>
<listitem>
<para>
Many new convenience methods have been added to the library,
most in <classname>QPDFObjectHandle</classname>. See
<filename>ChangeLog</filename> for a full list.
</para>
</listitem>
<listitem>
<para>
When building on a platform that supports ELF shared libraries
(such as Linux), symbol versions are enabled by default. They
can be disabled by passing
<option>--disable-ld-version-script</option> to
<command>./configure</command>.
</para>
</listitem>
<listitem>
<para>
The file <filename>libqpdf.pc</filename> is now installed to
support <command>pkg-config</command>.
</para>
</listitem>
<listitem>
<para>
Image comparison tests are off by default now since they are
not needed to verify a correct build or port of qpdf. They
are needed only when changing the actual PDF output generated
by qpdf. You should enable them if you are making deep
changes to qpdf itself. See <filename>README</filename> for
details.
</para>
</listitem>
<listitem>
<para>
Large file tests are off by default but can be turned on with
<command>./configure</command> or by setting an environment
variable before running the test suite. See
<filename>README</filename> for details.
</para>
</listitem>
<listitem>
<para>
When qpdf's test suite fails, failures are not printed to the
terminal anymore by default. Instead, find them in
<filename>build/qtest.log</filename>. For packagers who are
building with an autobuilder, you can add the
<option>--enable-show-failed-test-output</option> option to
<command>./configure</command> to restore the old behavior.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>2.3.1: December 28, 2011</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Fix thread-safety problem resulting from non-thread-safe use
of the PCRE library.
</para>
</listitem>
<listitem>
<para>
Made a few minor documentation fixes.
</para>
</listitem>
<listitem>
<para>
Add workaround for a bug that appears in some versions of
ghostscript to the test suite
</para>
</listitem>
<listitem>
<para>
Fix minor build issue for Visual C++ 2010.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>2.3.0: August 11, 2011</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Bug fix: when preserving existing encryption on encrypted
files with cleartext metadata, older qpdf versions would
generate password-protected files with no valid password.
This operation now works. This bug only affected files
created by copying existing encryption parameters; explicit
encryption with specification of cleartext metadata worked
before and continues to work.
</para>
</listitem>
<listitem>
<para>
Enhance <classname>QPDFWriter</classname> with a new
constructor that allows you to delay the specification of the
output file. When using this constructor, you may now call
<function>QPDFWriter::setOutputFilename</function> to specify
the output file, or you may use
<function>QPDFWriter::setOutputMemory</function> to cause
<classname>QPDFWriter</classname> to write the resulting PDF
file to a memory buffer. You may then use
<function>QPDFWriter::getBuffer</function> to retrieve the
memory buffer.
</para>
</listitem>
<listitem>
<para>
Add new API call <function>QPDF::replaceObject</function> for
replacing objects by object ID
</para>
</listitem>
<listitem>
<para>
Add new API call <function>QPDF::swapObjects</function> for
swapping two objects by object ID
</para>
</listitem>
<listitem>
<para>
Add <function>QPDFObjectHandle::getDictAsMap</function> and
<function>QPDFObjectHandle::getArrayAsVector</function> to
allow retrieval of dictionary objects as maps and array
objects as vectors.
</para>
</listitem>
<listitem>
<para>
Add functions <function>qpdf_get_info_key</function> and
<function>qpdf_set_info_key</function> to the C API for
manipulating string fields of the document's
<literal>/Info</literal> dictionary.
</para>
</listitem>
<listitem>
<para>
Add functions <function>qpdf_init_write_memory</function>,
<function>qpdf_get_buffer_length</function>, and
<function>qpdf_get_buffer</function> to the C API for writing
PDF files to a memory buffer instead of a file.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>2.2.4: June 25, 2011</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Fix installation and compilation issues; no functionality
changes.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>2.2.3: April 30, 2011</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Handle some damaged streams with incorrect characters
following the stream keyword.
</para>
</listitem>
<listitem>
<para>
Improve handling of inline images when normalizing content
streams.
</para>
</listitem>
<listitem>
<para>
Enhance error recovery to properly handle files that use
object 0 as a regular object, which is specifically disallowed
by the spec.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>2.2.2: October 4, 2010</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Add new function <function>qpdf_read_memory</function>
to the C API to call
<function>QPDF::processMemoryFile</function>. This was an
omission in qpdf 2.2.1.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>2.2.1: October 1, 2010</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Add new method <function>QPDF::setOutputStreams</function>
to replace <varname>std::cout</varname> and
<varname>std::cerr</varname> with other streams for generation
of diagnostic messages and error messages. This can be useful
for GUIs or other applications that want to capture any output
generated by the library to present to the user in some other
way. Note that QPDF does not write to
<varname>std::cout</varname> (or the specified output stream)
except where explicitly mentioned in
<filename>QPDF.hh</filename>, and that the only use of the
error stream is for warnings. Note also that output of
warnings is suppressed when
<literal>setSuppressWarnings(true)</literal> is called.
</para>
</listitem>
<listitem>
<para>
Add new method <function>QPDF::processMemoryFile</function>
for operating on PDF files that are loaded into memory rather
than in a file on disk.
</para>
</listitem>
<listitem>
<para>
Give a warning but otherwise ignore empty PDF objects by
treating them as null. Empty object are not permitted by the
PDF specification but have been known to appear in some actual
PDF files.
</para>
</listitem>
<listitem>
<para>
Handle inline image filter abbreviations when the appear as
stream filter abbreviations. The PDF specification does not
allow use of stream filter abbreviations in this way, but
Adobe Reader and some other PDF readers accept them since they
sometimes appear incorrectly in actual PDF files.
</para>
</listitem>
<listitem>
<para>
Implement miscellaneous enhancements to
<classname>PointerHolder</classname> and
<classname>Buffer</classname> to support other changes.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>2.2.0: August 14, 2010</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Add new methods to <classname>QPDFObjectHandle</classname>
(<function>newStream</function> and
<function>replaceStreamData</function> for creating new
streams and replacing stream data. This makes it possible to
perform a wide range of operations that were not previously
possible.
</para>
</listitem>
<listitem>
<para>
Add new helper method in
<classname>QPDFObjectHandle</classname>
(<function>addPageContents</function>) for appending or
prepending new content streams to a page. This method makes
it possible to manipulate content streams without having to be
concerned whether a page's contents are a single stream or an
array of streams.
</para>
</listitem>
<listitem>
<para>
Add new method in <classname>QPDFObjectHandle</classname>:
<function>replaceOrRemoveKey</function>, which replaces a
dictionary key
with a given value unless the value is null, in which case it
removes the key instead.
</para>
</listitem>
<listitem>
<para>
Add new method in <classname>QPDFObjectHandle</classname>:
<function>getRawStreamData</function>, which returns the raw
(unfiltered) stream data into a buffer. This complements the
<function>getStreamData</function> method, which returns the
filtered (uncompressed) stream data and can only be used when
the stream's data is filterable.
</para>
</listitem>
<listitem>
<para>
Provide two new examples:
<command>pdf-double-page-size</command> and
<command>pdf-invert-images</command> that illustrate the newly
added interfaces.
</para>
</listitem>
<listitem>
<para>
Fix a memory leak that would cause loss of a few bytes for
every object involved in a cycle of object references. Thanks
to Jian Ma for calling my attention to the leak.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>2.1.5: April 25, 2010</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Remove restriction of file identifier strings to 16 bytes.
This unnecessary restriction was preventing qpdf from being
able to encrypt or decrypt files with identifier strings that
were not exactly 16 bytes long. The specification imposes no
such restriction.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>2.1.4: April 18, 2010</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Apply the same padding calculation fix from version 2.1.2 to
the main cross reference stream as well.
</para>
</listitem>
<listitem>
<para>
Since <command>qpdf --check</command> only performs limited
checks, clarify the output to make it clear that there still
may be errors that qpdf can't check. This should make it less
surprising to people when another PDF reader is unable to read
a file that qpdf thinks is okay.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>2.1.3: March 27, 2010</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Fix bug that could cause a failure when rewriting PDF files
that contain object streams with unreferenced objects that in
turn reference indirect scalars.
</para>
</listitem>
<listitem>
<para>
Don't complain about (invalid) AES streams that aren't a
multiple of 16 bytes. Instead, pad them before decrypting.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>2.1.2: January 24, 2010</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Fix bug in padding around first half cross reference stream in
linearized files. The bug could cause an assertion failure
when linearizing certain unlucky files.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>2.1.1: December 14, 2009</term>
<listitem>
<itemizedlist>
<listitem>
<para>
No changes in functionality; insert missing include in an
internal library header file to support gcc 4.4, and update
test suite to ignore broken Adobe Reader installations.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>2.1: October 30, 2009</term>
<listitem>
<itemizedlist>
<listitem>
<para>
This is the first version of qpdf to include Windows support.
On Windows, it is possible to build a DLL. Additionally, a
partial C-language API has been introduced, which makes it
possible to call qpdf functions from non-C++ environments. I
am very grateful to <!-- Žarko Gajić --> Zarko Gagic (<ulink
url="http://delphi.about.com/">http://delphi.about.com/</ulink>)
for tirelessly testing numerous pre-release versions of this
DLL and providing many excellent suggestions on improving the
interface.
</para>
<para>
For programming to the C interface, please see the header file
<filename>qpdf/qpdf-c.h</filename> and the example
<filename>examples/pdf-linearize.c</filename>.
</para>
</listitem>
<listitem>
<para>
Zarko Gajic has written a Delphi wrapper for qpdf, which can
be downloaded from qpdf's download side. Zarko's Delphi
wrapper is released with the same licensing terms as qpdf
itself and comes with this disclaimer: &ldquo;Delphi wrapper
unit <filename>qpdf.pas</filename> created by Zarko Gajic
(<ulink
url="http://delphi.about.com/">http://delphi.about.com/</ulink>).
Use at your own risk and for whatever purpose you want. No
support is provided. Sample code is provided.&rdquo;
</para>
</listitem>
<listitem>
<para>
Support has been added for AES encryption and crypt filters.
Although qpdf does not presently support files that use
PKI-based encryption, with the addition of AES and crypt
filters, qpdf is now be able to open most encrypted files
created with newer versions of Acrobat or other PDF creation
software. Note that I have not been able to get very many
files encrypted in this way, so it's possible there could
still be some cases that qpdf can't handle. Please report
them if you find them.
</para>
</listitem>
<listitem>
<para>
Many error messages have been improved to include more
information in hopes of making qpdf a more useful tool for PDF
experts to use in manually recovering damaged PDF files.
</para>
</listitem>
<listitem>
<para>
Attempt to avoid compressing metadata streams if possible.
This is consistent with other PDF creation applications.
</para>
</listitem>
<listitem>
<para>
Provide new command-line options for AES encrypt, cleartext
metadata, and setting the minimum and forced PDF versions of
output files.
</para>
</listitem>
<listitem>
<para>
Add additional methods to the <classname>QPDF</classname>
object for querying the document's permissions. Although qpdf
does not enforce these permissions, it does make them
available so that applications that use qpdf can enforce
permissions.
</para>
</listitem>
<listitem>
<para>
The <option>--check</option> option to <command>qpdf</command>
has been extended to include some additional information.
</para>
</listitem>
<listitem>
<para>
There have been a handful of non-compatible API changes. For
details, see <xref linkend="ref.upgrading-to-2.1"/>.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>2.0.6: May 3, 2009</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Do not attempt to uncompress streams that have decode
parameters we don't recognize. Earlier versions of qpdf would
have rejected files with such streams.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>2.0.5: March 10, 2009</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Improve error handling in the LZW decoder, and fix a small
error introduced in the previous version with regard to
handling full tables. The LZW decoder has been more strongly
verified in this release.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>2.0.4: February 21, 2009</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Include proper support for LZW streams encoded without the
&ldquo;early code change&rdquo; flag. Special thanks to Atom
Smasher who reported the problem and provided an input file
compressed in this way, which I did not previously have.
</para>
</listitem>
<listitem>
<para>
Implement some improvements to file recovery logic.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>2.0.3: February 15, 2009</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Compile cleanly with gcc 4.4.
</para>
</listitem>
<listitem>
<para>
Handle strings encoded as UTF-16BE properly.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>2.0.2: June 30, 2008</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Update test suite to work properly with a
non-<command>bash</command> <filename>/bin/sh</filename> and
with Perl 5.10. No changes were made to the actual qpdf
source code itself for this release.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>2.0.1: May 6, 2008</term>
<listitem>
<itemizedlist>
<listitem>
<para>
No changes in functionality or interface. This release
includes fixes to the source code so that qpdf compiles
properly and passes its test suite on a broader range of
platforms. See <filename>ChangeLog</filename> in the source
distribution for details.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>2.0: April 29, 2008</term>
<listitem>
<itemizedlist>
<listitem>
<para>
First public release.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
</variablelist>
</appendix>
<appendix id="ref.upgrading-to-2.1">
<title>Upgrading from 2.0 to 2.1</title>
<para>
Although, as a general rule, we like to avoid introducing
source-level incompatibilities in qpdf's interface, there were a
few non-compatible changes made in this version. A considerable
amount of source code that uses qpdf will probably compile without
any changes, but in some cases, you may have to update your code.
The changes are enumerated here. There are also some new
interfaces; for those, please refer to the header files.
</para>
<itemizedlist>
<listitem>
<para>
QPDF's exception handling mechanism now uses
<classname>std::logic_error</classname> for internal errors and
<classname>std::runtime_error</classname> for runtime errors in
favor of the now removed <classname>QEXC</classname> classes used
in previous versions. The <classname>QEXC</classname> exception
classes predated the addition of the
<filename>&lt;stdexcept&gt;</filename> header file to the C++
standard library. Most of the exceptions thrown by the qpdf
library itself are still of type <classname>QPDFExc</classname>
which is now derived from
<classname>std::runtime_error</classname>. Programs that caught
an instance of <classname>std::exception</classname> and
displayed it by calling the <function>what()</function> method
will not need to be changed.
</para>
</listitem>
<listitem>
<para>
The <classname>QPDFExc</classname> class now internally
represents various fields of the error condition and provides
interfaces for querying them. Among the fields is a numeric
error code that can help applications act differently on (a small
number of) different error conditions. See
<filename>QPDFExc.hh</filename> for details.
</para>
</listitem>
<listitem>
<para>
Warnings can be retrieved from qpdf as instances of
<classname>QPDFExc</classname> instead of strings.
</para>
</listitem>
<listitem>
<para>
The nested <classname>QPDF::EncryptionData</classname> class's
constructor takes an additional argument. This class is
primarily intended to be used by
<classname>QPDFWriter</classname>. There's not really anything
useful an end-user application could do with it. It probably
shouldn't really be part of the public interface to begin with.
Likewise, some of the methods for computing internal encryption
dictionary parameters have changed to support
<literal>/R=4</literal> encryption.
</para>
</listitem>
<listitem>
<para>
The method <function>QPDF::getUserPassword</function> has been
removed since it didn't do what people would think it did. There
are now two new methods:
<function>QPDF::getPaddedUserPassword</function> and
<function>QPDF::getTrimmedUserPassword</function>. The first one
does what the old <function>QPDF::getUserPassword</function>
method used to do, which is to return the password with possible
binary padding as specified by the PDF specification. The second
one returns a human-readable password string.
</para>
</listitem>
<listitem>
<para>
The enumerated types that used to be nested in
<classname>QPDFWriter</classname> have moved to top-level
enumerated types and are now defined in the file
<filename>qpdf/Constants.h</filename>. This enables them to be
shared by both the C and C++ interfaces.
</para>
</listitem>
</itemizedlist>
</appendix>
<appendix id="ref.upgrading-to-3.0">
<title>Upgrading to 3.0</title>
<para>
For the most part, the API for qpdf version 3.0 is backward
compatible with versions 2.1 and later. There are two exceptions:
<itemizedlist>
<listitem>
<para>
The method
<function>QPDFObjectHandle::replaceStreamData</function> that
uses a <classname>StreamDataProvider</classname> to provide the
stream data no longer takes a <varname>length</varname>
parameter. While it would have been easy enough to keep the
parameter for backward compatibility, in this case, the
parameter was removed since this provides the user an
opportunity to simplify the calling code. This method was
introduced in version 2.2. At the time, the
<varname>length</varname> parameter was required in order to
ensure that calls to the stream data provider returned the same
length for a specific stream every time they were invoked. In
particular, the linearization code depends on this. Instead,
qpdf 3.0 and newer check for that constraint explicitly. The
first time the stream data provider is called for a specific
stream, the actual length is saved, and subsequent calls are
required to return the same number of bytes. This means the
calling code no longer has to compute the length in advance,
which can be a significant simplification. If your code fails
to compile because of the extra argument and you don't want to
make other changes to your code, just omit the argument.
</para>
</listitem>
<listitem>
<para>
Many methods take <type>long long</type> instead of other
integer types. Most if not all existing code should compile
fine with this change since such parameters had always
previously been smaller types. This change was required to
support files larger than two gigabytes in size.
</para>
</listitem>
</itemizedlist>
</para>
</appendix>
<appendix id="ref.upgrading-to-4.0">
<title>Upgrading to 4.0</title>
<para>
While version 4.0 includes a few non-compatible API changes, it is
very unlikely that anyone's code would have used any of those parts
of the API since they generally required information that would
only be available inside the library. In the unlikely event that
you should run into trouble, please see the ChangeLog. See also
<xref linkend="ref.release-notes"/> for a complete list of the
non-compatible API changes made in this version.
</para>
</appendix>
</book>
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