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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 "8.3.0">
<!ENTITY lastreleased "January 7, 2019">
]>
<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;2019</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 is licensed under <ulink
url="http://www.apache.org/licenses/LICENSE-2.0">the Apache
License, Version 2.0</ulink> (the "License"). Unless required by
applicable law or agreed to in writing, software distributed under
the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES
OR CONDITIONS OF ANY KIND, either express or implied. See the
License for the specific language governing permissions and
limitations under the License.
</para>
<para>
Versions of qpdf prior to version 7 were released under the terms
of <ulink url="https://opensource.org/licenses/Artistic-2.0">the
Artistic License, version 2.0</ulink>. At your option, you may
continue to consider qpdf to be licensed under those terms. The
Apache License 2.0 permits everything that the Artistic License 2.0
permits but is slightly less restrictive. Allowing the Artistic
License to continue being used is primary to help people who may
have to get specific approval to use qpdf in their products.
</para>
<para>
QPDF is intentionally released with a permissive license. However,
if there is some reason that the licensing terms don't work for
your requirements, please feel free to contact the copyright holder
to make other arrangements.
</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.md</filename> and
<filename>INSTALL</filename> files in the source distribution.
</para>
<sect1 id="ref.prerequisites">
<title>System Requirements</title>
<para>
The qpdf package has 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>
jpeg: <ulink
url="http://www.ijg.org/files/">http://www.ijg.org/files/</ulink>
or <ulink
url="https://libjpeg-turbo.org/">https://libjpeg-turbo.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, clang, 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.md</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.md</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.md</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>
<sect1 id="ref.packaging">
<title>Notes for Packagers</title>
<para>
If you are packaging qpdf for an operating system distribution,
here are some things you may want to keep in mind:
<itemizedlist>
<listitem>
<para>
Passing <option>--enable-show-failed-test-output</option> to
<command>./configure</command> will cause any failed test
output to be written to the console. This can be very useful
for seeing test failures generated by autobuilders where you
can't access qtest.log after the fact.
</para>
</listitem>
<listitem>
<para>
If qpdf's build environment detects the presence of autoconf
and related tools, it will check to ensure that automatically
generated files are up-to-date with recorded checksums and fail
if it detects a discrepancy. This feature is intended to
prevent you from accidentally forgetting to regenerate
automatic files after modifying their sources. If your
packaging environment automatically refreshes automatic files,
it can cause this check to fail. Supress qpdf's checks by
passing <option>--disable-check-autofiles</option> to
<command>/.configure</command>. This is safe since qpdf's
<command>autogen.sh</command> just runs autotools in the normal
way.
</para>
</listitem>
<listitem>
<para>
QPDF's <command>make install</command> does not install
completion files by default, but as a packager, it's good if
you install them wherever your distribution expects such files
to go. You can find completion files to install in the
<filename>completions</filename> directory.
</para>
</listitem>
<listitem>
<para>
Packagers are encouraged to install the source files from the
<filename>examples</filename> directory along with qpdf
development packages.
</para>
</listitem>
</itemizedlist>
</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>
If <option>@filename</option> appears anywhere in the
command-line, it will be read line by line, and each line will be
treated as a command-line argument. The <option>@-</option> option
allows arguments to be read from standard input. This allows qpdf
to be invoked with an arbitrary number of arbitrarily long
arguments. It is also very useful for avoiding having to pass
passwords on the command line.
</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. QPDF attempts
to detect this case and fail without overwriting 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.shell-completion">
<title>Shell Completion</title>
<para>
Starting in qpdf version 8.3.0, qpdf provides its own completion
support for zsh and bash. You can enable bash completion with
<command>eval $(qpdf --completion-bash)</command> and zsh
completion with <command>eval $(qpdf --completion-zsh)</command>.
If <command>qpdf</command> is not in your path, you should invoke
it above with an absolute path. If you invoke it with a relative
path, it will warn you, and the completion won't work if you're in
a different directory.
</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>--help</option></term>
<listitem>
<para>
Display command-line invocation help.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--version</option></term>
<listitem>
<para>
Display the current version of qpdf.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--copyright</option></term>
<listitem>
<para>
Show detailed copyright information.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--completion-bash</option></term>
<listitem>
<para>
Output a completion command you can eval to enable shell
completion from bash.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--completion-zsh</option></term>
<listitem>
<para>
Output a completion command you can eval to enable shell
completion from zsh.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--password=password</option></term>
<listitem>
<para>
Specifies a password for accessing encrypted files. Note that
you can use <option>@filename</option> or <option>@-</option>
as described above to put the password in a file or pass it
via standard input so you can avoid specifying it on the
command line.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--verbose</option></term>
<listitem>
<para>
Increase verbosity of output. For now, this just prints some
indication of any file that it creates.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--progress</option></term>
<listitem>
<para>
Indicate progress while writing files.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--no-warn</option></term>
<listitem>
<para>
Suppress writing of warnings to stderr. If warnings were
detected and suppressed, <command>qpdf</command> will still
exit with exit code 3.
</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>--password-is-hex-key</option></term>
<listitem>
<para>
Overrides the usual computation/retrieval of the PDF file's
encryption key from user/owner password with an explicit
specification of the encryption key. When this option is
specified, the argument to the <option>--password</option>
option is interpreted as a hexadecimal-encoded key value. This
only applies to the password used to open the main input file.
It does not apply to other files opened by
<option>--pages</option> or other options or to files being
written.
</para>
<para>
Most users will never have a need for this option, and no
standard viewers support this mode of operation, but it can be
useful for forensic or investigatory purposes. For example, if
a PDF file is encrypted with an unknown password, a
brute-force attack using the key directly is sometimes more
efficient than one using the password. Also, if a file is
heavily damaged, it may be possible to derive the encryption
key and recover parts of the file using it directly. To expose
the encryption key used by an encrypted file that you can open
normally, use the <option>--show-encryption-key</option>
option.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--rotate=[+|-]angle[:page-range]</option></term>
<listitem>
<para>
Apply rotation to specified pages. The
<option>page-range</option> portion of the option value has
the same format as page ranges in <xref
linkend="ref.page-selection"/>. If the page range is omitted,
the rotation is applied to all pages. The
<option>angle</option> portion of the parameter may be either
90, 180, or 270. If preceded by <option>+</option> or
<option>-</option>, the angle is added to or subtracted from
the specified pages' original rotations. Otherwise the pages'
rotations are set to the exact value. For example, the command
<command>qpdf in.pdf out.pdf --rotate=+90:2,4,6
--rotate=180:7-8</command> would rotate pages 2, 4, and 6 90
degrees clockwise from their original rotation and force the
rotation of pages 7 through 9 to 180 degrees regardless of
their original rotation, and the command <command>qpdf in.pdf
out.pdf --rotate=180</command> would rotate all pages by 180
degrees.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--keep-files-open=<replaceable>[yn]</replaceable></option></term>
<listitem>
<para>
This option controls whether qpdf keeps individual files open
while merging. Prior to version 8.1.0, qpdf always kept all
files open, but this meant that the number of files that could
be merged was limited by the operating system's open file
limit. Version 8.1.0 opened files as they were referenced and
closed them after each read, but this caused a major
performance impact. Version 8.2.0 optimized the performance
but did so in a way that, for local file systems, there was a
small but unavoidable performance hit, but for networked file
systems, the performance impact could be very high. Starting
with version 8.2.1, the default behavior is that files are
kept open if no more than 200 files are specified, but that
the behavior can be explicitly overridden with the
<option>--keep-files-open</option> flag. If you are merging
more than 200 files but less than the operating system's max
open files limit, you may want to use
<option>--keep-files-open=y</option>, especially if working
over a networked file system. If you are using a local file
system where the overhead is low and you might sometimes merge
more than the OS limit's number of files from a script and are
not worried about a few seconds additional processing time,
you may want to specify <option>--keep-files-open=n</option>.
</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>
<varlistentry>
<term><option>--collate</option></term>
<listitem>
<para>
When specified, collate rather than concatenate pages from
files specified with <option>--pages</option>. See <xref
linkend="ref.page-selection"/> for additional details.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--split-pages=[n]</option></term>
<listitem>
<para>
Write each group of <option>n</option> pages to a separate
output file. If <option>n</option> is not specified, create
single pages. Output file names are generated as follows:
<itemizedlist>
<listitem>
<para>
If the string <literal>%d</literal> appears in the output
file name, it is replaced with a range of zero-padded page
numbers starting from 1.
</para>
</listitem>
<listitem>
<para>
Otherwise, if the output file name ends in
<filename>.pdf</filename> (case insensitive), a zero-padded
page range, preceded by a dash, is inserted before the file
extension.
</para>
</listitem>
<listitem>
<para>
Otherwise, the file name is appended with a zero-padded
page range preceded by a dash.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Page ranges are a single number in the case of single-page
groups or two numbers separated by a dash otherwise.
For example, if <filename>infile.pdf</filename> has 12 pages
<itemizedlist>
<listitem>
<para>
<command>qpdf --split-pages infile.pdf %d-out</command>
would generate files <filename>01-out</filename> through
<filename>12-out</filename>
</para>
</listitem>
<listitem>
<para>
<command>qpdf --split-pages=2 infile.pdf
outfile.pdf</command> would generate files
<filename>outfile-01-02.pdf</filename> through
<filename>outfile-11-12.pdf</filename>
</para>
</listitem>
<listitem>
<para>
<command>qpdf --split-pages infile.pdf
something.else</command> would generate files
<filename>something.else-01</filename> through
<filename>something.else-12</filename>
</para>
</listitem>
</itemizedlist>
</para>
<para>
Note that outlines, threads, and other global features of the
original PDF file are not preserved. For each page of output,
this option creates an empty PDF and copies a single page from
the output into it. If you require the global data, you will
have to run <command>qpdf</command> with the
<option>--pages</option> option once for each file. Using
<option>--split-pages</option> is much faster if you don't
require the global data.
</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 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. The qpdf library disregards this field when AES is
used or when 256-bit encryption is used. You should really
never disable accessibility, but qpdf lets you do it in case
you need to configure a file this way for testing purposes.
The PDF spec says that conforming readers should disregard
this permission and always allow accessibility.
</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>--assemble=[yn]</option></term>
<listitem>
<para>
Determines whether document assembly (rotation and reordering
of pages) is allowed.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--annotate=[yn]</option></term>
<listitem>
<para>
Determines whether modifying annotations is allowed. This
includes adding comments and filling in form fields. Also
allows editing of form fields if
<option>--modify-other=y</option> is given.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--form=[yn]</option></term>
<listitem>
<para>
Determines whether filling form fields is allowed.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--modify-other=[yn]</option></term>
<listitem>
<para>
Allow all document editing except those controlled separately
by the <option>--assemble</option>,
<option>--annotate</option>, and <option>--form</option>
options.
</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. This way of controlling modify access
has less granularity than new options added in qpdf 8.4.
<option><replaceable>modify-opt</replaceable></option> may be
one of the following:
<itemizedlist>
<listitem>
<para>
<option>all</option>: allow full document modification
</para>
</listitem>
<listitem>
<para>
<option>annotate</option>: allow comment authoring, form
operations, and document assembly
</para>
</listitem>
<listitem>
<para>
<option>form</option>: allow form field fill-in and signing
and document assembly
</para>
</listitem>
<listitem>
<para>
<option>assembly</option>: allow document assembly only
</para>
</listitem>
<listitem>
<para>
<option>none</option>: allow no modifications
</para>
</listitem>
</itemizedlist>
Using the <option>--modify</option> option does not allow you
to create certain combinations of permissions such as allowing
form filling but not allowing document assembly. Starting with
qpdf 8.4, you can either just use the other options to control
fields individually, or you can use something like
<option>--modify=form --assembly=n</option> to fine tune.
</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>
Starting with qpf 8.4, the special input file name
&ldquo;<filename>.</filename>&rdquo; can be used shortcut for the
primary input filename.
</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>
Starting with qpdf 5.0.0, it is possible to omit the page range.
If qpdf sees a value in the place where it expects a page range
and that value is not a valid range but is a valid file name, qpdf
will implicitly use the range <literal>1-z</literal>, meaning that
it will include all pages in the file. This makes it possible to
easily combine all pages in a set of files with a command like
<command>qpdf --empty out.pdf --pages *.pdf --</command>.
</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. A number preceded by an
&ldquo;r&rdquo; indicates to count from the end, so
<literal>r3-r1</literal> would be the last three pages of the
document. 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, 3, 5, 6, 7, 8,
9, 15, 14, 13, and 12 in that order.
</para>
</listitem>
<listitem>
<para>
<literal>z-1</literal>: all pages in the document in reverse
</para>
</listitem>
<listitem>
<para>
<literal>r3-r1</literal>: the last three pages of the document
</para>
</listitem>
<listitem>
<para>
<literal>r1-r3</literal>: the last three pages of the document
in reverse order
</para>
</listitem>
</itemizedlist>
</para>
<para>
Starting in qpdf version 8.3, you can specify the
<option>--collate</option> option. Note that this option is
specified outside of <option>--pages&nbsp;...&nbsp;--</option>.
When <option>--collate</option> is specified, it changes the
meaning of <option>--pages</option> so that the specified files,
as modified by page ranges, are collated rather than concatenated.
For example, if you add the files <filename>odd.pdf</filename> and
<filename>even.pdf</filename> containing odd and even pages of a
document respectively, you could run <command>qpdf --collate
odd.pdf --pages odd.pdf even.pdf -- all.pdf</command> to collate
the pages. This would pick page 1 from odd, page 1 from even, page
2 from odd, page 2 from even, etc. until all pages have been
included. Any number of files and page ranges can be specified. If
any file has fewer pages, that file is just skipped when its pages
have all been included. For example, if you ran <command>qpdf
--collate --empty --pages a.pdf 1-5 b.pdf 6-4 c.pdf r1 --
out.pdf</command>, you would get the following pages in this
order:
<itemizedlist>
<listitem><para>a.pdf page 1</para></listitem>
<listitem><para>b.pdf page 6</para></listitem>
<listitem><para>c.pdf last page</para></listitem>
<listitem><para>a.pdf page 2</para></listitem>
<listitem><para>b.pdf page 5</para></listitem>
<listitem><para>a.pdf page 3</para></listitem>
<listitem><para>b.pdf page 4</para></listitem>
<listitem><para>a.pdf page 4</para></listitem>
<listitem><para>a.pdf page 5</para></listitem>
</itemizedlist>
</para>
<para>
Starting in qpdf version 8.3, when you split and merge files, any
page labels (page numbers) are preserved in the final file. It is
expected that more document features will be preserved by
splitting and merging. In the mean time, 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. 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 . 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 . 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 document-level metadata 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>
<para>
Prior to version 8.4, it was not possible to specify the same page
from the same file directly more than once, and the workaround of
specifying the same file in more than one way was required.
Version 8.4 removes this limitation.
</para>
</sect1>
<sect1 id="ref.advanced-parsing">
<title>Advanced Parsing Options</title>
<para>
These options control aspects of how qpdf reads PDF files. Mostly
these are of use to people who are working with damaged files.
There is little reason to use these options unless you are trying
to solve specific problems. The following options are available:
<variablelist>
<varlistentry>
<term><option>--suppress-recovery</option></term>
<listitem>
<para>
Prevents qpdf from attempting to recover damaged files.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--ignore-xref-streams</option></term>
<listitem>
<para>
Tells qpdf to ignore any cross-reference streams.
</para>
</listitem>
</varlistentry>
</variablelist>
</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>
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>
</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>--compress-streams=<replaceable>[yn]</replaceable></option></term>
<listitem>
<para>
By default, or with <option>--compress-streams=y</option>,
qpdf will compress any stream with no other filters applied to
it with the <literal>/FlateDecode</literal> filter when it
writes it. To suppress this behavior and preserve uncompressed
streams as uncompressed, use
<option>--compress-streams=n</option>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--decode-level=<replaceable>option</replaceable></option></term>
<listitem>
<para>
Controls which streams qpdf tries to decode. The default is
<option>generalized</option>. The following options are
available:
<itemizedlist>
<listitem>
<para>
<option>none</option>: do not attempt to decode any streams
</para>
</listitem>
<listitem>
<para>
<option>generalized</option>: decode streams filtered with
supported generalized filters: <option>/LZWDecode</option>,
<option>/FlateDecode</option>,
<option>/ASCII85Decode</option>, and
<option>/ASCIIHexDecode</option>. We define generalized
filters as those to be used for general-purpose compression
or encoding, as opposed to filters specifically designed
for image data.
</para>
</listitem>
<listitem>
<para>
<option>specialized</option>: in addition to generalized,
decode streams with supported non-lossy specialized
filters; currently this is just <option>/RunLengthDecode</option>
</para>
</listitem>
<listitem>
<para>
<option>all</option>: in addition to generalized and
specialized, decode streams with supported lossy filters;
currently this is just <option>/DCTDecode</option> (JPEG)
</para>
</listitem>
</itemizedlist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--stream-data=<replaceable>option</replaceable></option></term>
<listitem>
<para>
Controls transformation of stream data. This option predates
the <option>--compress-streams</option> and
<option>--decode-level</option> options. Those options can be
used to achieve the same affect with more control. 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); equivalent to
<option>--compress-streams=y</option>
<option>--decode-level=generalized</option>
</para>
</listitem>
<listitem>
<para>
<option>preserve</option>: leave all stream data as is;
equivalent to <option>--compress-streams=n</option>
<option>--decode-level=none</option>
</para>
</listitem>
<listitem>
<para>
<option>uncompress</option>: uncompress stream data
compressed with generalized filters when possible;
equivalent to <option>--compress-streams=n</option>
<option>--decode-level=generalized</option>
</para>
</listitem>
</itemizedlist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--normalize-content=[yn]</option></term>
<listitem>
<para>
Enables or disables normalization of content streams. Content
normalization is enabled by default in QDF mode. Please see
<xref linkend="ref.qdf"/> for additional discussion of QDF
mode.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--object-streams=<replaceable>mode</replaceable></option></term>
<listitem>
<para>
Controls handling 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>--preserve-unreferenced</option></term>
<listitem>
<para>
Tells qpdf to preserve objects that are not referenced when
writing the file. Ordinarily any object that is not referenced
in a traversal of the document from the trailer dictionary
will be discarded. This may be useful in working with some
damaged files or inspecting files with known unreferenced
objects.
</para>
<para>
This flag is ignored for linearized files and has the effect
of causing objects in the new file to be written in order by
object ID from the original file. This does not mean that
object numbers will be the same since qpdf may create stream
lengths as direct or indirect differently from the original
file, and the original file may have gaps in its numbering.
</para>
<para>
See also <option>--preserve-unreferenced-resources</option>,
which does something completely different.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--preserve-unreferenced-resources</option></term>
<listitem>
<para>
Starting with qpdf 8.1, when splitting pages, qpdf ordinarily
attempts to remove images and fonts that are not used by a
page even if they are referenced in the page's resources
dictionary. This option suppresses that behavior. The only
reason to use this is if you suspect that qpdf is removing
resources it shouldn't be removing. If you encounter that
case, please report it as a bug.
</para>
<para>
See also <option>--preserve-unreferenced-resources</option>,
which does something completely different.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--newline-before-endstream</option></term>
<listitem>
<para>
Tells qpdf to insert a newline before the
<literal>endstream</literal> keyword, not counted in the
length, after any stream content even if the last character of
the stream was a newline. This may result in two newlines in
some cases. This is a requirement of PDF/A. While qpdf doesn't
specifically know how to generate PDF/A-compliant PDFs, this
at least prevents it from removing compliance on already
compliant files.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--linearize-pass1=<replaceable>file</replaceable></option></term>
<listitem>
<para>
Write the first pass of linearization to the named file. The
resulting file is not a valid PDF file. This option is useful
only for debugging <classname>QPDFWriter</classname>'s
linearization code. When qpdf linearizes files, it writes the
file in two passes, using the first pass to calculate sizes
and offsets that are required for hint tables and the
linearization dictionary. Ordinarily, the first pass is
discarded. This option enables it to be captured.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--coalesce-contents</option></term>
<listitem>
<para>
When a page's contents are split across multiple streams, this
option causes qpdf to combine them into a single stream. Use
of this option is never necessary for ordinary usage, but it
can help when working with some files in some cases. For
example, some PDF writers split page contents into small
streams at arbitrary points that may fall in the middle of
lexical tokens within the content, and some PDF readers may
get confused on such files. If you use qpdf to coalesce the
content streams, such readers may be able to work with the
file more easily. This can also be combined with QDF mode or
content normalization to make it easier to look at all of a
page's contents at once.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--flatten-annotations=<replaceable>option</replaceable></option></term>
<listitem>
<para>
This option collapses annotations into the pages' contents
with special handling for form fields. Ordinarily, an
annotation is rendered separately and on top of the page.
Combining annotations into the page's contents effectively
freezes the placement of the annotations, making them look
right after various page transformations. The library
functionality backing this option was added for the benefit of
programs that want to create <emphasis>n-up</emphasis> page
layouts and other similar things that don't work well with
annotations. The <replaceable>option</replaceable> parameter
may be any of the following:
<itemizedlist>
<listitem>
<para>
<option>all</option>: include all annotations that are not
marked invisible or hidden
</para>
</listitem>
<listitem>
<para>
<option>print</option>: only include annotations that
indicate that they should appear when the page is printed
</para>
</listitem>
<listitem>
<para>
<option>screen</option>: omit annotations that indicate
they should not appear on the screen
</para>
</listitem>
</itemizedlist>
</para>
<para>
Note that form fields are special because the annotations that
are used to render filled-in form fields may become out of
date from the fields' values if the form is filled in by a
program that doesn't know how to update the appearances. If
qpdf detects this case, its default behavior is not to flatten
those annotations because doing so would cause the value of
the form field to be lost. This gives you a chance to go back
and resave the form with a program that knows how to generate
appearances. QPDF itself can generate appearances with some
limitations. See the <option>--generate-appearances</option>
option below.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--generate-appearances</option></term>
<listitem>
<para>
If a file contains interactive form fields and indicates that
the appearances are out of date with the values of the form,
this flag will regenerate appearances, subject to a few
limitations. Note that there is not usually a reason to do
this, but it can be necessary before using the
<option>--flatten-annotations</option> option. Most of these
are not a problem with well-behaved PDF files. The limitations
are as follows:
<itemizedlist>
<listitem>
<para>
Radio button and checkbox appearances use the pre-set
values in the PDF file. QPDF just makes sure that the
correct appearance is displayed based on the value of the
field. This is fine for PDF files that create their forms
properly. Some PDF writers save appearances for fields when
they change, which could cause some controls to have
inconsistent appearances.
</para>
</listitem>
</itemizedlist>
<itemizedlist>
<listitem>
<para>
For text fields and list boxes, any characters that fall
outside of US-ASCII or, if detected, &ldquo;Windows
ANSI&rdquo; or &ldquo;Mac Roman&rdquo; encoding, will be
replaced by the <literal>?</literal> character.
</para>
</listitem>
</itemizedlist>
<itemizedlist>
<listitem>
<para>
Quadding is ignored. Quadding is used to specify whether
the contents of a field should be left, center, or right
aligned with the field.
</para>
</listitem>
</itemizedlist>
<itemizedlist>
<listitem>
<para>
Rich text, multi-line, and other more elaborate formatting
directives are ignored.
</para>
</listitem>
</itemizedlist>
<itemizedlist>
<listitem>
<para>
There is no support for multi-select fields or signature
fields.
</para>
</listitem>
</itemizedlist>
If qpdf doesn't do a good enough job with your form, use an
external application to save your filled-in form before
processing it with qpdf.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--optimize-images</option></term>
<listitem>
<para>
This flag causes qpdf to recompress all images that are not
compressed with DCT (JPEG) using DCT compression as long as
doing so decreases the size in bytes of the image data and the
image does not fall below minimum specified dimensions. Useful
information is provided when used in combination with
<option>--verbose</option>. See also the
<option>--oi-min-width</option>,
<option>--oi-min-height</option>, and
<option>--oi-min-area</option> options.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--oi-min-width=<replaceable>width</replaceable></option></term>
<listitem>
<para>
Avoid optimizing images whose width is below the specified
amount. If omitted, the default is 128 pixels. Use 0 for no
minimum.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--oi-min-height=<replaceable>height</replaceable></option></term>
<listitem>
<para>
Avoid optimizing images whose height is below the specified
amount. If omitted, the default is 128 pixels. Use 0 for no
minimum.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--oi-min-area=<replaceable>area-in-pixels</replaceable></option></term>
<listitem>
<para>
Avoid optimizing images whose pixel count
(width&nbsp;×&nbsp;height) is below the specified amount. If
omitted, the default is 16,384 pixels. Use 0 for no minimum.
</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>--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>
This paragraph discusses edge cases of content normalization that
are not of concern to most users and are not relevant when content
normalization is not enabled. When normalizing content, if qpdf
runs into any lexical errors, it will print a warning indicating
that content may be damaged. The only situation in which qpdf is
known to cause damage during content normalization is when a
page's contents are split across multiple streams and streams are
split in the middle of a lexical token such as a string, name, or
inline image. There may be some pathological cases in which qpdf
could damage content without noticing this, such as if the partial
tokens at the end of one stream and the beginning of the next
stream are both valid, but usually qpdf will be able to detect
this case. For slightly increased safety, you can specify
<option>--coalesce-contents</option> in addition to
<option>--normalize-content</option> or <option>--qdf</option>.
This will cause qpdf to combine all the content streams into one,
thus recombining any split tokens. However doing this will prevent
you from being able to see the original layout of the content
streams. If you must inspect the original content streams in an
uncompressed format, you can always run with <option>--qdf
--normalize-content=n</option> for a QDF file without content
normalization, or alternatively
<option>--stream-data=uncompress</option> for a regular non-QDF
mode file with uncompressed streams. These will both uncompress
all the streams but will not attempt to normalize content. Please
note that if you are using content normalization or QDF mode for
the purpose of manually inspecting files, you don't have to care
about this.
</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>
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>--deterministic-id</option></term>
<listitem>
<para>
Causes generation of a deterministic value for /ID. This
prevents use of timestamp and output file name information in
the /ID generation. Instead, at some slight additional runtime
cost, the /ID field is generated to include a digest of the
significant parts of the content of the output PDF file. This
means that a given qpdf operation should generate the same /ID
each time it is run, which can be useful when caching results
or for generation of some test data. Use of this flag is not
compatible with creation of encrypted files.
</para>
</listitem>
</varlistentry>
<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. If you
are trying to get the same /ID each time for a given file and
you are not generating encrypted files, consider using the
<option>--deterministic-id</option> option.
</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>--show-encryption-key</option></term>
<listitem>
<para>
When encryption information is being displayed, as when
<option>--check</option> or <option>--show-encryption</option>
is given, display the computed or retrieved encryption key as
a hexadecimal string. This value is not ordinarily useful to
users, but it can be used as the argument to
<option>--password</option> if the
<option>--password-is-hex-key</option> is specified. Note
that, when PDF files are encrypted, passwords and other
metadata are used only to compute an encryption key, and the
encryption key is what is actually used for encryption. This
enables retrieval of that key.
</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=trailer|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-npages</option></term>
<listitem>
<para>
Prints the number of pages in the input file on a line by
itself. Since the number of pages appears by itself on a
line, this option can be useful for scripting if you need to
know the number of pages in a file.
</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>--json</option></term>
<listitem>
<para>
Generate a JSON representation of the file. This is described
in depth in <xref linkend="ref.json"/>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--json-help</option></term>
<listitem>
<para>
Describe the format of the JSON output.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--json-key=key</option></term>
<listitem>
<para>
This option is repeatable. If specified, only top-level keys
specified will be included in the JSON output. If not
specified, all keys will be shown.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--json-object=trailer|obj[,gen]</option></term>
<listitem>
<para>
This option is repeatable. If specified, only specified
objects will be shown in the
&ldquo;<literal>objects</literal>&rdquo; key of the JSON
output. If absent, all objects will be shown.
</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). When <option>--check</option> is combined
with other options, checks are always performed before any
other options are processed. For erroneous files,
<option>--check</option> will cause qpdf to attempt to
recover, after which other options are effectively operating
on the recovered file. Combining <option>--check</option> with
other options in this way can be useful for manually
recovering severely damaged files.
</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>
<sect1 id="ref.using.from-cxx">
<title>Using QPDF from C++</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/QPDFWriter.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>-lz -ljpeg</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>
</sect1>
<sect1 id="ref.using.other-languages">
<title>Using QPDF from other languages</title>
<para>
The qpdf library is implemented in C++, which makes it hard to use
directly in other languages. There are a few things that can help.
</para>
<variablelist>
<varlistentry>
<term>&ldquo;C&rdquo;</term>
<listitem>
<para>
The qpdf library includes a &ldquo;C&rdquo; language interface
that provides a subset of the overall capabilities. The header
file <filename>qpdf/qpdf-c.h</filename> includes information
about its use. As long as you use a C++ linker, you can link C
programs with qpdf and use the C API. For languages that can
directly load methods from a shared library, the C API can also
be useful. People have reported success using the C API from
other languages on Windows by directly calling functions in the
DLL.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Python</term>
<listitem>
<para>
A Python module called <ulink
url="https://pypi.org/project/pikepdf/">pikepdf</ulink>
provides a clean and highly functional set of Python bindings
to the qpdf library. Using pikepdf, you can work with PDF files
in a natural way and combine qpdf's capabilities with other
functionality provided by Python's rich standard library and
available modules.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Other Languages</term>
<listitem>
<para>
Starting with version 8.3.0, the <command>qpdf</command>
command-line tool can produce a JSON representation of the PDF
file's non-content data. This can facilitate interacting
programmatically with PDF files through qpdf's command line
interface. For more information, please see <xref
linkend="ref.json"/>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
</chapter>
<chapter id="ref.json">
<title>QPDF JSON</title>
<sect1 id="ref.json-overview">
<title>Overview</title>
<para>
Beginning with qpdf version 8.3.0, the <command>qpdf</command>
command-line program can produce a JSON representation of the
non-content data in a PDF file. It includes a dump in JSON format
of all objects in the PDF file excluding the content of streams.
This JSON representation makes it very easy to look in detail at
the structure of a given PDF file, and it also provides a great way
to work with PDF files programmatically from the command-line in
languages that can't call or link with the qpdf library directly.
Note that stream data can be extracted from PDF files using other
qpdf command-line options.
</para>
</sect1>
<sect1 id="ref.json-guarantees">
<title>JSON Guarantees</title>
<para>
The qpdf JSON representation includes a JSON serialization of the
raw objects in the PDF file as well as some computed information in
a more easily extracted format. QPDF provides some guarantees about
its JSON format. These guarantees are designed to simplify the
experience of a developer working with the JSON format.
<variablelist>
<varlistentry>
<term>Compatibility</term>
<listitem>
<para>
The top-level JSON object output is a dictionary. The JSON
output contains various nested dictionaries and arrays. With
the exception of dictionaries that are populated by the fields
of objects from the file, all instances of a dictionary are
guaranteed to have exactly the same keys. Future versions of
qpdf are free to add additional keys but not to remove keys or
change the type of object that a key points to. The qpdf
program validates this guarantee, and in the unlikely event
that a bug in qpdf should cause it to generate data that
doesn't conform to this rule, it will ask you to file a bug
report.
</para>
<para>
The top-level JSON structure contains a
&ldquo;<literal>version</literal>&rdquo; key whose value is
simple integer. The value of the <literal>version</literal> key
will be incremented if a non-compatible change is made. A
non-compatible change would be any change that involves removal
of a key, a change to the format of data pointed to by a key,
or a semantic change that requires a different interpretation
of a previously existing key. A strong effort will be made to
avoid breaking compatibility.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Documentation</term>
<listitem>
<para>
The <command>qpdf</command> command can be invoked with the
<option>--json-help</option> option. This will output a JSON
structure that has the same structure as the JSON output that
qpdf generates, except that each field in the help output is a
description of the corresponding field in the JSON output. The
specific guarantees are as follows:
<itemizedlist>
<listitem>
<para>
A dictionary in the help output means that the corresponding
location in the actual JSON output is also a dictionary with
exactly the same keys; that is, no keys present in help are
absent in the real output, and no keys will be present in
the real output that are not in help.
</para>
</listitem>
<listitem>
<para>
A string in the help output is a description of the item
that appears in the corresponding location of the actual
output. The corresponding output can have any format.
</para>
</listitem>
<listitem>
<para>
An array in the help output always contains a single
element. It indicates that the corresponding location in the
actual output is also an array, and that each element of the
array has whatever format is implied by the single element
of the help output's array.
</para>
</listitem>
</itemizedlist>
For example, the help output indicates includes a
&ldquo;<literal>pagelabels</literal>&rdquo; key whose value is
an array of one element. That element is a dictionary with keys
&ldquo;<literal>index</literal>&rdquo; and
&ldquo;<literal>label</literal>&rdquo;. In addition to
describing the meaning of those keys, this tells you that the
actual JSON output will contain a <literal>pagelabels</literal>
array, each of whose elements is a dictionary that contains an
<literal>index</literal> key, a <literal>label</literal> key,
and no other keys.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Directness and Simplicity</term>
<listitem>
<para>
The JSON output contains the value of every object in the file,
but it also contains some processed data. This is analogous to
how qpdf's library interface works. The processed data is
similar to the helper functions in that it allows you to look
at certain aspects of the PDF file without having to understand
all the nuances of the PDF specification, while the raw objects
allow you to mine the PDF for anything that the higher-level
interfaces are lacking.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</sect1>
<sect1 id="json.limitations">
<title>Limitations of JSON Representation</title>
<para>
There are a few limitations to be aware of with the JSON structure:
<itemizedlist>
<listitem>
<para>
Strings, names, and indirect object references in the original
PDF file are all converted to strings in the JSON
representation. In the case of a &ldquo;normal&rdquo; PDF file,
you can tell the difference because a name starts with a slash
(<literal>/</literal>), and an indirect object reference looks
like <literal>n n R</literal>, but if there were to be a string
that looked like a name or indirect object reference, there
would be no way to tell this from the JSON output. Note that
there are certain cases where you know for sure what something
is, such as knowing that dictionary keys in objects are always
names and that certain things in the higher-level computed data
are known to contain indirect object references.
</para>
</listitem>
<listitem>
<para>
The JSON format doesn't support binary data very well. Mostly
the details are not important, but they are presented here for
information. When qpdf outputs a string in the JSON
representation, it converts the string to UTF-8, assuming usual
PDF string semantics. Specifically, if the original string is
UTF-16, it is converted to UTF-8. Otherwise, it is assumed to
have PDF doc encoding, and is converted to UTF-8 with that
assumption. This causes strange things to happen to binary
strings. For example, if you had the binary string
<literal>&lt;038051&gt;</literal>, this would be output to the
JSON as <literal>\u0003•Q</literal> because
<literal>03</literal> is not a printable character and
<literal>80</literal> is the bullet character in PDF doc
encoding and is mapped to the Unicode value
<literal>2022</literal>. Since <literal>51</literal> is
<literal>Q</literal>, it is output as is. If you wanted to
convert back from here to a binary string, would have to
recognize Unicode values whose code points are higher than
<literal>0xFF</literal> and map those back to their
corresponding PDF doc encoding characters. There is no way to
tell the difference between a Unicode string that was originally
encoded as UTF-16 or one that was converted from PDF doc
encoding. In other words, it's best if you don't try to use the
JSON format to extract binary strings from the PDF file, but if
you really had to, it could be done. Note that qpdf's
<option>--show-object</option> option does not have this
limitation and will reveal the string as encoded in the original
file.
</para>
</listitem>
</itemizedlist>
</para>
</sect1>
<sect1 id="json.considerations">
<title>JSON: Special Considerations</title>
<para>
For the most part, the built-in JSON help tells you everything you
need to know about the JSON format, but there are a few
non-obvious things to be aware of:
<itemizedlist>
<listitem>
<para>
While qpdf guarantees that keys present in the help will be
present in the output, those fields may be null or empty if the
information is not known or absent in the file. Also, if you
specify <option>--json-keys</option>, the keys that are not
listed will be excluded entirely except for those that
<option>--json-help</option> says are always present.
</para>
</listitem>
<listitem>
<para>
In a few places, there are keys with names containing
<literal>pageposfrom1</literal>. The values of these keys are
null or an integer. If an integer, they point to a page index
within the file numbering from 1. Note that JSON indexes from
0, and you would also use 0-based indexing using the API.
However, 1-based indexing is easier in this case because the
command-line syntax for specifying page ranges is 1-based. If
you were going to write a program that looked through the JSON
for information about specific pages and then use the
command-line to extract those pages, 1-based indexing is
easier. Besides, it's more convenient to subtract 1 from a
program in a real programming language than it is to add 1 from
shell code.
</para>
</listitem>
<listitem>
<para>
The image information included in the <literal>page</literal>
section of the JSON output includes the key
&ldquo;<literal>filterable</literal>&rdquo;. Note that the
value of this field may depend on the
<option>--decode-level</option> that you invoke qpdf with. The
JSON output includes a top-level key
&ldquo;<literal>parameters</literal>&rdquo; that indicates the
decode level used for computing whether a stream was
filterable. For example, jpeg images will be shown as not
filterable by default, but they will be shown as filterable if
you run <command>qpdf --json --decode-level=all</command>.
</para>
</listitem>
</itemizedlist>
</para>
</sect1>
</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, though it is possible to determine whether an
object is direct or not if this information is needed. 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 C++-11's
<classname>std::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>
The primary class for interacting with PDF objects is
<classname>QPDFObjectHandle</classname>. Instances of this class
can be passed around by value, copied, stored in containers, etc.
with very low overhead. Instances of
<classname>QPDFObjectHandle</classname> created by reading from a
file will 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>
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 files
<filename>QPDF.hh</filename> and
<filename>QPDFObjectHandle.hh</filename>. There are also some
additional helper classes that provide higher level API functions
for certain document constructions. These are discussed in <xref
linkend="ref.helper-classes"/>.
</para>
</sect1>
<sect1 id="ref.helper-classes">
<title>Helper Classes</title>
<para>
QPDF version 8.1 introduced the concept of helper classes. Helper
classes are intended to contain higher level APIs that allow
developers to work with certain document constructs at an
abstraction level above that of
<classname>QPDFObjectHandle</classname> while staying true to
qpdf's philosophy of not hiding document structure from the
developer. As with qpdf in general, the goal is take away some of
the more tedious bookkeeping aspects of working with PDF files,
not to remove the need for the developer to understand how the PDF
construction in question works. The driving factor behind the
creation of helper classes was to allow the evolution of higher
level interfaces in qpdf without polluting the interfaces of the
main top-level classes <classname>QPDF</classname> and
<classname>QPDFObjectHandle</classname>.
</para>
<para>
There are two kinds of helper classes:
<emphasis>document</emphasis> helpers and
<emphasis>object</emphasis> helpers. Document helpers are
constructed with a reference to a <classname>QPDF</classname>
object and provide methods for working with structures that are at
the document level. Object helpers are constructed with an
instance of a <classname>QPDFObjectHandle</classname> and provide
methods for working with specific types of objects.
</para>
<para>
Examples of document helpers include
<classname>QPDFPageDocumentHelper</classname>, which contains
methods for operating on the document's page trees, such as
enumerating all pages of a document and adding and removing pages;
and <classname>QPDFAcroFormDocumentHelper</classname>, which
contains document-level methods related to interactive forms, such
as enumerating form fields and creating mappings between form
fields and annotations.
</para>
<para>
Examples of object helpers include
<classname>QPDFPageObjectHelper</classname> for performing
operations on pages such as page rotation and some operations on
content streams, <classname>QPDFFormFieldObjectHelper</classname>
for performing operations related to interactive form fields, and
<classname>QPDFAnnotationObjectHelper</classname> for working with
annotations.
</para>
<para>
It is always possible to retrieve the underlying
<classname>QPDF</classname> reference from a document helper and
the underlying <classname>QPDFObjectHandle</classname> reference
from an object helper. Helpers are designed to be helpers, not
wrappers. The intention is that, in general, it is safe to freely
intermix operations that use helpers with operations that use the
underlying objects. Document and object helpers do not attempt to
provide a complete interface for working with the things they are
helping with, nor do they attempt to encapsulate underlying
structures. They just provide a few methods to help with
error-prone, repetitive, or complex tasks. In some cases, a helper
object may cache some information that is expensive to gather. In
such cases, the helper classes are implemented so that their own
methods keep the cache consistent, and the header file will
provide a method to invalidate the cache and a description of what
kinds of operations would make the cache invalid. If in doubt, you
can always discard a helper class and create a new one with the
same underlying objects, which will ensure that you have discarded
any stale information.
</para>
<para>
By Convention, document helpers are called
<classname>QPDFSomethingDocumentHelper</classname> and are derived
from <classname>QPDFDocumentHelper</classname>, and object helpers
are called <classname>QPDFSomethingObjectHelper</classname> and
are derived from <classname>QPDFObjectHelper</classname>. For
details on specific helpers, please see their header files. You
can find them by looking at
<filename>include/qpdf/QPDF*DocumentHelper.hh</filename> and
<filename>include/qpdf/QPDF*ObjectHelper.hh</filename>.
</para>
<para>
In order to avoid creation of circular dependencies, the following
general guidelines are followed with helper classes:
<itemizedlist>
<listitem>
<para>
Core class interfaces do not know about helper classes. For
example, no methods of <classname>QPDF</classname> or
<classname>QPDFObjectHandle</classname> will include helper
classes in their interfaces.
</para>
</listitem>
<listitem>
<para>
Interfaces of object helpers will usually not use document
helpers in their interfaces. This is because it is much more
useful for document helpers to have methods that return object
helpers. Most operations in PDF files start at the document
level and go from there to the object level rather than the
other way around. It can sometimes be useful to map back from
object-level structures to document-level structures. If there
is a desire to do this, it will generally be provided by a
method in the document helper class.
</para>
</listitem>
<listitem>
<para>
Most of the time, object helpers don't know about other object
helpers. However, in some cases, one type of object may be a
container for another type of object, in which case it may make
sense for the outer object to know about the inner object. For
example, there are methods in the
<classname>QPDFPageObjectHelper</classname> that know
<classname>QPDFAnnotationObjectHelper</classname> because
references to annotations are contained in page dictionaries.
</para>
</listitem>
<listitem>
<para>
Any helper or core library class may use helpers in their
implementations.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Prior to qpdf version 8.1, higher level interfaces were added as
&ldquo;convenience functions&rdquo; in either
<classname>QPDF</classname> or
<classname>QPDFObjectHandle</classname>. For compatibility, older
convenience functions for operating with pages will remain in
those classes even as alternatives are provided in helper classes.
Going forward, new higher level interfaces will be provided using
helper classes.
</para>
</sect1>
<sect1 id="ref.implementation-notes">
<title>Implementation Notes</title>
<para>
This section contains a few notes about QPDF's internal
implementation, particularly around what it does when it first
processes a file. This section is a bit of a simplification of
what it actually does, but it could serve as a starting point to
someone trying to understand the implementation. There is nothing
in this section that you need to know to use the qpdf library.
</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>
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 iterates 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>
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 a PDF header. 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.random-numbers">
<title>Random Number Generation</title>
<para>
QPDF generates random numbers to support generation of encrypted
data. Versions prior to 5.0.1 used <function>random</function> or
<function>rand</function> from <filename>stdlib</filename> to
generate random numbers. Version 5.0.1, if available, used
operating system-provided secure random number generation instead,
enabling use of <filename>stdlib</filename> random number
generation only if enabled by a compile-time option. Starting in
version 5.1.0, use of insecure random numbers was disabled unless
enabled at compile time. Starting in version 5.1.0, it is also
possible for you to disable use of OS-provided secure random
numbers. This is especially useful on Windows if you want to
avoid a dependency on Microsoft's cryptography API. In this case,
you must provide your own random data provider. Regardless of how
you compile qpdf, starting in version 5.1.0, it is possible for
you to provide your own random data provider at runtime. This
would enable you to use some software-based secure pseudorandom
number generator and to avoid use of whatever the operating system
provides. For details on how to do this, please refer to the
top-level README.md file in the source distribution and to comments
in <filename>QUtil.hh</filename>.
</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 &ldquo;from&rdquo;
<classname>QPDF</classname> object must remain valid after the
copy as discussed in the note below. 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>
<para>
Please note: when you copy objects from one
<classname>QPDF</classname> to another, the source
<classname>QPDF</classname> object must remain valid until you
have finished with the destination object. This is because the
original object is still used to retrieve any referenced stream
data from the copied object.
</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>8.3.0: January 7, 2019</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Command-line Enhancements
</para>
<itemizedlist>
<listitem>
<para>
Shell completion: you can now use eval <command>$(qpdf
--completion-bash)</command> and eval <command>$(qpdf
--completion-zsh)</command> to enable shell completion for
bash and zsh.
</para>
</listitem>
<listitem>
<para>
Page numbers (also known as page labels) are now preserved
when merging and splitting files with the
<option>--pages</option> and <option>--split-pages</option>
options.
</para>
</listitem>
<listitem>
<para>
Bookmarks are partially preserved when splitting pages with
the <option>--split-pages</option> option. Specifically, the
outlines dictionary and some supporting metadata are copied
into the split files. The result is that all bookmarks from
the original file appear, those that point to pages that are
preserved work, and those that point to pages that are not
preserved don't do anything. This is an interim step toward
proper support for bookmarks in splitting and merging
operations.
</para>
</listitem>
<listitem>
<para>
Page collation: add new option <option>--collate</option>.
When specified, the semantics of <option>--pages</option>
change from concatenation to collation. See <xref
linkend="ref.page-selection"/> for examples and discussion.
</para>
</listitem>
<listitem>
<para>
Generation of information in JSON format, primarily to
facilitate use of qpdf from languages other than C++. Add
new options <option>--json</option>,
<option>--json-key</option>, and
<option>--json-object</option> to generate a JSON
representation of the PDF file. Run <command>qpdf
--json-help</command> to get a description of the JSON
format. For more information, see <xref linkend="ref.json"/>.
</para>
</listitem>
<listitem>
<para>
The <option>--generate-appearances</option> flag will cause
qpdf to generate appearances for form fields if the PDF file
indicates that form field appearances are out of date. This
can happen when PDF forms are filled in by a program that
doesn't know how to regenerate the appearances of the
filled-in fields.
</para>
</listitem>
<listitem>
<para>
The <option>--flatten-annotations</option> flag can be used
to <emphasis>flatten</emphasis> annotations, including form
fields. Ordinarily, annotations are drawn separately from
the page. Flattening annotations is the process of combining
their appearances into the page's contents. You might want
to do this if you are going to rotate or combine pages using
a tool that doesn't understand about annotations. You may
also want to use <option>--generate-appearances</option>
when using this flag since annotations for outdated form
fields are not flattened as that would cause loss of
information.
</para>
</listitem>
<listitem>
<para>
The <option>--optimize-images</option> flag tells qpdf to
recompresses every image using DCT (JPEG) compression as
long as the image is not already compressed with lossy
compression and recompressing the image reduces its size.
The additional options <option>--oi-min-width</option>,
<option>--oi-min-height</option>, and
<option>--oi-min-area</option> prevent recompression of
images whose width, height, or pixel area
(width&nbsp;&#xd7;&nbsp;height) are below a specified
threshold.
</para>
</listitem>
<listitem>
<para>
The <option>--show-object</option> option can now be given
as <option>--show-object=trailer</option> to show the
trailer dictionary.
</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>
Bug Fixes and Enhancements
</para>
<itemizedlist>
<listitem>
<para>
QPDF now automatically detects and recovers from dangling
references. If a PDF file contained an indirect reference to
a non-existent object, which is valid, when adding a new
object to the file, it was possible for the new object to
take the object ID of the dangling reference, thereby
causing the dangling reference to point to the new object.
This case is now prevented.
</para>
</listitem>
<listitem>
<para>
Fixes to form field setting code: strings are always written
in UTF-16 format, and checkboxes and radio buttons are
handled properly with respect to synchronization of values
and appearance states.
</para>
</listitem>
<listitem>
<para>
The <function>QPDF::checkLinearization()</function> no
longer causes the program to crash when it detects problems
with linearization data. Instead, it issues a normal warning
or error.
</para>
</listitem>
<listitem>
<para>
Ordinarily qpdf treats an argument of the form
<option>@file</option> to mean that command-line options
should be read from <filename>file</filename>. Now, if
<filename>file</filename> does not exist but
<filename>@file</filename> does, qpdf will treat
<filename>@file</filename> as a regular option. This makes
it possible to work more easily with PDF files whose names
happen to start with the <literal>@</literal> character.
</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>
Library Enhancements
</para>
<itemizedlist>
<listitem>
<para>
Remove the restriction in most cases that the source QPDF
object used in a
<function>QPDF::copyForeignObject</function> call has to
stick around until the destination QPDF is written. The
exceptional case is when the source stream gets is data
using a QPDFObjectHandle::StreamDataProvider. For a more
in-depth discussion, see comments around
<function>copyForeignObject</function> in
<filename>QPDF.hh</filename>.
</para>
</listitem>
<listitem>
<para>
Add new method
<function>QPDFWriter::getFinalVersion()</function>, which
returns the PDF version that will ultimately be written to
the final file. See comments in
<filename>QPDFWriter.hh</filename> for some restrictions on
its use.
</para>
</listitem>
<listitem>
<para>
Add several methods for transcoding strings to some of the
character sets used in PDF files:
<function>QUtil::utf8_to_ascii</function>,
<function>QUtil::utf8_to_win_ansi</function>,
<function>QUtil::utf8_to_mac_roman</function>, and
<function>QUtil::utf8_to_utf16</function>. For the
single-byte encodings that support only a limited character
sets, these methods replace unsupported characters with a
specified substitute.
</para>
</listitem>
<listitem>
<para>
Add new methods to
<classname>QPDFAnnotationObjectHelper</classname> and
<classname>QPDFFormFieldObjectHelper</classname> for
querying flags and interpretation of different field types.
Define constants in <filename>qpdf/Constants.h</filename> to
help with interpretation of flag values.
</para>
</listitem>
<listitem>
<para>
Add new methods
<function>QPDFAcroFormDocumentHelper::generateAppearancesIfNeeded</function>
and
<function>QPDFFormFieldObjectHelper::generateAppearance</function>
for generating appearance streams. See discussion in
<filename>QPDFFormFieldObjectHelper.hh</filename> for
limitations.
</para>
</listitem>
<listitem>
<para>
Add two new helper functions for dealing with resource
dictionaries:
<function>QPDFObjectHandle::getResourceNames()</function>
returns a list of all second-level keys, which correspond to
the names of resources, and
<function>QPDFObjectHandle::mergeResources()</function>
merges two resources dictionaries as long as they have
non-conflicting keys. These methods are useful for certain
types of objects that resolve resources from multiple places,
such as form fields.
</para>
</listitem>
<listitem>
<para>
Add methods
<function>QPDFPageDocumentHelper::flattenAnnotations()</function>
and
<function>QPDFAnnotationObjectHelper::getPageContentForAppearance()</function>
for handling low-level details of annotation flattening.
</para>
</listitem>
<listitem>
<para>
Add new helper classes:
<classname>QPDFOutlineDocumentHelper</classname>,
<classname>QPDFOutlineObjectHelper</classname>,
<classname>QPDFPageLabelDocumentHelper</classname>,
<classname>QPDFNameTreeObjectHelper</classname>, and
<classname>QPDFNumberTreeObjectHelper</classname>.
</para>
</listitem>
<listitem>
<para>
Add method <function>QPDFObjectHandle::getJSON()</function>
that returns a JSON representation of the object. Call
<function>serialize()</function> on the result to convert it
to a string.
</para>
</listitem>
<listitem>
<para>
Add a simple JSON serializer. This is not a complete or
general-purpose JSON library. It allows assembly and
serialization of JSON structures with some restrictions,
which are described in the header file. This is the
serializer used by qpdf's new JSON representation.
</para>
</listitem>
<listitem>
<para>
Add new <classname>QPDFObjectHandle::Matrix</classname>
class along with a few convenience methods for dealing with
six-element numerical arrays as matrices.
</para>
</listitem>
<listitem>
<para>
Add new method
<function>QPDFObjectHandle::wrapInArray</function>, which returns
the object itself if it is an array, or an array containing
the object otherwise. This is a common construct in PDF.
This method prevents you from having to explicitly test
whether something is a single element or an array.
</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>
Build Improvements
</para>
<itemizedlist>
<listitem>
<para>
It is no longer necessary to run
<command>autogen.sh</command> to build from a pristine
checkout. Automatically generated files are now committed so
that it is possible to build on platforms without autoconf
directly from a clean checkout of the repository. The
<command>configure</command> script detects if the files are
out of date when it also determines that the tools are
present to regenerate them.
</para>
</listitem>
<listitem>
<para>
Pull requests and the master branch are now built
automatically in <ulink
url="https://dev.azure.com/qpdf/qpdf/_build">Azure
Pipelines</ulink>, which is free for open source projects.
The build includes Linux, mac, Windows 32-bit and 64-bit
with mingw and MSVC, and an AppImage build. Official qpdf
releases are now built with Azure Pipelines.
</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>
Notes for Packagers
</para>
<itemizedlist>
<listitem>
<para>
A new section has been added to the documentation with notes
for packagers. Please see <xref linkend="ref.packaging"/>.
</para>
</listitem>
<listitem>
<para>
The qpdf detects out-of-date automatically generated files.
If your packaging system automatically refreshes libtool or
autoconf files, it could cause this check to fail. To avoid
this problem, pass
<option>--disable-check-autofiles</option> to
<command>configure</command>.
</para>
</listitem>
<listitem>
<para>
If you would like to have qpdf completion enabled
automatically, you can install completion files in the
distribution's default location. You can find sample
completion files to install in the
<filename>completions</filename> directory.
</para>
</listitem>
</itemizedlist>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>8.2.1: August 18, 2018</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Command-line Enhancements
</para>
<itemizedlist>
<listitem>
<para>
Add
<option>--keep-files-open=<replaceable>[yn]</replaceable></option>
to override default determination of whether to keep files
open when merging. Please see the discussion of
<option>--keep-files-open</option> in <xref
linkend="ref.basic-options"/> for additional details.
</para>
</listitem>
</itemizedlist>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>8.2.0: August 16, 2018</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Command-line Enhancements
</para>
<itemizedlist>
<listitem>
<para>
Add <option>--no-warn</option> option to suppress issuing
warning messages. If there are any conditions that would
have caused warnings to be issued, the exit status is still
3.
</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>
Bug Fixes and Optimizations
</para>
<itemizedlist>
<listitem>
<para>
Performance fix: optimize page merging operation to avoid
unnecessary open/close calls on files being merged. This
solves a dramatic slow-down that was observed when merging
certain types of files.
</para>
</listitem>
<listitem>
<para>
Optimize how memory was used for the TIFF predictor,
drastically improving performance and memory usage for files
containing high-resolution images compressed with Flate
using the TIFF predictor.
</para>
</listitem>
<listitem>
<para>
Bug fix: end of line characters were not properly handled
inside strings in some cases.
</para>
</listitem>
<listitem>
<para>
Bug fix: using <option>--progress</option> on very small
files could cause an infinite loop.
</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>
API enhancements
</para>
<itemizedlist>
<listitem>
<para>
Add new class <classname>QPDFSystemError</classname>, derived
from <classname>std::runtime_error</classname>, which is now
thrown by <function>QUtil::throw_system_error</function>.
This enables the triggering <classname>errno</classname>
value to be retrieved.
</para>
</listitem>
<listitem>
<para>
Add <function>ClosedFileInputSource::stayOpen</function>
method, enabling a
<classname>ClosedFileInputSource</classname> to stay open
during manually indicated periods of high activity, thus
reducing the overhead of frequent open/close operations.
</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>
Build Changes
</para>
<itemizedlist>
<listitem>
<para>
For the mingw builds, change the name of the DLL import
library from <filename>libqpdf.a</filename> to
<filename>libqpdf.dll.a</filename> to more accurately
reflect that it is an import library rather than a static
library. This potentially clears the way for supporting a
static library in the future, though presently, the qpdf
Windows build only builds the DLL and executables.
</para>
</listitem>
</itemizedlist>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>8.1.0: June 23, 2018</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Usability Improvements
</para>
<itemizedlist>
<listitem>
<para>
When splitting files, qpdf detects fonts and images that the
document metadata claims are referenced from a page but are
not actually referenced and omits them from the output file.
This change can cause a significant reduction in the size of
split PDF files for files created by some software packages.
Prior versions of qpdf would believe the document metadata
and sometimes include all the images from all the other
pages even though the pages were no longer present. In the
unlikely event that the old behavior should be desired, it
can be enabled by specifying
<option>--preserve-unreferenced-resources</option>. For
additional details, please see <xref
linkend="ref.advanced-transformation"/>.
</para>
</listitem>
<listitem>
<para>
When merging multiple PDF files, qpdf no longer leaves all
the files open. This makes it possible to merge numbers of
files that may exceed the operating system's limit for the
maximum number of open files.
</para>
</listitem>
<listitem>
<para>
The <option>--rotate</option> option's syntax has been
extended to make the page range optional. If you specify
<option>--rotate=<replaceable>angle</replaceable></option>
without specifying a page range, the rotation will be
applied to all pages. This can be especially useful for
adjusting a PDF created from a multi-page document that
was scanned upside down.
</para>
</listitem>
<listitem>
<para>
When merging multiple files, the <option>--verbose</option>
option now prints information about each file as it operates
on that file.
</para>
</listitem>
<listitem>
<para>
When the <option>--progress</option> option is specified,
qpdf will print a running indicator of its best guess at how
far through the writing process it is. Note that, as with
all progress meters, it's an approximation. This option is
implemented in a way that makes it useful for software that
uses the qpdf library; see API Enhancements below.
</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>
Bug Fixes
</para>
<itemizedlist>
<listitem>
<para>
Properly decrypt files that use revision 3 of the standard
security handler but use 40 bit keys (even though revision 3
supports 128-bit keys).
</para>
</listitem>
<listitem>
<para>
Limit depth of nested data structures to prevent crashes
from certain types of malformed (malicious) PDFs.
</para>
</listitem>
<listitem>
<para>
In &ldquo;newline before endstream&rdquo; mode, insert the
required extra newline before the
<literal>endstream</literal> at the end of object streams.
This one case was previously omitted.
</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>
API Enhancements
</para>
<itemizedlist>
<listitem>
<para>
The first round of higher level &ldquo;helper&rdquo;
interfaces has been introduced. These are designed to
provide a more convenient way of interacting with certain
document features than using
<classname>QPDFObjectHandle</classname> directly. For
details on helpers, see <xref
linkend="ref.helper-classes"/>. Specific additional
interfaces are described below.
</para>
</listitem>
<listitem>
<para>
Add two new document helper classes:
<classname>QPDFPageDocumentHelper</classname> for working
with pages, and
<classname>QPDFAcroFormDocumentHelper</classname> for
working with interactive forms. No old methods have been
removed, but <classname>QPDFPageDocumentHelper</classname>
is now the preferred way to perform operations on pages
rather than calling the old methods in
<classname>QPDFObjectHandle</classname> and
<classname>QPDF</classname> directly. Comments in the header
files direct you to the new interfaces. Please see the
header files and <filename>ChangeLog</filename> for
additional details.
</para>
</listitem>
<listitem>
<para>
Add three new object helper class:
<classname>QPDFPageObjectHelper</classname> for pages,
<classname>QPDFFormFieldObjectHelper</classname> for
interactive form fields, and
<classname>QPDFAnnotationObjectHelper</classname> for
annotations. All three classes are fairly sparse at the
moment, but they have some useful, basic functionality.
</para>
</listitem>
<listitem>
<para>
A new example program
<filename>examples/pdf-set-form-values.cc</filename> has
been added that illustrates use of the new document and
object helpers.
</para>
</listitem>
<listitem>
<para>
The method
<function>QPDFWriter::registerProgressReporter</function>
has been added. This method allows you to register a
function that is called by <classname>QPDFWriter</classname>
to update your idea of the percentage it thinks it is
through writing its output. Client programs can use this to
implement reasonably accurate progress meters. The
<command>qpdf</command> command line tool uses this to
implement its <option>--progress</option> option.
</para>
</listitem>
<listitem>
<para>
New methods
<function>QPDFObjectHandle::newUnicodeString</function> and
<function>QPDFObject::unparseBinary</function> have been
added to allow for more convenient creation of strings that
are explicitly encoded using big-endian UTF-16. This is
useful for creating strings that appear outside of content
streams, such as labels, form fields, outlines, document
metadata, etc.
</para>
</listitem>
<listitem>
<para>
A new class
<classname>QPDFObjectHandle::Rectangle</classname> has been
added to ease working with PDF rectangles, which are just
arrays of four numeric values.
</para>
</listitem>
</itemizedlist>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>8.0.2: March 6, 2018</term>
<listitem>
<itemizedlist>
<listitem>
<para>
When a loop is detected while following cross reference
streams or tables, treat this as damage instead of silently
ignoring the previous table. This prevents loss of otherwise
recoverable data in some damaged files.
</para>
</listitem>
</itemizedlist>
<itemizedlist>
<listitem>
<para>
Properly handle pages with no contents.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>8.0.1: March 4, 2018</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Disregard data check errors when uncompressing
<option>/FlateDecode</option> streams. This is consistent with
most other PDF readers and allows qpdf to recover data from
another class of malformed PDF files.
</para>
</listitem>
<listitem>
<para>
On the command line when specifying page ranges, support
preceding a page number by &ldquo;r&rdquo; to indicate that it
should be counted from the end. For example, the range
<literal>r3-r1</literal> would indicate the last three pages
of a document.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>8.0.0: February 25, 2018</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Packaging and Distribution Changes
</para>
<itemizedlist>
<listitem>
<para>
QPDF is now distributed as an <ulink
url="https://appimage.org/">AppImage</ulink> in addition to
all the other ways it is distributed. The AppImage can be
found in the download area with the other packages. Thanks
to Kurt Pfeifle and Simon Peter for their contributions.
</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>
Bug Fixes
</para>
<itemizedlist>
<listitem>
<para>
<function>QPDFObjectHandle::getUTF8Val</function> now
properly treats non-Unicode strings as encoded with PDF Doc
Encoding.
</para>
</listitem>
<listitem>
<para>
Improvements to handling of objects in PDF files that are
not of the expected type. In most cases, qpdf will be able
to warn for such cases rather than fail with an exception.
Previous versions of qpdf would sometimes fail with errors
such as &ldquo;operation for dictionary object attempted on
object of wrong type&rdquo;. This situation should be mostly
or entirely eliminated now.
</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>
Enhancements to the <command>qpdf</command> Command-line Tool.
All new options listed here are documented in more detail in
<xref linkend="ref.using"/>.
</para>
<itemizedlist>
<listitem>
<para>
The option
<option>--linearize-pass1=<replaceable>file</replaceable></option>
has been added for debugging qpdf's linearization code.
</para>
</listitem>
<listitem>
<para>
The option <option>--coalesce-contents</option> can be used
to combine content streams of a page whose contents are an
array of streams into a single stream.
</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>
API Enhancements. All new API calls are documented in their
respective classes' header files. There are no non-compatible
changes to the API.
</para>
<itemizedlist>
<listitem>
<para>
Add function <function>qpdf_check_pdf</function> to the C API.
This function does basic checking that is a subset of what
<command>qpdf --check</command> performs.
</para>
</listitem>
<listitem>
<para>
Major enhancements to the lexical layer of qpdf. For a
complete list of enhancements, please refer to the
<filename>ChangeLog</filename> file. Most of the changes
result in improvements to qpdf's ability handle erroneous
files. It is also possible for programs to handle
whitespace, comments, and inline images as tokens.
</para>
</listitem>
<listitem>
<para>
New API for working with PDF content streams at a lexical
level. The new class
<classname>QPDFObjectHandle::TokenFilter</classname> allows
the developer to provide token handlers. Token filters can be
used with several different methods in
<classname>QPDFObjectHandle</classname> as well as with a
lower-level interface. See comments in
<filename>QPDFObjectHandle.hh</filename> as well as the new
examples <filename>examples/pdf-filter-tokens.cc</filename>
and <filename>examples/pdf-count-strings.cc</filename> for
details.
</para>
</listitem>
</itemizedlist>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>7.1.1: February 4, 2018</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Bug fix: files whose /ID fields were other than 16 bytes long
can now be properly linearized
</para>
</listitem>
<listitem>
<para>
A few compile and link issues have been corrected for some
platforms.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>7.1.0: January 14, 2018</term>
<listitem>
<itemizedlist>
<listitem>
<para>
PDF files contain streams that may be compressed with various
compression algorithms which, in some cases, may be enhanced
by various predictor functions. Previously only the PNG up
predictor was supported. In this version, all the PNG
predictors as well as the TIFF predictor are supported. This
increases the range of files that qpdf is able to handle.
</para>
</listitem>
<listitem>
<para>
QPDF now allows a raw encryption key to be specified in place
of a password when opening encrypted files, and will
optionally display the encryption key used by a file. This is
a non-standard operation, but it can be useful in certain
situations. Please see the discussion of
<option>--password-is-hex-key</option> in <xref
linkend="ref.basic-options"/> or the comments around
<function>QPDF::setPasswordIsHexKey</function> in
<filename>QPDF.hh</filename> for additional details.
</para>
</listitem>
<listitem>
<para>
Bug fix: numbers ending with a trailing decimal point are now
properly recognized as numbers.
</para>
</listitem>
<listitem>
<para>
Bug fix: when building qpdf from source on some platforms
(especially MacOS), the build could get confused by older
versions of qpdf installed on the system. This has been
corrected.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>7.0.0: September 15, 2017</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Packaging and Distribution Changes
</para>
<itemizedlist>
<listitem>
<para>
QPDF's primary license is now <ulink
url="http://www.apache.org/licenses/LICENSE-2.0">version 2.0
of the Apache License</ulink> rather than version 2.0 of the
Artistic License. You may still, at your option, consider
qpdf to be licensed with version 2.0 of the Artistic
license.
</para>
</listitem>
<listitem>
<para>
QPDF no longer has a dependency on the PCRE (Perl-Compatible
Regular Expression) library. QPDF now has an added
dependency on the JPEG library.
</para>
</listitem>
</itemizedlist>
</listitem>
</itemizedlist>
<itemizedlist>
<listitem>
<para>
Bug Fixes
</para>
<itemizedlist>
<listitem>
<para>
This release contains many bug fixes for various infinite
loops, memory leaks, and other memory errors that could be
encountered with specially crafted or otherwise erroneous
PDF files.
</para>
</listitem>
</itemizedlist>
</listitem>
</itemizedlist>
<itemizedlist>
<listitem>
<para>
New Features
</para>
<itemizedlist>
<listitem>
<para>
QPDF now supports reading and writing streams encoded with
JPEG or RunLength encoding. Library API enhancements and
command-line options have been added to control this
behavior. See command-line options
<option>--compress-streams</option> and
<option>--decode-level</option> and methods
<function>QPDFWriter::setCompressStreams</function> and
<function>QPDFWriter::setDecodeLevel</function>.
</para>
</listitem>
<listitem>
<para>
QPDF is much better at recovering from broken files. In most
cases, qpdf will skip invalid objects and will preserve
broken stream data by not attempting to filter broken
streams. QPDF is now able to recover or at least not crash
on dozens of broken test files I have received over the past
few years.
</para>
</listitem>
<listitem>
<para>
Page rotation is now supported and accessible from both the
library and the command line.
</para>
</listitem>
<listitem>
<para>
<classname>QPDFWriter</classname> supports writing files in
a way that preserves PCLm compliance in support of
driverless printing. This is very specialized and is only
useful to applications that already know how to create PCLm
files.
</para>
</listitem>
</itemizedlist>
</listitem>
</itemizedlist>
<itemizedlist>
<listitem>
<para>
Enhancements to the <command>qpdf</command> Command-line Tool.
All new options listed here are documented in more detail in
<xref linkend="ref.using"/>.
</para>
<itemizedlist>
<listitem>
<para>
Command-line arguments can now be read from files or
standard input using <literal>@file</literal> or
<literal>@-</literal> syntax. Please see <xref
linkend="ref.invocation"/>.
</para>
</listitem>
<listitem>
<para>
<option>--rotate</option>: request page rotation
</para>
</listitem>
<listitem>
<para>
<option>--newline-before-endstream</option>: ensure that a
newline appears before every <literal>endstream</literal>
keyword in the file; used to prevent qpdf from breaking
PDF/A compliance on already compliant files.
</para>
</listitem>
<listitem>
<para>
<option>--preserve-unreferenced</option>: preserve
unreferenced objects in the input PDF
</para>
</listitem>
<listitem>
<para>
<option>--split-pages</option>: break output into chunks
with fixed numbers of pages
</para>
</listitem>
<listitem>
<para>
<option>--verbose</option>: print the name of each output
file that is created
</para>
</listitem>
<listitem>
<para>
<option>--compress-streams</option> and
<option>--decode-level</option> replace
<option>--stream-data</option> for improving granularity of
controlling compression and decompression of stream data.
The <option>--stream-data</option> option will remain
available.
</para>
</listitem>
<listitem>
<para>
When running <command>qpdf --check</command> with other
options, checks are always run first. This enables qpdf to
perform its full recovery logic before outputting other
information. This can be especially useful when manually
recovering broken files, looking at qpdf's regenerated cross
reference table, or other similar operations.
</para>
</listitem>
<listitem>
<para>
Process <command>--pages</command> earlier so that other
options like <option>--show-pages</option> or
<option>--split-pages</option> can operate on the file after
page splitting/merging has occurred.
</para>
</listitem>
</itemizedlist>
</listitem>
</itemizedlist>
<itemizedlist>
<listitem>
<para>
API Changes. All new API calls are documented in their
respective classes' header files.
</para>
<itemizedlist>
<listitem>
<para>
<function>QPDFObjectHandle::rotatePage</function>: apply
rotation to a page object
</para>
</listitem>
<listitem>
<para>
<function>QPDFWriter::setNewlineBeforeEndstream</function>:
force newline to appear before <literal>endstream</literal>
</para>
</listitem>
<listitem>
<para>
<function>QPDFWriter::setPreserveUnreferencedObjects</function>:
preserve unreferenced objects that appear in the input PDF.
The default behavior is to discard them.
</para>
</listitem>
<listitem>
<para>
New <classname>Pipeline</classname> types
<classname>Pl_RunLength</classname> and
<classname>Pl_DCT</classname> are available for developers
who wish to produce or consume RunLength or DCT stream data
directly. The <filename>examples/pdf-create.cc</filename>
example illustrates their use.
</para>
</listitem>
<listitem>
<para>
<function>QPDFWriter::setCompressStreams</function> and
<function>QPDFWriter::setDecodeLevel</function> methods
control handling of different types of stream compression.
</para>
</listitem>
<listitem>
<para>
Add new C API functions
<function>qpdf_set_compress_streams</function>,
<function>qpdf_set_decode_level</function>,
<function>qpdf_set_preserve_unreferenced_objects</function>,
and <function>qpdf_set_newline_before_endstream</function>
corresponding to the new <classname>QPDFWriter</classname>
methods.
</para>
</listitem>
</itemizedlist>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
<term>6.0.0: November 10, 2015</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Implement <option>--deterministic-id</option> command-line
option and <function>QPDFWriter::setDeterministicID</function>
as well as C API function
<function>qpdf_set_deterministic_ID</function> for generating
a deterministic ID for non-encrypted files. When this option
is selected, the ID of the file depends on the contents of the
output file, and not on transient items such as the timestamp
or output file name.
</para>
</listitem>
<listitem>
<para>
Make qpdf more tolerant of files whose xref table entries are
not the correct length.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>5.1.3: May 24, 2015</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Bug fix: fix-qdf was not properly handling files that
contained object streams with more than 255 objects in them.
</para>
</listitem>
<listitem>
<para>
Bug fix: qpdf was not properly initializing Microsoft's secure
crypto provider on fresh Windows installations that had not
had any keys created yet.
</para>
</listitem>
<listitem>
<para>
Fix a few errors found by Gynvael Coldwind and
Mateusz Jurczyk of the Google Security Team. Please see the
ChangeLog for details.
</para>
</listitem>
<listitem>
<para>
Properly handle pages that have no contents at all. There were
many cases in which qpdf handled this fine, but a few methods
blindly obtained page contents with handling the possibility
that there were no contents.
</para>
</listitem>
<listitem>
<para>
Make qpdf more robust for a few more kinds of problems that
may occur in invalid PDF files.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>5.1.2: June 7, 2014</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Bug fix: linearizing files could create a corrupted output
file under extremely unlikely file size circumstances. See
ChangeLog for details. The odds of getting hit by this are
very low, though one person did.
</para>
</listitem>
<listitem>
<para>
Bug fix: qpdf would fail to write files that had streams with
decode parameters referencing other streams.
</para>
</listitem>
<listitem>
<para>
New example program: <command>pdf-split-pages</command>:
efficiently split PDF files into individual pages. The example
program does this more efficiently than using <command>qpdf
--pages</command> to do it.
</para>
</listitem>
<listitem>
<para>
Packaging fix: Visual C++ binaries did not support Windows XP.
This has been rectified by updating the compilers used to
generate the release binaries.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>5.1.1: January 14, 2014</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Performance fix: copying foreign objects could be very slow
with certain types of files. This was most likely to be
visible during page splitting and was due to traversing the
same objects multiple times in some cases.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>5.1.0: December 17, 2013</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Added runtime option
(<function>QUtil::setRandomDataProvider</function>) to supply
your own random data provider. You can use this if you want
to avoid using the OS-provided secure random number generation
facility or stdlib's less secure version. See comments in
include/qpdf/QUtil.hh for details.
</para>
</listitem>
<listitem>
<para>
Fixed image comparison tests to not create 12-bit-per-pixel
images since some versions of tiffcmp have bugs in comparing
them in some cases. This increases the disk space required by
the image comparison tests, which are off by default anyway.
</para>
</listitem>
<listitem>
<para>
Introduce a number of small fixes for compilation on the
latest clang in MacOS and the latest Visual C++ in Windows.
</para>
</listitem>
<listitem>
<para>
Be able to handle broken files that end the xref table header
with a space instead of a newline.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>5.0.1: October 18, 2013</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Thanks to a detailed review by Florian Weimer and the Red Hat
Product Security Team, this release includes a number of
non-user-visible security hardening changes. Please see the
ChangeLog file in the source distribution for the complete
list.
</para>
</listitem>
<listitem>
<para>
When available, operating system-specific secure random number
generation is used for generating initialization vectors and
other random values used during encryption or file creation.
For the Windows build, this results in an added dependency on
Microsoft's cryptography API. To disable the OS-specific
cryptography and use the old version, pass the
<option>--enable-insecure-random</option> option to
<command>./configure</command>.
</para>
</listitem>
<listitem>
<para>
The <command>qpdf</command> command-line tool now issues a
warning when <option>-accessibility=n</option> is specified
for newer encryption versions stating that the option is
ignored. qpdf, per the spec, has always ignored this flag,
but it previously did so silently. This warning is issued
only by the command-line tool, not by the library. The
library's handling of this flag is unchanged.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>5.0.0: July 10, 2013</term>
<listitem>
<itemizedlist>
<listitem>
<para>
Bug fix: previous versions of qpdf would lose objects with
generation != 0 when generating object streams. Fixing this
required changes to the public API.
</para>
</listitem>
<listitem>
<para>
Removed methods from public API that were only supposed to be
called by QPDFWriter and couldn't realistically be called
anywhere else. See ChangeLog for details.
</para>
</listitem>
<listitem>
<para>
New <type>QPDFObjGen</type> class added to represent an object
ID/generation pair.
<function>QPDFObjectHandle::getObjGen()</function> is now
preferred over
<function>QPDFObjectHandle::getObjectID()</function> and
<function>QPDFObjectHandle::getGeneration()</function> as it
makes it less likely for people to accidentally write code
that ignores the generation number. See
<filename>QPDF.hh</filename> and
<filename>QPDFObjectHandle.hh</filename> for additional notes.
</para>
</listitem>
<listitem>
<para>
Add <option>--show-npages</option> command-line option to the
<command>qpdf</command> command to show the number of pages in
a file.
</para>
</listitem>
<listitem>
<para>
Allow omission of the page range within
<option>--pages</option> for the <command>qpdf</command>
command. When omitted, the page range is implicitly taken to
be all the pages in the file.
</para>
</listitem>
<listitem>
<para>
Various enhancements were made to support different types of
broken files or broken readers. Details can be found in
<filename>ChangeLog</filename>.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<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.md</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>
<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>
<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>
<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.md</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.md</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>
<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>
<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 --> Gajic (<ulink
url="http://zarko-gajic.iz.hr/">http://zarko-gajic.iz.hr/</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>
Žarko Gajic has written a Delphi wrapper for qpdf, which can
be downloaded from qpdf's download side. Žarko'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 Žarko Gajic
(<ulink
url="http://zarko-gajic.iz.hr/">http://zarko-gajic.iz.hr/</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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