path: root/manual/qpdf-manual.xml

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<!ENTITY swversion "8.2.1">
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<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;2018</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>
 </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.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><option>--extract=[yn]</option></term>
      <listitem>
       <para>
        Determines whether or not to allow text/graphic extraction.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><option>--print=<replaceable>print-opt</replaceable></option></term>
      <listitem>
       <para>
        Controls printing access.
        <option><replaceable>print-opt</replaceable></option> may be
        one of the following:
        <itemizedlist>
         <listitem>
          <para>
           <option>full</option>: allow full printing
          </para>
         </listitem>
         <listitem>
          <para>
           <option>low</option>: allow low-resolution printing only
          </para>
         </listitem>
         <listitem>
          <para>
           <option>none</option>: disallow printing
          </para>
         </listitem>
        </itemizedlist>
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><option>--modify=<replaceable>modify-opt</replaceable></option></term>
      <listitem>
       <para>
        Controls modify access.
        <option><replaceable>modify-opt</replaceable></option> may be
        one of the following, each of which implies all the options
        that follow it:
        <itemizedlist>
         <listitem>
          <para>
           <option>all</option>: allow full document modification
          </para>
         </listitem>
         <listitem>
          <para>
           <option>annotate</option>: allow comment authoring and form operations
          </para>
         </listitem>
         <listitem>
          <para>
           <option>form</option>: allow form field fill-in and signing
          </para>
         </listitem>
         <listitem>
          <para>
           <option>assembly</option>: allow document assembly only
          </para>
         </listitem>
         <listitem>
          <para>
           <option>none</option>: allow no modifications
          </para>
         </listitem>
        </itemizedlist>
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><option>--cleartext-metadata</option></term>
      <listitem>
       <para>
        If specified, any metadata stream in the document will be left
        unencrypted even if the rest of the document is encrypted.
        This also forces the PDF version to be at least 1.5.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><option>--use-aes=[yn]</option></term>
      <listitem>
       <para>
        If <option>--use-aes=y</option> is specified, AES encryption
        will be used instead of RC4 encryption.  This forces the PDF
        version to be at least 1.6.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><option>--force-V4</option></term>
      <listitem>
       <para>
        Use of this option forces the <literal>/V</literal> and
        <literal>/R</literal> parameters in the document's encryption
        dictionary to be set to the value <literal>4</literal>.  As
        qpdf will automatically do this when required, there is no
        reason to ever use this option.  It exists primarily for use
        in testing qpdf itself.  This option also forces the PDF
        version to be at least 1.5.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
    If <option><replaceable>key-length</replaceable></option> is 256,
    the minimum PDF version is 1.7 with extension level 8, and the
    AES-based encryption format used is the PDF 2.0 encryption method
    supported by Acrobat X.  the same options are available as with
    128 bits with the following exceptions:
    <variablelist>
     <varlistentry>
      <term><option>--use-aes</option></term>
      <listitem>
       <para>
        This option is not available with 256-bit keys.  AES is always
        used with 256-bit encryption keys.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><option>--force-V4</option></term>
      <listitem>
       <para>
        This option is not available with 256 keys.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><option>--force-R5</option></term>
      <listitem>
       <para>
        If specified, qpdf sets the minimum version to 1.7 at
        extension level 3 and writes the deprecated encryption format
        used by Acrobat version IX.  This option should not be used in
        practice to generate PDF files that will be in general use,
        but it can be useful to generate files if you are trying to
        test proper support in another application for PDF files
        encrypted in this way.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
    The default for each permission option is to be fully permissive.
   </para>
  </sect1>
  <sect1 id="ref.page-selection">
   <title>Page Selection Options</title>
   <para>
    Starting with qpdf 3.0, it is possible to split and merge PDF
    files by selecting pages from one or more input files.  Whatever
    file is given as the primary input file is used as the starting
    point, but its pages are replaced with pages as specified.

    <programlisting><option>--pages <replaceable>input-file</replaceable> [ <replaceable>--password=password</replaceable> ] [ <replaceable>page-range</replaceable> ] [ ... ] --</option>
</programlisting>
    Multiple input files may be specified.  Each one is given as the
    name of the input file, an optional password (if required to open
    the file), and the range of pages.  Note that
    &ldquo;<option>--</option>&rdquo; terminates parsing of page
    selection flags.
   </para>
   <para>
    For each file that pages should be taken from, specify the file, a
    password needed to open the file (if any), and a page range.  The
    password needs to be given only once per file.  If any of the
    input files are the same as the primary input file or the file
    used to copy encryption parameters (if specified), you do not need
    to repeat the password here.  The same file can be repeated
    multiple times.  If a file that is repeated has a password, the
    password only has to be given the first time.  All non-page data
    (info, outlines, page numbers, etc.) are taken from the primary
    input file.  To discard these, use <option>--empty</option> as the
    primary input.
   </para>
   <para>
    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>
    It is not presently possible to specify the same page from the
    same file directly more than once, but you can make this work by
    specifying two different paths to the same file (such as by
    putting <filename>./</filename> somewhere in the path).  This can
    also be used if you want to repeat a page from one of the input
    files in the output file.  This may be made more convenient in a
    future version of qpdf if there is enough demand for this feature.
   </para>
   <para>
    The page range is a set of numbers separated by commas, ranges of
    numbers separated dashes, or combinations of those. The character
    &ldquo;z&rdquo; represents the last page. 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 infile.pdf 1-5 -- outfile.pdf</option>
</programlisting>
    If you wanted pages 1 through 5 from
    <filename>infile.pdf</filename> but you wanted the rest of the
    metadata to be dropped, you could instead run

    <programlisting><command>qpdf</command> <option>--empty --pages infile.pdf 1-5 -- outfile.pdf</option>
</programlisting>
    If you wanted to take pages 1&ndash;5 from
    <filename>file1.pdf</filename> and pages 11&ndash;15 from
    <filename>file2.pdf</filename> in reverse, you would run

    <programlisting><command>qpdf</command> <option>file1.pdf --pages file1.pdf 1-5 file2.pdf 15-11 -- outfile.pdf</option>
</programlisting>
    If, for some reason, you wanted to take the first page of an
    encrypted file called <filename>encrypted.pdf</filename> with
    password <literal>pass</literal> and repeat it twice in an output
    file, and if you wanted to drop 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>
  </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>8.2.1: 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>