source: introduction/important/building-notes.xml@ db099a7

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<sect1 id="unpacking">
  <?dbhtml filename="notes-on-building.html"?>


  <title>Notes on Building Software</title>

  <para>Those people who have built an LFS system may be aware
  of the general principles of downloading and unpacking software. Some
  of that information is repeated here for those new to building
  their own software.</para>

  <para>Each set of installation instructions contains a URL from which you
  can download the package.  The patches; however, are stored on the LFS
  servers and are available via HTTP.  These are referenced as needed in the
  installation instructions.</para>

  <para>While you can keep the source files anywhere you like, we assume that
  you have unpacked the package and changed into the directory created by the
  unpacking process (the source directory).  We also assume you have
  uncompressed any required patches and they are in the directory
  immediately above the source directory.</para>

  <para>We can not emphasize strongly enough that you should start from a
  <emphasis>clean source tree</emphasis> each time. This means that if
  you have had an error during configuration or compilation, it's usually
  best to delete the source tree and
  re-unpack it <emphasis>before</emphasis> trying again. This obviously
  doesn't apply if you're an advanced user used to hacking
  <filename>Makefile</filename>s and C code, but if in doubt, start from a
  clean tree.</para>

  <sect2>
    <title>Building Software as an Unprivileged (non-root) User</title>

    <para>The golden rule of Unix System Administration is to use your
    superpowers only when necessary. Hence, BLFS recommends that you
    build software as an unprivileged user and only become the
    <systemitem class='username'>root</systemitem> user when installing the
    software. This philosophy is followed in all the packages in this book.
    Unless otherwise specified, all instructions should be executed as an
    unprivileged user. The book will advise you on instructions that need
    <systemitem class='username'>root</systemitem> privileges.</para>

  </sect2>

  <sect2>
    <title>Unpacking the Software</title>

    <para>If a file is in <filename class='extension'>.tar</filename> format
    and compressed, it is unpacked by running one of the following
    commands:</para>

<screen><userinput>tar -xvf filename.tar.gz
tar -xvf filename.tgz
tar -xvf filename.tar.Z
tar -xvf filename.tar.bz2</userinput></screen>

    <note>
      <para>You may omit using the <option>v</option> parameter in the commands
      shown above and below if you wish to suppress the verbose listing of all
      the files in the archive as they are extracted. This can help speed up the
      extraction as well as make any errors produced during the extraction
      more obvious to you.</para>
    </note>

    <para>You can also use a slightly different method:</para>

<screen><userinput>bzcat filename.tar.bz2 | tar -xv</userinput></screen>

    <para>
      Finally, sometimes we have a compressed patch file in
      <filename class='extension'>.patch.gz</filename> or
      <filename class='extension'>.patch.bz2</filename> format.
      The best way to apply the patch is piping the output of the
      decompressor to the <command>patch</command> utility.  For example:
    </para>

    <screen><userinput>gzip -cd ../patchname.patch.gz | patch -p1</userinput></screen>

    <para>
      Or for a patch compressed with <command>bzip2</command>:
    </para>

    <screen><userinput>bzcat ../patchname.patch.bz2 | patch -p1</userinput></screen>

  </sect2>

  <sect2>
    <title>Verifying File Integrity</title>

    <para>Generally, to verify that the downloaded file is complete,
    many package maintainers also distribute md5sums of the files. To verify the
    md5sum of the downloaded files, download both the file and the
    corresponding md5sum file to the same directory (preferably from different
    on-line locations), and (assuming <filename>file.md5sum</filename> is the
    md5sum file downloaded) run the following command:</para>

<screen><userinput>md5sum -c file.md5sum</userinput></screen>

    <para>If there are any errors, they will be reported. Note that the BLFS
    book includes md5sums for all the source files also. To use the BLFS
    supplied md5sums, you can create a <filename>file.md5sum</filename> (place
    the md5sum data and the exact name of the downloaded file on the same
    line of a file, separated by white space) and run the command shown above.
    Alternately, simply run the command shown below and compare the output
    to the md5sum data shown in the BLFS book.</para>

<screen><userinput>md5sum <replaceable>&lt;name_of_downloaded_file&gt;</replaceable></userinput></screen>

    <para>MD5 is not cryptographically secure, so the md5sums are only
    provided for detecting unmalicious changes to the file content.  For
    example, an error or truncation introduced during network transfer, or
    a <quote>stealth</quote> update to the package from the upstream
    (updating the content of a released tarball instead of making a new
    release properly).</para>

    <para>There is no <quote>100%</quote> secure way to make
    sure the genuity of the source files.  Assuming the upstream is managing
    their website correctly (the private key is not leaked and the domain is
    not hijacked), and the trust anchors have been set up correctly using
    <xref linkend="make-ca"/> on the BLFS system, we can reasonably trust
    download URLs to the upstream official website
    <emphasis role="bold">with https protocol</emphasis>.  Note that
    BLFS book itself is published on a website with https, so you should
    already have some confidence in https protocol or you wouldn't trust the
    book content.</para>

    <para>If the package is downloaded from an unofficial location (for
    example a local mirror), checksums generated by cryptographically secure
    digest algorithms (for example SHA256) can be used to verify the
    genuity of the package.  Download the checksum file from the upstream
    <emphasis role="bold">official</emphasis> website (or somewhere
    <emphasis role="bold">you can trust</emphasis>) and compare the
    checksum of the package from unofficial location with it.  For example,
    SHA256 checksum can be checked with the command:</para>

    <note>
      <para>If the checksum and the package are downloaded from the same
      untrusted location, you won't gain security enhancement by verifying
      the package with the checksum. The attacker can fake the checksum as
      well as compromising the package itself.</para>
    </note>

<screen><userinput>sha256sum -c <replaceable>file</replaceable>.sha256sum</userinput></screen>

    <para>If <xref linkend="gnupg2"/> is installed, you can also verify the
    genuity of the package with a GPG signature.  Import the upstream GPG
    public key with:</para>

<screen><userinput>gpg --recv-key <replaceable>keyID</replaceable></userinput></screen>

    <para><replaceable>keyID</replaceable> should be replaced with the key ID
    from somewhere <emphasis role="bold">you can trust</emphasis> (for
    example, copy it from the upstream official website using https).  Now
    you can verify the signature with:</para>

<screen><userinput>gpg --recv-key <replaceable>file</replaceable>.sig <replaceable>file</replaceable></userinput></screen>

    <para>The advantage of <application>GnuPG</application> signature is,
    once you imported a public key which can be trusted, you can download
    both the package and its signature from the same unofficial location and
    verify them with the public key.  So you won't need to connect to the
    official upstream website to retrieve a checksum for each new release.
    You only need to update the public key if it's expired or revoked.
    </para>

  </sect2>

  <sect2>
    <title>Creating Log Files During Installation</title>

    <para>For larger packages, it is convenient to create log files instead of
    staring at the screen hoping to catch a particular error or warning. Log
    files are also useful for debugging and keeping records. The following
    command allows you to create an installation log. Replace
    <replaceable>&lt;command&gt;</replaceable> with the command you intend to execute.</para>

<screen><userinput>( <replaceable>&lt;command&gt;</replaceable> 2&gt;&amp;1 | tee compile.log &amp;&amp; exit $PIPESTATUS )</userinput></screen>

    <para><option>2&gt;&amp;1</option> redirects error messages to the same
    location as standard output. The <command>tee</command> command allows
    viewing of the output while logging the results to a file. The parentheses
    around the command run the entire command in a subshell and finally the
    <command>exit $PIPESTATUS</command> command ensures the result of the
    <replaceable>&lt;command&gt;</replaceable> is returned as the result and not the
    result of the <command>tee</command> command.</para>

  </sect2>

  <sect2 id="parallel-builds" xreflabel="Using Multiple Processors">
    <title>Using Multiple Processors</title>

    <para>For many modern systems with multiple processors (or cores) the
    compilation time for a package can be reduced by performing a "parallel
    make" by either setting an environment variable or telling the make program
    to simultaneously execute multiple jobs.</para>

    <para>For instance, an Intel Core i9-13900K CPU contains 8 performance
    (P) cores and 16 efficiency (E) cores, and the P cores support SMT
    (Simultaneous MultiThreading, also known as
    <quote>Hyper-Threading</quote>) so each P core can run two threads
    simultaneously and the Linux kernel will treat each P core as two
    logical cores.  As a result, there are 32 logical cores in total.
    To utilize all these logical cores running <command>make</command>, we
    can set an environment variable to tell <command>make</command> to
    run 32 jobs simultaneously:</para>

    <screen><userinput>export MAKEFLAGS='-j32'</userinput></screen>

    <para>or just building with:</para>

    <screen><userinput>make -j32</userinput></screen>

    <para>
      If you have applied the optional <command>sed</command> when building
      <application>ninja</application> in LFS, you can use:
    </para>

    <screen><userinput>export NINJAJOBS=32</userinput></screen>

    <para>
      when a package uses <command>ninja</command>, or just:
    </para>

    <screen><userinput>ninja -j32</userinput></screen>

    <para>
      If you are not sure about the number of logical cores, run the
      <command>nproc</command> command.
    </para>

    <para>
      For <command>make</command>, the default number of jobs is 1.  But
      for <command>ninja</command>, the default number of jobs is N + 2 if
      the number of logical cores N is greater than 2; or N + 1 if
      N is 1 or 2.  The reason to use a number of jobs slightly greater
      than the number of logical cores is keeping all logical
      processors busy even if some jobs are performing I/O operations.
    </para>

    <para>
      Note that the <option>-j</option> switches only limits the parallel
      jobs started by <command>make</command> or <command>ninja</command>,
      but each job may still spawn its own processes or threads.  For
      example, <command>ld.gold</command> will use multiple threads for
      linking, and some tests of packages can spawn multiple threads for
      testing thread safety properties.  There is no generic way for the
      building system to know the number of processes or threads spawned by
      a job. So generally we should not consider the value passed with
      <option>-j</option> a hard limit of the number of logical cores to
      use.  Read <xref linkend='build-in-cgroup'/> if you want to set such
      a hard limit.
    </para>

    <para>Generally the number of processes should not exceed the number of
    cores supported by the CPU too much.  To list the processors on your
    system, issue: <userinput>grep processor /proc/cpuinfo</userinput>.
    </para>

    <para>In some cases, using multiple processes may result in a race
    condition where the success of the build depends on the order of the
    commands run by the <command>make</command> program.  For instance, if an
    executable needs File A and File B, attempting to link the program before
    one of the dependent components is available will result in a failure.
    This condition usually arises because the upstream developer has not
    properly designated all the prerequisites needed to accomplish a step in the
    Makefile.</para>

    <para>If this occurs, the best way to proceed is to drop back to a
    single processor build.  Adding <option>-j1</option> to a make command
    will override the similar setting in the <envar>MAKEFLAGS</envar>
    environment variable.</para>

    <important>
      <para>
        Another problem may occur with modern CPU's, which have a lot of cores.
        Each job started consumes memory, and if the sum of the needed
        memory for each job exceeds the available memory, you may encounter
        either an OOM (Out of Memory) kernel interrupt or intense swapping
        that will slow the build beyond reasonable limits.
      </para>

      <para>
        Some compilations with <command>g++</command> may consume up to 2.5 GB
        of memory, so to be safe, you should restrict the number of jobs
        to (Total Memory in GB)/2.5, at least for big packages such as LLVM,
        WebKitGtk, QtWebEngine, or libreoffice.
      </para>
    </important>
  </sect2>

  <sect2 id="build-in-cgroup">
    <title>Use Linux Control Group to Limit the Resource Usage</title>

    <para>
      Sometimes we want to limit the resource usage when we build a
      package.  For example, when we have 8 logical cores, we may want
      to use only 6 cores for building the package and reserve another
      2 cores for playing a movie.  The Linux kernel provides a feature
      called control groups (cgroup) for such a need.
    </para>

    <para>
      Enable control group in the kernel configuration, then rebuild the
      kernel and reboot if necessary:
    </para>

    <xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
      href="cgroup-kernel.xml"/>

    <!-- We need cgroup2 mounted at /sys/fs/cgroup.  It's done by
         systemd itself in LFS systemd, mountvirtfs script in LFS sysv. -->

    <para revision='systemd'>
      Ensure <xref linkend='systemd'/> and <xref linkend='shadow'/> have
      been rebuilt with <xref linkend='linux-pam'/> support (if you are
      interacting via a SSH or graphical session, also ensure the
      <xref linkend='openssh'/> server or the desktop manager has been
      built with <xref linkend='linux-pam'/>).  As the &root; user, create
      a configuration file to allow resource control without &root;
      privilege, and instruct <command>systemd</command> to reload the
      configuration:
    </para>

    <screen revision="systemd" role="nodump"><userinput>mkdir -pv /etc/systemd/system/user@.service.d &amp;&amp;
cat &gt; /etc/systemd/system/user@.service.d/delegate.conf &lt;&lt; EOF &amp;&amp;
<literal>[Service]
Delegate=memory cpuset</literal>
systemctl daemon-reload</userinput></screen>

    <para revision='systemd'>
      Then logout and login again.  Now to run <command>make -j5</command>
      with the first 4 logical cores and 8 GB of system memory, issue:
    </para>

    <screen revision="systemd" role="nodump"><userinput>systemctl   --user start dbus                &amp;&amp;
systemd-run --user --pty --pipe --wait -G -d \
            -p MemoryHigh=8G                 \
            -p AllowedCPUs=0-3               \
            make -j5</userinput></screen>

    <para revision='sysv'>
      Ensure <xref linkend='sudo'/> is installed.  To run
      <command>make -j5</command> with the first 4 logical cores and 8 GB
      of system memory, issue:
    </para>

    <!-- "\EOF" because we expect $$ to be expanded by the "bash -e"
         shell, not the current shell.

         TODO: can we use elogind to delegate the controllers (like
         systemd) to avoid relying on sudo?  -->
    <screen revision="sysv" role="nodump"><userinput>bash -e &lt;&lt; \EOF
  sudo mkdir /sys/fs/cgroup/$$
  sudo sh -c \
    "echo +memory +cpuset > /sys/fs/cgroup/cgroup.subtree_control"
  sudo sh -c \
    "echo 0-3 > /sys/fs/cgroup/$$/cpuset.cpus"
  sudo sh -c \
    "echo $(bc -e '8*2^30') > /sys/fs/cgroup/$$/memory.high"
  (
    sudo sh -c "echo $BASHPID > /sys/fs/cgroup/$$/cgroup.procs"
    exec make -j5
  )
  sudo rmdir /sys/fs/cgroup/$$
EOF</userinput></screen>

    <para>
      With
      <phrase revision='systemd'>
        <parameter>MemoryHigh=8G</parameter>
      </phrase>
      <phrase revision='sysv'>
        <literal>8589934592</literal> (the output of
        <userinput>bc -e '8*2^30'</userinput>, 2^30 represents
        2<superscript>30</superscript>, i.e. a Gigabyte) in the
        <filename>memory.high</filename> entry
      </phrase>, a soft limit of memory usage is set.
      If the processes in the cgroup (<command>make</command> and all the
      descendants of it) uses more than 8 GB of system memory in total,
      the kernel will throttle down the processes and try to reclaim the
      system memory from them.  But they can still use more than 8 GB of
      system memory.  If you want to make a hard limit instead, replace
      <phrase revision='systemd'>
        <parameter>MemoryHigh</parameter> with
        <parameter>MemoryMax</parameter>.
      </phrase>
      <phrase revision='sysv'>
        <filename>memory.high</filename> with
        <filename>memory.max</filename>.
      </phrase>
      But doing so will cause the processes killed if 8 GB is not enough
      for them.
    </para>

    <para>
      <phrase revision='systemd'>
        <parameter>AllowedCPUs=0-3</parameter>
      </phrase>
      <phrase revision='sysv'>
        <literal>0-3</literal> in the <filename>cpuset.cpus</filename>
        entry
      </phrase> makes the kernel only run the processes in the cgroup on
      the logical cores with numbers 0, 1, 2, or 3.  You may need to
      adjust this setting based the mapping between the logical cores and the
      physical cores.  For example, with an Intel Core i9-13900K CPU,
      the logical cores 0, 2, 4, ..., 14 are mapped to the first threads of
      the eight physical P cores, the logical cores 1, 3, 5, ..., 15 are
      mapped to the second threads of the physical P cores, and the logical
      cores 16, 17, ..., 31 are mapped to the 16 physical E cores.  So if
      we want to use four threads from four different P cores, we need to
      specify <literal>0,2,4,6</literal> instead of <literal>0-3</literal>.
      Note that the other CPU models may use a different mapping scheme.
      If you are not sure about the mapping between the logical cores
      and the physical cores, run <command>grep -E '^processor|^core'
      /proc/cpuinfo</command> which will output logical core IDs in the
      <computeroutput>processor</computeroutput> lines, and physical core
      IDs in the <computeroutput>core id</computeroutput> lines.
    </para>

    <para>
      When the <command>nproc</command> or <command>ninja</command> command
      runs in a cgroup, it will use the number of logical cores assigned to
      the cgroup as the <quote>system logical core count.</quote>  For
      example, in a cgroup with logical cores 0-3 assigned,
      <command>nproc</command> will print
      <computeroutput>4</computeroutput>, and <command>ninja</command>
      will run 6 (4 + 2) jobs simultaneously if no <option>-j</option>
      setting is explicitly given.
    </para>

    <para revision="systemd">
      Read the man pages <ulink role='man'
      url='&man;systemd-run.1'>systemd-run(1)</ulink> and
      <ulink role='man'
  url='&man;systemd.resource-control.5'>systemd.resource-control(5)</ulink>
      for the detailed explanation of parameters in the command.
    </para>

    <para revision="sysv">
      Read the <filename>Documentation/admin-guide/cgroup-v2.rst</filename>
      file in the Linux kernel source tree for the detailed explanation of
      <systemitem class="filesystem">cgroup2</systemitem> pseudo file
      system entries referred in the command.
    </para>

  </sect2>

  <sect2 id="automating-builds" xreflabel="Automated Building Procedures">
    <title>Automated Building Procedures</title>

    <para>There are times when automating the building of a package can come in
    handy. Everyone has their own reasons for wanting to automate building,
    and everyone goes about it in their own way. Creating
    <filename>Makefile</filename>s, <application>Bash</application> scripts,
    <application>Perl</application> scripts or simply a list of commands used
    to cut and paste are just some of the methods you can use to automate
    building BLFS packages. Detailing how and providing examples of the many
    ways you can automate the building of packages is beyond the scope of this
    section. This section will expose you to using file redirection and the
    <command>yes</command> command to help provide ideas on how to automate
    your builds.</para>

    <bridgehead renderas="sect3">File Redirection to Automate Input</bridgehead>

    <para>You will find times throughout your BLFS journey when you will come
    across a package that has a command prompting you for information. This
    information might be configuration details, a directory path, or a response
    to a license agreement. This can present a challenge to automate the
    building of that package. Occasionally, you will be prompted for different
    information in a series of questions. One method to automate this type of
    scenario requires putting the desired responses in a file and using
    redirection so that the program uses the data in the file as the answers to
    the questions.</para>
<!-- outdated
    <para>Building the <application>CUPS</application> package is a good
    example of how redirecting a file as input to prompts can help you automate
    the build. If you run the test suite, you are asked to respond to a series
    of questions regarding the type of test to run and if you have any
    auxiliary programs the test can use. You can create a file with your
    responses, one response per line, and use a command similar to the
    one shown below to automate running the test suite:</para>

<screen><userinput>make check &lt; ../cups-1.1.23-testsuite_parms</userinput></screen>
-->
    <para>This effectively makes the test suite use the responses in the file
    as the input to the questions. Occasionally you may end up doing a bit of
    trial and error determining the exact format of your input file for some
    things, but once figured out and documented you can use this to automate
    building the package.</para>

    <bridgehead renderas="sect3">Using <command>yes</command> to Automate
    Input</bridgehead>

    <para>Sometimes you will only need to provide one response, or provide the
    same response to many prompts. For these instances, the
    <command>yes</command> command works really well. The
    <command>yes</command> command can be used to provide a response (the same
    one) to one or more instances of questions. It can be used to simulate
    pressing just the <keycap>Enter</keycap> key, entering the
    <keycap>Y</keycap> key or entering a string of text. Perhaps the easiest
    way to show its use is in an example.</para>

    <para>First, create a short <application>Bash</application> script by
    entering the following commands:</para>

<screen><userinput>cat &gt; blfs-yes-test1 &lt;&lt; "EOF"
<literal>#!/bin/bash

echo -n -e "\n\nPlease type something (or nothing) and press Enter ---> "

read A_STRING

if test "$A_STRING" = ""; then A_STRING="Just the Enter key was pressed"
else A_STRING="You entered '$A_STRING'"
fi

echo -e "\n\n$A_STRING\n\n"</literal>
EOF
chmod 755 blfs-yes-test1</userinput></screen>

    <para>Now run the script by issuing <command>./blfs-yes-test1</command> from
    the command line. It will wait for a response, which can be anything (or
    nothing) followed by the <keycap>Enter</keycap> key. After entering
    something, the result will be echoed to the screen. Now use the
    <command>yes</command> command to automate the entering of a
    response:</para>

<screen><userinput>yes | ./blfs-yes-test1</userinput></screen>

    <para>Notice that piping <command>yes</command> by itself to the script
    results in <keycap>y</keycap> being passed to the script. Now try it with a
    string of text:</para>

<screen><userinput>yes 'This is some text' | ./blfs-yes-test1</userinput></screen>

    <para>The exact string was used as the response to the script. Finally,
    try it using an empty (null) string:</para>

<screen><userinput>yes '' | ./blfs-yes-test1</userinput></screen>

    <para>Notice this results in passing just the press of the
    <keycap>Enter</keycap> key to the script. This is useful for times when the
    default answer to the prompt is sufficient. This syntax is used in the
    <xref linkend="net-tools-automate-example"/> instructions to accept all the
    defaults to the many prompts during the configuration step. You may now
    remove the test script, if desired.</para>

    <bridgehead renderas="sect3">File Redirection to Automate Output</bridgehead>

    <para>In order to automate the building of some packages, especially those
    that require you to read a license agreement one page at a time, requires
    using a method that avoids having to press a key to display each page.
    Redirecting the output to a file can be used in these instances to assist
    with the automation. The previous section on this page touched on creating
    log files of the build output. The redirection method shown there used the
    <command>tee</command> command to redirect output to a file while also
    displaying the output to the screen. Here, the output will only be sent to
    a file.</para>

    <para>Again, the easiest way to demonstrate the technique is to show an
    example. First, issue the command:</para>

<screen><userinput>ls -l /usr/bin | less</userinput></screen>

    <para>Of course, you'll be required to view the output one page at a time
    because the <command>less</command> filter was used. Now try the same
    command, but this time redirect the output to a file. The special file
    <filename>/dev/null</filename> can be used instead of the filename shown,
    but you will have no log file to examine:</para>

<screen><userinput>ls -l /usr/bin | less &gt; redirect_test.log 2&gt;&amp;1</userinput></screen>

    <para>Notice that this time the command immediately returned to the shell
    prompt without having to page through the output. You may now remove the
    log file.</para>

    <para>The last example will use the <command>yes</command> command in
    combination with output redirection to bypass having to page through the
    output and then provide a <keycap>y</keycap> to a prompt. This technique
    could be used in instances when otherwise you would have to page through
    the output of a file (such as a license agreement) and then answer the
    question of <computeroutput>do you accept the above?</computeroutput>.
    For this example,
    another short <application>Bash</application> script is required:</para>

<screen><userinput>cat &gt; blfs-yes-test2 &lt;&lt; "EOF"
<literal>#!/bin/bash

ls -l /usr/bin | less

echo -n -e "\n\nDid you enjoy reading this? (y,n) "

read A_STRING

if test "$A_STRING" = "y"; then A_STRING="You entered the 'y' key"
else A_STRING="You did NOT enter the 'y' key"
fi

echo -e "\n\n$A_STRING\n\n"</literal>
EOF
chmod 755 blfs-yes-test2</userinput></screen>

    <para>This script can be used to simulate a program that requires you to
    read a license agreement, then respond appropriately to accept the
    agreement before the program will install anything. First, run the script
    without any automation techniques by issuing
    <command>./blfs-yes-test2</command>.</para>

    <para>Now issue the following command which uses two automation techniques,
    making it suitable for use in an automated build script:</para>

<screen><userinput>yes | ./blfs-yes-test2 &gt; blfs-yes-test2.log 2&gt;&amp;1</userinput></screen>

    <para>If desired, issue <command>tail blfs-yes-test2.log</command> to see
    the end of the paged output, and confirmation that <keycap>y</keycap> was
    passed through to the script. Once satisfied that it works as it should,
    you may remove the script and log file.</para>

    <para>Finally, keep in mind that there are many ways to automate and/or
    script the build commands. There is not a single <quote>correct</quote> way
    to do it. Your imagination is the only limit.</para>

  </sect2>

  <sect2>
    <title>Dependencies</title>

    <para>For each package described, BLFS lists the known dependencies.
    These are listed under several headings, whose meaning is as follows:</para>

    <itemizedlist>
      <listitem>
        <para><emphasis>Required</emphasis> means that the target package
        cannot be correctly built without the dependency having first been
        installed, except if the dependency is said to be
        <quote>runtime</quote> which means the target package can be built but
        cannot function without it.</para>
        <para>
          Note that a target package can start to <quote>function</quote>
          in many subtle ways: an installed configuration file can make the
          init system, cron daemon, or bus daemon to run a program
          automatically; another package using the target package as a
          dependency can run a program from the target package in the
          building system; and the configuration sections in the BLFS book
          may also run a program from a just installed package.  So if
          you are installing the target package without a
          <emphasis>Required (runtime)</emphasis> dependency installed,
          You should install the dependency as soon as possible after the
          installation of the target package.
        </para>
      </listitem>
      <listitem>
        <para><emphasis>Recommended</emphasis> means that BLFS strongly
        suggests this package is installed first (except if said to be
        <quote>runtime,</quote> see below) for a clean and trouble-free
        build, that won't have issues either during the build process, or at
        run-time.  The instructions in the book assume these packages are
        installed.  Some changes or workarounds may be required if these
        packages are not installed. If a recommended dependency is said
        to be <quote>runtime,</quote> it means that BLFS strongly suggests
        that this dependency is installed before using the package, for
        getting full functionality.</para>
      </listitem>
      <listitem>
        <para><emphasis>Optional</emphasis> means that this package might be
        installed for added functionality. Often BLFS will describe the
        dependency to explain the added functionality that will result.
        Some optional dependencies are automatically picked up by the target
        package if the dependency is installed, while others
        also need additional configuration options to be enabled
        when the target package is built.  Such additional options are
        often documented in the BLFS book.  If an optional dependency is
        said to be <quote>runtime,</quote> it means you may install
        the dependency after installing the target package to support some
        optional features of the target package if you need these
        features.</para>
        <para>An optional dependency may be out of BLFS.  If you need such
        an <emphasis>external</emphasis> optional dependency for some
        features you need, read <xref linkend='beyond'/> for the general
        hint about installing an out-of-BLFS package.</para>
      </listitem>
    </itemizedlist>

  </sect2>

  <sect2 id="package_updates">
    <title>Using the Most Current Package Sources</title>

    <para>On occasion you may run into a situation in the book when a package
    will not build or work properly. Though the Editors attempt to ensure
    that every package in the book builds and works properly, sometimes a
    package has been overlooked or was not tested with this particular version
    of BLFS.</para>

    <para>If you discover that a package will not build or work properly, you
    should see if there is a more current version of the package. Typically
    this means you go to the maintainer's web site and download the most current
    tarball and attempt to build the package. If you cannot determine the
    maintainer's web site by looking at the download URLs, use Google and query
    the package's name. For example, in the Google search bar type:
    'package_name download' (omit the quotes) or something similar. Sometimes
    typing: 'package_name home page' will result in you finding the
    maintainer's web site.</para>

  </sect2>

  <sect2 id="stripping">
    <title>Stripping One More Time</title>

    <para>
      In LFS, stripping of debugging symbols and unneeded symbol table
      entries was discussed a couple of times.  When building BLFS packages,
      there are generally no special instructions that discuss stripping
      again. Stripping can be done while installing a package, or
      afterwards.
    </para>

    <bridgehead renderas="sect3" id="stripping-install">Stripping while Installing a Package</bridgehead>

    <para>
      There are several ways to strip executables installed by a
      package. They depend on the build system used (see below <link
        linkend="buildsystems">the section about build systems</link>),
      so only some
      generalities can be listed here:
    </para>

    <note>
      <para>
        The following methods using the feature of a building system
        (autotools, meson, or cmake) will not strip static libraries if any
        is installed.  Fortunately there are not too many static libraries
        in BLFS, and a static library can always be stripped safely by
        running <command>strip --strip-unneeded</command> on it manually.
      </para>
    </note>

    <itemizedlist>
      <listitem>
        <para>
          The packages using autotools usually have an
          <parameter>install-strip</parameter> target in their generated
          <filename>Makefile</filename> files. So installing stripped
          executables is just a matter of using
          <command>make install-strip</command> instead of
          <command>make install</command>.
        </para>
      </listitem>
      <listitem>
        <para>
          The packages using the meson build system can accept
          <parameter>-D strip=true</parameter> when running
          <command>meson</command>.  If you've forgot to add this option
          running the <command>meson</command>, you can also run
          <command>meson install --strip</command> instead of
          <command>ninja install</command>.
        </para>
      </listitem>
      <listitem>
        <para>
          <command>cmake</command> generates
          <parameter>install/strip</parameter> targets for both the
          <parameter>Unix Makefiles</parameter> and
          <parameter>Ninja</parameter> generators (the default is
          <parameter>Unix Makefiles</parameter> on linux). So just run
          <command>make install/strip</command> or
          <command>ninja install/strip</command> instead of the
          <command>install</command> counterparts.
        </para>
      </listitem>
      <listitem>
        <para>
          Removing (or not generating) debug symbols can also be
          achieved by removing the
          <parameter>-g&lt;something&gt;</parameter> options
          in C/C++ calls. How to do that is very specific for each
          package.  And, it does not remove unneeded symbol table entries.
          So it will not be explained in detail here. See also below
          the paragraphs about optimization.
        </para>
      </listitem>
    </itemizedlist>

    <bridgehead renderas="sect3" id="stripping-installed">Stripping Installed Executables</bridgehead>

    <para>
      The <command>strip</command> utility changes files in place, which may
      break anything using it if it is loaded in memory. Note that if a file is
      in use but just removed from the disk (i.e. not overwritten nor
      modified), this is not a problem since the kernel can use
      <quote>deleted</quote> files.  Look at <filename>/proc/*/maps</filename>
      and it is likely that you'll see some <emphasis>(deleted)</emphasis>
      entries. The <command>mv</command> just removes the destination file from
      the directory but does not touch its content, so that it satisfies the
      condition for the kernel to use the old (deleted) file.
      But this approach can detach hard links into duplicated copies,
      causing a bloat which is obviously unwanted as we are stripping to
      reduce system size.  If two files in a same file system share the
      same inode number, they are hard links to each other and we should
      reconstruct the link.  The script below is just an example.
      It should be run as the &root; user:
    </para>

<screen><userinput>cat &gt; /usr/sbin/strip-all.sh &lt;&lt; "EOF"
<literal>#!/usr/bin/bash

if [ $EUID -ne 0 ]; then
  echo "Need to be root"
  exit 1
fi

last_fs_inode=
last_file=

{ find /usr/lib -type f -name '*.so*' ! -name '*dbg'
  find /usr/lib -type f -name '*.a'
  find /usr/{bin,sbin,libexec} -type f
} | xargs stat -c '%m %i %n' | sort | while read fs inode file; do
       if ! readelf -h $file >/dev/null 2>&amp;1; then continue; fi
       if file $file | grep --quiet --invert-match 'not stripped'; then continue; fi

       if [ "$fs $inode" = "$last_fs_inode" ]; then
         ln -f $last_file $file;
         continue;
       fi

       cp --preserve $file    ${file}.tmp
       strip --strip-unneeded ${file}.tmp
       mv ${file}.tmp $file

       last_fs_inode="$fs $inode"
       last_file=$file
done</literal>
EOF
chmod 744 /usr/sbin/strip-all.sh</userinput></screen>

    <para>
      If you install programs in other directories such as <filename
      class="directory">/opt</filename> or <filename
      class="directory">/usr/local</filename>, you may want to strip the files
      there too. Just add other directories to scan in the compound list of
      <command>find</command> commands between the braces.
    </para>

    <para>
      For more information on stripping, see <ulink
      url="https://www.technovelty.org/linux/stripping-shared-libraries.html"/>.
    </para>

  </sect2>

<!--
  <sect2 id="libtool">
    <title>Libtool files</title>

    <para>
      One of the side effects of packages that use Autotools, including
      libtool, is that they create many files with an .la extension.  These
      files are not needed in an LFS environment.  If there are conflicts with
      pkgconfig entries, they can actually prevent successful builds.  You
      may want to consider removing these files periodically:
    </para>

<screen><userinput>find /lib /usr/lib -not -path "*Image*" -a -name \*.la -delete</userinput></screen>

    <para>
      The above command removes all .la files with the exception of those that
      have <quote>Image</quote> or <quote>openldap</quote> as a part of the
      path.  These .la files are used by the ImageMagick and openldap programs,
      respectively.  There may be other exceptions by packages not in BLFS.
    </para>

  </sect2>
-->
  <sect2 id="buildsystems">
    <title>Working with different build systems</title>

    <para>
      There are now three different build systems in common use for
      converting C or C++ source code into compiled programs or
      libraries and their details (particularly, finding out about available
      options and their default values) differ. It may be easiest to understand
      the issues caused by some choices (typically slow execution or
      unexpected use of, or omission of, optimizations) by starting with
      the <envar>CFLAGS</envar>, <envar>CXXFLAGS</envar>, and
      <envar>LDFLAGS</envar> environment variables.  There are also some
      programs which use Rust.
    </para>

    <para>
      Most LFS and BLFS builders are probably aware of the basics of
      <envar>CFLAGS</envar> and <envar>CXXFLAGS</envar> for altering how a
      program is compiled. Typically, some form of optimization is used by
      upstream developers (<option>-O2</option> or <option>-O3</option>),
      sometimes with the creation of debug symbols (<option>-g</option>),
      as defaults.
    </para>

    <para>
      If there are contradictory flags (e.g. multiple different
      <option>-O</option> values),
      the <emphasis>last</emphasis> value will be used. Sometimes this means
      that flags specified in environment variables will be picked up before
      values hardcoded in the Makefile, and therefore ignored.  For example,
      where a user specifies <option>-O2</option> and that is followed by
      <option>-O3</option> the build will use <option>-O3</option>.
    </para>

    <para>
      There are various other things which can be passed in CFLAGS or
      CXXFLAGS, such as allowing using the instruction set extensions
      available with a specific microarchitecture (e.g.
      <option>-march=amdfam10</option> or <option>-march=native</option>),
      tune the generated code for a specific microarchitecture (e. g.
      <option>-mtune=tigerlake</option> or <option>-mtune=native</option>,
      if <option>-mtune=</option> is not used, the microarchitecture from
      <option>-march=</option> setting will be used), or specifying a
      specific standard for C or C++ (<option>-std=c++17</option> for
      example).  But one thing which has now come to light is that
      programmers might include debug assertions in their code, expecting
      them to be disabled in releases by using <option>-D NDEBUG</option>.
      Specifically, if <xref linkend="mesa"/> is built with these
      assertions enabled, some activities such as loading levels of games
      can take extremely long times, even on high-class video cards.
    </para>

    <bridgehead renderas="sect3" id="autotools-info">Autotools with Make</bridgehead>

      <para>
        This combination is often described as <quote>CMMI</quote>
        (configure, make, make install) and is used here to also cover
        the few packages which have a configure script that is not
        generated by autotools.
      </para>

      <para>
        Sometimes running <command>./configure --help</command> will produce
        useful options about switches which might be used.  At other times,
        after looking at the output from configure you may need to look
        at the details of the script to find out what it was actually searching
        for.
      </para>

      <para>
       Many configure scripts will pick up any CFLAGS or CXXFLAGS from the
       environment, but CMMI packages vary about how these will be mixed with
       any flags which would otherwise be used (<emphasis>variously</emphasis>:
       ignored, used to replace the programmer's suggestion, used before the
       programmer's suggestion, or used after the programmer's suggestion).
      </para>

      <para>
        In most CMMI packages, running <command>make</command> will list
        each command and run it, interspersed with any warnings. But some
        packages try to be <quote>silent</quote> and only show which file
        they are compiling or linking instead of showing the command line.
        If you need to inspect the command, either because of an error, or
        just to see what options and flags are being used, adding
        <option>V=1</option> to the make invocation may help.
      </para>

    <bridgehead renderas="sect3" id="cmake-info">CMake</bridgehead>

      <para>
        CMake works in a very different way, and it has two backends which
        can be used on BLFS: <command>make</command> and
        <command>ninja</command>. The default backend is make, but
        ninja can be faster on large packages with multiple processors. To
        use ninja, specify <option>-G Ninja</option> in the cmake command.
        However, there are some packages which create fatal errors in their
        ninja files but build successfully using the default of Unix
        Makefiles.
      </para>

      <para>
        The hardest part of using CMake is knowing what options you might wish
        to specify. The only way to get a list of what the package knows about
        is to run <command>cmake -LAH</command> and look at the output for that
        default configuration.
      </para>

      <para>
        Perhaps the most-important thing about CMake is that it has a variety
        of CMAKE_BUILD_TYPE values, and these affect the flags. The default
        is that this is not set and no flags are generated. Any
        <envar>CFLAGS</envar> or <envar>CXXFLAGS</envar> in the environment
        will be used. If the programmer has coded any debug assertions,
        those will be enabled unless -D NDEBUG is used. The following
        CMAKE_BUILD_TYPE values will generate the flags shown, and these
        will come <emphasis>after</emphasis> any flags in the environment
        and therefore take precedence.
      </para>

      <informaltable align="center">
        <tgroup cols="2">
          <colspec colnum="1" align="center"/>
          <colspec colnum="2" align="center"/>
          <thead>
            <row><entry>Value</entry><entry>Flags</entry></row>
          </thead>
          <tbody>
            <row>
              <entry>Debug</entry><entry><option>-g</option></entry>
            </row>
            <row>
              <entry>Release</entry><entry><option>-O3 -D NDEBUG</option></entry>
            </row>
            <row>
              <entry>RelWithDebInfo</entry><entry><option>-O2 -g -D NDEBUG</option></entry>
            </row>
            <row>
              <entry>MinSizeRel</entry><entry><option>-Os -D NDEBUG</option></entry>
            </row>
          </tbody>
        </tgroup>
      </informaltable>

      <para>
        CMake tries to produce quiet builds. To see the details of the commands
        which are being run, use <command>make VERBOSE=1</command> or
        <command>ninja -v</command>.
      </para>

      <para>
        By default, CMake treats file installation differently from the other
        build systems: if a file already exists and is not newer than a file
        that would overwrite it, then the file is not installed. This may be
        a problem if a user wants to record which file belongs to a package,
        either using <envar>LD_PRELOAD</envar>, or by listing files newer
        than a timestamp. The default can be changed by setting the variable
        <envar>CMAKE_INSTALL_ALWAYS</envar> to 1 in the
        <emphasis>environment</emphasis>, for example by
        <command>export</command>'ing it.
      </para>

    <bridgehead renderas="sect3" id="meson-info">Meson</bridgehead>

      <para>
        Meson has some similarities to CMake, but many differences. To get
        details of the defines that you may wish to change you can look at
        <filename>meson_options.txt</filename> which is usually in the
        top-level directory.
      </para>

      <para>
        If you have already configured the package by running
        <command>meson</command> and now wish to change one or more settings,
        you can either remove the build directory, recreate it, and use the
        altered options, or within the build directory run <command>meson
        configure</command>, e.g. to set an option:
      </para>

<screen><userinput>meson configure -D &lt;some_option&gt;=true</userinput></screen>

      <para>
        If you do that, the file <filename>meson-private/cmd_line.txt</filename>
        will show the <emphasis>last</emphasis> commands which were used.
      </para>

      <para>
        Meson provides the following buildtype values, and the flags they enable
        come <emphasis>after</emphasis> any flags supplied in the environment and
        therefore take precedence.
      </para>

      <itemizedlist>
        <listitem>
          <para>plain : no added flags. This is for distributors to supply their
          own <envar>CFLAGS</envar>, <envar>CXXFLAGS</envar> and
          <envar>LDFLAGS</envar>. There is no obvious reason to use
          this in BLFS.</para>
        </listitem>
        <listitem>
          <para>debug : <option>-g</option> - this is the default if
          nothing is specified in either <filename>meson.build</filename>
          or the command line. However it results large and slow binaries,
          so we should override it in BLFS.</para>
        </listitem>
        <listitem>
          <para>debugoptimized : <option>-O2 -g</option> - this is the
          default specified in <filename>meson.build</filename> of some
          packages.</para>
        </listitem>
        <listitem>
          <para>release : <option>-O3</option> (occasionally a package will
          force <option>-O2</option> here) - this is the buildtype we use
          for most packages with Meson build system in BLFS.</para>
        </listitem>
      </itemizedlist>

      <!-- From https://mesonbuild.com/Builtin-options.html#core-options:
           b_ndebug: Default value = false, Possible values are
           true, false, if-release.  Some packages sets it to if-release
           so we mistakenly believed if-release had been the default.  -->
      <para>
        The <option>-D NDEBUG</option> flag is implied by the release
        buildtype for some packages (for example <xref linkend='mesa'/>).
        It can also be provided explicitly by passing
        <option>-D b_ndebug=true</option>.
      </para>

      <para>
        To see the details of the commands which are being run in a package using
        meson, use <command>ninja -v</command>.
      </para>

    <bridgehead renderas="sect3" id="rust-info">Rustc and Cargo</bridgehead>

      <para>
        Most released rustc programs are provided as crates (source tarballs)
        which will query a server to check current versions of dependencies
        and then download them as necessary.  These packages are built using
        <command>cargo --release</command>. In theory, you can manipulate the
        RUSTFLAGS to change the optimize-level (default for
        <option>--release</option> is 3, i. e.
        <option>-Copt-level=3</option>, like <option>-O3</option>) or to
        force it to build for the machine it is being compiled on, using
        <option>-Ctarget-cpu=native</option> but in practice this seems to
        make no significant difference.
      </para>

      <para>
        If you are compiling a standalone Rust program (as an unpackaged
        <filename class='extension'>.rs</filename> file) by running
        <command>rustc</command> directly, you should specify
        <option>-O</option> (the abbreviation of
        <option>-Copt-level=2</option>) or <option>-Copt-level=3</option>
        otherwise it will do an unoptimized compile and run
        <emphasis>much</emphasis> slower.  If you are compiling the program
        for debugging it, replace the <option>-O</option> or
        <option>-Copt-level=</option> options with <option>-g</option> to
        produce an unoptimized program with debug info.
      </para>

      <para>
        Like <command>ninja</command>, by default <command>cargo</command>
        uses all logical cores.  This can often be worked around,
        either by exporting
        <envar>CARGO_BUILD_JOBS=<replaceable>&lt;N&gt;</replaceable></envar>
        or passing
        <option>--jobs <replaceable>&lt;N&gt;</replaceable></option> to
        <command>cargo</command>.
        For compiling rustc itself, specifying
        <option>--jobs <replaceable>&lt;N&gt;</replaceable></option> for
        invocations of <command>x.py</command>
        (together with the <envar>CARGO_BUILD_JOBS</envar> environment
        variable, which looks like a <quote>belt and braces</quote>
        approach but seems to be necessary) mostly works. The exception is
        running the tests when building rustc, some of them will
        nevertheless use all online CPUs, at least as of rustc-1.42.0.
      </para>

  </sect2>

  <sect2 id="optimizations">
    <title>Optimizing the build</title>

      <para>
        Many people will prefer to optimize compiles as they see fit, by providing
        <envar>CFLAGS</envar> or <envar>CXXFLAGS</envar>. For an
        introduction to the options available with gcc and g++ see <ulink
        url="https://gcc.gnu.org/onlinedocs/gcc-&gcc-version;/gcc/Optimize-Options.html"/>.
        The same content can be also found in <command>info gcc</command>.
      </para>

      <para>
        Some packages default to <option>-O2 -g</option>, others to
        <option>-O3 -g</option>, and if <envar>CFLAGS</envar> or
        <envar>CXXFLAGS</envar> are supplied they might be added to the
        package's defaults, replace the package's defaults, or even be
        ignored.  There are details on some desktop packages which were
        mostly current in April 2019 at
        <ulink url="https://www.linuxfromscratch.org/~ken/tuning/"/> - in
        particular, <filename>README.txt</filename>,
        <filename>tuning-1-packages-and-notes.txt</filename>, and
        <filename>tuning-notes-2B.txt</filename>. The particular thing to
        remember is that if you want to try some of the more interesting
        flags you may need to force verbose builds to confirm what is being
        used.
      </para>

      <para>
        Clearly, if you are optimizing your own program you can spend time to
        profile it and perhaps recode some of it if it is too slow. But for
        building a whole system that approach is impractical. In general,
        <option>-O3</option> usually produces faster programs than
        <option>-O2</option>.  Specifying
        <option>-march=native</option> is also beneficial, but means that
        you cannot move the binaries to an incompatible machine - this can
        also apply to newer machines, not just to older machines. For
        example programs compiled for <literal>amdfam10</literal> run on
        old Phenoms, Kaveris, and Ryzens : but programs compiled for a
        Kaveri will not run on a Ryzen because certain op-codes are not
        present.  Similarly, if you build for a Haswell not everything will
        run on a SandyBridge.
      </para>

      <note>
        <para>
          Be careful that the name of a <option>-march</option> setting
          does not always match the baseline of the microarchitecture
          with the same name.  For example, the Skylake-based Intel Celeron
          processors do not support AVX at all, but
          <option>-march=skylake</option> assumes AVX and even AVX2.
        </para>
      </note>

      <para>
        When a shared library is built by GCC, a feature named
        <quote>semantic interposition</quote> is enabled by default.  When
        the shared library refers to a symbol name with external linkage
        and default visibility, if the symbol exists in both the shared
        library and the main executable, semantic interposition guarantees
        the symbol in the main executable is always used.  This feature
        was invented in an attempt to make the behavior of linking a shared
        library and linking a static library as similar as possible.  Today
        only a small number of packages still depend on semantic
        interposition, but the feature is still on by the default of GCC,
        causing many optimizations disabled for shared libraries because
        they conflict with semantic interposition.  The
        <option>-fno-semantic-interposition</option> option can be passed
        to <command>gcc</command> or <command>g++</command> to disable
        semantic interposition and enable more optimizations for shared
        libraries.  This option is used as the default of some packages
        (for example <xref linkend='python3'/>), and it's also the default
        of Clang.
      </para>

      <para>
        There are also various other options which some people claim are
        beneficial. At worst, you get to recompile and test, and then
        discover that in your usage the options do not provide a benefit.
      </para>

      <para>
        If building Perl or Python modules,
        in general the <envar>CFLAGS</envar> and <envar>CXXFLAGS</envar>
        used are those which were used by those <quote>parent</quote>
        packages.
      </para>

      <para>
        For <envar>LDFLAGS</envar>, there are three options can be used
        for optimization.  They are quite safe to use and the building
        system of some packages use some of these options as the default.
      </para>

      <para>
        With <option>-Wl,-O1</option>, the linker will
        optimize the hash table to speed up the dynamic linking.
        Note that <option>-Wl,-O1</option> is completely unrelated to the
        compiler optimization flag <option>-O1</option>.
      </para>

      <para>
        With <option>-Wl,--as-needed</option>, the linker will disregard
        unnecessary <option>-l<replaceable>foo</replaceable></option> options
        from the command line, i. e. the shared library <systemitem
        class='library'>lib<replaceable>foo</replaceable></systemitem>
        will only be linked if a symbol in <systemitem
        class='library'>lib<replaceable>foo</replaceable></systemitem> is
        really referred from the executable or shared library being linked.
        This can sometimes mitigate the <quote>excessive dependencies to
        shared libraries</quote> issues caused by
        <application>libtool</application>.
      </para>

      <para>
        With <option>-Wl,-z,pack-relative-relocs</option>, the linker
        generates a more compacted form of the relative relocation entries
        for PIEs and shared libraries.  It reduces the size of the linked
        PIE or shared library, and speeds up the loading of the PIE or
        shared library.
      </para>

      <para>
        The <option>-Wl,</option> prefix is necessary because despite the
        variable is named <envar>LDFLAGS</envar>, its content is actually
        passed to <command>gcc</command> (or <command>g++</command>,
        <command>clang</command>, etc.) during the link stage, not directly
        passed to <command>ld</command>.
      </para>

  </sect2>

  <sect2 id="hardening">
    <title>Options for hardening the build</title>

      <para>
        Even on desktop systems, there are still a lot of exploitable
        vulnerabilities. For many of these, the attack comes via javascript
        in a browser. Often, a series of vulnerabilities are used to gain
        access to data (or sometimes to pwn, i.e. own, the machine and
        install rootkits).  Most commercial distros will apply various
        hardening measures.
      </para>

      <para>
        In the past, there was Hardened LFS where gcc (a much older version)
        was forced to use hardening (with options to turn some of it off on a
        per-package basis).  The current LFS and BLFS books are carrying
        forward a part of its spirit by enabling PIE
        (<option>-fPIE -pie</option>) and SSP
        (<option>-fstack-protector-strong</option>) as the defaults
        for GCC and clang.  What is being covered here is different - first
        you have to make sure that the package is indeed using your added
        flags and not over-riding them.
      </para>

      <para>
        For hardening options which are reasonably cheap, there is some
        discussion in the 'tuning' link above (occasionally, one or more
        of these options might be inappropriate for a package). These
        options are <option>-D _FORTIFY_SOURCE=2</option>
        (or <option>-D _FORTIFY_SOURCE=3</option> which is more secure but
        with a larger performance overhead) and
        (for C++) <option>-D _GLIBCXX_ASSERTIONS</option>. On modern
        machines these should only have a little impact on how fast things
        run, and often they will not be noticeable.
      </para>

      <para>
        The main distros use much more, such as RELRO (Relocation Read Only)
        and perhaps <option>-fstack-clash-protection</option>. You may also
        encounter the so-called <quote>userspace retpoline</quote>
        (<option>-mindirect-branch=thunk</option> etc.) which
        is the equivalent of the spectre mitigations applied to the linux
        kernel in late 2018. The kernel mitigations caused a lot of complaints
        about lost performance, if you have a production server you might wish
        to consider testing that, along with the other available options, to
        see if performance is still sufficient.
      </para>

      <para>
        Whilst gcc has many hardening options, clang/LLVM's strengths lie
        elsewhere. Some options which gcc provides are said to be less effective
        in clang/LLVM.
      </para>

  </sect2>

</sect1>