nixpkgs/doc/stdenv.xml

<chapter xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xml:id="chap-stdenv">
 <title>The Standard Environment</title>
 <para>
  The standard build environment in the Nix Packages collection provides an
  environment for building Unix packages that does a lot of common build tasks
  automatically. In fact, for Unix packages that use the standard
  <literal>./configure; make; make install</literal> build interface, you
  don’t need to write a build script at all; the standard environment does
  everything automatically. If <literal>stdenv</literal> doesn’t do what you
  need automatically, you can easily customise or override the various build
  phases.
 </para>
 <section xml:id="sec-using-stdenv">
  <title>Using <literal>stdenv</literal></title>

  <para>
   To build a package with the standard environment, you use the function
   <varname>stdenv.mkDerivation</varname>, instead of the primitive built-in
   function <varname>derivation</varname>, e.g.
<programlisting>
stdenv.mkDerivation {
  name = "libfoo-1.2.3";
  src = fetchurl {
    url = http://example.org/libfoo-1.2.3.tar.bz2;
    sha256 = "0x2g1jqygyr5wiwg4ma1nd7w4ydpy82z9gkcv8vh2v8dn3y58v5m";
  };
}</programlisting>
   (<varname>stdenv</varname> needs to be in scope, so if you write this in a
   separate Nix expression from <filename>pkgs/all-packages.nix</filename>, you
   need to pass it as a function argument.) Specifying a
   <varname>name</varname> and a <varname>src</varname> is the absolute minimum
   you need to do. Many packages have dependencies that are not provided in the
   standard environment. It’s usually sufficient to specify those
   dependencies in the <varname>buildInputs</varname> attribute:
<programlisting>
stdenv.mkDerivation {
  name = "libfoo-1.2.3";
  ...
  buildInputs = [libbar perl ncurses];
}</programlisting>
   This attribute ensures that the <filename>bin</filename> subdirectories of
   these packages appear in the <envar>PATH</envar> environment variable during
   the build, that their <filename>include</filename> subdirectories are
   searched by the C compiler, and so on. (See
   <xref linkend="ssec-setup-hooks"/> for details.)
  </para>

  <para>
   Often it is necessary to override or modify some aspect of the build. To
   make this easier, the standard environment breaks the package build into a
   number of <emphasis>phases</emphasis>, all of which can be overridden or
   modified individually: unpacking the sources, applying patches, configuring,
   building, and installing. (There are some others; see
   <xref linkend="sec-stdenv-phases"/>.) For instance, a package that doesn’t
   supply a makefile but instead has to be compiled “manually” could be
   handled like this:
<programlisting>
stdenv.mkDerivation {
  name = "fnord-4.5";
  ...
  buildPhase = ''
    gcc foo.c -o foo
  '';
  installPhase = ''
    mkdir -p $out/bin
    cp foo $out/bin
  '';
}</programlisting>
   (Note the use of <literal>''</literal>-style string literals, which are very
   convenient for large multi-line script fragments because they don’t need
   escaping of <literal>"</literal> and <literal>\</literal>, and because
   indentation is intelligently removed.)
  </para>

  <para>
   There are many other attributes to customise the build. These are listed in
   <xref linkend="ssec-stdenv-attributes"/>.
  </para>

  <para>
   While the standard environment provides a generic builder, you can still
   supply your own build script:
<programlisting>
stdenv.mkDerivation {
  name = "libfoo-1.2.3";
  ...
  builder = ./builder.sh;
}</programlisting>
   where the builder can do anything it wants, but typically starts with
<programlisting>
source $stdenv/setup
</programlisting>
   to let <literal>stdenv</literal> set up the environment (e.g., process the
   <varname>buildInputs</varname>). If you want, you can still use
   <literal>stdenv</literal>’s generic builder:
<programlisting>
source $stdenv/setup

buildPhase() {
  echo "... this is my custom build phase ..."
  gcc foo.c -o foo
}

installPhase() {
  mkdir -p $out/bin
  cp foo $out/bin
}

genericBuild
</programlisting>
  </para>
 </section>
 <section xml:id="sec-tools-of-stdenv">
  <title>Tools provided by <literal>stdenv</literal></title>

  <para>
   The standard environment provides the following packages:
   <itemizedlist>
    <listitem>
     <para>
      The GNU C Compiler, configured with C and C++ support.
     </para>
    </listitem>
    <listitem>
     <para>
      GNU coreutils (contains a few dozen standard Unix commands).
     </para>
    </listitem>
    <listitem>
     <para>
      GNU findutils (contains <command>find</command>).
     </para>
    </listitem>
    <listitem>
     <para>
      GNU diffutils (contains <command>diff</command>, <command>cmp</command>).
     </para>
    </listitem>
    <listitem>
     <para>
      GNU <command>sed</command>.
     </para>
    </listitem>
    <listitem>
     <para>
      GNU <command>grep</command>.
     </para>
    </listitem>
    <listitem>
     <para>
      GNU <command>awk</command>.
     </para>
    </listitem>
    <listitem>
     <para>
      GNU <command>tar</command>.
     </para>
    </listitem>
    <listitem>
     <para>
      <command>gzip</command>, <command>bzip2</command> and
      <command>xz</command>.
     </para>
    </listitem>
    <listitem>
     <para>
      GNU Make. It has been patched to provide <quote>nested</quote> output
      that can be fed into the <command>nix-log2xml</command> command and
      <command>log2html</command> stylesheet to create a structured, readable
      output of the build steps performed by Make.
     </para>
    </listitem>
    <listitem>
     <para>
      Bash. This is the shell used for all builders in the Nix Packages
      collection. Not using <command>/bin/sh</command> removes a large source
      of portability problems.
     </para>
    </listitem>
    <listitem>
     <para>
      The <command>patch</command> command.
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   On Linux, <literal>stdenv</literal> also includes the
   <command>patchelf</command> utility.
  </para>
 </section>
 <section xml:id="ssec-stdenv-dependencies">
  <title>Specifying dependencies</title>

  <para>
   As described in the Nix manual, almost any <filename>*.drv</filename> store
   path in a derivation's attribute set will induce a dependency on that
   derivation. <varname>mkDerivation</varname>, however, takes a few attributes
   intended to, between them, include all the dependencies of a package. This
   is done both for structure and consistency, but also so that certain other
   setup can take place. For example, certain dependencies need their bin
   directories added to the <envar>PATH</envar>. That is built-in, but other
   setup is done via a pluggable mechanism that works in conjunction with these
   dependency attributes. See <xref linkend="ssec-setup-hooks"/> for details.
  </para>

  <para>
   Dependencies can be broken down along three axes: their host and target
   platforms relative to the new derivation's, and whether they are propagated.
   The platform distinctions are motivated by cross compilation; see
   <xref linkend="chap-cross"/> for exactly what each platform means.
   <footnote>
    <para>
     The build platform is ignored because it is a mere implementation detail
     of the package satisfying the dependency: As a general programming
     principle, dependencies are always <emphasis>specified</emphasis> as
     interfaces, not concrete implementation.
    </para>
   </footnote>
   But even if one is not cross compiling, the platforms imply whether or not
   the dependency is needed at run-time or build-time, a concept that makes
   perfect sense outside of cross compilation. For now, the run-time/build-time
   distinction is just a hint for mental clarity, but in the future it perhaps
   could be enforced.
  </para>

  <para>
   The extension of <envar>PATH</envar> with dependencies, alluded to above,
   proceeds according to the relative platforms alone. The process is carried
   out only for dependencies whose host platform matches the new derivation's
   build platform–i.e. which run on the platform where the new derivation
   will be built.
   <footnote>
    <para>
     Currently, that means for native builds all dependencies are put on the
     <envar>PATH</envar>. But in the future that may not be the case for sake
     of matching cross: the platforms would be assumed to be unique for native
     and cross builds alike, so only the <varname>depsBuild*</varname> and
     <varname>nativeBuildDependencies</varname> dependencies would affect the
     <envar>PATH</envar>.
    </para>
   </footnote>
   For each dependency <replaceable>dep</replaceable> of those dependencies,
   <filename><replaceable>dep</replaceable>/bin</filename>, if present, is
   added to the <envar>PATH</envar> environment variable.
  </para>

  <para>
   The dependency is propagated when it forces some of its other-transitive
   (non-immediate) downstream dependencies to also take it on as an immediate
   dependency. Nix itself already takes a package's transitive dependencies
   into account, but this propagation ensures nixpkgs-specific infrastructure
   like setup hooks (mentioned above) also are run as if the propagated
   dependency.
  </para>

  <para>
   It is important to note dependencies are not necessary propagated as the
   same sort of dependency that they were before, but rather as the
   corresponding sort so that the platform rules still line up. The exact rules
   for dependency propagation can be given by assigning each sort of dependency
   two integers based one how it's host and target platforms are offset from
   the depending derivation's platforms. Those offsets are given are given
   below in the descriptions of each dependency list attribute.
   Algorithmically, we traverse propagated inputs, accumulating every
   propagated dep's propagated deps and adjusting them to account for the
   "shift in perspective" described by the current dep's platform offsets. This
   results in sort a transitive closure of the dependency relation, with the
   offsets being approximately summed when two dependency links are combined.
   We also prune transitive deps whose combined offsets go out-of-bounds, which
   can be viewed as a filter over that transitive closure removing dependencies
   that are blatantly absurd.
  </para>

  <para>
   We can define the process precisely with
   <link xlink:href="https://en.wikipedia.org/wiki/Natural_deduction">Natural
   Deduction</link> using the inference rules. This probably seems a bit
   obtuse, but so is the bash code that actually implements it!
   <footnote>
    <para>
     The <function>findInputs</function> function, currently residing in
     <filename>pkgs/stdenv/generic/setup.sh</filename>, implements the
     propagation logic.
    </para>
   </footnote>
   They're confusing in very different ways so...hopefully if something doesn't
   make sense in one presentation, it does in the other!
<programlisting>
let mapOffset(h, t, i) = i + (if i &lt;= 0 then h else t - 1)

propagated-dep(h0, t0, A, B)
propagated-dep(h1, t1, B, C)
h0 + h1 in {-1, 0, 1}
h0 + t1 in {-1, 0, 1}
-------------------------------------- Transitive property
propagated-dep(mapOffset(h0, t0, h1),
               mapOffset(h0, t0, t1),
               A, C)</programlisting>
<programlisting>
let mapOffset(h, t, i) = i + (if i &lt;= 0 then h else t - 1)

dep(h0, _, A, B)
propagated-dep(h1, t1, B, C)
h0 + h1 in {-1, 0, 1}
h0 + t1 in {-1, 0, -1}
-------------------------------------- Take immediate deps' propagated deps
propagated-dep(mapOffset(h0, t0, h1),
               mapOffset(h0, t0, t1),
               A, C)</programlisting>
<programlisting>
propagated-dep(h, t, A, B)
-------------------------------------- Propagated deps count as deps
dep(h, t, A, B)</programlisting>
   Some explanation of this monstrosity is in order. In the common case, the
   target offset of a dependency is the successor to the target offset:
   <literal>t = h + 1</literal>. That means that:
<programlisting>
let f(h, t, i) = i + (if i &lt;= 0 then h else t - 1)
let f(h, h + 1, i) = i + (if i &lt;= 0 then h else (h + 1) - 1)
let f(h, h + 1, i) = i + (if i &lt;= 0 then h else h)
let f(h, h + 1, i) = i + h
  </programlisting>
   This is where the "sum-like" comes from above: We can just sum all the host
   offset to get the host offset of the transitive dependency. The target
   offset is the transitive dep is simply the host offset + 1, just as it was
   with the dependencies composed to make this transitive one; it can be
   ignored as it doesn't add any new information.
  </para>

  <para>
   Because of the bounds checks, the uncommon cases are <literal>h =
   t</literal> and <literal>h + 2 = t</literal>. In the former case, the
   motivation for <function>mapOffset</function> is that since its host and
   target platforms are the same, no transitive dep of it should be able to
   "discover" an offset greater than its reduced target offsets.
   <function>mapOffset</function> effectively "squashes" all its transitive
   dependencies' offsets so that none will ever be greater than the target
   offset of the original <literal>h = t</literal> package. In the other case,
   <literal>h + 1</literal> is skipped over between the host and target
   offsets. Instead of squashing the offsets, we need to "rip" them apart so no
   transitive dependencies' offset is that one.
  </para>

  <para>
   Overall, the unifying theme here is that propagation shouldn't be
   introducing transitive dependencies involving platforms the needing package
   is unaware of. The offset bounds checking and definition of
   <function>mapOffset</function> together ensure that this is the case.
   Discovering a new offset is discovering a new platform, and since those
   platforms weren't in the derivation "spec" of the needing package, they
   cannot be relevant. From a capability perspective, we can imagine that the
   host and target platforms of a package are the capabilities a package
   requires, and the depending package must provide the capability to the
   dependency.
  </para>

  <variablelist>
   <title>Variables specifying dependencies</title>
   <varlistentry>
    <term><varname>depsBuildBuild</varname>
    </term>
    <listitem>
     <para>
      A list of dependencies whose host and target platforms are the new
      derivation's build platform. This means a <literal>-1</literal> host and
      <literal>-1</literal> target offset from the new derivation's platforms.
      They are programs/libraries used at build time that furthermore produce
      programs/libraries also used at build time. If the dependency doesn't
      care about the target platform (i.e. isn't a compiler or similar tool),
      put it in <varname>nativeBuildInputs</varname>instead. The most common
      use for this <literal>buildPackages.stdenv.cc</literal>, the default C
      compiler for this role. That example crops up more than one might think
      in old commonly used C libraries.
     </para>
     <para>
      Since these packages are able to be run at build time, that are always
      added to the <envar>PATH</envar>, as described above. But since these
      packages are only guaranteed to be able to run then, they shouldn't
      persist as run-time dependencies. This isn't currently enforced, but
      could be in the future.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><varname>nativeBuildInputs</varname>
    </term>
    <listitem>
     <para>
      A list of dependencies whose host platform is the new derivation's build
      platform, and target platform is the new derivation's host platform. This
      means a <literal>-1</literal> host offset and <literal>0</literal> target
      offset from the new derivation's platforms. They are programs/libraries
      used at build time that, if they are a compiler or similar tool, produce
      code to run at run time—i.e. tools used to build the new derivation. If
      the dependency doesn't care about the target platform (i.e. isn't a
      compiler or similar tool), put it here, rather than in
      <varname>depsBuildBuild</varname> or <varname>depsBuildTarget</varname>.
      This would be called <varname>depsBuildHost</varname> but for historical
      continuity.
     </para>
     <para>
      Since these packages are able to be run at build time, that are added to
      the <envar>PATH</envar>, as described above. But since these packages
      only are guaranteed to be able to run then, they shouldn't persist as
      run-time dependencies. This isn't currently enforced, but could be in the
      future.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><varname>depsBuildTarget</varname>
    </term>
    <listitem>
     <para>
      A list of dependencies whose host platform is the new derivation's build
      platform, and target platform is the new derivation's target platform.
      This means a <literal>-1</literal> host offset and <literal>1</literal>
      target offset from the new derivation's platforms. They are programs used
      at build time that produce code to run at run with code produced by the
      depending package. Most commonly, these would tools used to build the
      runtime or standard library the currently-being-built compiler will
      inject into any code it compiles. In many cases, the currently-being
      built compiler is itself employed for that task, but when that compiler
      won't run (i.e. its build and host platform differ) this is not possible.
      Other times, the compiler relies on some other tool, like binutils, that
      is always built separately so the dependency is unconditional.
     </para>
     <para>
      This is a somewhat confusing dependency to wrap ones head around, and for
      good reason. As the only one where the platform offsets are not adjacent
      integers, it requires thinking of a bootstrapping stage
      <emphasis>two</emphasis> away from the current one. It and it's use-case
      go hand in hand and are both considered poor form: try not to need this
      sort dependency, and try not avoid building standard libraries / runtimes
      in the same derivation as the compiler produces code using them. Instead
      strive to build those like a normal library, using the newly-built
      compiler just as a normal library would. In short, do not use this
      attribute unless you are packaging a compiler and are sure it is needed.
     </para>
     <para>
      Since these packages are able to be run at build time, that are added to
      the <envar>PATH</envar>, as described above. But since these packages
      only are guaranteed to be able to run then, they shouldn't persist as
      run-time dependencies. This isn't currently enforced, but could be in the
      future.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><varname>depsHostHost</varname>
    </term>
    <listitem>
     <para>
      A list of dependencies whose host and target platforms match the new
      derivation's host platform. This means a both <literal>0</literal> host
      offset and <literal>0</literal> target offset from the new derivation's
      host platform. These are packages used at run-time to generate code also
      used at run-time. In practice, that would usually be tools used by
      compilers for metaprogramming/macro systems, or libraries used by the
      macros/metaprogramming code itself. It's always preferable to use a
      <varname>depsBuildBuild</varname> dependency in the derivation being
      built than a <varname>depsHostHost</varname> on the tool doing the
      building for this purpose.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><varname>buildInputs</varname>
    </term>
    <listitem>
     <para>
      A list of dependencies whose host platform and target platform match the
      new derivation's. This means a <literal>0</literal> host offset and
      <literal>1</literal> target offset from the new derivation's host
      platform. This would be called <varname>depsHostTarget</varname> but for
      historical continuity. If the dependency doesn't care about the target
      platform (i.e. isn't a compiler or similar tool), put it here, rather
      than in <varname>depsBuildBuild</varname>.
     </para>
     <para>
      These often are programs/libraries used by the new derivation at
      <emphasis>run</emphasis>-time, but that isn't always the case. For
      example, the machine code in a statically linked library is only used at
      run time, but the derivation containing the library is only needed at
      build time. Even in the dynamic case, the library may also be needed at
      build time to appease the linker.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><varname>depsTargetTarget</varname>
    </term>
    <listitem>
     <para>
      A list of dependencies whose host platform matches the new derivation's
      target platform. This means a <literal>1</literal> offset from the new
      derivation's platforms. These are packages that run on the target
      platform, e.g. the standard library or run-time deps of standard library
      that a compiler insists on knowing about. It's poor form in almost all
      cases for a package to depend on another from a future stage [future
      stage corresponding to positive offset]. Do not use this attribute unless
      you are packaging a compiler and are sure it is needed.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><varname>depsBuildBuildPropagated</varname>
    </term>
    <listitem>
     <para>
      The propagated equivalent of <varname>depsBuildBuild</varname>. This
      perhaps never ought to be used, but it is included for consistency [see
      below for the others].
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><varname>propagatedNativeBuildInputs</varname>
    </term>
    <listitem>
     <para>
      The propagated equivalent of <varname>nativeBuildInputs</varname>. This
      would be called <varname>depsBuildHostPropagated</varname> but for
      historical continuity. For example, if package <varname>Y</varname> has
      <literal>propagatedNativeBuildInputs = [X]</literal>, and package
      <varname>Z</varname> has <literal>buildInputs = [Y]</literal>, then
      package <varname>Z</varname> will be built as if it included package
      <varname>X</varname> in its <varname>nativeBuildInputs</varname>. If
      instead, package <varname>Z</varname> has <literal>nativeBuildInputs =
      [Y]</literal>, then <varname>Z</varname> will be built as if it included
      <varname>X</varname> in the <varname>depsBuildBuild</varname> of package
      <varname>Z</varname>, because of the sum of the two <literal>-1</literal>
      host offsets.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><varname>depsBuildTargetPropagated</varname>
    </term>
    <listitem>
     <para>
      The propagated equivalent of <varname>depsBuildTarget</varname>. This is
      prefixed for the same reason of alerting potential users.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><varname>depsHostHostPropagated</varname>
    </term>
    <listitem>
     <para>
      The propagated equivalent of <varname>depsHostHost</varname>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><varname>propagatedBuildInputs</varname>
    </term>
    <listitem>
     <para>
      The propagated equivalent of <varname>buildInputs</varname>. This would
      be called <varname>depsHostTargetPropagated</varname> but for historical
      continuity.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><varname>depsTargetTarget</varname>
    </term>
    <listitem>
     <para>
      The propagated equivalent of <varname>depsTargetTarget</varname>. This is
      prefixed for the same reason of alerting potential users.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </section>
 <section xml:id="ssec-stdenv-attributes">
  <title>Attributes</title>

  <variablelist>
   <title>Variables affecting <literal>stdenv</literal> initialisation</title>
   <varlistentry>
    <term><varname>NIX_DEBUG</varname>
    </term>
    <listitem>
     <para>
      A natural number indicating how much information to log. If set to 1 or
      higher, <literal>stdenv</literal> will print moderate debug information
      during the build. In particular, the <command>gcc</command> and
      <command>ld</command> wrapper scripts will print out the complete command
      line passed to the wrapped tools. If set to 6 or higher, the
      <literal>stdenv</literal> setup script will be run with <literal>set
      -x</literal> tracing. If set to 7 or higher, the <command>gcc</command>
      and <command>ld</command> wrapper scripts will also be run with
      <literal>set -x</literal> tracing.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

  <variablelist>
   <title>Variables affecting build properties</title>
   <varlistentry>
    <term><varname>enableParallelBuilding</varname>
    </term>
    <listitem>
     <para>
      If set to <literal>true</literal>, <literal>stdenv</literal> will pass
      specific flags to <literal>make</literal> and other build tools to enable
      parallel building with up to <literal>build-cores</literal> workers.
     </para>
     <para>
      Unless set to <literal>false</literal>, some build systems with good
      support for parallel building including <literal>cmake</literal>,
      <literal>meson</literal>, and <literal>qmake</literal> will set it to
      <literal>true</literal>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><varname>preferLocalBuild</varname>
    </term>
    <listitem>
     <para>
      If set, specifies that the package is so lightweight in terms of build
      operations (e.g. write a text file from a Nix string to the store) that
      there's no need to look for it in binary caches -- it's faster to just
      build it locally. It also tells Hydra and other facilities that this
      package doesn't need to be exported in binary caches (noone would use it,
      after all).
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

  <variablelist>
   <title>Special variables</title>
   <varlistentry>
    <term><varname>passthru</varname>
    </term>
    <listitem>
     <para>
      This is an attribute set which can be filled with arbitrary values. For
      example:
<programlisting>
passthru = {
  foo = "bar";
  baz = {
    value1 = 4;
    value2 = 5;
  };
}
</programlisting>
     </para>
     <para>
      Values inside it are not passed to the builder, so you can change them
      without triggering a rebuild. However, they can be accessed outside of a
      derivation directly, as if they were set inside a derivation itself, e.g.
      <literal>hello.baz.value1</literal>. We don't specify any usage or schema
      of <literal>passthru</literal> - it is meant for values that would be
      useful outside the derivation in other parts of a Nix expression (e.g. in
      other derivations). An example would be to convey some specific
      dependency of your derivation which contains a program with plugins
      support. Later, others who make derivations with plugins can use
      passed-through dependency to ensure that their plugin would be
      binary-compatible with built program.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </section>
 <section xml:id="sec-stdenv-phases">
  <title>Phases</title>

  <para>
   The generic builder has a number of <emphasis>phases</emphasis>. Package
   builds are split into phases to make it easier to override specific parts of
   the build (e.g., unpacking the sources or installing the binaries).
   Furthermore, it allows a nicer presentation of build logs in the Nix build
   farm.
  </para>

  <para>
   Each phase can be overridden in its entirety either by setting the
   environment variable <varname><replaceable>name</replaceable>Phase</varname>
   to a string containing some shell commands to be executed, or by redefining
   the shell function <varname><replaceable>name</replaceable>Phase</varname>.
   The former is convenient to override a phase from the derivation, while the
   latter is convenient from a build script. However, typically one only wants
   to <emphasis>add</emphasis> some commands to a phase, e.g. by defining
   <literal>postInstall</literal> or <literal>preFixup</literal>, as skipping
   some of the default actions may have unexpected consequences.
  </para>

  <section xml:id="ssec-controlling-phases">
   <title>Controlling phases</title>

   <para>
    There are a number of variables that control what phases are executed and
    in what order:
    <variablelist>
     <title>Variables affecting phase control</title>
     <varlistentry>
      <term><varname>phases</varname>
      </term>
      <listitem>
       <para>
        Specifies the phases. You can change the order in which phases are
        executed, or add new phases, by setting this variable. If it’s not
        set, the default value is used, which is <literal>$prePhases
        unpackPhase patchPhase $preConfigurePhases configurePhase
        $preBuildPhases buildPhase checkPhase $preInstallPhases installPhase
        fixupPhase $preDistPhases distPhase $postPhases</literal>.
       </para>
       <para>
        Usually, if you just want to add a few phases, it’s more convenient
        to set one of the variables below (such as
        <varname>preInstallPhases</varname>), as you then don’t specify all
        the normal phases.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><varname>prePhases</varname>
      </term>
      <listitem>
       <para>
        Additional phases executed before any of the default phases.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><varname>preConfigurePhases</varname>
      </term>
      <listitem>
       <para>
        Additional phases executed just before the configure phase.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><varname>preBuildPhases</varname>
      </term>
      <listitem>
       <para>
        Additional phases executed just before the build phase.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><varname>preInstallPhases</varname>
      </term>
      <listitem>
       <para>
        Additional phases executed just before the install phase.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><varname>preFixupPhases</varname>
      </term>
      <listitem>
       <para>
        Additional phases executed just before the fixup phase.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><varname>preDistPhases</varname>
      </term>
      <listitem>
       <para>
        Additional phases executed just before the distribution phase.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><varname>postPhases</varname>
      </term>
      <listitem>
       <para>
        Additional phases executed after any of the default phases.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </section>

  <section xml:id="ssec-unpack-phase">
   <title>The unpack phase</title>

   <para>
    The unpack phase is responsible for unpacking the source code of the
    package. The default implementation of <function>unpackPhase</function>
    unpacks the source files listed in the <envar>src</envar> environment
    variable to the current directory. It supports the following files by
    default:
    <variablelist>
     <varlistentry>
      <term>Tar files</term>
      <listitem>
       <para>
        These can optionally be compressed using <command>gzip</command>
        (<filename>.tar.gz</filename>, <filename>.tgz</filename> or
        <filename>.tar.Z</filename>), <command>bzip2</command>
        (<filename>.tar.bz2</filename> or <filename>.tbz2</filename>) or
        <command>xz</command> (<filename>.tar.xz</filename> or
        <filename>.tar.lzma</filename>).
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term>Zip files</term>
      <listitem>
       <para>
        Zip files are unpacked using <command>unzip</command>. However,
        <command>unzip</command> is not in the standard environment, so you
        should add it to <varname>buildInputs</varname> yourself.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term>Directories in the Nix store</term>
      <listitem>
       <para>
        These are simply copied to the current directory. The hash part of the
        file name is stripped, e.g.
        <filename>/nix/store/1wydxgby13cz...-my-sources</filename> would be
        copied to <filename>my-sources</filename>.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
    Additional file types can be supported by setting the
    <varname>unpackCmd</varname> variable (see below).
   </para>

   <para></para>

   <variablelist>
    <title>Variables controlling the unpack phase</title>
    <varlistentry>
     <term><varname>srcs</varname> / <varname>src</varname>
     </term>
     <listitem>
      <para>
       The list of source files or directories to be unpacked or copied. One of
       these must be set.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>sourceRoot</varname>
     </term>
     <listitem>
      <para>
       After running <function>unpackPhase</function>, the generic builder
       changes the current directory to the directory created by unpacking the
       sources. If there are multiple source directories, you should set
       <varname>sourceRoot</varname> to the name of the intended directory.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>setSourceRoot</varname>
     </term>
     <listitem>
      <para>
       Alternatively to setting <varname>sourceRoot</varname>, you can set
       <varname>setSourceRoot</varname> to a shell command to be evaluated by
       the unpack phase after the sources have been unpacked. This command must
       set <varname>sourceRoot</varname>.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>preUnpack</varname>
     </term>
     <listitem>
      <para>
       Hook executed at the start of the unpack phase.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>postUnpack</varname>
     </term>
     <listitem>
      <para>
       Hook executed at the end of the unpack phase.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>dontMakeSourcesWritable</varname>
     </term>
     <listitem>
      <para>
       If set to <literal>1</literal>, the unpacked sources are
       <emphasis>not</emphasis> made writable. By default, they are made
       writable to prevent problems with read-only sources. For example, copied
       store directories would be read-only without this.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>unpackCmd</varname>
     </term>
     <listitem>
      <para>
       The unpack phase evaluates the string <literal>$unpackCmd</literal> for
       any unrecognised file. The path to the current source file is contained
       in the <varname>curSrc</varname> variable.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </section>

  <section xml:id="ssec-patch-phase">
   <title>The patch phase</title>

   <para>
    The patch phase applies the list of patches defined in the
    <varname>patches</varname> variable.
   </para>

   <variablelist>
    <title>Variables controlling the patch phase</title>
    <varlistentry>
     <term><varname>patches</varname>
     </term>
     <listitem>
      <para>
       The list of patches. They must be in the format accepted by the
       <command>patch</command> command, and may optionally be compressed using
       <command>gzip</command> (<filename>.gz</filename>),
       <command>bzip2</command> (<filename>.bz2</filename>) or
       <command>xz</command> (<filename>.xz</filename>).
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>patchFlags</varname>
     </term>
     <listitem>
      <para>
       Flags to be passed to <command>patch</command>. If not set, the argument
       <option>-p1</option> is used, which causes the leading directory
       component to be stripped from the file names in each patch.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>prePatch</varname>
     </term>
     <listitem>
      <para>
       Hook executed at the start of the patch phase.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>postPatch</varname>
     </term>
     <listitem>
      <para>
       Hook executed at the end of the patch phase.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </section>

  <section xml:id="ssec-configure-phase">
   <title>The configure phase</title>

   <para>
    The configure phase prepares the source tree for building. The default
    <function>configurePhase</function> runs <filename>./configure</filename>
    (typically an Autoconf-generated script) if it exists.
   </para>

   <variablelist>
    <title>Variables controlling the configure phase</title>
    <varlistentry>
     <term><varname>configureScript</varname>
     </term>
     <listitem>
      <para>
       The name of the configure script. It defaults to
       <filename>./configure</filename> if it exists; otherwise, the configure
       phase is skipped. This can actually be a command (like <literal>perl
       ./Configure.pl</literal>).
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>configureFlags</varname>
     </term>
     <listitem>
      <para>
       A list of strings passed as additional arguments to the configure
       script.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>configureFlagsArray</varname>
     </term>
     <listitem>
      <para>
       A shell array containing additional arguments passed to the configure
       script. You must use this instead of <varname>configureFlags</varname>
       if the arguments contain spaces.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>dontAddPrefix</varname>
     </term>
     <listitem>
      <para>
       By default, the flag <literal>--prefix=$prefix</literal> is added to the
       configure flags. If this is undesirable, set this variable to true.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>prefix</varname>
     </term>
     <listitem>
      <para>
       The prefix under which the package must be installed, passed via the
       <option>--prefix</option> option to the configure script. It defaults to
       <option>$out</option>.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>dontAddDisableDepTrack</varname>
     </term>
     <listitem>
      <para>
       By default, the flag <literal>--disable-dependency-tracking</literal> is
       added to the configure flags to speed up Automake-based builds. If this
       is undesirable, set this variable to true.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>dontFixLibtool</varname>
     </term>
     <listitem>
      <para>
       By default, the configure phase applies some special hackery to all
       files called <filename>ltmain.sh</filename> before running the configure
       script in order to improve the purity of Libtool-based packages
       <footnote>
        <para>
         It clears the
         <varname>sys_lib_<replaceable>*</replaceable>search_path</varname>
         variables in the Libtool script to prevent Libtool from using
         libraries in <filename>/usr/lib</filename> and such.
        </para>
       </footnote>
       . If this is undesirable, set this variable to true.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>dontDisableStatic</varname>
     </term>
     <listitem>
      <para>
       By default, when the configure script has
       <option>--enable-static</option>, the option
       <option>--disable-static</option> is added to the configure flags.
      </para>
      <para>
       If this is undesirable, set this variable to true.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>configurePlatforms</varname>
     </term>
     <listitem>
      <para>
       By default, when cross compiling, the configure script has
       <option>--build=...</option> and <option>--host=...</option> passed.
       Packages can instead pass <literal>[ "build" "host" "target" ]</literal>
       or a subset to control exactly which platform flags are passed.
       Compilers and other tools should use this to also pass the target
       platform, for example.
       <footnote>
        <para>
         Eventually these will be passed when in native builds too, to improve
         determinism: build-time guessing, as is done today, is a risk of
         impurity.
        </para>
       </footnote>
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>preConfigure</varname>
     </term>
     <listitem>
      <para>
       Hook executed at the start of the configure phase.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>postConfigure</varname>
     </term>
     <listitem>
      <para>
       Hook executed at the end of the configure phase.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </section>

  <section xml:id="build-phase">
   <title>The build phase</title>

   <para>
    The build phase is responsible for actually building the package (e.g.
    compiling it). The default <function>buildPhase</function> simply calls
    <command>make</command> if a file named <filename>Makefile</filename>,
    <filename>makefile</filename> or <filename>GNUmakefile</filename> exists in
    the current directory (or the <varname>makefile</varname> is explicitly
    set); otherwise it does nothing.
   </para>

   <variablelist>
    <title>Variables controlling the build phase</title>
    <varlistentry>
     <term><varname>dontBuild</varname>
     </term>
     <listitem>
      <para>
       Set to true to skip the build phase.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>makefile</varname>
     </term>
     <listitem>
      <para>
       The file name of the Makefile.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
      <term><varname>checkInputs</varname>
      </term>
      <listitem><para>
        A list of dependencies used by the phase. This gets included in
        <varname>buildInputs</varname> when <varname>doCheck</varname> is set.
      </para></listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>makeFlags</varname>
     </term>
     <listitem>
      <para>
       A list of strings passed as additional flags to <command>make</command>.
       These flags are also used by the default install and check phase. For
       setting make flags specific to the build phase, use
       <varname>buildFlags</varname> (see below).
<programlisting>
makeFlags = [ "PREFIX=$(out)" ];
</programlisting>
       <note>
        <para>
         The flags are quoted in bash, but environment variables can be
         specified by using the make syntax.
        </para>
       </note>
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>makeFlagsArray</varname>
     </term>
     <listitem>
      <para>
       A shell array containing additional arguments passed to
       <command>make</command>. You must use this instead of
       <varname>makeFlags</varname> if the arguments contain spaces, e.g.
<programlisting>
makeFlagsArray=(CFLAGS="-O0 -g" LDFLAGS="-lfoo -lbar")
</programlisting>
       Note that shell arrays cannot be passed through environment variables,
       so you cannot set <varname>makeFlagsArray</varname> in a derivation
       attribute (because those are passed through environment variables): you
       have to define them in shell code.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>buildFlags</varname> / <varname>buildFlagsArray</varname>
     </term>
     <listitem>
      <para>
       A list of strings passed as additional flags to <command>make</command>.
       Like <varname>makeFlags</varname> and <varname>makeFlagsArray</varname>,
       but only used by the build phase.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>preBuild</varname>
     </term>
     <listitem>
      <para>
       Hook executed at the start of the build phase.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>postBuild</varname>
     </term>
     <listitem>
      <para>
       Hook executed at the end of the build phase.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>

   <para>
    You can set flags for <command>make</command> through the
    <varname>makeFlags</varname> variable.
   </para>

   <para>
    Before and after running <command>make</command>, the hooks
    <varname>preBuild</varname> and <varname>postBuild</varname> are called,
    respectively.
   </para>
  </section>

  <section xml:id="ssec-check-phase">
   <title>The check phase</title>

   <para>
    The check phase checks whether the package was built correctly by running
    its test suite. The default <function>checkPhase</function> calls
    <command>make check</command>, but only if the <varname>doCheck</varname>
    variable is enabled.
   </para>

   <variablelist>
    <title>Variables controlling the check phase</title>
    <varlistentry>
     <term><varname>doCheck</varname>
     </term>
     <listitem>
      <para>
       Controls whether the check phase is executed. By default it is skipped,
       but if <varname>doCheck</varname> is set to true, the check phase is
       usually executed. Thus you should set
<programlisting>doCheck = true;</programlisting>
       in the derivation to enable checks. The exception is cross compilation.
       Cross compiled builds never run tests, no matter how
       <varname>doCheck</varname> is set, as the newly-built program won't run
       on the platform used to build it.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>makeFlags</varname> /
    <varname>makeFlagsArray</varname> /
    <varname>makefile</varname>
     </term>
     <listitem>
      <para>
       See the build phase for details.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>checkTarget</varname>
     </term>
     <listitem>
      <para>
       The make target that runs the tests. Defaults to
       <literal>check</literal>.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>checkFlags</varname> / <varname>checkFlagsArray</varname>
     </term>
     <listitem>
      <para>
       A list of strings passed as additional flags to <command>make</command>.
       Like <varname>makeFlags</varname> and <varname>makeFlagsArray</varname>,
       but only used by the check phase.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>preCheck</varname>
     </term>
     <listitem>
      <para>
       Hook executed at the start of the check phase.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>postCheck</varname>
     </term>
     <listitem>
      <para>
       Hook executed at the end of the check phase.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </section>

  <section xml:id="ssec-install-phase">
   <title>The install phase</title>

   <para>
    The install phase is responsible for installing the package in the Nix
    store under <envar>out</envar>. The default
    <function>installPhase</function> creates the directory
    <literal>$out</literal> and calls <command>make install</command>.
   </para>

   <variablelist>
    <title>Variables controlling the install phase</title>
    <varlistentry>
     <term><varname>makeFlags</varname> /
    <varname>makeFlagsArray</varname> /
    <varname>makefile</varname>
     </term>
     <listitem>
      <para>
       See the build phase for details.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>installTargets</varname>
     </term>
     <listitem>
      <para>
       The make targets that perform the installation. Defaults to
       <literal>install</literal>. Example:
<programlisting>
installTargets = "install-bin install-doc";</programlisting>
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>installFlags</varname> / <varname>installFlagsArray</varname>
     </term>
     <listitem>
      <para>
       A list of strings passed as additional flags to <command>make</command>.
       Like <varname>makeFlags</varname> and <varname>makeFlagsArray</varname>,
       but only used by the install phase.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>preInstall</varname>
     </term>
     <listitem>
      <para>
       Hook executed at the start of the install phase.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>postInstall</varname>
     </term>
     <listitem>
      <para>
       Hook executed at the end of the install phase.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </section>

  <section xml:id="ssec-fixup-phase">
   <title>The fixup phase</title>

   <para>
    The fixup phase performs some (Nix-specific) post-processing actions on the
    files installed under <filename>$out</filename> by the install phase. The
    default <function>fixupPhase</function> does the following:
    <itemizedlist>
     <listitem>
      <para>
       It moves the <filename>man/</filename>, <filename>doc/</filename> and
       <filename>info/</filename> subdirectories of <envar>$out</envar> to
       <filename>share/</filename>.
      </para>
     </listitem>
     <listitem>
      <para>
       It strips libraries and executables of debug information.
      </para>
     </listitem>
     <listitem>
      <para>
       On Linux, it applies the <command>patchelf</command> command to ELF
       executables and libraries to remove unused directories from the
       <literal>RPATH</literal> in order to prevent unnecessary runtime
       dependencies.
      </para>
     </listitem>
     <listitem>
      <para>
       It rewrites the interpreter paths of shell scripts to paths found in
       <envar>PATH</envar>. E.g., <filename>/usr/bin/perl</filename> will be
       rewritten to
       <filename>/nix/store/<replaceable>some-perl</replaceable>/bin/perl</filename>
       found in <envar>PATH</envar>.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <variablelist>
    <title>Variables controlling the fixup phase</title>
    <varlistentry>
     <term><varname>dontStrip</varname>
     </term>
     <listitem>
      <para>
       If set, libraries and executables are not stripped. By default, they
       are.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>dontStripHost</varname>
     </term>
     <listitem>
      <para>
       Like <varname>dontStripHost</varname>, but only affects the
       <command>strip</command> command targetting the package's host platform.
       Useful when supporting cross compilation, but otherwise feel free to
       ignore.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>dontStripTarget</varname>
     </term>
     <listitem>
      <para>
       Like <varname>dontStripHost</varname>, but only affects the
       <command>strip</command> command targetting the packages' target
       platform. Useful when supporting cross compilation, but otherwise feel
       free to ignore.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>dontMoveSbin</varname>
     </term>
     <listitem>
      <para>
       If set, files in <filename>$out/sbin</filename> are not moved to
       <filename>$out/bin</filename>. By default, they are.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>stripAllList</varname>
     </term>
     <listitem>
      <para>
       List of directories to search for libraries and executables from which
       <emphasis>all</emphasis> symbols should be stripped. By default, it’s
       empty. Stripping all symbols is risky, since it may remove not just
       debug symbols but also ELF information necessary for normal execution.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>stripAllFlags</varname>
     </term>
     <listitem>
      <para>
       Flags passed to the <command>strip</command> command applied to the
       files in the directories listed in <varname>stripAllList</varname>.
       Defaults to <option>-s</option> (i.e. <option>--strip-all</option>).
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>stripDebugList</varname>
     </term>
     <listitem>
      <para>
       List of directories to search for libraries and executables from which
       only debugging-related symbols should be stripped. It defaults to
       <literal>lib bin sbin</literal>.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>stripDebugFlags</varname>
     </term>
     <listitem>
      <para>
       Flags passed to the <command>strip</command> command applied to the
       files in the directories listed in <varname>stripDebugList</varname>.
       Defaults to <option>-S</option> (i.e. <option>--strip-debug</option>).
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>dontPatchELF</varname>
     </term>
     <listitem>
      <para>
       If set, the <command>patchelf</command> command is not used to remove
       unnecessary <literal>RPATH</literal> entries. Only applies to Linux.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>dontPatchShebangs</varname>
     </term>
     <listitem>
      <para>
       If set, scripts starting with <literal>#!</literal> do not have their
       interpreter paths rewritten to paths in the Nix store.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>forceShare</varname>
     </term>
     <listitem>
      <para>
       The list of directories that must be moved from
       <filename>$out</filename> to <filename>$out/share</filename>. Defaults
       to <literal>man doc info</literal>.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>setupHook</varname>
     </term>
     <listitem>
      <para>
       A package can export a <link
    linkend="ssec-setup-hooks">setup
       hook</link> by setting this variable. The setup hook, if defined, is
       copied to <filename>$out/nix-support/setup-hook</filename>. Environment
       variables are then substituted in it using
       <function
    linkend="fun-substituteAll">substituteAll</function>.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>preFixup</varname>
     </term>
     <listitem>
      <para>
       Hook executed at the start of the fixup phase.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>postFixup</varname>
     </term>
     <listitem>
      <para>
       Hook executed at the end of the fixup phase.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry xml:id="stdenv-separateDebugInfo">
     <term><varname>separateDebugInfo</varname>
     </term>
     <listitem>
      <para>
       If set to <literal>true</literal>, the standard environment will enable
       debug information in C/C++ builds. After installation, the debug
       information will be separated from the executables and stored in the
       output named <literal>debug</literal>. (This output is enabled
       automatically; you don’t need to set the <varname>outputs</varname>
       attribute explicitly.) To be precise, the debug information is stored in
       <filename><replaceable>debug</replaceable>/lib/debug/.build-id/<replaceable>XX</replaceable>/<replaceable>YYYY…</replaceable></filename>,
       where <replaceable>XXYYYY…</replaceable> is the <replaceable>build
       ID</replaceable> of the binary — a SHA-1 hash of the contents of the
       binary. Debuggers like GDB use the build ID to look up the separated
       debug information.
      </para>
      <para>
       For example, with GDB, you can add
<programlisting>
set debug-file-directory ~/.nix-profile/lib/debug
</programlisting>
       to <filename>~/.gdbinit</filename>. GDB will then be able to find debug
       information installed via <literal>nix-env -i</literal>.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </section>

  <section xml:id="ssec-installCheck-phase">
   <title>The installCheck phase</title>

   <para>
    The installCheck phase checks whether the package was installed correctly
    by running its test suite against the installed directories. The default
    <function>installCheck</function> calls <command>make
    installcheck</command>.
   </para>

   <variablelist>
    <title>Variables controlling the installCheck phase</title>
    <varlistentry>
     <term><varname>doInstallCheck</varname>
     </term>
     <listitem>
      <para>
       Controls whether the installCheck phase is executed. By default it is
       skipped, but if <varname>doInstallCheck</varname> is set to true, the
       installCheck phase is usually executed. Thus you should set
<programlisting>doInstallCheck = true;</programlisting>
       in the derivation to enable install checks. The exception is cross
       compilation. Cross compiled builds never run tests, no matter how
       <varname>doInstallCheck</varname> is set, as the newly-built program
       won't run on the platform used to build it.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
      <term><varname>installCheckInputs</varname>
      </term>
      <listitem><para>
        A list of dependencies used by the phase. This gets included in
        <varname>buildInputs</varname> when <varname>doInstallCheck</varname>
        is set.
      </para></listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>preInstallCheck</varname>
     </term>
     <listitem>
      <para>
       Hook executed at the start of the installCheck phase.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>postInstallCheck</varname>
     </term>
     <listitem>
      <para>
       Hook executed at the end of the installCheck phase.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </section>

  <section xml:id="ssec-distribution-phase">
   <title>The distribution phase</title>

   <para>
    The distribution phase is intended to produce a source distribution of the
    package. The default <function>distPhase</function> first calls
    <command>make dist</command>, then it copies the resulting source tarballs
    to <filename>$out/tarballs/</filename>. This phase is only executed if the
    attribute <varname>doDist</varname> is set.
   </para>

   <variablelist>
    <title>Variables controlling the distribution phase</title>
    <varlistentry>
     <term><varname>distTarget</varname>
     </term>
     <listitem>
      <para>
       The make target that produces the distribution. Defaults to
       <literal>dist</literal>.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>distFlags</varname> / <varname>distFlagsArray</varname>
     </term>
     <listitem>
      <para>
       Additional flags passed to <command>make</command>.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>tarballs</varname>
     </term>
     <listitem>
      <para>
       The names of the source distribution files to be copied to
       <filename>$out/tarballs/</filename>. It can contain shell wildcards. The
       default is <filename>*.tar.gz</filename>.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>dontCopyDist</varname>
     </term>
     <listitem>
      <para>
       If set, no files are copied to <filename>$out/tarballs/</filename>.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>preDist</varname>
     </term>
     <listitem>
      <para>
       Hook executed at the start of the distribution phase.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><varname>postDist</varname>
     </term>
     <listitem>
      <para>
       Hook executed at the end of the distribution phase.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </section>
 </section>
 <section xml:id="ssec-stdenv-functions">
  <title>Shell functions</title>

  <para>
   The standard environment provides a number of useful functions.
  </para>

  <variablelist>
   <varlistentry xml:id='fun-makeWrapper'>
    <term><function>makeWrapper</function><replaceable>executable</replaceable><replaceable>wrapperfile</replaceable><replaceable>args</replaceable>
    </term>
    <listitem>
     <para>
      Constructs a wrapper for a program with various possible arguments. For
      example:
<programlisting>
# adds `FOOBAR=baz` to `$out/bin/foo`’s environment
makeWrapper $out/bin/foo $wrapperfile --set FOOBAR baz

# prefixes the binary paths of `hello` and `git`
# Be advised that paths often should be patched in directly
# (via string replacements or in `configurePhase`).
makeWrapper $out/bin/foo $wrapperfile --prefix PATH : ${lib.makeBinPath [ hello git ]}
</programlisting>
      There’s many more kinds of arguments, they are documented in
      <literal>nixpkgs/pkgs/build-support/setup-hooks/make-wrapper.sh</literal>.
     </para>
     <para>
      <literal>wrapProgram</literal> is a convenience function you probably
      want to use most of the time.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry xml:id='fun-substitute'>
    <term><function>substitute</function><replaceable>infile</replaceable><replaceable>outfile</replaceable><replaceable>subs</replaceable>
    </term>
    <listitem>
     <para>
      Performs string substitution on the contents of
      <replaceable>infile</replaceable>, writing the result to
      <replaceable>outfile</replaceable>. The substitutions in
      <replaceable>subs</replaceable> are of the following form:
      <variablelist>
       <varlistentry>
        <term><option>--replace</option><replaceable>s1</replaceable><replaceable>s2</replaceable>
        </term>
        <listitem>
         <para>
          Replace every occurrence of the string <replaceable>s1</replaceable>
          by <replaceable>s2</replaceable>.
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term><option>--subst-var</option><replaceable>varName</replaceable>
        </term>
        <listitem>
         <para>
          Replace every occurrence of
          <literal>@<replaceable>varName</replaceable>@</literal> by the
          contents of the environment variable
          <replaceable>varName</replaceable>. This is useful for generating
          files from templates, using
          <literal>@<replaceable>...</replaceable>@</literal> in the template
          as placeholders.
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term><option>--subst-var-by</option><replaceable>varName</replaceable><replaceable>s</replaceable>
        </term>
        <listitem>
         <para>
          Replace every occurrence of
          <literal>@<replaceable>varName</replaceable>@</literal> by the string
          <replaceable>s</replaceable>.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </para>
     <para>
      Example:
<programlisting>
substitute ./foo.in ./foo.out \
    --replace /usr/bin/bar $bar/bin/bar \
    --replace "a string containing spaces" "some other text" \
    --subst-var someVar
</programlisting>
     </para>
     <para>
      <function>substitute</function> is implemented using the
      <command
      xlink:href="http://replace.richardlloyd.org.uk/">replace</command>
      command. Unlike with the <command>sed</command> command, you don’t have
      to worry about escaping special characters. It supports performing
      substitutions on binary files (such as executables), though there
      you’ll probably want to make sure that the replacement string is as
      long as the replaced string.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry xml:id='fun-substituteInPlace'>
    <term><function>substituteInPlace</function><replaceable>file</replaceable><replaceable>subs</replaceable>
    </term>
    <listitem>
     <para>
      Like <function>substitute</function>, but performs the substitutions in
      place on the file <replaceable>file</replaceable>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry xml:id='fun-substituteAll'>
    <term><function>substituteAll</function><replaceable>infile</replaceable><replaceable>outfile</replaceable>
    </term>
    <listitem>
     <para>
      Replaces every occurrence of
      <literal>@<replaceable>varName</replaceable>@</literal>, where
      <replaceable>varName</replaceable> is any environment variable, in
      <replaceable>infile</replaceable>, writing the result to
      <replaceable>outfile</replaceable>. For instance, if
      <replaceable>infile</replaceable> has the contents
<programlisting>
#! @bash@/bin/sh
PATH=@coreutils@/bin
echo @foo@
</programlisting>
      and the environment contains
      <literal>bash=/nix/store/bmwp0q28cf21...-bash-3.2-p39</literal> and
      <literal>coreutils=/nix/store/68afga4khv0w...-coreutils-6.12</literal>,
      but does not contain the variable <varname>foo</varname>, then the output
      will be
<programlisting>
#! /nix/store/bmwp0q28cf21...-bash-3.2-p39/bin/sh
PATH=/nix/store/68afga4khv0w...-coreutils-6.12/bin
echo @foo@
</programlisting>
      That is, no substitution is performed for undefined variables.
     </para>
     <para>
      Environment variables that start with an uppercase letter or an
      underscore are filtered out, to prevent global variables (like
      <literal>HOME</literal>) or private variables (like
      <literal>__ETC_PROFILE_DONE</literal>) from accidentally getting
      substituted. The variables also have to be valid bash “names”, as
      defined in the bash manpage (alphanumeric or <literal>_</literal>, must
      not start with a number).
     </para>
    </listitem>
   </varlistentry>
   <varlistentry xml:id='fun-substituteAllInPlace'>
    <term><function>substituteAllInPlace</function><replaceable>file</replaceable>
    </term>
    <listitem>
     <para>
      Like <function>substituteAll</function>, but performs the substitutions
      in place on the file <replaceable>file</replaceable>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry xml:id='fun-stripHash'>
    <term><function>stripHash</function><replaceable>path</replaceable>
    </term>
    <listitem>
     <para>
      Strips the directory and hash part of a store path, outputting the name
      part to <literal>stdout</literal>. For example:
<programlisting>
# prints coreutils-8.24
stripHash "/nix/store/9s9r019176g7cvn2nvcw41gsp862y6b4-coreutils-8.24"
</programlisting>
      If you wish to store the result in another variable, then the following
      idiom may be useful:
<programlisting>
name="/nix/store/9s9r019176g7cvn2nvcw41gsp862y6b4-coreutils-8.24"
someVar=$(stripHash $name)
</programlisting>
     </para>
    </listitem>
   </varlistentry>
   <varlistentry xml:id='fun-wrapProgram'>
    <term><function>wrapProgram</function><replaceable>executable</replaceable><replaceable>makeWrapperArgs</replaceable>
    </term>
    <listitem>
     <para>
      Convenience function for <literal>makeWrapper</literal> that
      automatically creates a sane wrapper file It takes all the same arguments
      as <literal>makeWrapper</literal>, except for <literal>--argv0</literal>.
     </para>
     <para>
      It cannot be applied multiple times, since it will overwrite the wrapper
      file.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </section>
 <section xml:id="ssec-setup-hooks">
  <title>Package setup hooks</title>

  <para>
   Nix itself considers a build-time dependency merely something that should
   previously be built and accessible at build time—packages themselves are
   on their own to perform any additional setup. In most cases, that is fine,
   and the downstream derivation can deal with it's own dependencies. But for a
   few common tasks, that would result in almost every package doing the same
   sort of setup work---depending not on the package itself, but entirely on
   which dependencies were used.
  </para>

  <para>
   In order to alleviate this burden, the <firstterm>setup
   hook></firstterm>mechanism was written, where any package can include a
   shell script that [by convention rather than enforcement by Nix], any
   downstream reverse-dependency will source as part of its build process. That
   allows the downstream dependency to merely specify its dependencies, and
   lets those dependencies effectively initialize themselves. No boilerplate
   mirroring the list of dependencies is needed.
  </para>

  <para>
   The Setup hook mechanism is a bit of a sledgehammer though: a powerful
   feature with a broad and indiscriminate area of effect. The combination of
   its power and implicit use may be expedient, but isn't without costs. Nix
   itself is unchanged, but the spirit of adding dependencies being effect-free
   is violated even if the letter isn't. For example, if a derivation path is
   mentioned more than once, Nix itself doesn't care and simply makes sure the
   dependency derivation is already built just the same—depending is just
   needing something to exist, and needing is idempotent. However, a dependency
   specified twice will have its setup hook run twice, and that could easily
   change the build environment (though a well-written setup hook will
   therefore strive to be idempotent so this is in fact not observable). More
   broadly, setup hooks are anti-modular in that multiple dependencies, whether
   the same or different, should not interfere and yet their setup hooks may
   well do so.
  </para>

  <para>
   The most typical use of the setup hook is actually to add other hooks which
   are then run (i.e. after all the setup hooks) on each dependency. For
   example, the C compiler wrapper's setup hook feeds itself flags for each
   dependency that contains relevant libaries and headers. This is done by
   defining a bash function, and appending its name to one of
   <envar>envBuildBuildHooks</envar>`, <envar>envBuildHostHooks</envar>`,
   <envar>envBuildTargetHooks</envar>`, <envar>envHostHostHooks</envar>`,
   <envar>envHostTargetHooks</envar>`, or <envar>envTargetTargetHooks</envar>`.
   These 6 bash variables correspond to the 6 sorts of dependencies by platform
   (there's 12 total but we ignore the propagated/non-propagated axis).
  </para>

  <para>
   Packages adding a hook should not hard code a specific hook, but rather
   choose a variable <emphasis>relative</emphasis> to how they are included.
   Returning to the C compiler wrapper example, if it itself is an
   <literal>n</literal> dependency, then it only wants to accumulate flags from
   <literal>n + 1</literal> dependencies, as only those ones match the
   compiler's target platform. The <envar>hostOffset</envar> variable is
   defined with the current dependency's host offset
   <envar>targetOffset</envar> with its target offset, before it's setup hook
   is sourced. Additionally, since most environment hooks don't care about the
   target platform, That means the setup hook can append to the right bash
   array by doing something like
<programlisting language="bash">
addEnvHooks "$hostOffset" myBashFunction
  </programlisting>
  </para>

  <para>
   The <emphasis>existence</emphasis> of setups hooks has long been documented
   and packages inside Nixpkgs are free to use these mechanism. Other packages,
   however, should not rely on these mechanisms not changing between Nixpkgs
   versions. Because of the existing issues with this system, there's little
   benefit from mandating it be stable for any period of time.
  </para>

  <para>
   Here are some packages that provide a setup hook. Since the mechanism is
   modular, this probably isn't an exhaustive list. Then again, since the
   mechanism is only to be used as a last resort, it might be.
   <variablelist>
    <varlistentry>
     <term>Bintools Wrapper</term>
     <listitem>
      <para>
       Bintools Wrapper wraps the binary utilities for a bunch of miscellaneous
       purposes. These are GNU Binutils when targetting Linux, and a mix of
       cctools and GNU binutils for Darwin. [The "Bintools" name is supposed to
       be a compromise between "Binutils" and "cctools" not denoting any
       specific implementation.] Specifically, the underlying bintools package,
       and a C standard library (glibc or Darwin's libSystem, just for the
       dynamic loader) are all fed in, and dependency finding, hardening (see
       below), and purity checks for each are handled by Bintools Wrapper.
       Packages typically depend on CC Wrapper, which in turn (at run time)
       depends on Bintools Wrapper.
      </para>
      <para>
       Bintools Wrapper was only just recently split off from CC Wrapper, so
       the division of labor is still being worked out. For example, it
       shouldn't care about about the C standard library, but just take a
       derivation with the dynamic loader (which happens to be the glibc on
       linux). Dependency finding however is a task both wrappers will continue
       to need to share, and probably the most important to understand. It is
       currently accomplished by collecting directories of host-platform
       dependencies (i.e. <varname>buildInputs</varname> and
       <varname>nativeBuildInputs</varname>) in environment variables. Bintools
       Wrapper's setup hook causes any <filename>lib</filename> and
       <filename>lib64</filename> subdirectories to be added to
       <envar>NIX_LDFLAGS</envar>. Since CC Wrapper and Bintools Wrapper use
       the same strategy, most of the Bintools Wrapper code is sparsely
       commented and refers to CC Wrapper. But CC Wrapper's code, by contrast,
       has quite lengthy comments. Bintools Wrapper merely cites those, rather
       than repeating them, to avoid falling out of sync.
      </para>
      <para>
       A final task of the setup hook is defining a number of standard
       environment variables to tell build systems which executables full-fill
       which purpose. They are defined to just be the base name of the tools,
       under the assumption that Bintools Wrapper's binaries will be on the
       path. Firstly, this helps poorly-written packages, e.g. ones that look
       for just <command>gcc</command> when <envar>CC</envar> isn't defined yet
       <command>clang</command> is to be used. Secondly, this helps packages
       not get confused when cross-compiling, in which case multiple Bintools
       Wrappers may simultaneously be in use.
       <footnote>
        <para>
         Each wrapper targets a single platform, so if binaries for multiple
         platforms are needed, the underlying binaries must be wrapped multiple
         times. As this is a property of the wrapper itself, the multiple
         wrappings are needed whether or not the same underlying binaries can
         target multiple platforms.
        </para>
       </footnote>
       <envar>BUILD_</envar>- and <envar>TARGET_</envar>-prefixed versions of
       the normal environment variable are defined for the additional Bintools
       Wrappers, properly disambiguating them.
      </para>
      <para>
       A problem with this final task is that Bintools Wrapper is honest and
       defines <envar>LD</envar> as <command>ld</command>. Most packages,
       however, firstly use the C compiler for linking, secondly use
       <envar>LD</envar> anyways, defining it as the C compiler, and thirdly,
       only so define <envar>LD</envar> when it is undefined as a fallback.
       This triple-threat means Bintools Wrapper will break those packages, as
       LD is already defined as the actual linker which the package won't
       override yet doesn't want to use. The workaround is to define, just for
       the problematic package, <envar>LD</envar> as the C compiler. A good way
       to do this would be <command>preConfigure = "LD=$CC"</command>.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>CC Wrapper</term>
     <listitem>
      <para>
       CC Wrapper wraps a C toolchain for a bunch of miscellaneous purposes.
       Specifically, a C compiler (GCC or Clang), wrapped binary tools, and a C
       standard library (glibc or Darwin's libSystem, just for the dynamic
       loader) are all fed in, and dependency finding, hardening (see below),
       and purity checks for each are handled by CC Wrapper. Packages typically
       depend on CC Wrapper, which in turn (at run time) depends on Bintools
       Wrapper.
      </para>
      <para>
       Dependency finding is undoubtedly the main task of CC Wrapper. This
       works just like Bintools Wrapper, except that any
       <filename>include</filename> subdirectory of any relevant dependency is
       added to <envar>NIX_CFLAGS_COMPILE</envar>. The setup hook itself
       contains some lengthy comments describing the exact convoluted mechanism
       by which this is accomplished.
      </para>
      <para>
       CC Wrapper also like Bintools Wrapper defines standard environment
       variables with the names of the tools it wraps, for the same reasons
       described above. Importantly, while it includes a <command>cc</command>
       symlink to the c compiler for portability, the <envar>CC</envar> will be
       defined using the compiler's "real name" (i.e. <command>gcc</command> or
       <command>clang</command>). This helps lousy build systems that inspect
       on the name of the compiler rather than run it.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>Perl</term>
     <listitem>
      <para>
       Adds the <filename>lib/site_perl</filename> subdirectory of each build
       input to the <envar>PERL5LIB</envar> environment variable. For instance,
       if <varname>buildInputs</varname> contains Perl, then the
       <filename>lib/site_perl</filename> subdirectory of each input is added
       to the <envar>PERL5LIB</envar> environment variable.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>Python</term>
     <listitem>
      <para>
       Adds the <filename>lib/${python.libPrefix}/site-packages</filename>
       subdirectory of each build input to the <envar>PYTHONPATH</envar>
       environment variable.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>pkg-config</term>
     <listitem>
      <para>
       Adds the <filename>lib/pkgconfig</filename> and
       <filename>share/pkgconfig</filename> subdirectories of each build input
       to the <envar>PKG_CONFIG_PATH</envar> environment variable.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>Automake</term>
     <listitem>
      <para>
       Adds the <filename>share/aclocal</filename> subdirectory of each build
       input to the <envar>ACLOCAL_PATH</envar> environment variable.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>Autoconf</term>
     <listitem>
      <para>
       The <varname>autoreconfHook</varname> derivation adds
       <varname>autoreconfPhase</varname>, which runs autoreconf, libtoolize
       and automake, essentially preparing the configure script in
       autotools-based builds.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>libxml2</term>
     <listitem>
      <para>
       Adds every file named <filename>catalog.xml</filename> found under the
       <filename>xml/dtd</filename> and <filename>xml/xsl</filename>
       subdirectories of each build input to the
       <envar>XML_CATALOG_FILES</envar> environment variable.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>teTeX / TeX Live</term>
     <listitem>
      <para>
       Adds the <filename>share/texmf-nix</filename> subdirectory of each build
       input to the <envar>TEXINPUTS</envar> environment variable.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>Qt 4</term>
     <listitem>
      <para>
       Sets the <envar>QTDIR</envar> environment variable to Qt’s path.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>gdk-pixbuf</term>
     <listitem>
      <para>
       Exports <envar>GDK_PIXBUF_MODULE_FILE</envar> environment variable the
       the builder. Add librsvg package to <varname>buildInputs</varname> to
       get svg support.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>GHC</term>
     <listitem>
      <para>
       Creates a temporary package database and registers every Haskell build
       input in it (TODO: how?).
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>GStreamer</term>
     <listitem>
      <para>
       Adds the GStreamer plugins subdirectory of each build input to the
       <envar>GST_PLUGIN_SYSTEM_PATH_1_0</envar> or
       <envar>GST_PLUGIN_SYSTEM_PATH</envar> environment variable.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>paxctl</term>
     <listitem>
      <para>
       Defines the <varname>paxmark</varname> helper for setting per-executable
       PaX flags on Linux (where it is available by default; on all other
       platforms, <varname>paxmark</varname> is a no-op). For example, to
       disable secure memory protections on the executable
       <replaceable>foo</replaceable>:
<programlisting>
      postFixup = ''
        paxmark m $out/bin/<replaceable>foo</replaceable>
      '';
    </programlisting>
       The <literal>m</literal> flag is the most common flag and is typically
       required for applications that employ JIT compilation or otherwise need
       to execute code generated at run-time. Disabling PaX protections should
       be considered a last resort: if possible, problematic features should be
       disabled or patched to work with PaX.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>autoPatchelfHook</term>
     <listitem>
      <para>
       This is a special setup hook which helps in packaging proprietary
       software in that it automatically tries to find missing shared library
       dependencies of ELF files. All packages within the
       <envar>runtimeDependencies</envar> environment variable are
       unconditionally added to executables, which is useful for programs that
       use <citerefentry>
       <refentrytitle>dlopen</refentrytitle>
       <manvolnum>3</manvolnum> </citerefentry> to load libraries at runtime.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </section>
 <section xml:id="sec-purity-in-nixpkgs">
  <title>Purity in Nixpkgs</title>

  <para>
   [measures taken to prevent dependencies on packages outside the store, and
   what you can do to prevent them]
  </para>

  <para>
   GCC doesn't search in locations such as <filename>/usr/include</filename>.
   In fact, attempts to add such directories through the <option>-I</option>
   flag are filtered out. Likewise, the linker (from GNU binutils) doesn't
   search in standard locations such as <filename>/usr/lib</filename>. Programs
   built on Linux are linked against a GNU C Library that likewise doesn't
   search in the default system locations.
  </para>
 </section>
 <section xml:id="sec-hardening-in-nixpkgs">
  <title>Hardening in Nixpkgs</title>

  <para>
   There are flags available to harden packages at compile or link-time. These
   can be toggled using the <varname>stdenv.mkDerivation</varname> parameters
   <varname>hardeningDisable</varname> and <varname>hardeningEnable</varname>.
  </para>

  <para>
   Both parameters take a list of flags as strings. The special
   <varname>"all"</varname> flag can be passed to
   <varname>hardeningDisable</varname> to turn off all hardening. These flags
   can also be used as environment variables for testing or development
   purposes.
  </para>

  <para>
   The following flags are enabled by default and might require disabling with
   <varname>hardeningDisable</varname> if the program to package is
   incompatible.
  </para>

  <variablelist>
   <varlistentry>
    <term><varname>format</varname>
    </term>
    <listitem>
     <para>
      Adds the <option>-Wformat -Wformat-security
      -Werror=format-security</option> compiler options. At present, this warns
      about calls to <varname>printf</varname> and <varname>scanf</varname>
      functions where the format string is not a string literal and there are
      no format arguments, as in <literal>printf(foo);</literal>. This may be a
      security hole if the format string came from untrusted input and contains
      <literal>%n</literal>.
     </para>
     <para>
      This needs to be turned off or fixed for errors similar to:
     </para>
<programlisting>
/tmp/nix-build-zynaddsubfx-2.5.2.drv-0/zynaddsubfx-2.5.2/src/UI/guimain.cpp:571:28: error: format not a string literal and no format arguments [-Werror=format-security]
         printf(help_message);
                            ^
cc1plus: some warnings being treated as errors
    </programlisting>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><varname>stackprotector</varname>
    </term>
    <listitem>
     <para>
      Adds the <option>-fstack-protector-strong --param
      ssp-buffer-size=4</option> compiler options. This adds safety checks
      against stack overwrites rendering many potential code injection attacks
      into aborting situations. In the best case this turns code injection
      vulnerabilities into denial of service or into non-issues (depending on
      the application).
     </para>
     <para>
      This needs to be turned off or fixed for errors similar to:
     </para>
<programlisting>
bin/blib.a(bios_console.o): In function `bios_handle_cup':
/tmp/nix-build-ipxe-20141124-5cbdc41.drv-0/ipxe-5cbdc41/src/arch/i386/firmware/pcbios/bios_console.c:86: undefined reference to `__stack_chk_fail'
    </programlisting>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><varname>fortify</varname>
    </term>
    <listitem>
     <para>
      Adds the <option>-O2 -D_FORTIFY_SOURCE=2</option> compiler options.
      During code generation the compiler knows a great deal of information
      about buffer sizes (where possible), and attempts to replace insecure
      unlimited length buffer function calls with length-limited ones. This is
      especially useful for old, crufty code. Additionally, format strings in
      writable memory that contain '%n' are blocked. If an application depends
      on such a format string, it will need to be worked around.
     </para>
     <para>
      Additionally, some warnings are enabled which might trigger build
      failures if compiler warnings are treated as errors in the package build.
      In this case, set <option>NIX_CFLAGS_COMPILE</option> to
      <option>-Wno-error=warning-type</option>.
     </para>
     <para>
      This needs to be turned off or fixed for errors similar to:
     </para>
<programlisting>
malloc.c:404:15: error: return type is an incomplete type
malloc.c:410:19: error: storage size of 'ms' isn't known
    </programlisting>
<programlisting>
strdup.h:22:1: error: expected identifier or '(' before '__extension__'
    </programlisting>
<programlisting>
strsep.c:65:23: error: register name not specified for 'delim'
    </programlisting>
<programlisting>
installwatch.c:3751:5: error: conflicting types for '__open_2'
    </programlisting>
<programlisting>
fcntl2.h:50:4: error: call to '__open_missing_mode' declared with attribute error: open with O_CREAT or O_TMPFILE in second argument needs 3 arguments
    </programlisting>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><varname>pic</varname>
    </term>
    <listitem>
     <para>
      Adds the <option>-fPIC</option> compiler options. This options adds
      support for position independent code in shared libraries and thus making
      ASLR possible.
     </para>
     <para>
      Most notably, the Linux kernel, kernel modules and other code not running
      in an operating system environment like boot loaders won't build with PIC
      enabled. The compiler will is most cases complain that PIC is not
      supported for a specific build.
     </para>
     <para>
      This needs to be turned off or fixed for assembler errors similar to:
     </para>
<programlisting>
ccbLfRgg.s: Assembler messages:
ccbLfRgg.s:33: Error: missing or invalid displacement expression `private_key_len@GOTOFF'
    </programlisting>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><varname>strictoverflow</varname>
    </term>
    <listitem>
     <para>
      Signed integer overflow is undefined behaviour according to the C
      standard. If it happens, it is an error in the program as it should check
      for overflow before it can happen, not afterwards. GCC provides built-in
      functions to perform arithmetic with overflow checking, which are correct
      and faster than any custom implementation. As a workaround, the option
      <option>-fno-strict-overflow</option> makes gcc behave as if signed
      integer overflows were defined.
     </para>
     <para>
      This flag should not trigger any build or runtime errors.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><varname>relro</varname>
    </term>
    <listitem>
     <para>
      Adds the <option>-z relro</option> linker option. During program load,
      several ELF memory sections need to be written to by the linker, but can
      be turned read-only before turning over control to the program. This
      prevents some GOT (and .dtors) overwrite attacks, but at least the part
      of the GOT used by the dynamic linker (.got.plt) is still vulnerable.
     </para>
     <para>
      This flag can break dynamic shared object loading. For instance, the
      module systems of Xorg and OpenCV are incompatible with this flag. In
      almost all cases the <varname>bindnow</varname> flag must also be
      disabled and incompatible programs typically fail with similar errors at
      runtime.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><varname>bindnow</varname>
    </term>
    <listitem>
     <para>
      Adds the <option>-z bindnow</option> linker option. During program load,
      all dynamic symbols are resolved, allowing for the complete GOT to be
      marked read-only (due to <varname>relro</varname>). This prevents GOT
      overwrite attacks. For very large applications, this can incur some
      performance loss during initial load while symbols are resolved, but this
      shouldn't be an issue for daemons.
     </para>
     <para>
      This flag can break dynamic shared object loading. For instance, the
      module systems of Xorg and PHP are incompatible with this flag. Programs
      incompatible with this flag often fail at runtime due to missing symbols,
      like:
     </para>
<programlisting>
intel_drv.so: undefined symbol: vgaHWFreeHWRec
    </programlisting>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   The following flags are disabled by default and should be enabled with
   <varname>hardeningEnable</varname> for packages that take untrusted input
   like network services.
  </para>

  <variablelist>
   <varlistentry>
    <term><varname>pie</varname>
    </term>
    <listitem>
     <para>
      Adds the <option>-fPIE</option> compiler and <option>-pie</option> linker
      options. Position Independent Executables are needed to take advantage of
      Address Space Layout Randomization, supported by modern kernel versions.
      While ASLR can already be enforced for data areas in the stack and heap
      (brk and mmap), the code areas must be compiled as position-independent.
      Shared libraries already do this with the <varname>pic</varname> flag, so
      they gain ASLR automatically, but binary .text regions need to be build
      with <varname>pie</varname> to gain ASLR. When this happens, ROP attacks
      are much harder since there are no static locations to bounce off of
      during a memory corruption attack.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   For more in-depth information on these hardening flags and hardening in
   general, refer to the
   <link xlink:href="https://wiki.debian.org/Hardening">Debian Wiki</link>,
   <link xlink:href="https://wiki.ubuntu.com/Security/Features">Ubuntu
   Wiki</link>,
   <link xlink:href="https://wiki.gentoo.org/wiki/Project:Hardened">Gentoo
   Wiki</link>, and the
   <link xlink:href="https://wiki.archlinux.org/index.php/DeveloperWiki:Security">
   Arch Wiki</link>.
  </para>
 </section>
</chapter>