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
   Nix requires. For convenience, you can also use <varname>pname</varname> and
   <varname>version</varname> attributes and <literal>mkDerivation</literal>
   will automatically set <varname>name</varname> to
   <literal>"${pname}-${version}"</literal> by default. Since
   <link xlink:href="https://github.com/NixOS/rfcs/pull/35">RFC 0035</link>,
   this is preferred for packages in Nixpkgs, as it allows us to reuse the
   version easily:
<programlisting>
stdenv.mkDerivation rec {
  pname = "libfoo";
  version = "1.2.3";
  src = fetchurl {
    url = "http://example.org/libfoo-source-${version}.tar.bz2";
    sha256 = "0x2g1jqygyr5wiwg4ma1nd7w4ydpy82z9gkcv8vh2v8dn3y58v5m";
  };
}</programlisting>
  </para>

  <para>
   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 xml:id="footnote-stdenv-ignored-build-platform">
    <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. By default, the
   run-time/build-time distinction is just a hint for mental clarity, but with
   <varname>strictDeps</varname> set it is mostly enforced even in the native
   case.
  </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. dependencies which run on the platform where the new
   derivation will be built.
   <footnote xml:id="footnote-stdenv-native-dependencies-in-path">
    <para>
     Currently, this 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>nativeBuildInputs</varname> would be added to 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 that dependencies are not necessarily 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 to each dependency two
   integers based one how its host and target platforms are offset from the
   depending derivation's platforms. Those offsets are given below in the
   descriptions of each dependency list attribute. Algorithmically, we traverse
   propagated inputs, accumulating every propagated dependency's propagated
   dependencies and adjusting them to account for the "shift in perspective"
   described by the current dependency'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 dependencies 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 xml:id="footnote-stdenv-find-inputs-location">
    <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 will 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 dependencies' propagated dependencies
propagated-dep(mapOffset(h0, t0, h1),
               mapOffset(h0, t0, t1),
               A, C)</programlisting>
<programlisting>
propagated-dep(h, t, A, B)
----------------------------- Propagated dependencies count as dependencies
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 "sum-like" comes in from above: We can just sum all of the
   host offsets to get the host offset of the transitive dependency. The target
   offset is the transitive dependency 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 dependency 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 depending
   package is unaware of. [One can imagine the dependending package asking for
   dependencies with the platforms it knows about; other platforms it doesn't
   know how to ask for. The platform description in that scenario is a kind of
   unforagable capability.] 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.
      These are programs and libraries used at build time that produce programs
      and 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 of
      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, they 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. These are programs and
      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 could be called <varname>depsBuildHost</varname> but
      <varname>nativeBuildInputs</varname> is used for historical continuity.
     </para>
     <para>
      Since these packages are able to be run at build-time, they are 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>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. These are programs
      used at build time that produce code to run with code produced by the
      depending package. Most commonly, these are tools used to build the
      runtime or standard library that 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 that the dependency is
      unconditional.
     </para>
     <para>
      This is a somewhat confusing concept to wrap one’s head around, and for
      good reason. As the only dependency type 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 its use-case
      go hand in hand and are both considered poor form: try to not need this
      sort of dependency, and try to avoid building standard libraries and
      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 run at build time, they are 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>depsHostHost</varname>
    </term>
    <listitem>
     <para>
      A list of dependencies whose host and target platforms match the new
      derivation's host platform. This means a <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, this would usually be tools used by compilers
      for macros or a metaprogramming system, or libraries used by the macros
      or metaprogramming code itself. It's always preferable to use a
      <varname>depsBuildBuild</varname> dependency in the derivation being
      built over 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 a
      <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 are often programs and 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>depsTargetTargetPropagated</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 debugging
      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>Attributes 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>
  </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>
   <varlistentry>
    <term>
     <varname>passthru.updateScript</varname>
    </term>
    <listitem>
     <para>
      A script to be run by <filename>maintainers/scripts/update.nix</filename>
      when the package is matched. It needs to be an executable file, either on
      the file system:
<programlisting>
passthru.updateScript = ./update.sh;
</programlisting>
      or inside the expression itself:
<programlisting>
passthru.updateScript = writeScript "update-zoom-us" ''
  #!/usr/bin/env nix-shell
  #!nix-shell -i bash -p curl pcre common-updater-scripts

  set -eu -o pipefail

  version="$(curl -sI https://zoom.us/client/latest/zoom_x86_64.tar.xz | grep -Fi 'Location:' | pcregrep -o1 '/(([0-9]\.?)+)/')"
  update-source-version zoom-us "$version"
'';
</programlisting>
      The attribute can also contain a list, a script followed by arguments to
      be passed to it:
<programlisting>
passthru.updateScript = [ ../../update.sh pname "--requested-release=unstable" ];
</programlisting>
     </para>
     <para>
      The script will be usually run from the root of the Nixpkgs repository
      but you should not rely on that. Also note that the update scripts will
      be run in parallel by default; you should avoid running <command>git
      commit</command> or any other commands that cannot handle that.
     </para>
     <para>
      For information about how to run the updates, execute
      <command>nix-shell maintainers/scripts/update.nix</command>.
     </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. The default
   script for each phase is defined in the file <filename>pkgs/stdenv/generic/setup.sh</filename>.
  </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>, <filename>.tbz2</filename> or
        <filename>.tbz</filename>) or <command>xz</command>
        (<filename>.tar.xz</filename>, <filename>.tar.lzma</filename> or
        <filename>.txz</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>nativeBuildInputs</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>dontUnpack</varname>
     </term>
     <listitem>
      <para>
       Set to true to skip 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>dontConfigure</varname>
     </term>
     <listitem>
      <para>
       Set to true to skip the configure phase.
      </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>prefixKey</varname>
     </term>
     <listitem>
      <para>
       The key to use when specifying the prefix. By default, this is set to
       <option>--prefix=</option> as that is used by the majority of packages.
      </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 xml:id="footnote-stdenv-sys-lib-search-path">
        <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 can use this to also pass the target platform.
       <footnote xml:id="footnote-stdenv-build-time-guessing-impurity">
        <para>
         Eventually these will be passed building natively as well, 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>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>
preBuild = ''
  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>checkInputs</varname>
     </term>
     <listitem>
      <para>
       A list of dependencies used by the phase. This gets included in
       <varname>nativeBuildInputs</varname> when <varname>doCheck</varname> is
       set.
      </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>dontStrip</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>dontStrip</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>dontPruneLibtoolFiles</varname>
     </term>
     <listitem>
      <para>
       If set, libtool <literal>.la</literal> files associated with shared
       libraries won't have their <literal>dependency_libs</literal> field
       cleared.
      </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>installCheckTarget</varname>
     </term>
     <listitem>
      <para>
       The make target that runs the install tests. Defaults to
       <literal>installcheck</literal>.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      <varname>installCheckFlags</varname> / <varname>installCheckFlagsArray</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 installCheck phase.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      <varname>installCheckInputs</varname>
     </term>
     <listitem>
      <para>
       A list of dependencies used by the phase. This gets included in
       <varname>nativeBuildInputs</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 as 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 its 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 added 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 libraries 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 the wrapper 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 its 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 this 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>
   First, let’s cover some setup hooks that are part of Nixpkgs default
   stdenv. This means that they are run for every package built using
   <function>stdenv.mkDerivation</function>. Some of these are platform
   specific, so they may run on Linux but not Darwin or vice-versa.
   <variablelist>
    <varlistentry>
     <term>
      <literal>move-docs.sh</literal>
     </term>
     <listitem>
      <para>
       This setup hook moves any installed documentation to the
       <literal>/share</literal> subdirectory directory. This includes the man,
       doc and info directories. This is needed for legacy programs that do not
       know how to use the <literal>share</literal> subdirectory.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      <literal>compress-man-pages.sh</literal>
     </term>
     <listitem>
      <para>
       This setup hook compresses any man pages that have been installed. The
       compression is done using the gzip program. This helps to reduce the
       installed size of packages.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      <literal>strip.sh</literal>
     </term>
     <listitem>
      <para>
       This runs the strip command on installed binaries and libraries. This
       removes unnecessary information like debug symbols when they are not
       needed. This also helps to reduce the installed size of packages.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      <literal>patch-shebangs.sh</literal>
     </term>
     <listitem>
      <para>
       This setup hook patches installed scripts to use the full path to the
       shebang interpreter. A shebang interpreter is the first commented line
       of a script telling the operating system which program will run the
       script (e.g <literal>#!/bin/bash</literal>). In Nix, we want an exact
       path to that interpreter to be used. This often replaces
       <literal>/bin/sh</literal> with a path in the Nix store.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      <literal>audit-tmpdir.sh</literal>
     </term>
     <listitem>
      <para>
       This verifies that no references are left from the install binaries to
       the directory used to build those binaries. This ensures that the
       binaries do not need things outside the Nix store. This is currently
       supported in Linux only.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      <literal>multiple-outputs.sh</literal>
     </term>
     <listitem>
      <para>
       This setup hook adds configure flags that tell packages to install files
       into any one of the proper outputs listed in <literal>outputs</literal>.
       This behavior can be turned off by setting
       <literal>setOutputFlags</literal> to false in the derivation
       environment. See <xref linkend="chap-multiple-output"/> for more
       information.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      <literal>move-sbin.sh</literal>
     </term>
     <listitem>
      <para>
       This setup hook moves any binaries installed in the sbin subdirectory
       into bin. In addition, a link is provided from sbin to bin for
       compatibility.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      <literal>move-lib64.sh</literal>
     </term>
     <listitem>
      <para>
       This setup hook moves any libraries installed in the lib64 subdirectory
       into lib. In addition, a link is provided from lib64 to lib for
       compatibility.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      <literal>set-source-date-epoch-to-latest.sh</literal>
     </term>
     <listitem>
      <para>
       This sets <literal>SOURCE_DATE_EPOCH</literal> to the modification time
       of the most recent file.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      Bintools Wrapper
     </term>
     <listitem>
      <para>
       The 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 the
       Bintools Wrapper. Packages typically depend on CC Wrapper, which in turn
       (at run time) depends on the Bintools Wrapper.
      </para>
      <para>
       The 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. The
       Bintools Wrapper's setup hook causes any <filename>lib</filename> and
       <filename>lib64</filename> subdirectories to be added to
       <envar>NIX_LDFLAGS</envar>. Since the CC Wrapper and the Bintools
       Wrapper use the same strategy, most of the Bintools Wrapper code is
       sparsely commented and refers to the CC Wrapper. But the CC Wrapper's
       code, by contrast, has quite lengthy comments. The 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 fulfill
       which purpose. They are defined to just be the base name of the tools,
       under the assumption that the 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 xml:id="footnote-stdenv-per-platform-wrapper">
        <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 additional Bintools
       Wrappers, properly disambiguating them.
      </para>
      <para>
       A problem with this final task is that the 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>
       The 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 the CC Wrapper.
       Packages typically depend on the CC Wrapper, which in turn (at run-time)
       depends on the Bintools Wrapper.
      </para>
      <para>
       Dependency finding is undoubtedly the main task of the CC Wrapper. This
       works just like the 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>
       Similarly, the CC Wrapper follows the Bintools Wrapper in defining
       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>
   </variablelist>
  </para>

  <para>
   Here are some more packages that provide a setup hook. Since the list of
   hooks is extensible, this is not an exhaustive list the mechanism is only to
   be used as a last resort, it might cover most uses.
   <variablelist>
    <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. Most autotools-based packages come with the
       configure script pre-generated, but this hook is necessary for a few
       packages and when you need to patch the package’s configure scripts.
      </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 to
       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>
      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 based on the given
       <varname>buildInputs</varname> and <varname>nativeBuildInputs</varname>.
      </para>
      <para>
       You can also specify a <envar>runtimeDependencies</envar> environment
       variable which lists dependencies that are unconditionally added to all
       executables.
      </para>
      <para>
       This is useful for programs that use <citerefentry>
       <refentrytitle>dlopen</refentrytitle>
       <manvolnum>3</manvolnum> </citerefentry> to load libraries at runtime.
      </para>
      <para>
       In certain situations you may want to run the main command
       (<command>autoPatchelf</command>) of the setup hook on a file or a set
       of directories instead of unconditionally patching all outputs. This can
       be done by setting the <envar>dontAutoPatchelf</envar> environment
       variable to a non-empty value.
      </para>
      <para>
       The <command>autoPatchelf</command> command also recognizes a
       <parameter class="command">--no-recurse</parameter> command line flag,
       which prevents it from recursing into subdirectories.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      breakpointHook
     </term>
     <listitem>
      <para>
       This hook will make a build pause instead of stopping when a failure
       happens. It prevents nix from cleaning up the build environment
       immediately and allows the user to attach to a build environment using
       the <command>cntr</command> command. Upon build error it will print
       instructions on how to use <command>cntr</command>, which can be used to
       enter the environment for debugging. Installing cntr and running the
       command will provide shell access to the build sandbox of failed build.
       At <filename>/var/lib/cntr</filename> the sandboxed filesystem is
       mounted. All commands and files of the system are still accessible
       within the shell. To execute commands from the sandbox use the cntr exec
       subcommand. <command>cntr</command> is only supported
       on Linux-based platforms. To use it first add <literal>cntr</literal> to
       your <literal>environment.systemPackages</literal> on NixOS or
       alternatively to the root user on non-NixOS systems. Then in the package
       that is supposed to be inspected, add <literal>breakpointHook</literal>
       to <literal>nativeBuildInputs</literal>.
<programlisting>
nativeBuildInputs = [ breakpointHook ];
</programlisting>
       When a build failure happens there will be an instruction printed that
       shows how to attach with <literal>cntr</literal> to the build sandbox.
      </para>
      <note>
       <title>Caution with remote builds</title>
       <para>
        This won't work with remote builds as the build environment is on
        a different machine and can't be accessed by <command>cntr</command>.
        Remote builds can be turned off by setting <literal>--option builders ''</literal>
        for <command>nix-build</command> or <literal>--builders ''</literal> for
        <command>nix build</command>.
       </para>
      </note>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      libiconv, libintl
     </term>
     <listitem>
      <para>
       A few libraries automatically add to <literal>NIX_LDFLAGS</literal>
       their library, making their symbols automatically available to the
       linker. This includes libiconv and libintl (gettext). This is done to
       provide compatibility between GNU Linux, where libiconv and libintl are
       bundled in, and other systems where that might not be the case.
       Sometimes, this behavior is not desired. To disable this behavior, set
       <literal>dontAddExtraLibs</literal>.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      cmake
     </term>
     <listitem>
      <para>
       Overrides the default configure phase to run the CMake command. By
       default, we use the Make generator of CMake. In addition, dependencies
       are added automatically to CMAKE_PREFIX_PATH so that packages are
       correctly detected by CMake. Some additional flags are passed in to give
       similar behavior to configure-based packages. You can disable this
       hook’s behavior by setting configurePhase to a custom value, or by
       setting dontUseCmakeConfigure. cmakeFlags controls flags passed only to
       CMake. By default, parallel building is enabled as CMake supports
       parallel building almost everywhere. When Ninja is also in use, CMake
       will detect that and use the ninja generator.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      xcbuildHook
     </term>
     <listitem>
      <para>
       Overrides the build and install phases to run the “xcbuild” command.
       This hook is needed when a project only comes with build files for the
       XCode build system. You can disable this behavior by setting buildPhase
       and configurePhase to a custom value. xcbuildFlags controls flags passed
       only to xcbuild.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      meson
     </term>
     <listitem>
      <para>
       Overrides the configure phase to run meson to generate Ninja files. You
       can disable this behavior by setting configurePhase to a custom value,
       or by setting dontUseMesonConfigure. To run these files, you should
       accompany meson with ninja. mesonFlags controls only the flags passed to
       meson. By default, parallel building is enabled as Meson supports
       parallel building almost everywhere.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      ninja
     </term>
     <listitem>
      <para>
       Overrides the build, install, and check phase to run ninja instead of
       make. You can disable this behavior with the dontUseNinjaBuild,
       dontUseNinjaInstall, and dontUseNinjaCheck, respectively. Parallel
       building is enabled by default in Ninja.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      unzip
     </term>
     <listitem>
      <para>
       This setup hook will allow you to unzip .zip files specified in $src.
       There are many similar packages like unrar, undmg, etc.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      wafHook
     </term>
     <listitem>
      <para>
       Overrides the configure, build, and install phases. This will run the
       "waf" script used by many projects. If wafPath (default ./waf) doesn’t
       exist, it will copy the version of waf available in Nixpkgs. wafFlags can
       be used to pass flags to the waf script.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      scons
     </term>
     <listitem>
      <para>
       Overrides the build, install, and check phases. This uses the scons
       build system as a replacement for make. scons does not provide a
       configure phase, so everything is managed at build and install time.
      </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>