<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id="chap-stdenv">
<title>The Standard Environment</title>
<para>
The standard build environment in the Nix Packages collection provides an
environment for building Unix packages that does a lot of common build tasks
automatically. In fact, for Unix packages that use the standard
<literal>./configure; make; make install</literal> build interface, you
don’t need to write a build script at all; the standard environment does
everything automatically. If <literal>stdenv</literal> doesn’t do what you
need automatically, you can easily customise or override the various build
phases.
</para>
<section xml:id="sec-using-stdenv">
<title>Using <literal>stdenv</literal></title>
<para>
To build a package with the standard environment, you use the function
<varname>stdenv.mkDerivation</varname>, instead of the primitive built-in
function <varname>derivation</varname>, e.g.
<programlisting>
stdenv.mkDerivation {
name = "libfoo-1.2.3";
src = fetchurl {
url = http://example.org/libfoo-1.2.3.tar.bz2;
sha256 = "0x2g1jqygyr5wiwg4ma1nd7w4ydpy82z9gkcv8vh2v8dn3y58v5m";
};
}</programlisting>
(<varname>stdenv</varname> needs to be in scope, so if you write this in a
separate Nix expression from <filename>pkgs/all-packages.nix</filename>, you
need to pass it as a function argument.) Specifying a
<varname>name</varname> and a <varname>src</varname> is the absolute minimum
you need to do. Many packages have dependencies that are not provided in the
standard environment. It’s usually sufficient to specify those
dependencies in the <varname>buildInputs</varname> attribute:
<programlisting>
stdenv.mkDerivation {
name = "libfoo-1.2.3";
...
buildInputs = [libbar perl ncurses];
}</programlisting>
This attribute ensures that the <filename>bin</filename> subdirectories of
these packages appear in the <envar>PATH</envar> environment variable during
the build, that their <filename>include</filename> subdirectories are
searched by the C compiler, and so on. (See
<xref linkend="ssec-setup-hooks"/> for details.)
</para>
<para>
Often it is necessary to override or modify some aspect of the build. To
make this easier, the standard environment breaks the package build into a
number of <emphasis>phases</emphasis>, all of which can be overridden or
modified individually: unpacking the sources, applying patches, configuring,
building, and installing. (There are some others; see
<xref linkend="sec-stdenv-phases"/>.) For instance, a package that doesn’t
supply a makefile but instead has to be compiled “manually” could be
handled like this:
<programlisting>
stdenv.mkDerivation {
name = "fnord-4.5";
...
buildPhase = ''
gcc foo.c -o foo
'';
installPhase = ''
mkdir -p $out/bin
cp foo $out/bin
'';
}</programlisting>
(Note the use of <literal>''</literal>-style string literals, which are very
convenient for large multi-line script fragments because they don’t need
escaping of <literal>"</literal> and <literal>\</literal>, and because
indentation is intelligently removed.)
</para>
<para>
There are many other attributes to customise the build. These are listed in
<xref linkend="ssec-stdenv-attributes"/>.
</para>
<para>
While the standard environment provides a generic builder, you can still
supply your own build script:
<programlisting>
stdenv.mkDerivation {
name = "libfoo-1.2.3";
...
builder = ./builder.sh;
}</programlisting>
where the builder can do anything it wants, but typically starts with
<programlisting>
source $stdenv/setup
</programlisting>
to let <literal>stdenv</literal> set up the environment (e.g., process the
<varname>buildInputs</varname>). If you want, you can still use
<literal>stdenv</literal>’s generic builder:
<programlisting>
source $stdenv/setup
buildPhase() {
echo "... this is my custom build phase ..."
gcc foo.c -o foo
}
installPhase() {
mkdir -p $out/bin
cp foo $out/bin
}
genericBuild
</programlisting>
</para>
</section>
<section xml:id="sec-tools-of-stdenv">
<title>Tools provided by <literal>stdenv</literal></title>
<para>
The standard environment provides the following packages:
<itemizedlist>
<listitem>
<para>
The GNU C Compiler, configured with C and C++ support.
</para>
</listitem>
<listitem>
<para>
GNU coreutils (contains a few dozen standard Unix commands).
</para>
</listitem>
<listitem>
<para>
GNU findutils (contains <command>find</command>).
</para>
</listitem>
<listitem>
<para>
GNU diffutils (contains <command>diff</command>, <command>cmp</command>).
</para>
</listitem>
<listitem>
<para>
GNU <command>sed</command>.
</para>
</listitem>
<listitem>
<para>
GNU <command>grep</command>.
</para>
</listitem>
<listitem>
<para>
GNU <command>awk</command>.
</para>
</listitem>
<listitem>
<para>
GNU <command>tar</command>.
</para>
</listitem>
<listitem>
<para>
<command>gzip</command>, <command>bzip2</command> and
<command>xz</command>.
</para>
</listitem>
<listitem>
<para>
GNU Make. It has been patched to provide <quote>nested</quote> output
that can be fed into the <command>nix-log2xml</command> command and
<command>log2html</command> stylesheet to create a structured, readable
output of the build steps performed by Make.
</para>
</listitem>
<listitem>
<para>
Bash. This is the shell used for all builders in the Nix Packages
collection. Not using <command>/bin/sh</command> removes a large source
of portability problems.
</para>
</listitem>
<listitem>
<para>
The <command>patch</command> command.
</para>
</listitem>
</itemizedlist>
</para>
<para>
On Linux, <literal>stdenv</literal> also includes the
<command>patchelf</command> utility.
</para>
</section>
<section xml:id="ssec-stdenv-dependencies">
<title>Specifying dependencies</title>
<para>
As described in the Nix manual, almost any <filename>*.drv</filename> store
path in a derivation's attribute set will induce a dependency on that
derivation. <varname>mkDerivation</varname>, however, takes a few attributes
intended to, between them, include all the dependencies of a package. This
is done both for structure and consistency, but also so that certain other
setup can take place. For example, certain dependencies need their bin
directories added to the <envar>PATH</envar>. That is built-in, but other
setup is done via a pluggable mechanism that works in conjunction with these
dependency attributes. See <xref linkend="ssec-setup-hooks"/> for details.
</para>
<para>
Dependencies can be broken down along three axes: their host and target
platforms relative to the new derivation's, and whether they are propagated.
The platform distinctions are motivated by cross compilation; see
<xref linkend="chap-cross"/> for exactly what each platform means.
<footnote>
<para>
The build platform is ignored because it is a mere implementation detail
of the package satisfying the dependency: As a general programming
principle, dependencies are always <emphasis>specified</emphasis> as
interfaces, not concrete implementation.
</para>
</footnote>
But even if one is not cross compiling, the platforms imply whether or not
the dependency is needed at run-time or build-time, a concept that makes
perfect sense outside of cross compilation. For now, the run-time/build-time
distinction is just a hint for mental clarity, but in the future it perhaps
could be enforced.
</para>
<para>
The extension of <envar>PATH</envar> with dependencies, alluded to above,
proceeds according to the relative platforms alone. The process is carried
out only for dependencies whose host platform matches the new derivation's
build platform–i.e. which run on the platform where the new derivation
will be built.
<footnote>
<para>
Currently, that means for native builds all dependencies are put on the
<envar>PATH</envar>. But in the future that may not be the case for sake
of matching cross: the platforms would be assumed to be unique for native
and cross builds alike, so only the <varname>depsBuild*</varname> and
<varname>nativeBuildDependencies</varname> dependencies would affect the
<envar>PATH</envar>.
</para>
</footnote>
For each dependency <replaceable>dep</replaceable> of those dependencies,
<filename><replaceable>dep</replaceable>/bin</filename>, if present, is
added to the <envar>PATH</envar> environment variable.
</para>
<para>
The dependency is propagated when it forces some of its other-transitive
(non-immediate) downstream dependencies to also take it on as an immediate
dependency. Nix itself already takes a package's transitive dependencies
into account, but this propagation ensures nixpkgs-specific infrastructure
like setup hooks (mentioned above) also are run as if the propagated
dependency.
</para>
<para>
It is important to note dependencies are not necessary propagated as the
same sort of dependency that they were before, but rather as the
corresponding sort so that the platform rules still line up. The exact rules
for dependency propagation can be given by assigning each sort of dependency
two integers based one how it's host and target platforms are offset from
the depending derivation's platforms. Those offsets are given are given
below in the descriptions of each dependency list attribute.
Algorithmically, we traverse propagated inputs, accumulating every
propagated dep's propagated deps and adjusting them to account for the
"shift in perspective" described by the current dep's platform offsets. This
results in sort a transitive closure of the dependency relation, with the
offsets being approximately summed when two dependency links are combined.
We also prune transitive deps whose combined offsets go out-of-bounds, which
can be viewed as a filter over that transitive closure removing dependencies
that are blatantly absurd.
</para>
<para>
We can define the process precisely with
<link xlink:href="https://en.wikipedia.org/wiki/Natural_deduction">Natural
Deduction</link> using the inference rules. This probably seems a bit
obtuse, but so is the bash code that actually implements it!
<footnote>
<para>
The <function>findInputs</function> function, currently residing in
<filename>pkgs/stdenv/generic/setup.sh</filename>, implements the
propagation logic.
</para>
</footnote>
They're confusing in very different ways so...hopefully if something doesn't
make sense in one presentation, it does in the other!
<programlisting>
let mapOffset(h, t, i) = i + (if i <= 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 <= 0 then h else t - 1)
dep(h0, _, A, B)
propagated-dep(h1, t1, B, C)
h0 + h1 in {-1, 0, 1}
h0 + t1 in {-1, 0, -1}
-------------------------------------- Take immediate deps' propagated deps
propagated-dep(mapOffset(h0, t0, h1),
mapOffset(h0, t0, t1),
A, C)</programlisting>
<programlisting>
propagated-dep(h, t, A, B)
-------------------------------------- Propagated deps count as deps
dep(h, t, A, B)</programlisting>
Some explanation of this monstrosity is in order. In the common case, the
target offset of a dependency is the successor to the target offset:
<literal>t = h + 1</literal>. That means that:
<programlisting>
let f(h, t, i) = i + (if i <= 0 then h else t - 1)
let f(h, h + 1, i) = i + (if i <= 0 then h else (h + 1) - 1)
let f(h, h + 1, i) = i + (if i <= 0 then h else h)
let f(h, h + 1, i) = i + h
</programlisting>
This is where the "sum-like" comes from above: We can just sum all the host
offset to get the host offset of the transitive dependency. The target
offset is the transitive dep is simply the host offset + 1, just as it was
with the dependencies composed to make this transitive one; it can be
ignored as it doesn't add any new information.
</para>
<para>
Because of the bounds checks, the uncommon cases are <literal>h =
t</literal> and <literal>h + 2 = t</literal>. In the former case, the
motivation for <function>mapOffset</function> is that since its host and
target platforms are the same, no transitive dep of it should be able to
"discover" an offset greater than its reduced target offsets.
<function>mapOffset</function> effectively "squashes" all its transitive
dependencies' offsets so that none will ever be greater than the target
offset of the original <literal>h = t</literal> package. In the other case,
<literal>h + 1</literal> is skipped over between the host and target
offsets. Instead of squashing the offsets, we need to "rip" them apart so no
transitive dependencies' offset is that one.
</para>
<para>
Overall, the unifying theme here is that propagation shouldn't be
introducing transitive dependencies involving platforms the needing package
is unaware of. The offset bounds checking and definition of
<function>mapOffset</function> together ensure that this is the case.
Discovering a new offset is discovering a new platform, and since those
platforms weren't in the derivation "spec" of the needing package, they
cannot be relevant. From a capability perspective, we can imagine that the
host and target platforms of a package are the capabilities a package
requires, and the depending package must provide the capability to the
dependency.
</para>
<variablelist>
<title>Variables specifying dependencies</title>
<varlistentry>
<term>
<varname>depsBuildBuild</varname>
</term>
<listitem>
<para>
A list of dependencies whose host and target platforms are the new
derivation's build platform. This means a <literal>-1</literal> host and
<literal>-1</literal> target offset from the new derivation's platforms.
They are programs/libraries used at build time that furthermore produce
programs/libraries also used at build time. If the dependency doesn't
care about the target platform (i.e. isn't a compiler or similar tool),
put it in <varname>nativeBuildInputs</varname>instead. The most common
use for this <literal>buildPackages.stdenv.cc</literal>, the default C
compiler for this role. That example crops up more than one might think
in old commonly used C libraries.
</para>
<para>
Since these packages are able to be run at build time, that are always
added to the <envar>PATH</envar>, as described above. But since these
packages are only guaranteed to be able to run then, they shouldn't
persist as run-time dependencies. This isn't currently enforced, but
could be in the future.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>nativeBuildInputs</varname>
</term>
<listitem>
<para>
A list of dependencies whose host platform is the new derivation's build
platform, and target platform is the new derivation's host platform. This
means a <literal>-1</literal> host offset and <literal>0</literal> target
offset from the new derivation's platforms. They are programs/libraries
used at build time that, if they are a compiler or similar tool, produce
code to run at run time—i.e. tools used to build the new derivation. If
the dependency doesn't care about the target platform (i.e. isn't a
compiler or similar tool), put it here, rather than in
<varname>depsBuildBuild</varname> or <varname>depsBuildTarget</varname>.
This would be called <varname>depsBuildHost</varname> but for historical
continuity.
</para>
<para>
Since these packages are able to be run at build time, that are added to
the <envar>PATH</envar>, as described above. But since these packages
only are guaranteed to be able to run then, they shouldn't persist as
run-time dependencies. This isn't currently enforced, but could be in the
future.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>depsBuildTarget</varname>
</term>
<listitem>
<para>
A list of dependencies whose host platform is the new derivation's build
platform, and target platform is the new derivation's target platform.
This means a <literal>-1</literal> host offset and <literal>1</literal>
target offset from the new derivation's platforms. They are programs used
at build time that produce code to run at run with code produced by the
depending package. Most commonly, these would tools used to build the
runtime or standard library the currently-being-built compiler will
inject into any code it compiles. In many cases, the currently-being
built compiler is itself employed for that task, but when that compiler
won't run (i.e. its build and host platform differ) this is not possible.
Other times, the compiler relies on some other tool, like binutils, that
is always built separately so the dependency is unconditional.
</para>
<para>
This is a somewhat confusing dependency to wrap ones head around, and for
good reason. As the only one where the platform offsets are not adjacent
integers, it requires thinking of a bootstrapping stage
<emphasis>two</emphasis> away from the current one. It and it's use-case
go hand in hand and are both considered poor form: try not to need this
sort dependency, and try not avoid building standard libraries / runtimes
in the same derivation as the compiler produces code using them. Instead
strive to build those like a normal library, using the newly-built
compiler just as a normal library would. In short, do not use this
attribute unless you are packaging a compiler and are sure it is needed.
</para>
<para>
Since these packages are able to be run at build time, that are added to
the <envar>PATH</envar>, as described above. But since these packages
only are guaranteed to be able to run then, they shouldn't persist as
run-time dependencies. This isn't currently enforced, but could be in the
future.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>depsHostHost</varname>
</term>
<listitem>
<para>
A list of dependencies whose host and target platforms match the new
derivation's host platform. This means a both <literal>0</literal> host
offset and <literal>0</literal> target offset from the new derivation's
host platform. These are packages used at run-time to generate code also
used at run-time. In practice, that would usually be tools used by
compilers for metaprogramming/macro systems, or libraries used by the
macros/metaprogramming code itself. It's always preferable to use a
<varname>depsBuildBuild</varname> dependency in the derivation being
built than a <varname>depsHostHost</varname> on the tool doing the
building for this purpose.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>buildInputs</varname>
</term>
<listitem>
<para>
A list of dependencies whose host platform and target platform match the
new derivation's. This means a <literal>0</literal> host offset and
<literal>1</literal> target offset from the new derivation's host
platform. This would be called <varname>depsHostTarget</varname> but for
historical continuity. If the dependency doesn't care about the target
platform (i.e. isn't a compiler or similar tool), put it here, rather
than in <varname>depsBuildBuild</varname>.
</para>
<para>
These often are programs/libraries used by the new derivation at
<emphasis>run</emphasis>-time, but that isn't always the case. For
example, the machine code in a statically linked library is only used at
run time, but the derivation containing the library is only needed at
build time. Even in the dynamic case, the library may also be needed at
build time to appease the linker.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>depsTargetTarget</varname>
</term>
<listitem>
<para>
A list of dependencies whose host platform matches the new derivation's
target platform. This means a <literal>1</literal> offset from the new
derivation's platforms. These are packages that run on the target
platform, e.g. the standard library or run-time deps of standard library
that a compiler insists on knowing about. It's poor form in almost all
cases for a package to depend on another from a future stage [future
stage corresponding to positive offset]. Do not use this attribute unless
you are packaging a compiler and are sure it is needed.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>depsBuildBuildPropagated</varname>
</term>
<listitem>
<para>
The propagated equivalent of <varname>depsBuildBuild</varname>. This
perhaps never ought to be used, but it is included for consistency [see
below for the others].
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>propagatedNativeBuildInputs</varname>
</term>
<listitem>
<para>
The propagated equivalent of <varname>nativeBuildInputs</varname>. This
would be called <varname>depsBuildHostPropagated</varname> but for
historical continuity. For example, if package <varname>Y</varname> has
<literal>propagatedNativeBuildInputs = [X]</literal>, and package
<varname>Z</varname> has <literal>buildInputs = [Y]</literal>, then
package <varname>Z</varname> will be built as if it included package
<varname>X</varname> in its <varname>nativeBuildInputs</varname>. If
instead, package <varname>Z</varname> has <literal>nativeBuildInputs =
[Y]</literal>, then <varname>Z</varname> will be built as if it included
<varname>X</varname> in the <varname>depsBuildBuild</varname> of package
<varname>Z</varname>, because of the sum of the two <literal>-1</literal>
host offsets.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>depsBuildTargetPropagated</varname>
</term>
<listitem>
<para>
The propagated equivalent of <varname>depsBuildTarget</varname>. This is
prefixed for the same reason of alerting potential users.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>depsHostHostPropagated</varname>
</term>
<listitem>
<para>
The propagated equivalent of <varname>depsHostHost</varname>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>propagatedBuildInputs</varname>
</term>
<listitem>
<para>
The propagated equivalent of <varname>buildInputs</varname>. This would
be called <varname>depsHostTargetPropagated</varname> but for historical
continuity.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>depsTargetTarget</varname>
</term>
<listitem>
<para>
The propagated equivalent of <varname>depsTargetTarget</varname>. This is
prefixed for the same reason of alerting potential users.
</para>
</listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="ssec-stdenv-attributes">
<title>Attributes</title>
<variablelist>
<title>Variables affecting <literal>stdenv</literal> initialisation</title>
<varlistentry>
<term>
<varname>NIX_DEBUG</varname>
</term>
<listitem>
<para>
A natural number indicating how much information to log. If set to 1 or
higher, <literal>stdenv</literal> will print moderate debug information
during the build. In particular, the <command>gcc</command> and
<command>ld</command> wrapper scripts will print out the complete command
line passed to the wrapped tools. If set to 6 or higher, the
<literal>stdenv</literal> setup script will be run with <literal>set
-x</literal> tracing. If set to 7 or higher, the <command>gcc</command>
and <command>ld</command> wrapper scripts will also be run with
<literal>set -x</literal> tracing.
</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<title>Variables affecting build properties</title>
<varlistentry>
<term>
<varname>enableParallelBuilding</varname>
</term>
<listitem>
<para>
If set to <literal>true</literal>, <literal>stdenv</literal> will pass
specific flags to <literal>make</literal> and other build tools to enable
parallel building with up to <literal>build-cores</literal> workers.
</para>
<para>
Unless set to <literal>false</literal>, some build systems with good
support for parallel building including <literal>cmake</literal>,
<literal>meson</literal>, and <literal>qmake</literal> will set it to
<literal>true</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>preferLocalBuild</varname>
</term>
<listitem>
<para>
If set, specifies that the package is so lightweight in terms of build
operations (e.g. write a text file from a Nix string to the store) that
there's no need to look for it in binary caches -- it's faster to just
build it locally. It also tells Hydra and other facilities that this
package doesn't need to be exported in binary caches (noone would use it,
after all).
</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<title>Special variables</title>
<varlistentry>
<term>
<varname>passthru</varname>
</term>
<listitem>
<para>
This is an attribute set which can be filled with arbitrary values. For
example:
<programlisting>
passthru = {
foo = "bar";
baz = {
value1 = 4;
value2 = 5;
};
}
</programlisting>
</para>
<para>
Values inside it are not passed to the builder, so you can change them
without triggering a rebuild. However, they can be accessed outside of a
derivation directly, as if they were set inside a derivation itself, e.g.
<literal>hello.baz.value1</literal>. We don't specify any usage or schema
of <literal>passthru</literal> - it is meant for values that would be
useful outside the derivation in other parts of a Nix expression (e.g. in
other derivations). An example would be to convey some specific
dependency of your derivation which contains a program with plugins
support. Later, others who make derivations with plugins can use
passed-through dependency to ensure that their plugin would be
binary-compatible with built program.
</para>
</listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="sec-stdenv-phases">
<title>Phases</title>
<para>
The generic builder has a number of <emphasis>phases</emphasis>. Package
builds are split into phases to make it easier to override specific parts of
the build (e.g., unpacking the sources or installing the binaries).
Furthermore, it allows a nicer presentation of build logs in the Nix build
farm.
</para>
<para>
Each phase can be overridden in its entirety either by setting the
environment variable <varname><replaceable>name</replaceable>Phase</varname>
to a string containing some shell commands to be executed, or by redefining
the shell function <varname><replaceable>name</replaceable>Phase</varname>.
The former is convenient to override a phase from the derivation, while the
latter is convenient from a build script. However, typically one only wants
to <emphasis>add</emphasis> some commands to a phase, e.g. by defining
<literal>postInstall</literal> or <literal>preFixup</literal>, as skipping
some of the default actions may have unexpected consequences.
</para>
<section xml:id="ssec-controlling-phases">
<title>Controlling phases</title>
<para>
There are a number of variables that control what phases are executed and
in what order:
<variablelist>
<title>Variables affecting phase control</title>
<varlistentry>
<term>
<varname>phases</varname>
</term>
<listitem>
<para>
Specifies the phases. You can change the order in which phases are
executed, or add new phases, by setting this variable. If it’s not
set, the default value is used, which is <literal>$prePhases
unpackPhase patchPhase $preConfigurePhases configurePhase
$preBuildPhases buildPhase checkPhase $preInstallPhases installPhase
fixupPhase $preDistPhases distPhase $postPhases</literal>.
</para>
<para>
Usually, if you just want to add a few phases, it’s more convenient
to set one of the variables below (such as
<varname>preInstallPhases</varname>), as you then don’t specify all
the normal phases.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>prePhases</varname>
</term>
<listitem>
<para>
Additional phases executed before any of the default phases.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>preConfigurePhases</varname>
</term>
<listitem>
<para>
Additional phases executed just before the configure phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>preBuildPhases</varname>
</term>
<listitem>
<para>
Additional phases executed just before the build phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>preInstallPhases</varname>
</term>
<listitem>
<para>
Additional phases executed just before the install phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>preFixupPhases</varname>
</term>
<listitem>
<para>
Additional phases executed just before the fixup phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>preDistPhases</varname>
</term>
<listitem>
<para>
Additional phases executed just before the distribution phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>postPhases</varname>
</term>
<listitem>
<para>
Additional phases executed after any of the default phases.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</section>
<section xml:id="ssec-unpack-phase">
<title>The unpack phase</title>
<para>
The unpack phase is responsible for unpacking the source code of the
package. The default implementation of <function>unpackPhase</function>
unpacks the source files listed in the <envar>src</envar> environment
variable to the current directory. It supports the following files by
default:
<variablelist>
<varlistentry>
<term>
Tar files
</term>
<listitem>
<para>
These can optionally be compressed using <command>gzip</command>
(<filename>.tar.gz</filename>, <filename>.tgz</filename> or
<filename>.tar.Z</filename>), <command>bzip2</command>
(<filename>.tar.bz2</filename> or <filename>.tbz2</filename>) or
<command>xz</command> (<filename>.tar.xz</filename> or
<filename>.tar.lzma</filename>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
Zip files
</term>
<listitem>
<para>
Zip files are unpacked using <command>unzip</command>. However,
<command>unzip</command> is not in the standard environment, so you
should add it to <varname>buildInputs</varname> yourself.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
Directories in the Nix store
</term>
<listitem>
<para>
These are simply copied to the current directory. The hash part of the
file name is stripped, e.g.
<filename>/nix/store/1wydxgby13cz...-my-sources</filename> would be
copied to <filename>my-sources</filename>.
</para>
</listitem>
</varlistentry>
</variablelist>
Additional file types can be supported by setting the
<varname>unpackCmd</varname> variable (see below).
</para>
<para></para>
<variablelist>
<title>Variables controlling the unpack phase</title>
<varlistentry>
<term>
<varname>srcs</varname> / <varname>src</varname>
</term>
<listitem>
<para>
The list of source files or directories to be unpacked or copied. One of
these must be set.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>sourceRoot</varname>
</term>
<listitem>
<para>
After running <function>unpackPhase</function>, the generic builder
changes the current directory to the directory created by unpacking the
sources. If there are multiple source directories, you should set
<varname>sourceRoot</varname> to the name of the intended directory.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>setSourceRoot</varname>
</term>
<listitem>
<para>
Alternatively to setting <varname>sourceRoot</varname>, you can set
<varname>setSourceRoot</varname> to a shell command to be evaluated by
the unpack phase after the sources have been unpacked. This command must
set <varname>sourceRoot</varname>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>preUnpack</varname>
</term>
<listitem>
<para>
Hook executed at the start of the unpack phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>postUnpack</varname>
</term>
<listitem>
<para>
Hook executed at the end of the unpack phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>dontMakeSourcesWritable</varname>
</term>
<listitem>
<para>
If set to <literal>1</literal>, the unpacked sources are
<emphasis>not</emphasis> made writable. By default, they are made
writable to prevent problems with read-only sources. For example, copied
store directories would be read-only without this.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>unpackCmd</varname>
</term>
<listitem>
<para>
The unpack phase evaluates the string <literal>$unpackCmd</literal> for
any unrecognised file. The path to the current source file is contained
in the <varname>curSrc</varname> variable.
</para>
</listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="ssec-patch-phase">
<title>The patch phase</title>
<para>
The patch phase applies the list of patches defined in the
<varname>patches</varname> variable.
</para>
<variablelist>
<title>Variables controlling the patch phase</title>
<varlistentry>
<term>
<varname>patches</varname>
</term>
<listitem>
<para>
The list of patches. They must be in the format accepted by the
<command>patch</command> command, and may optionally be compressed using
<command>gzip</command> (<filename>.gz</filename>),
<command>bzip2</command> (<filename>.bz2</filename>) or
<command>xz</command> (<filename>.xz</filename>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>patchFlags</varname>
</term>
<listitem>
<para>
Flags to be passed to <command>patch</command>. If not set, the argument
<option>-p1</option> is used, which causes the leading directory
component to be stripped from the file names in each patch.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>prePatch</varname>
</term>
<listitem>
<para>
Hook executed at the start of the patch phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>postPatch</varname>
</term>
<listitem>
<para>
Hook executed at the end of the patch phase.
</para>
</listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="ssec-configure-phase">
<title>The configure phase</title>
<para>
The configure phase prepares the source tree for building. The default
<function>configurePhase</function> runs <filename>./configure</filename>
(typically an Autoconf-generated script) if it exists.
</para>
<variablelist>
<title>Variables controlling the configure phase</title>
<varlistentry>
<term>
<varname>configureScript</varname>
</term>
<listitem>
<para>
The name of the configure script. It defaults to
<filename>./configure</filename> if it exists; otherwise, the configure
phase is skipped. This can actually be a command (like <literal>perl
./Configure.pl</literal>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>configureFlags</varname>
</term>
<listitem>
<para>
A list of strings passed as additional arguments to the configure
script.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>configureFlagsArray</varname>
</term>
<listitem>
<para>
A shell array containing additional arguments passed to the configure
script. You must use this instead of <varname>configureFlags</varname>
if the arguments contain spaces.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>dontAddPrefix</varname>
</term>
<listitem>
<para>
By default, the flag <literal>--prefix=$prefix</literal> is added to the
configure flags. If this is undesirable, set this variable to true.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>prefix</varname>
</term>
<listitem>
<para>
The prefix under which the package must be installed, passed via the
<option>--prefix</option> option to the configure script. It defaults to
<option>$out</option>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>dontAddDisableDepTrack</varname>
</term>
<listitem>
<para>
By default, the flag <literal>--disable-dependency-tracking</literal> is
added to the configure flags to speed up Automake-based builds. If this
is undesirable, set this variable to true.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>dontFixLibtool</varname>
</term>
<listitem>
<para>
By default, the configure phase applies some special hackery to all
files called <filename>ltmain.sh</filename> before running the configure
script in order to improve the purity of Libtool-based packages
<footnote>
<para>
It clears the
<varname>sys_lib_<replaceable>*</replaceable>search_path</varname>
variables in the Libtool script to prevent Libtool from using
libraries in <filename>/usr/lib</filename> and such.
</para>
</footnote>
. If this is undesirable, set this variable to true.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>dontDisableStatic</varname>
</term>
<listitem>
<para>
By default, when the configure script has
<option>--enable-static</option>, the option
<option>--disable-static</option> is added to the configure flags.
</para>
<para>
If this is undesirable, set this variable to true.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>configurePlatforms</varname>
</term>
<listitem>
<para>
By default, when cross compiling, the configure script has
<option>--build=...</option> and <option>--host=...</option> passed.
Packages can instead pass <literal>[ "build" "host" "target" ]</literal>
or a subset to control exactly which platform flags are passed.
Compilers and other tools should use this to also pass the target
platform, for example.
<footnote>
<para>
Eventually these will be passed when in native builds too, to improve
determinism: build-time guessing, as is done today, is a risk of
impurity.
</para>
</footnote>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>preConfigure</varname>
</term>
<listitem>
<para>
Hook executed at the start of the configure phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>postConfigure</varname>
</term>
<listitem>
<para>
Hook executed at the end of the configure phase.
</para>
</listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="build-phase">
<title>The build phase</title>
<para>
The build phase is responsible for actually building the package (e.g.
compiling it). The default <function>buildPhase</function> simply calls
<command>make</command> if a file named <filename>Makefile</filename>,
<filename>makefile</filename> or <filename>GNUmakefile</filename> exists in
the current directory (or the <varname>makefile</varname> is explicitly
set); otherwise it does nothing.
</para>
<variablelist>
<title>Variables controlling the build phase</title>
<varlistentry>
<term>
<varname>dontBuild</varname>
</term>
<listitem>
<para>
Set to true to skip the build phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>makefile</varname>
</term>
<listitem>
<para>
The file name of the Makefile.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>checkInputs</varname>
</term>
<listitem>
<para>
A list of dependencies used by the phase. This gets included in
<varname>buildInputs</varname> when <varname>doCheck</varname> is set.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>makeFlags</varname>
</term>
<listitem>
<para>
A list of strings passed as additional flags to <command>make</command>.
These flags are also used by the default install and check phase. For
setting make flags specific to the build phase, use
<varname>buildFlags</varname> (see below).
<programlisting>
makeFlags = [ "PREFIX=$(out)" ];
</programlisting>
<note>
<para>
The flags are quoted in bash, but environment variables can be
specified by using the make syntax.
</para>
</note>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>makeFlagsArray</varname>
</term>
<listitem>
<para>
A shell array containing additional arguments passed to
<command>make</command>. You must use this instead of
<varname>makeFlags</varname> if the arguments contain spaces, e.g.
<programlisting>
makeFlagsArray=(CFLAGS="-O0 -g" LDFLAGS="-lfoo -lbar")
</programlisting>
Note that shell arrays cannot be passed through environment variables,
so you cannot set <varname>makeFlagsArray</varname> in a derivation
attribute (because those are passed through environment variables): you
have to define them in shell code.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>buildFlags</varname> / <varname>buildFlagsArray</varname>
</term>
<listitem>
<para>
A list of strings passed as additional flags to <command>make</command>.
Like <varname>makeFlags</varname> and <varname>makeFlagsArray</varname>,
but only used by the build phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>preBuild</varname>
</term>
<listitem>
<para>
Hook executed at the start of the build phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>postBuild</varname>
</term>
<listitem>
<para>
Hook executed at the end of the build phase.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
You can set flags for <command>make</command> through the
<varname>makeFlags</varname> variable.
</para>
<para>
Before and after running <command>make</command>, the hooks
<varname>preBuild</varname> and <varname>postBuild</varname> are called,
respectively.
</para>
</section>
<section xml:id="ssec-check-phase">
<title>The check phase</title>
<para>
The check phase checks whether the package was built correctly by running
its test suite. The default <function>checkPhase</function> calls
<command>make check</command>, but only if the <varname>doCheck</varname>
variable is enabled.
</para>
<variablelist>
<title>Variables controlling the check phase</title>
<varlistentry>
<term>
<varname>doCheck</varname>
</term>
<listitem>
<para>
Controls whether the check phase is executed. By default it is skipped,
but if <varname>doCheck</varname> is set to true, the check phase is
usually executed. Thus you should set
<programlisting>doCheck = true;</programlisting>
in the derivation to enable checks. The exception is cross compilation.
Cross compiled builds never run tests, no matter how
<varname>doCheck</varname> is set, as the newly-built program won't run
on the platform used to build it.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>makeFlags</varname> / <varname>makeFlagsArray</varname> / <varname>makefile</varname>
</term>
<listitem>
<para>
See the build phase for details.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>checkTarget</varname>
</term>
<listitem>
<para>
The make target that runs the tests. Defaults to
<literal>check</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>checkFlags</varname> / <varname>checkFlagsArray</varname>
</term>
<listitem>
<para>
A list of strings passed as additional flags to <command>make</command>.
Like <varname>makeFlags</varname> and <varname>makeFlagsArray</varname>,
but only used by the check phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>preCheck</varname>
</term>
<listitem>
<para>
Hook executed at the start of the check phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>postCheck</varname>
</term>
<listitem>
<para>
Hook executed at the end of the check phase.
</para>
</listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="ssec-install-phase">
<title>The install phase</title>
<para>
The install phase is responsible for installing the package in the Nix
store under <envar>out</envar>. The default
<function>installPhase</function> creates the directory
<literal>$out</literal> and calls <command>make install</command>.
</para>
<variablelist>
<title>Variables controlling the install phase</title>
<varlistentry>
<term>
<varname>makeFlags</varname> / <varname>makeFlagsArray</varname> / <varname>makefile</varname>
</term>
<listitem>
<para>
See the build phase for details.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>installTargets</varname>
</term>
<listitem>
<para>
The make targets that perform the installation. Defaults to
<literal>install</literal>. Example:
<programlisting>
installTargets = "install-bin install-doc";</programlisting>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>installFlags</varname> / <varname>installFlagsArray</varname>
</term>
<listitem>
<para>
A list of strings passed as additional flags to <command>make</command>.
Like <varname>makeFlags</varname> and <varname>makeFlagsArray</varname>,
but only used by the install phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>preInstall</varname>
</term>
<listitem>
<para>
Hook executed at the start of the install phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>postInstall</varname>
</term>
<listitem>
<para>
Hook executed at the end of the install phase.
</para>
</listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="ssec-fixup-phase">
<title>The fixup phase</title>
<para>
The fixup phase performs some (Nix-specific) post-processing actions on the
files installed under <filename>$out</filename> by the install phase. The
default <function>fixupPhase</function> does the following:
<itemizedlist>
<listitem>
<para>
It moves the <filename>man/</filename>, <filename>doc/</filename> and
<filename>info/</filename> subdirectories of <envar>$out</envar> to
<filename>share/</filename>.
</para>
</listitem>
<listitem>
<para>
It strips libraries and executables of debug information.
</para>
</listitem>
<listitem>
<para>
On Linux, it applies the <command>patchelf</command> command to ELF
executables and libraries to remove unused directories from the
<literal>RPATH</literal> in order to prevent unnecessary runtime
dependencies.
</para>
</listitem>
<listitem>
<para>
It rewrites the interpreter paths of shell scripts to paths found in
<envar>PATH</envar>. E.g., <filename>/usr/bin/perl</filename> will be
rewritten to
<filename>/nix/store/<replaceable>some-perl</replaceable>/bin/perl</filename>
found in <envar>PATH</envar>.
</para>
</listitem>
</itemizedlist>
</para>
<variablelist>
<title>Variables controlling the fixup phase</title>
<varlistentry>
<term>
<varname>dontStrip</varname>
</term>
<listitem>
<para>
If set, libraries and executables are not stripped. By default, they
are.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>dontStripHost</varname>
</term>
<listitem>
<para>
Like <varname>dontStripHost</varname>, but only affects the
<command>strip</command> command targetting the package's host platform.
Useful when supporting cross compilation, but otherwise feel free to
ignore.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>dontStripTarget</varname>
</term>
<listitem>
<para>
Like <varname>dontStripHost</varname>, but only affects the
<command>strip</command> command targetting the packages' target
platform. Useful when supporting cross compilation, but otherwise feel
free to ignore.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>dontMoveSbin</varname>
</term>
<listitem>
<para>
If set, files in <filename>$out/sbin</filename> are not moved to
<filename>$out/bin</filename>. By default, they are.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>stripAllList</varname>
</term>
<listitem>
<para>
List of directories to search for libraries and executables from which
<emphasis>all</emphasis> symbols should be stripped. By default, it’s
empty. Stripping all symbols is risky, since it may remove not just
debug symbols but also ELF information necessary for normal execution.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>stripAllFlags</varname>
</term>
<listitem>
<para>
Flags passed to the <command>strip</command> command applied to the
files in the directories listed in <varname>stripAllList</varname>.
Defaults to <option>-s</option> (i.e. <option>--strip-all</option>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>stripDebugList</varname>
</term>
<listitem>
<para>
List of directories to search for libraries and executables from which
only debugging-related symbols should be stripped. It defaults to
<literal>lib bin sbin</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>stripDebugFlags</varname>
</term>
<listitem>
<para>
Flags passed to the <command>strip</command> command applied to the
files in the directories listed in <varname>stripDebugList</varname>.
Defaults to <option>-S</option> (i.e. <option>--strip-debug</option>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>dontPatchELF</varname>
</term>
<listitem>
<para>
If set, the <command>patchelf</command> command is not used to remove
unnecessary <literal>RPATH</literal> entries. Only applies to Linux.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>dontPatchShebangs</varname>
</term>
<listitem>
<para>
If set, scripts starting with <literal>#!</literal> do not have their
interpreter paths rewritten to paths in the Nix store.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>forceShare</varname>
</term>
<listitem>
<para>
The list of directories that must be moved from
<filename>$out</filename> to <filename>$out/share</filename>. Defaults
to <literal>man doc info</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>setupHook</varname>
</term>
<listitem>
<para>
A package can export a <link
linkend="ssec-setup-hooks">setup
hook</link> by setting this variable. The setup hook, if defined, is
copied to <filename>$out/nix-support/setup-hook</filename>. Environment
variables are then substituted in it using
<function
linkend="fun-substituteAll">substituteAll</function>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>preFixup</varname>
</term>
<listitem>
<para>
Hook executed at the start of the fixup phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>postFixup</varname>
</term>
<listitem>
<para>
Hook executed at the end of the fixup phase.
</para>
</listitem>
</varlistentry>
<varlistentry xml:id="stdenv-separateDebugInfo">
<term>
<varname>separateDebugInfo</varname>
</term>
<listitem>
<para>
If set to <literal>true</literal>, the standard environment will enable
debug information in C/C++ builds. After installation, the debug
information will be separated from the executables and stored in the
output named <literal>debug</literal>. (This output is enabled
automatically; you don’t need to set the <varname>outputs</varname>
attribute explicitly.) To be precise, the debug information is stored in
<filename><replaceable>debug</replaceable>/lib/debug/.build-id/<replaceable>XX</replaceable>/<replaceable>YYYY…</replaceable></filename>,
where <replaceable>XXYYYY…</replaceable> is the <replaceable>build
ID</replaceable> of the binary — a SHA-1 hash of the contents of the
binary. Debuggers like GDB use the build ID to look up the separated
debug information.
</para>
<para>
For example, with GDB, you can add
<programlisting>
set debug-file-directory ~/.nix-profile/lib/debug
</programlisting>
to <filename>~/.gdbinit</filename>. GDB will then be able to find debug
information installed via <literal>nix-env -i</literal>.
</para>
</listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="ssec-installCheck-phase">
<title>The installCheck phase</title>
<para>
The installCheck phase checks whether the package was installed correctly
by running its test suite against the installed directories. The default
<function>installCheck</function> calls <command>make
installcheck</command>.
</para>
<variablelist>
<title>Variables controlling the installCheck phase</title>
<varlistentry>
<term>
<varname>doInstallCheck</varname>
</term>
<listitem>
<para>
Controls whether the installCheck phase is executed. By default it is
skipped, but if <varname>doInstallCheck</varname> is set to true, the
installCheck phase is usually executed. Thus you should set
<programlisting>doInstallCheck = true;</programlisting>
in the derivation to enable install checks. The exception is cross
compilation. Cross compiled builds never run tests, no matter how
<varname>doInstallCheck</varname> is set, as the newly-built program
won't run on the platform used to build it.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>installCheckInputs</varname>
</term>
<listitem>
<para>
A list of dependencies used by the phase. This gets included in
<varname>buildInputs</varname> when <varname>doInstallCheck</varname> is
set.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>preInstallCheck</varname>
</term>
<listitem>
<para>
Hook executed at the start of the installCheck phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>postInstallCheck</varname>
</term>
<listitem>
<para>
Hook executed at the end of the installCheck phase.
</para>
</listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="ssec-distribution-phase">
<title>The distribution phase</title>
<para>
The distribution phase is intended to produce a source distribution of the
package. The default <function>distPhase</function> first calls
<command>make dist</command>, then it copies the resulting source tarballs
to <filename>$out/tarballs/</filename>. This phase is only executed if the
attribute <varname>doDist</varname> is set.
</para>
<variablelist>
<title>Variables controlling the distribution phase</title>
<varlistentry>
<term>
<varname>distTarget</varname>
</term>
<listitem>
<para>
The make target that produces the distribution. Defaults to
<literal>dist</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>distFlags</varname> / <varname>distFlagsArray</varname>
</term>
<listitem>
<para>
Additional flags passed to <command>make</command>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>tarballs</varname>
</term>
<listitem>
<para>
The names of the source distribution files to be copied to
<filename>$out/tarballs/</filename>. It can contain shell wildcards. The
default is <filename>*.tar.gz</filename>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>dontCopyDist</varname>
</term>
<listitem>
<para>
If set, no files are copied to <filename>$out/tarballs/</filename>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>preDist</varname>
</term>
<listitem>
<para>
Hook executed at the start of the distribution phase.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>postDist</varname>
</term>
<listitem>
<para>
Hook executed at the end of the distribution phase.
</para>
</listitem>
</varlistentry>
</variablelist>
</section>
</section>
<section xml:id="ssec-stdenv-functions">
<title>Shell functions</title>
<para>
The standard environment provides a number of useful functions.
</para>
<variablelist>
<varlistentry xml:id='fun-makeWrapper'>
<term>
<function>makeWrapper</function> <replaceable>executable</replaceable> <replaceable>wrapperfile</replaceable> <replaceable>args</replaceable>
</term>
<listitem>
<para>
Constructs a wrapper for a program with various possible arguments. For
example:
<programlisting>
# adds `FOOBAR=baz` to `$out/bin/foo`’s environment
makeWrapper $out/bin/foo $wrapperfile --set FOOBAR baz
# prefixes the binary paths of `hello` and `git`
# Be advised that paths often should be patched in directly
# (via string replacements or in `configurePhase`).
makeWrapper $out/bin/foo $wrapperfile --prefix PATH : ${lib.makeBinPath [ hello git ]}
</programlisting>
There’s many more kinds of arguments, they are documented in
<literal>nixpkgs/pkgs/build-support/setup-hooks/make-wrapper.sh</literal>.
</para>
<para>
<literal>wrapProgram</literal> is a convenience function you probably
want to use most of the time.
</para>
</listitem>
</varlistentry>
<varlistentry xml:id='fun-substitute'>
<term>
<function>substitute</function> <replaceable>infile</replaceable> <replaceable>outfile</replaceable> <replaceable>subs</replaceable>
</term>
<listitem>
<para>
Performs string substitution on the contents of
<replaceable>infile</replaceable>, writing the result to
<replaceable>outfile</replaceable>. The substitutions in
<replaceable>subs</replaceable> are of the following form:
<variablelist>
<varlistentry>
<term>
<option>--replace</option> <replaceable>s1</replaceable> <replaceable>s2</replaceable>
</term>
<listitem>
<para>
Replace every occurrence of the string <replaceable>s1</replaceable>
by <replaceable>s2</replaceable>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--subst-var</option> <replaceable>varName</replaceable>
</term>
<listitem>
<para>
Replace every occurrence of
<literal>@<replaceable>varName</replaceable>@</literal> by the
contents of the environment variable
<replaceable>varName</replaceable>. This is useful for generating
files from templates, using
<literal>@<replaceable>...</replaceable>@</literal> in the template
as placeholders.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--subst-var-by</option> <replaceable>varName</replaceable> <replaceable>s</replaceable>
</term>
<listitem>
<para>
Replace every occurrence of
<literal>@<replaceable>varName</replaceable>@</literal> by the string
<replaceable>s</replaceable>.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
Example:
<programlisting>
substitute ./foo.in ./foo.out \
--replace /usr/bin/bar $bar/bin/bar \
--replace "a string containing spaces" "some other text" \
--subst-var someVar
</programlisting>
</para>
<para>
<function>substitute</function> is implemented using the
<command
xlink:href="http://replace.richardlloyd.org.uk/">replace</command>
command. Unlike with the <command>sed</command> command, you don’t have
to worry about escaping special characters. It supports performing
substitutions on binary files (such as executables), though there
you’ll probably want to make sure that the replacement string is as
long as the replaced string.
</para>
</listitem>
</varlistentry>
<varlistentry xml:id='fun-substituteInPlace'>
<term>
<function>substituteInPlace</function> <replaceable>file</replaceable> <replaceable>subs</replaceable>
</term>
<listitem>
<para>
Like <function>substitute</function>, but performs the substitutions in
place on the file <replaceable>file</replaceable>.
</para>
</listitem>
</varlistentry>
<varlistentry xml:id='fun-substituteAll'>
<term>
<function>substituteAll</function> <replaceable>infile</replaceable> <replaceable>outfile</replaceable>
</term>
<listitem>
<para>
Replaces every occurrence of
<literal>@<replaceable>varName</replaceable>@</literal>, where
<replaceable>varName</replaceable> is any environment variable, in
<replaceable>infile</replaceable>, writing the result to
<replaceable>outfile</replaceable>. For instance, if
<replaceable>infile</replaceable> has the contents
<programlisting>
#! @bash@/bin/sh
PATH=@coreutils@/bin
echo @foo@
</programlisting>
and the environment contains
<literal>bash=/nix/store/bmwp0q28cf21...-bash-3.2-p39</literal> and
<literal>coreutils=/nix/store/68afga4khv0w...-coreutils-6.12</literal>,
but does not contain the variable <varname>foo</varname>, then the output
will be
<programlisting>
#! /nix/store/bmwp0q28cf21...-bash-3.2-p39/bin/sh
PATH=/nix/store/68afga4khv0w...-coreutils-6.12/bin
echo @foo@
</programlisting>
That is, no substitution is performed for undefined variables.
</para>
<para>
Environment variables that start with an uppercase letter or an
underscore are filtered out, to prevent global variables (like
<literal>HOME</literal>) or private variables (like
<literal>__ETC_PROFILE_DONE</literal>) from accidentally getting
substituted. The variables also have to be valid bash “names”, as
defined in the bash manpage (alphanumeric or <literal>_</literal>, must
not start with a number).
</para>
</listitem>
</varlistentry>
<varlistentry xml:id='fun-substituteAllInPlace'>
<term>
<function>substituteAllInPlace</function> <replaceable>file</replaceable>
</term>
<listitem>
<para>
Like <function>substituteAll</function>, but performs the substitutions
in place on the file <replaceable>file</replaceable>.
</para>
</listitem>
</varlistentry>
<varlistentry xml:id='fun-stripHash'>
<term>
<function>stripHash</function> <replaceable>path</replaceable>
</term>
<listitem>
<para>
Strips the directory and hash part of a store path, outputting the name
part to <literal>stdout</literal>. For example:
<programlisting>
# prints coreutils-8.24
stripHash "/nix/store/9s9r019176g7cvn2nvcw41gsp862y6b4-coreutils-8.24"
</programlisting>
If you wish to store the result in another variable, then the following
idiom may be useful:
<programlisting>
name="/nix/store/9s9r019176g7cvn2nvcw41gsp862y6b4-coreutils-8.24"
someVar=$(stripHash $name)
</programlisting>
</para>
</listitem>
</varlistentry>
<varlistentry xml:id='fun-wrapProgram'>
<term>
<function>wrapProgram</function> <replaceable>executable</replaceable> <replaceable>makeWrapperArgs</replaceable>
</term>
<listitem>
<para>
Convenience function for <literal>makeWrapper</literal> that
automatically creates a sane wrapper file It takes all the same arguments
as <literal>makeWrapper</literal>, except for <literal>--argv0</literal>.
</para>
<para>
It cannot be applied multiple times, since it will overwrite the wrapper
file.
</para>
</listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="ssec-setup-hooks">
<title>Package setup hooks</title>
<para>
Nix itself considers a build-time dependency merely something that should
previously be built and accessible at build time—packages themselves are
on their own to perform any additional setup. In most cases, that is fine,
and the downstream derivation can deal with it's own dependencies. But for a
few common tasks, that would result in almost every package doing the same
sort of setup work---depending not on the package itself, but entirely on
which dependencies were used.
</para>
<para>
In order to alleviate this burden, the <firstterm>setup
hook></firstterm>mechanism was written, where any package can include a
shell script that [by convention rather than enforcement by Nix], any
downstream reverse-dependency will source as part of its build process. That
allows the downstream dependency to merely specify its dependencies, and
lets those dependencies effectively initialize themselves. No boilerplate
mirroring the list of dependencies is needed.
</para>
<para>
The Setup hook mechanism is a bit of a sledgehammer though: a powerful
feature with a broad and indiscriminate area of effect. The combination of
its power and implicit use may be expedient, but isn't without costs. Nix
itself is unchanged, but the spirit of adding dependencies being effect-free
is violated even if the letter isn't. For example, if a derivation path is
mentioned more than once, Nix itself doesn't care and simply makes sure the
dependency derivation is already built just the same—depending is just
needing something to exist, and needing is idempotent. However, a dependency
specified twice will have its setup hook run twice, and that could easily
change the build environment (though a well-written setup hook will
therefore strive to be idempotent so this is in fact not observable). More
broadly, setup hooks are anti-modular in that multiple dependencies, whether
the same or different, should not interfere and yet their setup hooks may
well do so.
</para>
<para>
The most typical use of the setup hook is actually to add other hooks which
are then run (i.e. after all the setup hooks) on each dependency. For
example, the C compiler wrapper's setup hook feeds itself flags for each
dependency that contains relevant libaries and headers. This is done by
defining a bash function, and appending its name to one of
<envar>envBuildBuildHooks</envar>`, <envar>envBuildHostHooks</envar>`,
<envar>envBuildTargetHooks</envar>`, <envar>envHostHostHooks</envar>`,
<envar>envHostTargetHooks</envar>`, or <envar>envTargetTargetHooks</envar>`.
These 6 bash variables correspond to the 6 sorts of dependencies by platform
(there's 12 total but we ignore the propagated/non-propagated axis).
</para>
<para>
Packages adding a hook should not hard code a specific hook, but rather
choose a variable <emphasis>relative</emphasis> to how they are included.
Returning to the C compiler wrapper example, if it itself is an
<literal>n</literal> dependency, then it only wants to accumulate flags from
<literal>n + 1</literal> dependencies, as only those ones match the
compiler's target platform. The <envar>hostOffset</envar> variable is
defined with the current dependency's host offset
<envar>targetOffset</envar> with its target offset, before it's setup hook
is sourced. Additionally, since most environment hooks don't care about the
target platform, That means the setup hook can append to the right bash
array by doing something like
<programlisting language="bash">
addEnvHooks "$hostOffset" myBashFunction
</programlisting>
</para>
<para>
The <emphasis>existence</emphasis> of setups hooks has long been documented
and packages inside Nixpkgs are free to use these mechanism. Other packages,
however, should not rely on these mechanisms not changing between Nixpkgs
versions. Because of the existing issues with this system, there's little
benefit from mandating it be stable for any period of time.
</para>
<para>
Here are some packages that provide a setup hook. Since the mechanism is
modular, this probably isn't an exhaustive list. Then again, since the
mechanism is only to be used as a last resort, it might be.
<variablelist>
<varlistentry>
<term>
Bintools Wrapper
</term>
<listitem>
<para>
Bintools Wrapper wraps the binary utilities for a bunch of miscellaneous
purposes. These are GNU Binutils when targetting Linux, and a mix of
cctools and GNU binutils for Darwin. [The "Bintools" name is supposed to
be a compromise between "Binutils" and "cctools" not denoting any
specific implementation.] Specifically, the underlying bintools package,
and a C standard library (glibc or Darwin's libSystem, just for the
dynamic loader) are all fed in, and dependency finding, hardening (see
below), and purity checks for each are handled by Bintools Wrapper.
Packages typically depend on CC Wrapper, which in turn (at run time)
depends on Bintools Wrapper.
</para>
<para>
Bintools Wrapper was only just recently split off from CC Wrapper, so
the division of labor is still being worked out. For example, it
shouldn't care about about the C standard library, but just take a
derivation with the dynamic loader (which happens to be the glibc on
linux). Dependency finding however is a task both wrappers will continue
to need to share, and probably the most important to understand. It is
currently accomplished by collecting directories of host-platform
dependencies (i.e. <varname>buildInputs</varname> and
<varname>nativeBuildInputs</varname>) in environment variables. Bintools
Wrapper's setup hook causes any <filename>lib</filename> and
<filename>lib64</filename> subdirectories to be added to
<envar>NIX_LDFLAGS</envar>. Since CC Wrapper and Bintools Wrapper use
the same strategy, most of the Bintools Wrapper code is sparsely
commented and refers to CC Wrapper. But CC Wrapper's code, by contrast,
has quite lengthy comments. Bintools Wrapper merely cites those, rather
than repeating them, to avoid falling out of sync.
</para>
<para>
A final task of the setup hook is defining a number of standard
environment variables to tell build systems which executables full-fill
which purpose. They are defined to just be the base name of the tools,
under the assumption that Bintools Wrapper's binaries will be on the
path. Firstly, this helps poorly-written packages, e.g. ones that look
for just <command>gcc</command> when <envar>CC</envar> isn't defined yet
<command>clang</command> is to be used. Secondly, this helps packages
not get confused when cross-compiling, in which case multiple Bintools
Wrappers may simultaneously be in use.
<footnote>
<para>
Each wrapper targets a single platform, so if binaries for multiple
platforms are needed, the underlying binaries must be wrapped multiple
times. As this is a property of the wrapper itself, the multiple
wrappings are needed whether or not the same underlying binaries can
target multiple platforms.
</para>
</footnote>
<envar>BUILD_</envar>- and <envar>TARGET_</envar>-prefixed versions of
the normal environment variable are defined for the additional Bintools
Wrappers, properly disambiguating them.
</para>
<para>
A problem with this final task is that Bintools Wrapper is honest and
defines <envar>LD</envar> as <command>ld</command>. Most packages,
however, firstly use the C compiler for linking, secondly use
<envar>LD</envar> anyways, defining it as the C compiler, and thirdly,
only so define <envar>LD</envar> when it is undefined as a fallback.
This triple-threat means Bintools Wrapper will break those packages, as
LD is already defined as the actual linker which the package won't
override yet doesn't want to use. The workaround is to define, just for
the problematic package, <envar>LD</envar> as the C compiler. A good way
to do this would be <command>preConfigure = "LD=$CC"</command>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
CC Wrapper
</term>
<listitem>
<para>
CC Wrapper wraps a C toolchain for a bunch of miscellaneous purposes.
Specifically, a C compiler (GCC or Clang), wrapped binary tools, and a C
standard library (glibc or Darwin's libSystem, just for the dynamic
loader) are all fed in, and dependency finding, hardening (see below),
and purity checks for each are handled by CC Wrapper. Packages typically
depend on CC Wrapper, which in turn (at run time) depends on Bintools
Wrapper.
</para>
<para>
Dependency finding is undoubtedly the main task of CC Wrapper. This
works just like Bintools Wrapper, except that any
<filename>include</filename> subdirectory of any relevant dependency is
added to <envar>NIX_CFLAGS_COMPILE</envar>. The setup hook itself
contains some lengthy comments describing the exact convoluted mechanism
by which this is accomplished.
</para>
<para>
CC Wrapper also like Bintools Wrapper defines standard environment
variables with the names of the tools it wraps, for the same reasons
described above. Importantly, while it includes a <command>cc</command>
symlink to the c compiler for portability, the <envar>CC</envar> will be
defined using the compiler's "real name" (i.e. <command>gcc</command> or
<command>clang</command>). This helps lousy build systems that inspect
on the name of the compiler rather than run it.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
Perl
</term>
<listitem>
<para>
Adds the <filename>lib/site_perl</filename> subdirectory of each build
input to the <envar>PERL5LIB</envar> environment variable. For instance,
if <varname>buildInputs</varname> contains Perl, then the
<filename>lib/site_perl</filename> subdirectory of each input is added
to the <envar>PERL5LIB</envar> environment variable.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
Python
</term>
<listitem>
<para>
Adds the <filename>lib/${python.libPrefix}/site-packages</filename>
subdirectory of each build input to the <envar>PYTHONPATH</envar>
environment variable.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
pkg-config
</term>
<listitem>
<para>
Adds the <filename>lib/pkgconfig</filename> and
<filename>share/pkgconfig</filename> subdirectories of each build input
to the <envar>PKG_CONFIG_PATH</envar> environment variable.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
Automake
</term>
<listitem>
<para>
Adds the <filename>share/aclocal</filename> subdirectory of each build
input to the <envar>ACLOCAL_PATH</envar> environment variable.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
Autoconf
</term>
<listitem>
<para>
The <varname>autoreconfHook</varname> derivation adds
<varname>autoreconfPhase</varname>, which runs autoreconf, libtoolize
and automake, essentially preparing the configure script in
autotools-based builds.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
libxml2
</term>
<listitem>
<para>
Adds every file named <filename>catalog.xml</filename> found under the
<filename>xml/dtd</filename> and <filename>xml/xsl</filename>
subdirectories of each build input to the
<envar>XML_CATALOG_FILES</envar> environment variable.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
teTeX / TeX Live
</term>
<listitem>
<para>
Adds the <filename>share/texmf-nix</filename> subdirectory of each build
input to the <envar>TEXINPUTS</envar> environment variable.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
Qt 4
</term>
<listitem>
<para>
Sets the <envar>QTDIR</envar> environment variable to Qt’s path.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
gdk-pixbuf
</term>
<listitem>
<para>
Exports <envar>GDK_PIXBUF_MODULE_FILE</envar> environment variable the
the builder. Add librsvg package to <varname>buildInputs</varname> to
get svg support.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
GHC
</term>
<listitem>
<para>
Creates a temporary package database and registers every Haskell build
input in it (TODO: how?).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
GStreamer
</term>
<listitem>
<para>
Adds the GStreamer plugins subdirectory of each build input to the
<envar>GST_PLUGIN_SYSTEM_PATH_1_0</envar> or
<envar>GST_PLUGIN_SYSTEM_PATH</envar> environment variable.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
paxctl
</term>
<listitem>
<para>
Defines the <varname>paxmark</varname> helper for setting per-executable
PaX flags on Linux (where it is available by default; on all other
platforms, <varname>paxmark</varname> is a no-op). For example, to
disable secure memory protections on the executable
<replaceable>foo</replaceable>:
<programlisting>
postFixup = ''
paxmark m $out/bin/<replaceable>foo</replaceable>
'';
</programlisting>
The <literal>m</literal> flag is the most common flag and is typically
required for applications that employ JIT compilation or otherwise need
to execute code generated at run-time. Disabling PaX protections should
be considered a last resort: if possible, problematic features should be
disabled or patched to work with PaX.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
autoPatchelfHook
</term>
<listitem>
<para>
This is a special setup hook which helps in packaging proprietary
software in that it automatically tries to find missing shared library
dependencies of ELF files. All packages within the
<envar>runtimeDependencies</envar> environment variable are
unconditionally added to executables, which is useful for programs that
use <citerefentry>
<refentrytitle>dlopen</refentrytitle>
<manvolnum>3</manvolnum> </citerefentry> to load libraries at runtime.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</section>
<section xml:id="sec-purity-in-nixpkgs">
<title>Purity in Nixpkgs</title>
<para>
[measures taken to prevent dependencies on packages outside the store, and
what you can do to prevent them]
</para>
<para>
GCC doesn't search in locations such as <filename>/usr/include</filename>.
In fact, attempts to add such directories through the <option>-I</option>
flag are filtered out. Likewise, the linker (from GNU binutils) doesn't
search in standard locations such as <filename>/usr/lib</filename>. Programs
built on Linux are linked against a GNU C Library that likewise doesn't
search in the default system locations.
</para>
</section>
<section xml:id="sec-hardening-in-nixpkgs">
<title>Hardening in Nixpkgs</title>
<para>
There are flags available to harden packages at compile or link-time. These
can be toggled using the <varname>stdenv.mkDerivation</varname> parameters
<varname>hardeningDisable</varname> and <varname>hardeningEnable</varname>.
</para>
<para>
Both parameters take a list of flags as strings. The special
<varname>"all"</varname> flag can be passed to
<varname>hardeningDisable</varname> to turn off all hardening. These flags
can also be used as environment variables for testing or development
purposes.
</para>
<para>
The following flags are enabled by default and might require disabling with
<varname>hardeningDisable</varname> if the program to package is
incompatible.
</para>
<variablelist>
<varlistentry>
<term>
<varname>format</varname>
</term>
<listitem>
<para>
Adds the <option>-Wformat -Wformat-security
-Werror=format-security</option> compiler options. At present, this warns
about calls to <varname>printf</varname> and <varname>scanf</varname>
functions where the format string is not a string literal and there are
no format arguments, as in <literal>printf(foo);</literal>. This may be a
security hole if the format string came from untrusted input and contains
<literal>%n</literal>.
</para>
<para>
This needs to be turned off or fixed for errors similar to:
</para>
<programlisting>
/tmp/nix-build-zynaddsubfx-2.5.2.drv-0/zynaddsubfx-2.5.2/src/UI/guimain.cpp:571:28: error: format not a string literal and no format arguments [-Werror=format-security]
printf(help_message);
^
cc1plus: some warnings being treated as errors
</programlisting>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>stackprotector</varname>
</term>
<listitem>
<para>
Adds the <option>-fstack-protector-strong --param
ssp-buffer-size=4</option> compiler options. This adds safety checks
against stack overwrites rendering many potential code injection attacks
into aborting situations. In the best case this turns code injection
vulnerabilities into denial of service or into non-issues (depending on
the application).
</para>
<para>
This needs to be turned off or fixed for errors similar to:
</para>
<programlisting>
bin/blib.a(bios_console.o): In function `bios_handle_cup':
/tmp/nix-build-ipxe-20141124-5cbdc41.drv-0/ipxe-5cbdc41/src/arch/i386/firmware/pcbios/bios_console.c:86: undefined reference to `__stack_chk_fail'
</programlisting>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>fortify</varname>
</term>
<listitem>
<para>
Adds the <option>-O2 -D_FORTIFY_SOURCE=2</option> compiler options.
During code generation the compiler knows a great deal of information
about buffer sizes (where possible), and attempts to replace insecure
unlimited length buffer function calls with length-limited ones. This is
especially useful for old, crufty code. Additionally, format strings in
writable memory that contain '%n' are blocked. If an application depends
on such a format string, it will need to be worked around.
</para>
<para>
Additionally, some warnings are enabled which might trigger build
failures if compiler warnings are treated as errors in the package build.
In this case, set <option>NIX_CFLAGS_COMPILE</option> to
<option>-Wno-error=warning-type</option>.
</para>
<para>
This needs to be turned off or fixed for errors similar to:
</para>
<programlisting>
malloc.c:404:15: error: return type is an incomplete type
malloc.c:410:19: error: storage size of 'ms' isn't known
</programlisting>
<programlisting>
strdup.h:22:1: error: expected identifier or '(' before '__extension__'
</programlisting>
<programlisting>
strsep.c:65:23: error: register name not specified for 'delim'
</programlisting>
<programlisting>
installwatch.c:3751:5: error: conflicting types for '__open_2'
</programlisting>
<programlisting>
fcntl2.h:50:4: error: call to '__open_missing_mode' declared with attribute error: open with O_CREAT or O_TMPFILE in second argument needs 3 arguments
</programlisting>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>pic</varname>
</term>
<listitem>
<para>
Adds the <option>-fPIC</option> compiler options. This options adds
support for position independent code in shared libraries and thus making
ASLR possible.
</para>
<para>
Most notably, the Linux kernel, kernel modules and other code not running
in an operating system environment like boot loaders won't build with PIC
enabled. The compiler will is most cases complain that PIC is not
supported for a specific build.
</para>
<para>
This needs to be turned off or fixed for assembler errors similar to:
</para>
<programlisting>
ccbLfRgg.s: Assembler messages:
ccbLfRgg.s:33: Error: missing or invalid displacement expression `private_key_len@GOTOFF'
</programlisting>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>strictoverflow</varname>
</term>
<listitem>
<para>
Signed integer overflow is undefined behaviour according to the C
standard. If it happens, it is an error in the program as it should check
for overflow before it can happen, not afterwards. GCC provides built-in
functions to perform arithmetic with overflow checking, which are correct
and faster than any custom implementation. As a workaround, the option
<option>-fno-strict-overflow</option> makes gcc behave as if signed
integer overflows were defined.
</para>
<para>
This flag should not trigger any build or runtime errors.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>relro</varname>
</term>
<listitem>
<para>
Adds the <option>-z relro</option> linker option. During program load,
several ELF memory sections need to be written to by the linker, but can
be turned read-only before turning over control to the program. This
prevents some GOT (and .dtors) overwrite attacks, but at least the part
of the GOT used by the dynamic linker (.got.plt) is still vulnerable.
</para>
<para>
This flag can break dynamic shared object loading. For instance, the
module systems of Xorg and OpenCV are incompatible with this flag. In
almost all cases the <varname>bindnow</varname> flag must also be
disabled and incompatible programs typically fail with similar errors at
runtime.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>bindnow</varname>
</term>
<listitem>
<para>
Adds the <option>-z bindnow</option> linker option. During program load,
all dynamic symbols are resolved, allowing for the complete GOT to be
marked read-only (due to <varname>relro</varname>). This prevents GOT
overwrite attacks. For very large applications, this can incur some
performance loss during initial load while symbols are resolved, but this
shouldn't be an issue for daemons.
</para>
<para>
This flag can break dynamic shared object loading. For instance, the
module systems of Xorg and PHP are incompatible with this flag. Programs
incompatible with this flag often fail at runtime due to missing symbols,
like:
</para>
<programlisting>
intel_drv.so: undefined symbol: vgaHWFreeHWRec
</programlisting>
</listitem>
</varlistentry>
</variablelist>
<para>
The following flags are disabled by default and should be enabled with
<varname>hardeningEnable</varname> for packages that take untrusted input
like network services.
</para>
<variablelist>
<varlistentry>
<term>
<varname>pie</varname>
</term>
<listitem>
<para>
Adds the <option>-fPIE</option> compiler and <option>-pie</option> linker
options. Position Independent Executables are needed to take advantage of
Address Space Layout Randomization, supported by modern kernel versions.
While ASLR can already be enforced for data areas in the stack and heap
(brk and mmap), the code areas must be compiled as position-independent.
Shared libraries already do this with the <varname>pic</varname> flag, so
they gain ASLR automatically, but binary .text regions need to be build
with <varname>pie</varname> to gain ASLR. When this happens, ROP attacks
are much harder since there are no static locations to bounce off of
during a memory corruption attack.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
For more in-depth information on these hardening flags and hardening in
general, refer to the
<link xlink:href="https://wiki.debian.org/Hardening">Debian Wiki</link>,
<link xlink:href="https://wiki.ubuntu.com/Security/Features">Ubuntu
Wiki</link>,
<link xlink:href="https://wiki.gentoo.org/wiki/Project:Hardened">Gentoo
Wiki</link>, and the
<link xlink:href="https://wiki.archlinux.org/index.php/DeveloperWiki:Security">
Arch Wiki</link>.
</para>
</section>
</chapter>