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 ./configure; make; make install build interface, you don’t need to write a build script at all; the standard environment does everything automatically. If stdenv doesn’t do what you need automatically, you can easily customise or override the various build phases.

To build a package with the standard environment, you use the function stdenv.mkDerivation, instead of the primitive built-in function derivation, e.g. stdenv.mkDerivation { name = "libfoo-1.2.3"; src = fetchurl { url = http://example.org/libfoo-1.2.3.tar.bz2; sha256 = "0x2g1jqygyr5wiwg4ma1nd7w4ydpy82z9gkcv8vh2v8dn3y58v5m"; }; } (stdenv needs to be in scope, so if you write this in a separate Nix expression from pkgs/all-packages.nix, you need to pass it as a function argument.) Specifying a name and a src 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 buildInputs attribute: stdenv.mkDerivation { name = "libfoo-1.2.3"; ... buildInputs = [libbar perl ncurses]; } This attribute ensures that the bin subdirectories of these packages appear in the PATH environment variable during the build, that their include subdirectories are searched by the C compiler, and so on. (See for details.) 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 phases, all of which can be overridden or modified individually: unpacking the sources, applying patches, configuring, building, and installing. (There are some others; see .) For instance, a package that doesn’t supply a makefile but instead has to be compiled “manually” could be handled like this: stdenv.mkDerivation { name = "fnord-4.5"; ... buildPhase = '' gcc foo.c -o foo ''; installPhase = '' mkdir -p $out/bin cp foo $out/bin ''; } (Note the use of ''-style string literals, which are very convenient for large multi-line script fragments because they don’t need escaping of " and \, and because indentation is intelligently removed.) There are many other attributes to customise the build. These are listed in . While the standard environment provides a generic builder, you can still supply your own build script: stdenv.mkDerivation { name = "libfoo-1.2.3"; ... builder = ./builder.sh; } where the builder can do anything it wants, but typically starts with source $stdenv/setup to let stdenv set up the environment (e.g., process the buildInputs). If you want, you can still use stdenv’s generic builder: 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

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

NIX_DEBUG If set, stdenv will print some debug information during the build. In particular, the gcc and ld wrapper scripts will print out the complete command line passed to the wrapped tools. nativeBuildInputs A list of dependencies used by the new derivation at build-time. I.e. these dependencies should not make it into the package's runtime-closure, though this is currently not checked. For each dependency dir, the directory dir/bin, if it exists, is added to the PATH environment variable. Other environment variables are also set up via a pluggable mechanism. For instance, if buildInputs contains Perl, then the lib/site_perl subdirectory of each input is added to the PERL5LIB environment variable. See for details. buildInputs A list of dependencies used by the new derivation at run-time. Currently, the build-time environment is modified in the exact same way as with nativeBuildInputs. This is problematic in that when cross-compiling, foreign executables can clobber native ones on the PATH. Even more confusing is static-linking. A statically-linked library should be listed here because ultimately that generated machine code will be used at run-time, even though a derivation containing the object files or static archives will only be used at build-time. A less confusing solution to this would be nice. propagatedNativeBuildInputs Like nativeBuildInputs, but these dependencies are propagated: that is, the dependencies listed here are added to the nativeBuildInputs of any package that uses this package as a dependency. So if package Y has propagatedBuildInputs = [X], and package Z has buildInputs = [Y], then package X will appear in Z’s build environment automatically. propagatedBuildInputs Like buildInputs, but propagated just like propagatedNativeBuildInputs. This inherits buildInputs's flaws of clobbering native executables when cross-compiling and being confusing for static linking. enableParallelBuilding If set, stdenv will pass specific flags to make and other build tools to enable parallel building with up to build-cores workers. preferLocalBuild 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). passthru This is an attribute set which can be filled with arbitrary values. For example: passthru = { foo = "bar"; baz = { value1 = 4; value2 = 5; }; } 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. hello.baz.value1. We don't specify any usage or schema of passthru - 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.

The generic builder has a number of phases. 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. Each phase can be overridden in its entirety either by setting the environment variable namePhase to a string containing some shell commands to be executed, or by redefining the shell function namePhase. The former is convenient to override a phase from the derivation, while the latter is convenient from a build script.

There are a number of variables that control what phases are executed and in what order: phases 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 $prePhases unpackPhase patchPhase $preConfigurePhases configurePhase $preBuildPhases buildPhase checkPhase $preInstallPhases installPhase fixupPhase $preDistPhases distPhase $postPhases. Usually, if you just want to add a few phases, it’s more convenient to set one of the variables below (such as preInstallPhases), as you then don’t specify all the normal phases. prePhases Additional phases executed before any of the default phases. preConfigurePhases Additional phases executed just before the configure phase. preBuildPhases Additional phases executed just before the build phase. preInstallPhases Additional phases executed just before the install phase. preFixupPhases Additional phases executed just before the fixup phase. preDistPhases Additional phases executed just before the distribution phase. postPhases Additional phases executed after any of the default phases.

The unpack phase is responsible for unpacking the source code of the package. The default implementation of unpackPhase unpacks the source files listed in the src environment variable to the current directory. It supports the following files by default: Tar files These can optionally be compressed using gzip (.tar.gz, .tgz or .tar.Z), bzip2 (.tar.bz2 or .tbz2) or xz (.tar.xz or .tar.lzma). Zip files Zip files are unpacked using unzip. However, unzip is not in the standard environment, so you should add it to buildInputs yourself. Directories in the Nix store These are simply copied to the current directory. The hash part of the file name is stripped, e.g. /nix/store/1wydxgby13cz...-my-sources would be copied to my-sources. Additional file types can be supported by setting the unpackCmd variable (see below). srcs / src The list of source files or directories to be unpacked or copied. One of these must be set. sourceRoot After running unpackPhase, the generic builder changes the current directory to the directory created by unpacking the sources. If there are multiple source directories, you should set sourceRoot to the name of the intended directory. setSourceRoot Alternatively to setting sourceRoot, you can set setSourceRoot to a shell command to be evaluated by the unpack phase after the sources have been unpacked. This command must set sourceRoot. preUnpack Hook executed at the start of the unpack phase. postUnpack Hook executed at the end of the unpack phase. dontMakeSourcesWritable If set to 1, the unpacked sources are not 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. unpackCmd The unpack phase evaluates the string $unpackCmd for any unrecognised file. The path to the current source file is contained in the curSrc variable.

The patch phase applies the list of patches defined in the patches variable. patches The list of patches. They must be in the format accepted by the patch command, and may optionally be compressed using gzip (.gz), bzip2 (.bz2) or xz (.xz). patchFlags Flags to be passed to patch. If not set, the argument -p1 is used, which causes the leading directory component to be stripped from the file names in each patch. prePatch Hook executed at the start of the patch phase. postPatch Hook executed at the end of the patch phase.

The configure phase prepares the source tree for building. The default configurePhase runs ./configure (typically an Autoconf-generated script) if it exists. configureScript The name of the configure script. It defaults to ./configure if it exists; otherwise, the configure phase is skipped. This can actually be a command (like perl ./Configure.pl). configureFlags A list of strings passed as additional arguments to the configure script. configureFlagsArray A shell array containing additional arguments passed to the configure script. You must use this instead of configureFlags if the arguments contain spaces. dontAddPrefix By default, the flag --prefix=$prefix is added to the configure flags. If this is undesirable, set this variable to true. prefix The prefix under which the package must be installed, passed via the --prefix option to the configure script. It defaults to $out. dontAddDisableDepTrack By default, the flag --disable-dependency-tracking is added to the configure flags to speed up Automake-based builds. If this is undesirable, set this variable to true. dontFixLibtool By default, the configure phase applies some special hackery to all files called ltmain.sh before running the configure script in order to improve the purity of Libtool-based packagesIt clears the sys_lib_*search_path variables in the Libtool script to prevent Libtool from using libraries in /usr/lib and such.. If this is undesirable, set this variable to true. dontDisableStatic By default, when the configure script has --enable-static, the option --disable-static is added to the configure flags. If this is undesirable, set this variable to true. preConfigure Hook executed at the start of the configure phase. postConfigure Hook executed at the end of the configure phase.

The build phase is responsible for actually building the package (e.g. compiling it). The default buildPhase simply calls make if a file named Makefile, makefile or GNUmakefile exists in the current directory (or the makefile is explicitly set); otherwise it does nothing. dontBuild Set to true to skip the build phase. makefile The file name of the Makefile. makeFlags A list of strings passed as additional flags to make. These flags are also used by the default install and check phase. For setting make flags specific to the build phase, use buildFlags (see below). makeFlagsArray A shell array containing additional arguments passed to make. You must use this instead of makeFlags if the arguments contain spaces, e.g. makeFlagsArray=(CFLAGS="-O0 -g" LDFLAGS="-lfoo -lbar") Note that shell arrays cannot be passed through environment variables, so you cannot set makeFlagsArray in a derivation attribute (because those are passed through environment variables): you have to define them in shell code. buildFlags / buildFlagsArray A list of strings passed as additional flags to make. Like makeFlags and makeFlagsArray, but only used by the build phase. preBuild Hook executed at the start of the build phase. postBuild Hook executed at the end of the build phase. You can set flags for make through the makeFlags variable. Before and after running make, the hooks preBuild and postBuild are called, respectively.

The check phase checks whether the package was built correctly by running its test suite. The default checkPhase calls make check, but only if the doCheck variable is enabled. doCheck If set to a non-empty string, the check phase is executed, otherwise it is skipped (default). Thus you should set doCheck = true; in the derivation to enable checks. makeFlags / makeFlagsArray / makefile See the build phase for details. checkTarget The make target that runs the tests. Defaults to check. checkFlags / checkFlagsArray A list of strings passed as additional flags to make. Like makeFlags and makeFlagsArray, but only used by the check phase. preCheck Hook executed at the start of the check phase. postCheck Hook executed at the end of the check phase.

The install phase is responsible for installing the package in the Nix store under out. The default installPhase creates the directory $out and calls make install. makeFlags / makeFlagsArray / makefile See the build phase for details. installTargets The make targets that perform the installation. Defaults to install. Example: installTargets = "install-bin install-doc"; installFlags / installFlagsArray A list of strings passed as additional flags to make. Like makeFlags and makeFlagsArray, but only used by the install phase. preInstall Hook executed at the start of the install phase. postInstall Hook executed at the end of the install phase.

The fixup phase performs some (Nix-specific) post-processing actions on the files installed under $out by the install phase. The default fixupPhase does the following: It moves the man/, doc/ and info/ subdirectories of $out to share/. It strips libraries and executables of debug information. On Linux, it applies the patchelf command to ELF executables and libraries to remove unused directories from the RPATH in order to prevent unnecessary runtime dependencies. It rewrites the interpreter paths of shell scripts to paths found in PATH. E.g., /usr/bin/perl will be rewritten to /nix/store/some-perl/bin/perl found in PATH. dontStrip If set, libraries and executables are not stripped. By default, they are. dontMoveSbin If set, files in $out/sbin are not moved to $out/bin. By default, they are. stripAllList List of directories to search for libraries and executables from which all 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. stripAllFlags Flags passed to the strip command applied to the files in the directories listed in stripAllList. Defaults to -s (i.e. --strip-all). stripDebugList List of directories to search for libraries and executables from which only debugging-related symbols should be stripped. It defaults to lib bin sbin. stripDebugFlags Flags passed to the strip command applied to the files in the directories listed in stripDebugList. Defaults to -S (i.e. --strip-debug). dontPatchELF If set, the patchelf command is not used to remove unnecessary RPATH entries. Only applies to Linux. dontPatchShebangs If set, scripts starting with #! do not have their interpreter paths rewritten to paths in the Nix store. forceShare The list of directories that must be moved from $out to $out/share. Defaults to man doc info. setupHook A package can export a setup hook by setting this variable. The setup hook, if defined, is copied to $out/nix-support/setup-hook. Environment variables are then substituted in it using substituteAll. preFixup Hook executed at the start of the fixup phase. postFixup Hook executed at the end of the fixup phase. separateDebugInfo If set to true, 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 debug. (This output is enabled automatically; you don’t need to set the outputs attribute explicitly.) To be precise, the debug information is stored in debug/lib/debug/.build-id/XX/YYYY…, where XXYYYY… is the build ID 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. For example, with GDB, you can add set debug-file-directory ~/.nix-profile/lib/debug to ~/.gdbinit. GDB will then be able to find debug information installed via nix-env -i.

The installCheck phase checks whether the package was installed correctly by running its test suite against the installed directories. The default installCheck calls make installcheck. doInstallCheck If set to a non-empty string, the installCheck phase is executed, otherwise it is skipped (default). Thus you should set doInstallCheck = true; in the derivation to enable install checks. preInstallCheck Hook executed at the start of the installCheck phase. postInstallCheck Hook executed at the end of the installCheck phase.

The distribution phase is intended to produce a source distribution of the package. The default distPhase first calls make dist, then it copies the resulting source tarballs to $out/tarballs/. This phase is only executed if the attribute doDist is set. distTarget The make target that produces the distribution. Defaults to dist. distFlags / distFlagsArray Additional flags passed to make. tarballs The names of the source distribution files to be copied to $out/tarballs/. It can contain shell wildcards. The default is *.tar.gz. dontCopyDist If set, no files are copied to $out/tarballs/. preDist Hook executed at the start of the distribution phase. postDist Hook executed at the end of the distribution phase.

The standard environment provides a number of useful functions. substitute infile outfile subs Performs string substitution on the contents of infile, writing the result to outfile. The substitutions in subs are of the following form: --replace s1 s2 Replace every occurence of the string s1 by s2. --subst-var varName Replace every occurence of @varName@ by the contents of the environment variable varName. This is useful for generating files from templates, using @...@ in the template as placeholders. --subst-var-by varName s Replace every occurence of @varName@ by the string s. Example: substitute ./foo.in ./foo.out \ --replace /usr/bin/bar $bar/bin/bar \ --replace "a string containing spaces" "some other text" \ --subst-var someVar substitute is implemented using the replace command. Unlike with the sed command, you don’t have to worry about escaping special characters. It supports performing substitutions on binary files (such as executables), though there you’ll probably want to make sure that the replacement string is as long as the replaced string. substituteInPlace file subs Like substitute, but performs the substitutions in place on the file file. substituteAll infile outfile Replaces every occurence of @varName@, where varName is any environment variable, in infile, writing the result to outfile. For instance, if infile has the contents #! @bash@/bin/sh PATH=@coreutils@/bin echo @foo@ and the environment contains bash=/nix/store/bmwp0q28cf21...-bash-3.2-p39 and coreutils=/nix/store/68afga4khv0w...-coreutils-6.12, but does not contain the variable foo, then the output will be #! /nix/store/bmwp0q28cf21...-bash-3.2-p39/bin/sh PATH=/nix/store/68afga4khv0w...-coreutils-6.12/bin echo @foo@ That is, no substitution is performed for undefined variables. Environment variables that start with an uppercase letter or an underscore are filtered out, to prevent global variables (like HOME) or private variables (like __ETC_PROFILE_DONE) from accidentally getting substituted. The variables also have to be valid bash “names”, as defined in the bash manpage (alphanumeric or _, must not start with a number). substituteAllInPlace file Like substituteAll, but performs the substitutions in place on the file file. stripHash path Strips the directory and hash part of a store path, outputting the name part to stdout. For example: # prints coreutils-8.24 stripHash "/nix/store/9s9r019176g7cvn2nvcw41gsp862y6b4-coreutils-8.24" If you wish to store the result in another variable, then the following idiom may be useful: name="/nix/store/9s9r019176g7cvn2nvcw41gsp862y6b4-coreutils-8.24" someVar=$(stripHash $name)

The following packages provide a setup hook: GCC wrapper Adds the include subdirectory of each build input to the NIX_CFLAGS_COMPILE environment variable, and the lib and lib64 subdirectories to NIX_LDFLAGS. Perl Adds the lib/site_perl subdirectory of each build input to the PERL5LIB environment variable. Python Adds the lib/${python.libPrefix}/site-packages subdirectory of each build input to the PYTHONPATH environment variable. pkg-config Adds the lib/pkgconfig and share/pkgconfig subdirectories of each build input to the PKG_CONFIG_PATH environment variable. Automake Adds the share/aclocal subdirectory of each build input to the ACLOCAL_PATH environment variable. Autoconf The autoreconfHook derivation adds autoreconfPhase, which runs autoreconf, libtoolize and automake, essentially preparing the configure script in autotools-based builds. libxml2 Adds every file named catalog.xml found under the xml/dtd and xml/xsl subdirectories of each build input to the XML_CATALOG_FILES environment variable. teTeX / TeX Live Adds the share/texmf-nix subdirectory of each build input to the TEXINPUTS environment variable. Qt 4 Sets the QTDIR environment variable to Qt’s path. gdk-pixbuf Exports GDK_PIXBUF_MODULE_FILE environment variable the the builder. Add librsvg package to buildInputs to get svg support. GHC Creates a temporary package database and registers every Haskell build input in it (TODO: how?). GStreamer Adds the GStreamer plugins subdirectory of each build input to the GST_PLUGIN_SYSTEM_PATH_1_0 or GST_PLUGIN_SYSTEM_PATH environment variable. paxctl Defines the paxmark helper for setting per-executable PaX flags on Linux (where it is available by default; on all other platforms, paxmark is a no-op). For example, to disable secure memory protections on the executable foo: postFixup = '' paxmark m $out/bin/foo ''; The m 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.

[measures taken to prevent dependencies on packages outside the store, and what you can do to prevent them] GCC doesn't search in locations such as /usr/include. In fact, attempts to add such directories through the -I flag are filtered out. Likewise, the linker (from GNU binutils) doesn't search in standard locations such as /usr/lib. Programs built on Linux are linked against a GNU C Library that likewise doesn't search in the default system locations.

There are flags available to harden packages at compile or link-time. These can be toggled using the stdenv.mkDerivation parameters hardeningDisable and hardeningEnable. Both parameters take a list of flags as strings. The special "all" flag can be passed to hardeningDisable to turn off all hardening. These flags can also be used as environment variables for testing or development purposes. The following flags are enabled by default and might require disabling with hardeningDisable if the program to package is incompatible. format Adds the -Wformat -Wformat-security -Werror=format-security compiler options. At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, as in printf(foo);. This may be a security hole if the format string came from untrusted input and contains %n. This needs to be turned off or fixed for errors similar to: /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 stackprotector Adds the -fstack-protector-strong --param ssp-buffer-size=4 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). This needs to be turned off or fixed for errors similar to: 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' fortify Adds the -O2 -D_FORTIFY_SOURCE=2 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. Addtionally, some warnings are enabled which might trigger build failures if compiler warnings are treated as errors in the package build. In this case, set NIX_CFLAGS_COMPILE to -Wno-error=warning-type. This needs to be turned off or fixed for errors similar to: malloc.c:404:15: error: return type is an incomplete type malloc.c:410:19: error: storage size of 'ms' isn't known strdup.h:22:1: error: expected identifier or '(' before '__extension__' strsep.c:65:23: error: register name not specified for 'delim' installwatch.c:3751:5: error: conflicting types for '__open_2' 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 pic Adds the -fPIC compiler options. This options adds support for position independant code in shared libraries and thus making ASLR possible. 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. This needs to be turned off or fixed for assembler errors similar to: ccbLfRgg.s: Assembler messages: ccbLfRgg.s:33: Error: missing or invalid displacement expression `private_key_len@GOTOFF' strictoverflow 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 -fno-strict-overflow makes gcc behave as if signed integer overflows were defined. This flag should not trigger any build or runtime errors. relro Adds the -z relro 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. 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 bindnow flag must also be disabled and incompatible programs typically fail with similar errors at runtime. bindnow Adds the -z bindnow linker option. During program load, all dynamic symbols are resolved, allowing for the complete GOT to be marked read-only (due to relro). 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. 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: intel_drv.so: undefined symbol: vgaHWFreeHWRec The following flags are disabled by default and should be enabled with hardeningEnable for packages that take untrusted input like network services. pie Adds the -fPIE compiler and -pie 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 pic flag, so they gain ASLR automatically, but binary .text regions need to be build with pie 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. For more in-depth information on these hardening flags and hardening in general, refer to the Debian Wiki, Ubuntu Wiki, Gentoo Wiki, and the Arch Wiki.