pyrox.dev
1# The Standard Environment {#chap-stdenv}
2
3The 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.
4
5## Using `stdenv` {#sec-using-stdenv}
6
7To build a package with the standard environment, you use the function `stdenv.mkDerivation`, instead of the primitive built-in function `derivation`, e.g.
8
9```nix
10stdenv.mkDerivation {
11 name = "libfoo-1.2.3";
12 src = fetchurl {
13 url = "http://example.org/libfoo-1.2.3.tar.bz2";
14 hash = "sha256-tWxU/LANbQE32my+9AXyt3nCT7NBVfJ45CX757EMT3Q=";
15 };
16}
17```
18
19(`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 Nix requires. For convenience, you can also use `pname` and `version` attributes and `mkDerivation` will automatically set `name` to `"${pname}-${version}"` by default.
20**Since [RFC 0035](https://github.com/NixOS/rfcs/pull/35), this is preferred for packages in Nixpkgs**, as it allows us to reuse the version easily:
21
22```nix
23stdenv.mkDerivation rec {
24 pname = "libfoo";
25 version = "1.2.3";
26 src = fetchurl {
27 url = "http://example.org/libfoo-source-${version}.tar.bz2";
28 hash = "sha256-tWxU/LANbQE32my+9AXyt3nCT7NBVfJ45CX757EMT3Q=";
29 };
30}
31```
32
33Many packages have dependencies that are not provided in the standard environment. It’s usually sufficient to specify those dependencies in the `buildInputs` attribute:
34
35```nix
36stdenv.mkDerivation {
37 pname = "libfoo";
38 version = "1.2.3";
39 ...
40 buildInputs = [libbar perl ncurses];
41}
42```
43
44This 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 [](#ssec-setup-hooks) for details.)
45
46Often 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 [](#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:
47
48```nix
49stdenv.mkDerivation {
50 pname = "fnord";
51 version = "4.5";
52 ...
53 buildPhase = ''
54 gcc foo.c -o foo
55 '';
56 installPhase = ''
57 mkdir -p $out/bin
58 cp foo $out/bin
59 '';
60}
61```
62
63(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.)
64
65There are many other attributes to customise the build. These are listed in [](#ssec-stdenv-attributes).
66
67While the standard environment provides a generic builder, you can still supply your own build script:
68
69```nix
70stdenv.mkDerivation {
71 pname = "libfoo";
72 version = "1.2.3";
73 ...
74 builder = ./builder.sh;
75}
76```
77
78where the builder can do anything it wants, but typically starts with
79
80```bash
81source $stdenv/setup
82```
83
84to let `stdenv` set up the environment (e.g. by resetting `PATH` and populating it from build inputs). If you want, you can still use `stdenv`’s generic builder:
85
86```bash
87source $stdenv/setup
88
89buildPhase() {
90 echo "... this is my custom build phase ..."
91 gcc foo.c -o foo
92}
93
94installPhase() {
95 mkdir -p $out/bin
96 cp foo $out/bin
97}
98
99genericBuild
100```
101
102### Building a `stdenv` package in `nix-shell` {#sec-building-stdenv-package-in-nix-shell}
103
104To build a `stdenv` package in a [`nix-shell`](https://nixos.org/manual/nix/unstable/command-ref/nix-shell.html), enter a shell, find the [phases](#sec-stdenv-phases) you wish to build, then invoke `genericBuild` manually:
105
106Go to an empty directory, invoke `nix-shell` with the desired package, and from inside the shell, set the output variables to a writable directory:
107
108```bash
109cd "$(mktemp -d)"
110nix-shell '<nixpkgs>' -A some_package
111export out=$(pwd)/out
112```
113
114Next, invoke the desired parts of the build.
115First, run the phases that generate a working copy of the sources, which will change directory to the sources for you:
116
117```bash
118phases="${prePhases[*]:-} unpackPhase patchPhase" genericBuild
119```
120
121Then, run more phases up until the failure is reached.
122If the failure is in the build or check phase, the following phases would be required:
123
124```bash
125phases="${preConfigurePhases[*]:-} configurePhase ${preBuildPhases[*]:-} buildPhase checkPhase" genericBuild
126```
127
128Use this command to run all install phases:
129```bash
130phases="${preInstallPhases[*]:-} installPhase ${preFixupPhases[*]:-} fixupPhase installCheckPhase" genericBuild
131```
132
133Single phase can be re-run as many times as necessary to examine the failure like so:
134
135```bash
136phases="buildPhase" genericBuild
137```
138
139To modify a [phase](#sec-stdenv-phases), first print it with
140
141```bash
142echo "$buildPhase"
143```
144
145Or, if that is empty, for instance, if it is using a function:
146
147```bash
148type buildPhase
149```
150
151then change it in a text editor, and paste it back to the terminal.
152
153::: {.note}
154This method may have some inconsistencies in environment variables and behaviour compared to a normal build within the [Nix build sandbox](https://nixos.org/manual/nix/unstable/language/derivations#builder-execution).
155The following is a non-exhaustive list of such differences:
156
157- `TMP`, `TMPDIR`, and similar variables likely point to non-empty directories that the build might conflict with files in.
158- Output store paths are not writable, so the variables for outputs need to be overridden to writable paths.
159- Other environment variables may be inconsistent with a `nix-build` either due to `nix-shell`'s initialization script or due to the use of `nix-shell` without the `--pure` option.
160
161If the build fails differently inside the shell than in the sandbox, consider using [`breakpointHook`](#breakpointhook) and invoking `nix-build` instead.
162The [`--keep-failed`](https://nixos.org/manual/nix/unstable/command-ref/conf-file#opt--keep-failed) option for `nix-build` may also be useful to examine the build directory of a failed build.
163:::
164
165## Tools provided by `stdenv` {#sec-tools-of-stdenv}
166
167The standard environment provides the following packages:
168
169- The GNU C Compiler, configured with C and C++ support.
170- GNU coreutils (contains a few dozen standard Unix commands).
171- GNU findutils (contains `find`).
172- GNU diffutils (contains `diff`, `cmp`).
173- GNU `sed`.
174- GNU `grep`.
175- GNU `awk`.
176- GNU `tar`.
177- `gzip`, `bzip2` and `xz`.
178- GNU Make.
179- 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.
180- The `patch` command.
181
182On Linux, `stdenv` also includes the `patchelf` utility.
183
184## Specifying dependencies {#ssec-stdenv-dependencies}
185
186Build systems often require more dependencies than just what `stdenv` provides. This section describes attributes accepted by `stdenv.mkDerivation` that can be used to make these dependencies available to the build system.
187
188### Overview {#ssec-stdenv-dependencies-overview}
189
190A full reference of the different kinds of dependencies is provided in [](#ssec-stdenv-dependencies-reference), but here is an overview of the most common ones.
191It should cover most use cases.
192
193Add dependencies to `nativeBuildInputs` if they are executed during the build:
194- those which are needed on `$PATH` during the build, for example `cmake` and `pkg-config`
195- [setup hooks](#ssec-setup-hooks), for example [`makeWrapper`](#fun-makeWrapper)
196- interpreters needed by [`patchShebangs`](#patch-shebangs.sh) for build scripts (with the `--build` flag), which can be the case for e.g. `perl`
197
198Add dependencies to `buildInputs` if they will end up copied or linked into the final output or otherwise used at runtime:
199- libraries used by compilers, for example `zlib`,
200- interpreters needed by [`patchShebangs`](#patch-shebangs.sh) for scripts which are installed, which can be the case for e.g. `perl`
201
202::: {.note}
203These criteria are independent.
204
205For example, software using Wayland usually needs the `wayland` library at runtime, so `wayland` should be added to `buildInputs`.
206But it also executes the `wayland-scanner` program as part of the build to generate code, so `wayland` should also be added to `nativeBuildInputs`.
207:::
208
209Dependencies needed only to run tests are similarly classified between native (executed during build) and non-native (executed at runtime):
210- `nativeCheckInputs` for test tools needed on `$PATH` (such as `ctest`) and [setup hooks](#ssec-setup-hooks) (for example [`pytestCheckHook`](#python))
211- `checkInputs` for libraries linked into test executables (for example the `qcheck` OCaml package)
212
213These dependencies are only injected when [`doCheck`](#var-stdenv-doCheck) is set to `true`.
214
215#### Example {#ssec-stdenv-dependencies-overview-example}
216
217Consider for example this simplified derivation for `solo5`, a sandboxing tool:
218```nix
219stdenv.mkDerivation rec {
220 pname = "solo5";
221 version = "0.7.5";
222
223 src = fetchurl {
224 url = "https://github.com/Solo5/solo5/releases/download/v${version}/solo5-v${version}.tar.gz";
225 hash = "sha256-viwrS9lnaU8sTGuzK/+L/PlMM/xRRtgVuK5pixVeDEw=";
226 };
227
228 nativeBuildInputs = [ makeWrapper pkg-config ];
229 buildInputs = [ libseccomp ];
230
231 postInstall = ''
232 substituteInPlace $out/bin/solo5-virtio-mkimage \
233 --replace "/usr/lib/syslinux" "${syslinux}/share/syslinux" \
234 --replace "/usr/share/syslinux" "${syslinux}/share/syslinux" \
235 --replace "cp " "cp --no-preserve=mode "
236
237 wrapProgram $out/bin/solo5-virtio-mkimage \
238 --prefix PATH : ${lib.makeBinPath [ dosfstools mtools parted syslinux ]}
239 '';
240
241 doCheck = true;
242 nativeCheckInputs = [ util-linux qemu ];
243 checkPhase = '' [elided] '';
244}
245```
246
247- `makeWrapper` is a setup hook, i.e., a shell script sourced by the generic builder of `stdenv`.
248 It is thus executed during the build and must be added to `nativeBuildInputs`.
249- `pkg-config` is a build tool which the configure script of `solo5` expects to be on `$PATH` during the build:
250 therefore, it must be added to `nativeBuildInputs`.
251- `libseccomp` is a library linked into `$out/bin/solo5-elftool`.
252 As it is used at runtime, it must be added to `buildInputs`.
253- Tests need `qemu` and `getopt` (from `util-linux`) on `$PATH`, these must be added to `nativeCheckInputs`.
254- Some dependencies are injected directly in the shell code of phases: `syslinux`, `dosfstools`, `mtools`, and `parted`.
255In this specific case, they will end up in the output of the derivation (`$out` here).
256As Nix marks dependencies whose absolute path is present in the output as runtime dependencies, adding them to `buildInputs` is not required.
257
258For more complex cases, like libraries linked into an executable which is then executed as part of the build system, see [](#ssec-stdenv-dependencies-reference).
259
260### Reference {#ssec-stdenv-dependencies-reference}
261
262As described in the Nix manual, almost any `*.drv` store path in a derivation’s attribute set will induce a dependency on that derivation. `mkDerivation`, however, takes a few attributes intended to 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 `PATH`. That is built-in, but other setup is done via a pluggable mechanism that works in conjunction with these dependency attributes. See [](#ssec-setup-hooks) for details.
263
264Dependencies 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 [](#chap-cross) for exactly what each platform means. [^footnote-stdenv-ignored-build-platform] But even if one is not cross compiling, the platforms imply whether or not the dependency is needed at run-time or build-time, a concept that makes perfect sense outside of cross compilation. By default, the run-time/build-time distinction is just a hint for mental clarity, but with `strictDeps` set it is mostly enforced even in the native case.
265
266The extension of `PATH` with dependencies, alluded to above, proceeds according to the relative platforms alone. The process is carried out only for dependencies whose host platform matches the new derivation’s build platform i.e. dependencies which run on the platform where the new derivation will be built. [^footnote-stdenv-native-dependencies-in-path] For each dependency \<dep\> of those dependencies, `dep/bin`, if present, is added to the `PATH` environment variable.
267
268A dependency is said to be **propagated** when some of its other-transitive (non-immediate) downstream dependencies also need it as an immediate dependency.
269[^footnote-stdenv-propagated-dependencies]
270
271It is important to note that dependencies are not necessarily propagated as the same sort of dependency that they were before, but rather as the corresponding sort so that the platform rules still line up. To determine the exact rules for dependency propagation, we start by assigning to each dependency a couple of ternary numbers (`-1` for `build`, `0` for `host`, and `1` for `target`) representing its [dependency type](#possible-dependency-types), which captures how its host and target platforms are each "offset" from the depending derivation’s host and target platforms. The following table summarize the different combinations that can be obtained:
272
273| `host → target` | attribute name | offset |
274| ------------------- | ------------------- | -------- |
275| `build --> build` | `depsBuildBuild` | `-1, -1` |
276| `build --> host` | `nativeBuildInputs` | `-1, 0` |
277| `build --> target` | `depsBuildTarget` | `-1, 1` |
278| `host --> host` | `depsHostHost` | `0, 0` |
279| `host --> target` | `buildInputs` | `0, 1` |
280| `target --> target` | `depsTargetTarget` | `1, 1` |
281
282Algorithmically, we traverse propagated inputs, accumulating every propagated dependency’s propagated dependencies and adjusting them to account for the “shift in perspective” described by the current dependency’s platform offsets. This results is sort of a transitive closure of the dependency relation, with the offsets being approximately summed when two dependency links are combined. We also prune transitive dependencies whose combined offsets go out-of-bounds, which can be viewed as a filter over that transitive closure removing dependencies that are blatantly absurd.
283
284We can define the process precisely with [Natural Deduction](https://en.wikipedia.org/wiki/Natural_deduction) using the inference rules. This probably seems a bit obtuse, but so is the bash code that actually implements it! [^footnote-stdenv-find-inputs-location] They’re confusing in very different ways so… hopefully if something doesn’t make sense in one presentation, it will in the other!
285
286```
287let mapOffset(h, t, i) = i + (if i <= 0 then h else t - 1)
288
289propagated-dep(h0, t0, A, B)
290propagated-dep(h1, t1, B, C)
291h0 + h1 in {-1, 0, 1}
292h0 + t1 in {-1, 0, 1}
293-------------------------------------- Transitive property
294propagated-dep(mapOffset(h0, t0, h1),
295 mapOffset(h0, t0, t1),
296 A, C)
297```
298
299```
300let mapOffset(h, t, i) = i + (if i <= 0 then h else t - 1)
301
302dep(h0, t0, A, B)
303propagated-dep(h1, t1, B, C)
304h0 + h1 in {-1, 0, 1}
305h0 + t1 in {-1, 0, -1}
306----------------------------- Take immediate dependencies' propagated dependencies
307propagated-dep(mapOffset(h0, t0, h1),
308 mapOffset(h0, t0, t1),
309 A, C)
310```
311
312```
313propagated-dep(h, t, A, B)
314----------------------------- Propagated dependencies count as dependencies
315dep(h, t, A, B)
316```
317
318Some explanation of this monstrosity is in order. In the common case, the target offset of a dependency is the successor to the target offset: `t = h + 1`. That means that:
319
320```
321let f(h, t, i) = i + (if i <= 0 then h else t - 1)
322let f(h, h + 1, i) = i + (if i <= 0 then h else (h + 1) - 1)
323let f(h, h + 1, i) = i + (if i <= 0 then h else h)
324let f(h, h + 1, i) = i + h
325```
326
327This is where “sum-like” comes in from above: We can just sum all of the host offsets to get the host offset of the transitive dependency. The target offset is the transitive dependency is 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.
328
329Because of the bounds checks, the uncommon cases are `h = t` and `h + 2 = t`. In the former case, the motivation for `mapOffset` is that since its host and target platforms are the same, no transitive dependency of it should be able to “discover” an offset greater than its reduced target offsets. `mapOffset` effectively “squashes” all its transitive dependencies’ offsets so that none will ever be greater than the target offset of the original `h = t` package. In the other case, `h + 1` 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.
330
331Overall, the unifying theme here is that propagation shouldn’t be introducing transitive dependencies involving platforms the depending package is unaware of. \[One can imagine the depending package asking for dependencies with the platforms it knows about; other platforms it doesn’t know how to ask for. The platform description in that scenario is a kind of unforgeable capability.\] The offset bounds checking and definition of `mapOffset` 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.
332
333#### Variables specifying dependencies {#variables-specifying-dependencies}
334
335##### `depsBuildBuild` {#var-stdenv-depsBuildBuild}
336
337A list of dependencies whose host and target platforms are the new derivation’s build platform. These are programs and libraries used at build time that produce programs and libraries also used at build time. If the dependency doesn’t care about the target platform (i.e. isn’t a compiler or similar tool), put it in `nativeBuildInputs` instead. The most common use of this `buildPackages.stdenv.cc`, the default C compiler for this role. That example crops up more than one might think in old commonly used C libraries.
338
339Since these packages are able to be run at build-time, they are always added to the `PATH`, 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.
340
341##### `nativeBuildInputs` {#var-stdenv-nativeBuildInputs}
342
343A list of dependencies whose host platform is the new derivation’s build platform, and target platform is the new derivation’s host platform. These are programs and libraries used at build-time that, if they are a compiler or similar tool, produce code to run at run-time—i.e. tools used to build the new derivation. If the dependency doesn’t care about the target platform (i.e. isn’t a compiler or similar tool), put it here, rather than in `depsBuildBuild` or `depsBuildTarget`. This could be called `depsBuildHost` but `nativeBuildInputs` is used for historical continuity.
344
345Since these packages are able to be run at build-time, they are added to the `PATH`, 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.
346
347##### `depsBuildTarget` {#var-stdenv-depsBuildTarget}
348
349A list of dependencies whose host platform is the new derivation’s build platform, and target platform is the new derivation’s target platform. These are programs used at build time that produce code to run with code produced by the depending package. Most commonly, these are tools used to build the runtime or standard library that the currently-being-built compiler will inject into any code it compiles. In many cases, the currently-being-built-compiler is itself employed for that task, but when that compiler won’t run (i.e. its build and host platform differ) this is not possible. Other times, the compiler relies on some other tool, like binutils, that is always built separately so that the dependency is unconditional.
350
351This is a somewhat confusing concept to wrap one’s head around, and for good reason. As the only dependency type where the platform offsets, `-1` and `1`, are not adjacent integers, it requires thinking of a bootstrapping stage *two* away from the current one. It and its use-case go hand in hand and are both considered poor form: try to not need this sort of dependency, and try to avoid building standard libraries and runtimes in the same derivation as the compiler produces code using them. Instead strive to build those like a normal library, using the newly-built compiler just as a normal library would. In short, do not use this attribute unless you are packaging a compiler and are sure it is needed.
352
353Since these packages are able to run at build time, they are added to the `PATH`, 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.
354
355##### `depsHostHost` {#var-stdenv-depsHostHost}
356
357A list of dependencies whose host and target platforms match the new derivation’s host platform. In practice, this would usually be tools used by compilers for macros or a metaprogramming system, or libraries used by the macros or metaprogramming code itself. It’s always preferable to use a `depsBuildBuild` dependency in the derivation being built over a `depsHostHost` on the tool doing the building for this purpose.
358
359##### `buildInputs` {#var-stdenv-buildInputs}
360
361A list of dependencies whose host platform and target platform match the new derivation’s. This would be called `depsHostTarget` 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 `depsBuildBuild`.
362
363These are often programs and libraries used by the new derivation at *run*-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.
364
365##### `depsTargetTarget` {#var-stdenv-depsTargetTarget}
366
367A list of dependencies whose host platform matches the new derivation’s target platform. 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.
368
369##### `depsBuildBuildPropagated` {#var-stdenv-depsBuildBuildPropagated}
370
371The propagated equivalent of `depsBuildBuild`. This perhaps never ought to be used, but it is included for consistency \[see below for the others\].
372
373##### `propagatedNativeBuildInputs` {#var-stdenv-propagatedNativeBuildInputs}
374
375The propagated equivalent of `nativeBuildInputs`. This would be called `depsBuildHostPropagated` but for historical continuity. For example, if package `Y` has `propagatedNativeBuildInputs = [X]`, and package `Z` has `buildInputs = [Y]`, then package `Z` will be built as if it included package `X` in its `nativeBuildInputs`. If instead, package `Z` has `nativeBuildInputs = [Y]`, then `Z` will be built as if it included `X` in the `depsBuildBuild` of package `Z`, because of the sum of the two `-1` host offsets.
376
377##### `depsBuildTargetPropagated` {#var-stdenv-depsBuildTargetPropagated}
378
379The propagated equivalent of `depsBuildTarget`. This is prefixed for the same reason of alerting potential users.
380
381##### `depsHostHostPropagated` {#var-stdenv-depsHostHostPropagated}
382
383The propagated equivalent of `depsHostHost`.
384
385##### `propagatedBuildInputs` {#var-stdenv-propagatedBuildInputs}
386
387The propagated equivalent of `buildInputs`. This would be called `depsHostTargetPropagated` but for historical continuity.
388
389##### `depsTargetTargetPropagated` {#var-stdenv-depsTargetTargetPropagated}
390
391The propagated equivalent of `depsTargetTarget`. This is prefixed for the same reason of alerting potential users.
392
393## Attributes {#ssec-stdenv-attributes}
394
395### Variables affecting `stdenv` initialisation {#variables-affecting-stdenv-initialisation}
396
397#### `NIX_DEBUG` {#var-stdenv-NIX_DEBUG}
398
399A number between 0 and 7 indicating how much information to log. If set to 1 or higher, `stdenv` will print moderate debugging information during the build. In particular, the `gcc` and `ld` wrapper scripts will print out the complete command line passed to the wrapped tools. If set to 6 or higher, the `stdenv` setup script will be run with `set -x` tracing. If set to 7 or higher, the `gcc` and `ld` wrapper scripts will also be run with `set -x` tracing.
400
401### Attributes affecting build properties {#attributes-affecting-build-properties}
402
403#### `enableParallelBuilding` {#var-stdenv-enableParallelBuilding}
404
405If set to `true`, `stdenv` will pass specific flags to `make` and other build tools to enable parallel building with up to `build-cores` workers.
406
407Unless set to `false`, some build systems with good support for parallel building including `cmake`, `meson`, and `qmake` will set it to `true`.
408
409### Special variables {#special-variables}
410
411#### `passthru` {#var-stdenv-passthru}
412
413This is an attribute set which can be filled with arbitrary values. For example:
414
415```nix
416passthru = {
417 foo = "bar";
418 baz = {
419 value1 = 4;
420 value2 = 5;
421 };
422}
423```
424
425Values 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.
426
427#### `passthru.updateScript` {#var-passthru-updateScript}
428
429A script to be run by `maintainers/scripts/update.nix` when the package is matched. The attribute can contain one of the following:
430
431- []{#var-passthru-updateScript-command} an executable file, either on the file system:
432
433 ```nix
434 passthru.updateScript = ./update.sh;
435 ```
436
437 or inside the expression itself:
438
439 ```nix
440 passthru.updateScript = writeScript "update-zoom-us" ''
441 #!/usr/bin/env nix-shell
442 #!nix-shell -i bash -p curl pcre common-updater-scripts
443
444 set -eu -o pipefail
445
446 version="$(curl -sI https://zoom.us/client/latest/zoom_x86_64.tar.xz | grep -Fi 'Location:' | pcregrep -o1 '/(([0-9]\.?)+)/')"
447 update-source-version zoom-us "$version"
448 '';
449 ```
450
451- a list, a script followed by arguments to be passed to it:
452
453 ```nix
454 passthru.updateScript = [ ../../update.sh pname "--requested-release=unstable" ];
455 ```
456
457- an attribute set containing:
458 - [`command`]{#var-passthru-updateScript-set-command} – a string or list in the [format expected by `passthru.updateScript`](#var-passthru-updateScript-command).
459 - [`attrPath`]{#var-passthru-updateScript-set-attrPath} (optional) – a string containing the canonical attribute path for the package. If present, it will be passed to the update script instead of the attribute path on which the package was discovered during Nixpkgs traversal.
460 - [`supportedFeatures`]{#var-passthru-updateScript-set-supportedFeatures} (optional) – a list of the [extra features](#var-passthru-updateScript-supported-features) the script supports.
461
462 ```nix
463 passthru.updateScript = {
464 command = [ ../../update.sh pname ];
465 attrPath = pname;
466 supportedFeatures = [ … ];
467 };
468 ```
469
470::: {.tip}
471A common pattern is to use the [`nix-update-script`](https://github.com/NixOS/nixpkgs/blob/master/pkgs/common-updater/nix-update.nix) attribute provided in Nixpkgs, which runs [`nix-update`](https://github.com/Mic92/nix-update):
472
473```nix
474passthru.updateScript = nix-update-script { };
475```
476
477For simple packages, this is often enough, and will ensure that the package is updated automatically by [`nixpkgs-update`](https://ryantm.github.io/nixpkgs-update) when a new version is released. The [update bot](https://nix-community.org/update-bot) runs periodically to attempt to automatically update packages, and will run `passthru.updateScript` if set. While not strictly necessary if the project is listed on [Repology](https://repology.org), using `nix-update-script` allows the package to update via many more sources (e.g. GitHub releases).
478:::
479
480##### How update scripts are executed? {#var-passthru-updateScript-execution}
481
482Update scripts are to be invoked by `maintainers/scripts/update.nix` script. You can run `nix-shell maintainers/scripts/update.nix` in the root of Nixpkgs repository for information on how to use it. `update.nix` offers several modes for selecting packages to update (e.g. select by attribute path, traverse Nixpkgs and filter by maintainer, etc.), and it will execute update scripts for all matched packages that have an `updateScript` attribute.
483
484Each update script will be passed the following environment variables:
485
486- [`UPDATE_NIX_NAME`]{#var-passthru-updateScript-env-UPDATE_NIX_NAME} – content of the `name` attribute of the updated package.
487- [`UPDATE_NIX_PNAME`]{#var-passthru-updateScript-env-UPDATE_NIX_PNAME} – content of the `pname` attribute of the updated package.
488- [`UPDATE_NIX_OLD_VERSION`]{#var-passthru-updateScript-env-UPDATE_NIX_OLD_VERSION} – content of the `version` attribute of the updated package.
489- [`UPDATE_NIX_ATTR_PATH`]{#var-passthru-updateScript-env-UPDATE_NIX_ATTR_PATH} – attribute path the `update.nix` discovered the package on (or the [canonical `attrPath`](#var-passthru-updateScript-set-attrPath) when available). Example: `pantheon.elementary-terminal`
490
491::: {.note}
492An update script will be usually run from the root of the Nixpkgs repository but you should not rely on that. Also note that `update.nix` executes update scripts in parallel by default so you should avoid running `git commit` or any other commands that cannot handle that.
493:::
494
495::: {.tip}
496While update scripts should not create commits themselves, `maintainers/scripts/update.nix` supports automatically creating commits when running it with `--argstr commit true`. If you need to customize commit message, you can have the update script implement [`commit`](#var-passthru-updateScript-commit) feature.
497:::
498
499##### Supported features {#var-passthru-updateScript-supported-features}
500###### `commit` {#var-passthru-updateScript-commit}
501
502This feature allows update scripts to *ask* `update.nix` to create Git commits.
503
504When support of this feature is declared, whenever the update script exits with `0` return status, it is expected to print a JSON list containing an object described below for each updated attribute to standard output.
505
506When `update.nix` is run with `--argstr commit true` arguments, it will create a separate commit for each of the objects. An empty list can be returned when the script did not update any files, for example, when the package is already at the latest version.
507
508The commit object contains the following values:
509
510- [`attrPath`]{#var-passthru-updateScript-commit-attrPath} – a string containing attribute path.
511- [`oldVersion`]{#var-passthru-updateScript-commit-oldVersion} – a string containing old version.
512- [`newVersion`]{#var-passthru-updateScript-commit-newVersion} – a string containing new version.
513- [`files`]{#var-passthru-updateScript-commit-files} – a non-empty list of file paths (as strings) to add to the commit.
514- [`commitBody`]{#var-passthru-updateScript-commit-commitBody} (optional) – a string with extra content to be appended to the default commit message (useful for adding changelog links).
515- [`commitMessage`]{#var-passthru-updateScript-commit-commitMessage} (optional) – a string to use instead of the default commit message.
516
517If the returned array contains exactly one object (e.g. `[{}]`), all values are optional and will be determined automatically.
518
519::: {.example #var-passthru-updateScript-example-commit}
520# Standard output of an update script using commit feature
521
522```json
523[
524 {
525 "attrPath": "volume_key",
526 "oldVersion": "0.3.11",
527 "newVersion": "0.3.12",
528 "files": [
529 "/path/to/nixpkgs/pkgs/development/libraries/volume-key/default.nix"
530 ]
531 }
532]
533```
534:::
535
536### Fixed-point arguments of `mkDerivation` {#mkderivation-recursive-attributes}
537
538If you pass a function to `mkDerivation`, it will receive as its argument the final arguments, including the overrides when reinvoked via `overrideAttrs`. For example:
539
540```nix
541mkDerivation (finalAttrs: {
542 pname = "hello";
543 withFeature = true;
544 configureFlags =
545 lib.optionals finalAttrs.withFeature ["--with-feature"];
546})
547```
548
549Note that this does not use the `rec` keyword to reuse `withFeature` in `configureFlags`.
550The `rec` keyword works at the syntax level and is unaware of overriding.
551
552Instead, the definition references `finalAttrs`, allowing users to change `withFeature`
553consistently with `overrideAttrs`.
554
555`finalAttrs` also contains the attribute `finalPackage`, which includes the output paths, etc.
556
557Let's look at a more elaborate example to understand the differences between
558various bindings:
559
560```nix
561# `pkg` is the _original_ definition (for illustration purposes)
562let pkg =
563 mkDerivation (finalAttrs: {
564 # ...
565
566 # An example attribute
567 packages = [];
568
569 # `passthru.tests` is a commonly defined attribute.
570 passthru.tests.simple = f finalAttrs.finalPackage;
571
572 # An example of an attribute containing a function
573 passthru.appendPackages = packages':
574 finalAttrs.finalPackage.overrideAttrs (newSelf: super: {
575 packages = super.packages ++ packages';
576 });
577
578 # For illustration purposes; referenced as
579 # `(pkg.overrideAttrs(x)).finalAttrs` etc in the text below.
580 passthru.finalAttrs = finalAttrs;
581 passthru.original = pkg;
582 });
583in pkg
584```
585
586Unlike the `pkg` binding in the above example, the `finalAttrs` parameter always references the final attributes. For instance `(pkg.overrideAttrs(x)).finalAttrs.finalPackage` is identical to `pkg.overrideAttrs(x)`, whereas `(pkg.overrideAttrs(x)).original` is the same as the original `pkg`.
587
588See also the section about [`passthru.tests`](#var-meta-tests).
589
590## Phases {#sec-stdenv-phases}
591
592`stdenv.mkDerivation` sets the Nix [derivation](https://nixos.org/manual/nix/stable/expressions/derivations.html#derivations)'s builder to a script that loads the stdenv `setup.sh` bash library and calls `genericBuild`. Most packaging functions rely on this default builder.
593
594This generic command invokes 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).
595
596Each 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. However, typically one only wants to *add* some commands to a phase, e.g. by defining `postInstall` or `preFixup`, as skipping some of the default actions may have unexpected consequences. The default script for each phase is defined in the file `pkgs/stdenv/generic/setup.sh`.
597
598When overriding a phase, for example `installPhase`, it is important to start with `runHook preInstall` and end it with `runHook postInstall`, otherwise `preInstall` and `postInstall` will not be run. Even if you don't use them directly, it is good practice to do so anyways for downstream users who would want to add a `postInstall` by overriding your derivation.
599
600While inside an interactive `nix-shell`, if you wanted to run all phases in the order they would be run in an actual build, you can invoke `genericBuild` yourself.
601
602### Controlling phases {#ssec-controlling-phases}
603
604There are a number of variables that control what phases are executed and in what order:
605
606#### Variables affecting phase control {#variables-affecting-phase-control}
607
608##### `phases` {#var-stdenv-phases}
609
610Specifies 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 installCheckPhase $preDistPhases distPhase $postPhases`.
611
612It is discouraged to set this variable, as it is easy to miss some important functionality hidden in some of the less obviously needed phases (like `fixupPhase` which patches the shebang of scripts).
613Usually, if you just want to add a few phases, it’s more convenient to set one of the variables below (such as `preInstallPhases`).
614
615##### `prePhases` {#var-stdenv-prePhases}
616
617Additional phases executed before any of the default phases.
618
619##### `preConfigurePhases` {#var-stdenv-preConfigurePhases}
620
621Additional phases executed just before the configure phase.
622
623##### `preBuildPhases` {#var-stdenv-preBuildPhases}
624
625Additional phases executed just before the build phase.
626
627##### `preInstallPhases` {#var-stdenv-preInstallPhases}
628
629Additional phases executed just before the install phase.
630
631##### `preFixupPhases` {#var-stdenv-preFixupPhases}
632
633Additional phases executed just before the fixup phase.
634
635##### `preDistPhases` {#var-stdenv-preDistPhases}
636
637Additional phases executed just before the distribution phase.
638
639##### `postPhases` {#var-stdenv-postPhases}
640
641Additional phases executed after any of the default phases.
642
643### The unpack phase {#ssec-unpack-phase}
644
645The 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:
646
647#### Tar files {#tar-files}
648
649These can optionally be compressed using `gzip` (`.tar.gz`, `.tgz` or `.tar.Z`), `bzip2` (`.tar.bz2`, `.tbz2` or `.tbz`) or `xz` (`.tar.xz`, `.tar.lzma` or `.txz`).
650
651#### Zip files {#zip-files}
652
653Zip files are unpacked using `unzip`. However, `unzip` is not in the standard environment, so you should add it to `nativeBuildInputs` yourself.
654
655#### Directories in the Nix store {#directories-in-the-nix-store}
656
657These are 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`.
658
659Additional file types can be supported by setting the `unpackCmd` variable (see below).
660
661#### Variables controlling the unpack phase {#variables-controlling-the-unpack-phase}
662
663##### `srcs` / `src` {#var-stdenv-src}
664
665The list of source files or directories to be unpacked or copied. One of these must be set. Note that if you use `srcs`, you should also set `sourceRoot` or `setSourceRoot`.
666
667##### `sourceRoot` {#var-stdenv-sourceRoot}
668
669After unpacking all of `src` and `srcs`, if neither of `sourceRoot` and `setSourceRoot` are set, `unpackPhase` of the generic builder checks that the unpacking produced a single directory and moves the current working directory into it.
670
671If `unpackPhase` produces multiple source directories, you should set `sourceRoot` to the name of the intended directory.
672You can also set `sourceRoot = ".";` if you want to control it yourself in a later phase.
673
674For example, if your want your build to start in a sub-directory inside your sources, and you are using `fetchzip`-derived `src` (like `fetchFromGitHub` or similar), you need to set `sourceRoot = "${src.name}/my-sub-directory"`.
675
676##### `setSourceRoot` {#var-stdenv-setSourceRoot}
677
678Alternatively 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`.
679
680For example, if you are using `fetchurl` on an archive file that gets unpacked into a single directory the name of which changes between package versions, and you want your build to start in its sub-directory, you need to set `setSourceRoot = "sourceRoot=$(echo */my-sub-directory)";`, or in the case of multiple sources, you could use something more specific, like `setSourceRoot = "sourceRoot=$(echo ${pname}-*/my-sub-directory)";`.
681
682##### `preUnpack` {#var-stdenv-preUnpack}
683
684Hook executed at the start of the unpack phase.
685
686##### `postUnpack` {#var-stdenv-postUnpack}
687
688Hook executed at the end of the unpack phase.
689
690##### `dontUnpack` {#var-stdenv-dontUnpack}
691
692Set to true to skip the unpack phase.
693
694##### `dontMakeSourcesWritable` {#var-stdenv-dontMakeSourcesWritable}
695
696If set to `1`, the unpacked sources are *not* made writable. By default, they are made writable to prevent problems with read-only sources. For example, copied store directories would be read-only without this.
697
698##### `unpackCmd` {#var-stdenv-unpackCmd}
699
700The unpack phase evaluates the string `$unpackCmd` for any unrecognised file. The path to the current source file is contained in the `curSrc` variable.
701
702### The patch phase {#ssec-patch-phase}
703
704The patch phase applies the list of patches defined in the `patches` variable.
705
706#### Variables controlling the patch phase {#variables-controlling-the-patch-phase}
707
708##### `dontPatch` {#var-stdenv-dontPatch}
709
710Set to true to skip the patch phase.
711
712##### `patches` {#var-stdenv-patches}
713
714The 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`).
715
716##### `patchFlags` {#var-stdenv-patchFlags}
717
718Flags 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.
719
720##### `prePatch` {#var-stdenv-prePatch}
721
722Hook executed at the start of the patch phase.
723
724##### `postPatch` {#var-stdenv-postPatch}
725
726Hook executed at the end of the patch phase.
727
728### The configure phase {#ssec-configure-phase}
729
730The configure phase prepares the source tree for building. The default `configurePhase` runs `./configure` (typically an Autoconf-generated script) if it exists.
731
732#### Variables controlling the configure phase {#variables-controlling-the-configure-phase}
733
734##### `configureScript` {#var-stdenv-configureScript}
735
736The 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`).
737
738##### `configureFlags` {#var-stdenv-configureFlags}
739
740A list of strings passed as additional arguments to the configure script.
741
742##### `dontConfigure` {#var-stdenv-dontConfigure}
743
744Set to true to skip the configure phase.
745
746##### `configureFlagsArray` {#var-stdenv-configureFlagsArray}
747
748A shell array containing additional arguments passed to the configure script. You must use this instead of `configureFlags` if the arguments contain spaces.
749
750##### `dontAddPrefix` {#var-stdenv-dontAddPrefix}
751
752By default, the flag `--prefix=$prefix` is added to the configure flags. If this is undesirable, set this variable to true.
753
754##### `prefix` {#var-stdenv-prefix}
755
756The prefix under which the package must be installed, passed via the `--prefix` option to the configure script. It defaults to `$out`.
757
758##### `prefixKey` {#var-stdenv-prefixKey}
759
760The key to use when specifying the prefix. By default, this is set to `--prefix=` as that is used by the majority of packages.
761
762##### `dontAddStaticConfigureFlags` {#var-stdenv-dontAddStaticConfigureFlags}
763
764By default, when building statically, stdenv will try to add build system appropriate configure flags to try to enable static builds.
765
766If this is undesirable, set this variable to true.
767
768##### `dontAddDisableDepTrack` {#var-stdenv-dontAddDisableDepTrack}
769
770By 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.
771
772##### `dontFixLibtool` {#var-stdenv-dontFixLibtool}
773
774By 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 packages [^footnote-stdenv-sys-lib-search-path] . If this is undesirable, set this variable to true.
775
776##### `dontDisableStatic` {#var-stdenv-dontDisableStatic}
777
778By default, when the configure script has `--enable-static`, the option `--disable-static` is added to the configure flags.
779
780If this is undesirable, set this variable to true. It is automatically set to true when building statically, for example through `pkgsStatic`.
781
782##### `configurePlatforms` {#var-stdenv-configurePlatforms}
783
784By default, when cross compiling, the configure script has `--build=...` and `--host=...` passed. Packages can instead pass `[ "build" "host" "target" ]` or a subset to control exactly which platform flags are passed. Compilers and other tools can use this to also pass the target platform. [^footnote-stdenv-build-time-guessing-impurity]
785
786##### `preConfigure` {#var-stdenv-preConfigure}
787
788Hook executed at the start of the configure phase.
789
790##### `postConfigure` {#var-stdenv-postConfigure}
791
792Hook executed at the end of the configure phase.
793
794### The build phase {#build-phase}
795
796The build phase is responsible for actually building the package (e.g. compiling it). The default `buildPhase` 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.
797
798#### Variables controlling the build phase {#variables-controlling-the-build-phase}
799
800##### `dontBuild` {#var-stdenv-dontBuild}
801
802Set to true to skip the build phase.
803
804##### `makefile` {#var-stdenv-makefile}
805
806The file name of the Makefile.
807
808##### `makeFlags` {#var-stdenv-makeFlags}
809
810A 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).
811
812```nix
813makeFlags = [ "PREFIX=$(out)" ];
814```
815
816::: {.note}
817The flags are quoted in bash, but environment variables can be specified by using the make syntax.
818:::
819
820##### `makeFlagsArray` {#var-stdenv-makeFlagsArray}
821
822A shell array containing additional arguments passed to `make`. You must use this instead of `makeFlags` if the arguments contain spaces, e.g.
823
824```nix
825preBuild = ''
826 makeFlagsArray+=(CFLAGS="-O0 -g" LDFLAGS="-lfoo -lbar")
827'';
828```
829
830Note 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.
831
832##### `buildFlags` / `buildFlagsArray` {#var-stdenv-buildFlags}
833
834A list of strings passed as additional flags to `make`. Like `makeFlags` and `makeFlagsArray`, but only used by the build phase.
835
836##### `preBuild` {#var-stdenv-preBuild}
837
838Hook executed at the start of the build phase.
839
840##### `postBuild` {#var-stdenv-postBuild}
841
842Hook executed at the end of the build phase.
843
844You can set flags for `make` through the `makeFlags` variable.
845
846Before and after running `make`, the hooks `preBuild` and `postBuild` are called, respectively.
847
848### The check phase {#ssec-check-phase}
849
850The check phase checks whether the package was built correctly by running its test suite. The default `checkPhase` calls `make $checkTarget`, but only if the [`doCheck` variable](#var-stdenv-doCheck) is enabled.
851
852#### Variables controlling the check phase {#variables-controlling-the-check-phase}
853
854##### `doCheck` {#var-stdenv-doCheck}
855
856Controls whether the check phase is executed. By default it is skipped, but if `doCheck` is set to true, the check phase is usually executed. Thus you should set
857
858```nix
859doCheck = true;
860```
861
862in the derivation to enable checks. The exception is cross compilation. Cross compiled builds never run tests, no matter how `doCheck` is set, as the newly-built program won’t run on the platform used to build it.
863
864##### `makeFlags` / `makeFlagsArray` / `makefile` {#makeflags-makeflagsarray-makefile}
865
866See the [build phase](#var-stdenv-makeFlags) for details.
867
868##### `checkTarget` {#var-stdenv-checkTarget}
869
870The `make` target that runs the tests.
871If unset, use `check` if it exists, otherwise `test`; if neither is found, do nothing.
872
873##### `checkFlags` / `checkFlagsArray` {#var-stdenv-checkFlags}
874
875A list of strings passed as additional flags to `make`. Like `makeFlags` and `makeFlagsArray`, but only used by the check phase.
876
877##### `checkInputs` {#var-stdenv-checkInputs}
878
879A list of host dependencies used by the phase, usually libraries linked into executables built during tests. This gets included in `buildInputs` when `doCheck` is set.
880
881##### `nativeCheckInputs` {#var-stdenv-nativeCheckInputs}
882
883A list of native dependencies used by the phase, notably tools needed on `$PATH`. This gets included in `nativeBuildInputs` when `doCheck` is set.
884
885##### `preCheck` {#var-stdenv-preCheck}
886
887Hook executed at the start of the check phase.
888
889##### `postCheck` {#var-stdenv-postCheck}
890
891Hook executed at the end of the check phase.
892
893### The install phase {#ssec-install-phase}
894
895The install phase is responsible for installing the package in the Nix store under `out`. The default `installPhase` creates the directory `$out` and calls `make install`.
896
897#### Variables controlling the install phase {#variables-controlling-the-install-phase}
898
899##### `dontInstall` {#var-stdenv-dontInstall}
900
901Set to true to skip the install phase.
902
903##### `makeFlags` / `makeFlagsArray` / `makefile` {#makeflags-makeflagsarray-makefile-1}
904
905See the [build phase](#var-stdenv-makeFlags) for details.
906
907##### `installTargets` {#var-stdenv-installTargets}
908
909The make targets that perform the installation. Defaults to `install`. Example:
910
911```nix
912installTargets = "install-bin install-doc";
913```
914
915##### `installFlags` / `installFlagsArray` {#var-stdenv-installFlags}
916
917A list of strings passed as additional flags to `make`. Like `makeFlags` and `makeFlagsArray`, but only used by the install phase.
918
919##### `preInstall` {#var-stdenv-preInstall}
920
921Hook executed at the start of the install phase.
922
923##### `postInstall` {#var-stdenv-postInstall}
924
925Hook executed at the end of the install phase.
926
927### The fixup phase {#ssec-fixup-phase}
928
929The fixup phase performs (Nix-specific) post-processing actions on the files installed under `$out` by the install phase. The default `fixupPhase` does the following:
930
931- It moves the `man/`, `doc/` and `info/` subdirectories of `$out` to `share/`.
932- It strips libraries and executables of debug information.
933- On Linux, it applies the `patchelf` command to ELF executables and libraries to remove unused directories from the `RPATH` in order to prevent unnecessary runtime dependencies.
934- It rewrites the interpreter paths of shell scripts to paths found in `PATH`. E.g., `/usr/bin/perl` will be rewritten to `/nix/store/some-perl/bin/perl` found in `PATH`. See [](#patch-shebangs.sh) for details.
935
936#### Variables controlling the fixup phase {#variables-controlling-the-fixup-phase}
937
938##### `dontFixup` {#var-stdenv-dontFixup}
939
940Set to true to skip the fixup phase.
941
942##### `dontStrip` {#var-stdenv-dontStrip}
943
944If set, libraries and executables are not stripped. By default, they are.
945
946##### `dontStripHost` {#var-stdenv-dontStripHost}
947
948Like `dontStrip`, but only affects the `strip` command targeting the package’s host platform. Useful when supporting cross compilation, but otherwise feel free to ignore.
949
950##### `dontStripTarget` {#var-stdenv-dontStripTarget}
951
952Like `dontStrip`, but only affects the `strip` command targeting the packages’ target platform. Useful when supporting cross compilation, but otherwise feel free to ignore.
953
954##### `dontMoveSbin` {#var-stdenv-dontMoveSbin}
955
956If set, files in `$out/sbin` are not moved to `$out/bin`. By default, they are.
957
958##### `stripAllList` {#var-stdenv-stripAllList}
959
960List 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.
961
962##### `stripAllListTarget` {#var-stdenv-stripAllListTarget}
963
964Like `stripAllList`, but only applies to packages’ target platform. By default, it’s empty. Useful when supporting cross compilation.
965
966##### `stripAllFlags` {#var-stdenv-stripAllFlags}
967
968Flags passed to the `strip` command applied to the files in the directories listed in `stripAllList`. Defaults to `-s` (i.e. `--strip-all`).
969
970##### `stripDebugList` {#var-stdenv-stripDebugList}
971
972List of directories to search for libraries and executables from which only debugging-related symbols should be stripped. It defaults to `lib lib32 lib64 libexec bin sbin`.
973
974##### `stripDebugListTarget` {#var-stdenv-stripDebugListTarget}
975
976Like `stripDebugList`, but only applies to packages’ target platform. By default, it’s empty. Useful when supporting cross compilation.
977
978##### `stripDebugFlags` {#var-stdenv-stripDebugFlags}
979
980Flags passed to the `strip` command applied to the files in the directories listed in `stripDebugList`. Defaults to `-S` (i.e. `--strip-debug`).
981
982##### `stripExclude` {#var-stdenv-stripExclude}
983
984A list of filenames or path patterns to avoid stripping. A file is excluded if its name _or_ path (from the derivation root) matches.
985
986This example prevents all `*.rlib` files from being stripped:
987
988```nix
989stdenv.mkDerivation {
990 # ...
991 stripExclude = [ "*.rlib" ]
992}
993```
994
995This example prevents files within certain paths from being stripped:
996
997```nix
998stdenv.mkDerivation {
999 # ...
1000 stripExclude = [ "lib/modules/*/build/* ]
1001}
1002```
1003
1004##### `dontPatchELF` {#var-stdenv-dontPatchELF}
1005
1006If set, the `patchelf` command is not used to remove unnecessary `RPATH` entries. Only applies to Linux.
1007
1008##### `dontPatchShebangs` {#var-stdenv-dontPatchShebangs}
1009
1010If set, scripts starting with `#!` do not have their interpreter paths rewritten to paths in the Nix store. See [](#patch-shebangs.sh) on how patching shebangs works.
1011
1012##### `dontPruneLibtoolFiles` {#var-stdenv-dontPruneLibtoolFiles}
1013
1014If set, libtool `.la` files associated with shared libraries won’t have their `dependency_libs` field cleared.
1015
1016##### `forceShare` {#var-stdenv-forceShare}
1017
1018The list of directories that must be moved from `$out` to `$out/share`. Defaults to `man doc info`.
1019
1020##### `setupHook` {#var-stdenv-setupHook}
1021
1022A package can export a [setup hook](#ssec-setup-hooks) by setting this variable. The setup hook, if defined, is copied to `$out/nix-support/setup-hook`. Environment variables are then substituted in it using `substituteAll`.
1023
1024##### `preFixup` {#var-stdenv-preFixup}
1025
1026Hook executed at the start of the fixup phase.
1027
1028##### `postFixup` {#var-stdenv-postFixup}
1029
1030Hook executed at the end of the fixup phase.
1031
1032##### `separateDebugInfo` {#stdenv-separateDebugInfo}
1033
1034If 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.
1035
1036:::{.example #ex-gdb-debug-symbols-socat}
1037
1038# Enable debug symbols for use with GDB
1039
1040To make GDB find debug information for the `socat` package and its dependencies, you can use the following `shell.nix`:
1041
1042```nix
1043let
1044 pkgs = import ./. {
1045 config = {};
1046 overlays = [
1047 (final: prev: {
1048 ncurses = prev.ncurses.overrideAttrs { separateDebugInfo = true; };
1049 readline = prev.readline.overrideAttrs { separateDebugInfo = true; };
1050 })
1051 ];
1052 };
1053
1054 myDebugInfoDirs = pkgs.symlinkJoin {
1055 name = "myDebugInfoDirs";
1056 paths = with pkgs; [
1057 glibc.debug
1058 ncurses.debug
1059 openssl.debug
1060 readline.debug
1061 ];
1062 };
1063in
1064 pkgs.mkShell {
1065
1066 NIX_DEBUG_INFO_DIRS = "${pkgs.lib.getLib myDebugInfoDirs}/lib/debug";
1067
1068 packages = [
1069 pkgs.gdb
1070 pkgs.socat
1071 ];
1072
1073 shellHook = ''
1074 ${pkgs.lib.getBin pkgs.gdb}/bin/gdb ${pkgs.lib.getBin pkgs.socat}/bin/socat
1075 '';
1076 }
1077```
1078
1079This setup works as follows:
1080- Add [`overlays`](#chap-overlays) to the package set, since debug symbols are disabled for `ncurses` and `readline` by default.
1081- Create a derivation to combine all required debug symbols under one path with [`symlinkJoin`](#trivial-builder-symlinkJoin).
1082- Set the environment variable `NIX_DEBUG_INFO_DIRS` in the shell. Nixpkgs patches `gdb` to use it for looking up debug symbols.
1083- Run `gdb` on the `socat` binary on shell startup in the [`shellHook`](#sec-pkgs-mkShell). Here we use [`lib.getBin`](#function-library-lib.attrsets.getBin) to ensure that the correct derivation output is selected rather than the default one.
1084
1085:::
1086
1087### The installCheck phase {#ssec-installCheck-phase}
1088
1089The installCheck phase checks whether the package was installed correctly by running its test suite against the installed directories. The default `installCheck` calls `make installcheck`.
1090
1091It is often better to add tests that are not part of the source distribution to `passthru.tests` (see
1092[](#var-meta-tests)). This avoids adding overhead to every build and enables us to run them independently.
1093
1094#### Variables controlling the installCheck phase {#variables-controlling-the-installcheck-phase}
1095
1096##### `doInstallCheck` {#var-stdenv-doInstallCheck}
1097
1098Controls whether the installCheck phase is executed. By default it is skipped, but if `doInstallCheck` is set to true, the installCheck phase is usually executed. Thus you should set
1099
1100```nix
1101doInstallCheck = true;
1102```
1103
1104in the derivation to enable install checks. The exception is cross compilation. Cross compiled builds never run tests, no matter how `doInstallCheck` is set, as the newly-built program won’t run on the platform used to build it.
1105
1106##### `installCheckTarget` {#var-stdenv-installCheckTarget}
1107
1108The make target that runs the install tests. Defaults to `installcheck`.
1109
1110##### `installCheckFlags` / `installCheckFlagsArray` {#var-stdenv-installCheckFlags}
1111
1112A list of strings passed as additional flags to `make`. Like `makeFlags` and `makeFlagsArray`, but only used by the installCheck phase.
1113
1114##### `installCheckInputs` {#var-stdenv-installCheckInputs}
1115
1116A list of host dependencies used by the phase, usually libraries linked into executables built during tests. This gets included in `buildInputs` when `doInstallCheck` is set.
1117
1118##### `nativeInstallCheckInputs` {#var-stdenv-nativeInstallCheckInputs}
1119
1120A list of native dependencies used by the phase, notably tools needed on `$PATH`. This gets included in `nativeBuildInputs` when `doInstallCheck` is set.
1121
1122##### `preInstallCheck` {#var-stdenv-preInstallCheck}
1123
1124Hook executed at the start of the installCheck phase.
1125
1126##### `postInstallCheck` {#var-stdenv-postInstallCheck}
1127
1128Hook executed at the end of the installCheck phase.
1129
1130### The distribution phase {#ssec-distribution-phase}
1131
1132The 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 `$out/tarballs/`. This phase is only executed if the attribute `doDist` is set.
1133
1134#### Variables controlling the distribution phase {#variables-controlling-the-distribution-phase}
1135
1136##### `doDist` {#var-stdenv-doDist}
1137
1138If set, the distribution phase is executed.
1139
1140##### `distTarget` {#var-stdenv-distTarget}
1141
1142The make target that produces the distribution. Defaults to `dist`.
1143
1144##### `distFlags` / `distFlagsArray` {#var-stdenv-distFlags}
1145
1146Additional flags passed to `make`.
1147
1148##### `tarballs` {#var-stdenv-tarballs}
1149
1150The names of the source distribution files to be copied to `$out/tarballs/`. It can contain shell wildcards. The default is `*.tar.gz`.
1151
1152##### `dontCopyDist` {#var-stdenv-dontCopyDist}
1153
1154If set, no files are copied to `$out/tarballs/`.
1155
1156##### `preDist` {#var-stdenv-preDist}
1157
1158Hook executed at the start of the distribution phase.
1159
1160##### `postDist` {#var-stdenv-postDist}
1161
1162Hook executed at the end of the distribution phase.
1163
1164## Shell functions and utilities {#ssec-stdenv-functions}
1165
1166The standard environment provides a number of useful functions.
1167
1168### `makeWrapper` \<executable\> \<wrapperfile\> \<args\> {#fun-makeWrapper}
1169
1170Constructs a wrapper for a program with various possible arguments. It is defined as part of 2 setup-hooks named `makeWrapper` and `makeBinaryWrapper` that implement the same bash functions. Hence, to use it you have to add `makeWrapper` to your `nativeBuildInputs`. Here's an example usage:
1171
1172```bash
1173# adds `FOOBAR=baz` to `$out/bin/foo`’s environment
1174makeWrapper $out/bin/foo $wrapperfile --set FOOBAR baz
1175
1176# Prefixes the binary paths of `hello` and `git`
1177# and suffixes the binary path of `xdg-utils`.
1178# Be advised that paths often should be patched in directly
1179# (via string replacements or in `configurePhase`).
1180makeWrapper $out/bin/foo $wrapperfile \
1181 --prefix PATH : ${lib.makeBinPath [ hello git ]} \
1182 --suffix PATH : ${lib.makeBinPath [ xdg-utils ]}
1183```
1184
1185Packages may expect or require other utilities to be available at runtime.
1186`makeWrapper` can be used to add packages to a `PATH` environment variable local to a wrapper.
1187
1188Use `--prefix` to explicitly set dependencies in `PATH`.
1189
1190::: {.note}
1191`--prefix` essentially hard-codes dependencies into the wrapper.
1192They cannot be overridden without rebuilding the package.
1193:::
1194
1195If dependencies should be resolved at runtime, use `--suffix` to append fallback values to `PATH`.
1196
1197There’s many more kinds of arguments, they are documented in `nixpkgs/pkgs/build-support/setup-hooks/make-wrapper.sh` for the `makeWrapper` implementation and in `nixpkgs/pkgs/build-support/setup-hooks/make-binary-wrapper/make-binary-wrapper.sh` for the `makeBinaryWrapper` implementation.
1198
1199`wrapProgram` is a convenience function you probably want to use most of the time, implemented by both `makeWrapper` and `makeBinaryWrapper`.
1200
1201Using the `makeBinaryWrapper` implementation is usually preferred, as it creates a tiny _compiled_ wrapper executable, that can be used as a shebang interpreter. This is needed mostly on Darwin, where shebangs cannot point to scripts, [due to a limitation with the `execve`-syscall](https://stackoverflow.com/questions/67100831/macos-shebang-with-absolute-path-not-working). Compiled wrappers generated by `makeBinaryWrapper` can be inspected with `less <path-to-wrapper>` - by scrolling past the binary data you should be able to see the shell command that generated the executable and there see the environment variables that were injected into the wrapper.
1202
1203### `remove-references-to -t` \<storepath\> [ `-t` \<storepath\> ... ] \<file\> ... {#fun-remove-references-to}
1204
1205Removes the references of the specified files to the specified store files. This is done without changing the size of the file by replacing the hash by `eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee`, and should work on compiled executables. This is meant to be used to remove the dependency of the output on inputs that are known to be unnecessary at runtime. Of course, reckless usage will break the patched programs.
1206To use this, add `removeReferencesTo` to `nativeBuildInputs`.
1207
1208As `remove-references-to` is an actual executable and not a shell function, it can be used with `find`.
1209Example removing all references to the compiler in the output:
1210```nix
1211postInstall = ''
1212 find "$out" -type f -exec remove-references-to -t ${stdenv.cc} '{}' +
1213'';
1214```
1215
1216### `substitute` \<infile\> \<outfile\> \<subs\> {#fun-substitute}
1217
1218Performs string substitution on the contents of \<infile\>, writing the result to \<outfile\>. The substitutions in \<subs\> are of the following form:
1219
1220#### `--replace` \<s1\> \<s2\> {#fun-substitute-replace}
1221
1222Replace every occurrence of the string \<s1\> by \<s2\>.
1223
1224#### `--subst-var` \<varName\> {#fun-substitute-subst-var}
1225
1226Replace every occurrence of `@varName@` by the contents of the environment variable \<varName\>. This is useful for generating files from templates, using `@...@` in the template as placeholders.
1227
1228#### `--subst-var-by` \<varName\> \<s\> {#fun-substitute-subst-var-by}
1229
1230Replace every occurrence of `@varName@` by the string \<s\>.
1231
1232Example:
1233
1234```shell
1235substitute ./foo.in ./foo.out \
1236 --replace /usr/bin/bar $bar/bin/bar \
1237 --replace "a string containing spaces" "some other text" \
1238 --subst-var someVar
1239```
1240
1241### `substituteInPlace` \<multiple files\> \<subs\> {#fun-substituteInPlace}
1242
1243Like `substitute`, but performs the substitutions in place on the files passed.
1244
1245### `substituteAll` \<infile\> \<outfile\> {#fun-substituteAll}
1246
1247Replaces every occurrence of `@varName@`, where \<varName\> is any environment variable, in \<infile\>, writing the result to \<outfile\>. For instance, if \<infile\> has the contents
1248
1249```bash
1250#! @bash@/bin/sh
1251PATH=@coreutils@/bin
1252echo @foo@
1253```
1254
1255and 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
1256
1257```bash
1258#! /nix/store/bmwp0q28cf21...-bash-3.2-p39/bin/sh
1259PATH=/nix/store/68afga4khv0w...-coreutils-6.12/bin
1260echo @foo@
1261```
1262
1263That is, no substitution is performed for undefined variables.
1264
1265Environment 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).
1266
1267### `substituteAllInPlace` \<file\> {#fun-substituteAllInPlace}
1268
1269Like `substituteAll`, but performs the substitutions in place on the file \<file\>.
1270
1271### `stripHash` \<path\> {#fun-stripHash}
1272
1273Strips the directory and hash part of a store path, outputting the name part to `stdout`. For example:
1274
1275```bash
1276# prints coreutils-8.24
1277stripHash "/nix/store/9s9r019176g7cvn2nvcw41gsp862y6b4-coreutils-8.24"
1278```
1279
1280If you wish to store the result in another variable, then the following idiom may be useful:
1281
1282```bash
1283name="/nix/store/9s9r019176g7cvn2nvcw41gsp862y6b4-coreutils-8.24"
1284someVar=$(stripHash $name)
1285```
1286
1287### `wrapProgram` \<executable\> \<makeWrapperArgs\> {#fun-wrapProgram}
1288
1289Convenience function for `makeWrapper` that replaces `<executable>` with a wrapper that executes the original program. It takes all the same arguments as `makeWrapper`, except for `--inherit-argv0` (used by the `makeBinaryWrapper` implementation) and `--argv0` (used by both `makeWrapper` and `makeBinaryWrapper` wrapper implementations).
1290
1291If you will apply it multiple times, it will overwrite the wrapper file and you will end up with double wrapping, which should be avoided.
1292
1293### `prependToVar` \<variableName\> \<elements...\> {#fun-prependToVar}
1294
1295Prepend elements to a variable.
1296
1297Example:
1298
1299```shellSession
1300$ configureFlags="--disable-static"
1301$ prependToVar configureFlags --disable-dependency-tracking --enable-foo
1302$ echo $configureFlags
1303--disable-dependency-tracking --enable-foo --disable-static
1304```
1305
1306### `appendToVar` \<variableName\> \<elements...\> {#fun-appendToVar}
1307
1308Append elements to a variable.
1309
1310Example:
1311
1312```shellSession
1313$ configureFlags="--disable-static"
1314$ appendToVar configureFlags --disable-dependency-tracking --enable-foo
1315$ echo $configureFlags
1316--disable-static --disable-dependency-tracking --enable-foo
1317```
1318
1319## Package setup hooks {#ssec-setup-hooks}
1320
1321Nix itself considers a build-time dependency as merely something that should previously be built and accessible at build time—packages themselves are on their own to perform any additional setup. In most cases, that is fine, and the downstream derivation can deal with its own dependencies. But for a few common tasks, that would result in almost every package doing the same sort of setup work—depending not on the package itself, but entirely on which dependencies were used.
1322
1323In order to alleviate this burden, the setup hook 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.
1324
1325The setup hook mechanism is a bit of a sledgehammer though: a powerful feature with a broad and indiscriminate area of effect. The combination of its power and implicit use may be expedient, but isn’t without costs. Nix itself is unchanged, but the spirit of added dependencies being effect-free is violated even if the latter isn’t. For example, if a derivation path is mentioned more than once, Nix itself doesn’t care and 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.
1326
1327The most typical use of the setup hook is actually to add other hooks which are then run (i.e. after all the setup hooks) on each dependency. For example, the C compiler wrapper’s setup hook feeds itself flags for each dependency that contains relevant libraries and headers. This is done by defining a bash function, and appending its name to one of `envBuildBuildHooks`, `envBuildHostHooks`, `envBuildTargetHooks`, `envHostHostHooks`, `envHostTargetHooks`, or `envTargetTargetHooks`. 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).
1328
1329Packages adding a hook should not hard code a specific hook, but rather choose a variable *relative* to how they are included. Returning to the C compiler wrapper example, if the wrapper itself is an `n` dependency, then it only wants to accumulate flags from `n + 1` dependencies, as only those ones match the compiler’s target platform. The `hostOffset` variable is defined with the current dependency’s host offset `targetOffset` with its target offset, before its setup hook is sourced. Additionally, since most environment hooks don’t care about the target platform, that means the setup hook can append to the right bash array by doing something like
1330
1331```bash
1332addEnvHooks "$hostOffset" myBashFunction
1333```
1334
1335The *existence* of setups hooks has long been documented and packages inside Nixpkgs are free to use this mechanism. Other packages, however, should not rely on these mechanisms not changing between Nixpkgs versions. Because of the existing issues with this system, there’s little benefit from mandating it be stable for any period of time.
1336
1337First, let’s cover some setup hooks that are part of Nixpkgs default `stdenv`. This means that they are run for every package built using `stdenv.mkDerivation` or when using a custom builder that has `source $stdenv/setup`. Some of these are platform specific, so they may run on Linux but not Darwin or vice-versa.
1338
1339### `move-docs.sh` {#move-docs.sh}
1340
1341This setup hook moves any installed documentation to the `/share` subdirectory directory. This includes the man, doc and info directories. This is needed for legacy programs that do not know how to use the `share` subdirectory.
1342
1343### `compress-man-pages.sh` {#compress-man-pages.sh}
1344
1345This setup hook compresses any man pages that have been installed. The compression is done using the gzip program. This helps to reduce the installed size of packages.
1346
1347### `strip.sh` {#strip.sh}
1348
1349This runs the strip command on installed binaries and libraries. This removes unnecessary information like debug symbols when they are not needed. This also helps to reduce the installed size of packages.
1350
1351### `patch-shebangs.sh` {#patch-shebangs.sh}
1352
1353This setup hook patches installed scripts to add Nix store paths to their shebang interpreter as found in the build environment. The [shebang](https://en.wikipedia.org/wiki/Shebang_(Unix)) line tells a Unix-like operating system which interpreter to use to execute the script's contents.
1354
1355::: {.note}
1356The [generic builder][generic-builder] populates `PATH` from inputs of the derivation.
1357:::
1358
1359[generic-builder]: https://github.com/NixOS/nixpkgs/blob/19d4f7dc485f74109bd66ef74231285ff797a823/pkgs/stdenv/generic/builder.sh
1360
1361#### Invocation {#patch-shebangs.sh-invocation}
1362
1363Multiple paths can be specified.
1364
1365```
1366patchShebangs [--build | --host] PATH...
1367```
1368
1369##### Flags {#patch-shebangs.sh-invocation-flags}
1370
1371`--build`
1372: Look up commands available at build time
1373
1374`--host`
1375: Look up commands available at run time
1376
1377##### Examples {#patch-shebangs.sh-invocation-examples}
1378
1379```sh
1380patchShebangs --host /nix/store/<hash>-hello-1.0/bin
1381```
1382
1383```sh
1384patchShebangs --build configure
1385```
1386
1387`#!/bin/sh` will be rewritten to `#!/nix/store/<hash>-some-bash/bin/sh`.
1388
1389`#!/usr/bin/env` gets special treatment: `#!/usr/bin/env python` is rewritten to `/nix/store/<hash>/bin/python`.
1390
1391Interpreter paths that point to a valid Nix store location are not changed.
1392
1393::: {.note}
1394A script file must be marked as executable, otherwise it will not be
1395considered.
1396:::
1397
1398This mechanism ensures that the interpreter for a given script is always found and is exactly the one specified by the build.
1399
1400It can be disabled by setting [`dontPatchShebangs`](#var-stdenv-dontPatchShebangs):
1401
1402```nix
1403stdenv.mkDerivation {
1404 # ...
1405 dontPatchShebangs = true;
1406 # ...
1407}
1408```
1409
1410The file [`patch-shebangs.sh`][patch-shebangs.sh] defines the [`patchShebangs`][patchShebangs] function. It is used to implement [`patchShebangsAuto`][patchShebangsAuto], the [setup hook](#ssec-setup-hooks) that is registered to run during the [fixup phase](#ssec-fixup-phase) by default.
1411
1412If you need to run `patchShebangs` at build time, it must be called explicitly within [one of the build phases](#sec-stdenv-phases).
1413
1414[patch-shebangs.sh]: https://github.com/NixOS/nixpkgs/blob/19d4f7dc485f74109bd66ef74231285ff797a823/pkgs/build-support/setup-hooks/patch-shebangs.sh
1415[patchShebangs]: https://github.com/NixOS/nixpkgs/blob/19d4f7dc485f74109bd66ef74231285ff797a823/pkgs/build-support/setup-hooks/patch-shebangs.sh#L24-L105
1416[patchShebangsAuto]: https://github.com/NixOS/nixpkgs/blob/19d4f7dc485f74109bd66ef74231285ff797a823/pkgs/build-support/setup-hooks/patch-shebangs.sh#L107-L119
1417
1418### `audit-tmpdir.sh` {#audit-tmpdir.sh}
1419
1420This verifies that no references are left from the install binaries to the directory used to build those binaries. This ensures that the binaries do not need things outside the Nix store. This is currently supported in Linux only.
1421
1422### `multiple-outputs.sh` {#multiple-outputs.sh}
1423
1424This setup hook adds configure flags that tell packages to install files into any one of the proper outputs listed in `outputs`. This behavior can be turned off by setting `setOutputFlags` to false in the derivation environment. See [](#chap-multiple-output) for more information.
1425
1426### `move-sbin.sh` {#move-sbin.sh}
1427
1428This setup hook moves any binaries installed in the `sbin/` subdirectory into `bin/`. In addition, a link is provided from `sbin/` to `bin/` for compatibility.
1429
1430### `move-lib64.sh` {#move-lib64.sh}
1431
1432This setup hook moves any libraries installed in the `lib64/` subdirectory into `lib/`. In addition, a link is provided from `lib64/` to `lib/` for compatibility.
1433
1434### `move-systemd-user-units.sh` {#move-systemd-user-units.sh}
1435
1436This setup hook moves any systemd user units installed in the `lib/` subdirectory into `share/`. In addition, a link is provided from `share/` to `lib/` for compatibility. This is needed for systemd to find user services when installed into the user profile.
1437
1438This hook only runs when compiling for Linux.
1439
1440### `set-source-date-epoch-to-latest.sh` {#set-source-date-epoch-to-latest.sh}
1441
1442This sets `SOURCE_DATE_EPOCH` to the modification time of the most recent file.
1443
1444### Bintools Wrapper and hook {#bintools-wrapper}
1445
1446The Bintools Wrapper wraps the binary utilities for a bunch of miscellaneous purposes. These are GNU Binutils when targeting Linux, and a mix of cctools and GNU binutils for Darwin. \[The “Bintools” name is supposed to be a compromise between “Binutils” and “cctools” not denoting any specific implementation.\] Specifically, the underlying bintools package, and a C standard library (glibc or Darwin’s libSystem, just for the dynamic loader) are all fed in, and dependency finding, hardening (see below), and purity checks for each are handled by the Bintools Wrapper. Packages typically depend on CC Wrapper, which in turn (at run time) depends on the Bintools Wrapper.
1447
1448The 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 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. `buildInputs` and `nativeBuildInputs`) in environment variables. The Bintools Wrapper’s setup hook causes any `lib` and `lib64` subdirectories to be added to `NIX_LDFLAGS`. Since the CC Wrapper and the Bintools Wrapper use the same strategy, most of the Bintools Wrapper code is sparsely commented and refers to the CC Wrapper. But the CC Wrapper’s code, by contrast, has quite lengthy comments. The Bintools Wrapper merely cites those, rather than repeating them, to avoid falling out of sync.
1449
1450A final task of the setup hook is defining a number of standard environment variables to tell build systems which executables fulfill which purpose. They are defined to just be the base name of the tools, under the assumption that the Bintools Wrapper’s binaries will be on the path. Firstly, this helps poorly-written packages, e.g. ones that look for just `gcc` when `CC` isn’t defined yet `clang` 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-stdenv-per-platform-wrapper] `BUILD_`- and `TARGET_`-prefixed versions of the normal environment variable are defined for additional Bintools Wrappers, properly disambiguating them.
1451
1452A problem with this final task is that the Bintools Wrapper is honest and defines `LD` as `ld`. Most packages, however, firstly use the C compiler for linking, secondly use `LD` anyways, defining it as the C compiler, and thirdly, only so define `LD` 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, `LD` as the C compiler. A good way to do this would be `preConfigure = "LD=$CC"`.
1453
1454### CC Wrapper and hook {#cc-wrapper}
1455
1456The CC Wrapper wraps a C toolchain for a bunch of miscellaneous purposes. Specifically, a C compiler (GCC or Clang), wrapped binary tools, and a C standard library (glibc or Darwin’s libSystem, just for the dynamic loader) are all fed in, and dependency finding, hardening (see below), and purity checks for each are handled by the CC Wrapper. Packages typically depend on the CC Wrapper, which in turn (at run-time) depends on the Bintools Wrapper.
1457
1458Dependency finding is undoubtedly the main task of the CC Wrapper. This works just like the Bintools Wrapper, except that any `include` subdirectory of any relevant dependency is added to `NIX_CFLAGS_COMPILE`. The setup hook itself contains elaborate comments describing the exact mechanism by which this is accomplished.
1459
1460Similarly, the CC Wrapper follows the Bintools Wrapper in defining standard environment variables with the names of the tools it wraps, for the same reasons described above. Importantly, while it includes a `cc` symlink to the c compiler for portability, the `CC` will be defined using the compiler’s “real name” (i.e. `gcc` or `clang`). This helps lousy build systems that inspect on the name of the compiler rather than run it.
1461
1462Here are some more packages that provide a setup hook. Since the list of hooks is extensible, this is not an exhaustive list. The mechanism is only to be used as a last resort, so it might cover most uses.
1463
1464### Other hooks {#stdenv-other-hooks}
1465
1466Many other packages provide hooks, that are not part of `stdenv`. You can find
1467these in the [Hooks Reference](#chap-hooks).
1468
1469### Compiler and Linker wrapper hooks {#compiler-linker-wrapper-hooks}
1470
1471If the file `${cc}/nix-support/cc-wrapper-hook` exists, it will be run at the end of the [compiler wrapper](#cc-wrapper).
1472If the file `${binutils}/nix-support/post-link-hook` exists, it will be run at the end of the linker wrapper.
1473These hooks allow a user to inject code into the wrappers.
1474As an example, these hooks can be used to extract `extraBefore`, `params` and `extraAfter` which store all the command line arguments passed to the compiler and linker respectively.
1475
1476## Purity in Nixpkgs {#sec-purity-in-nixpkgs}
1477
1478*Measures taken to prevent dependencies on packages outside the store, and what you can do to prevent them.*
1479
1480GCC 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.
1481
1482## Hardening in Nixpkgs {#sec-hardening-in-nixpkgs}
1483
1484There are flags available to harden packages at compile or link-time. These can be toggled using the `stdenv.mkDerivation` parameters `hardeningDisable` and `hardeningEnable`.
1485
1486Both 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.
1487
1488For more in-depth information on these hardening flags and hardening in general, refer to the [Debian Wiki](https://wiki.debian.org/Hardening), [Ubuntu Wiki](https://wiki.ubuntu.com/Security/Features), [Gentoo Wiki](https://wiki.gentoo.org/wiki/Project:Hardened), and the [Arch Wiki](https://wiki.archlinux.org/title/Security).
1489
1490### Hardening flags enabled by default {#sec-hardening-flags-enabled-by-default}
1491
1492The following flags are enabled by default and might require disabling with `hardeningDisable` if the program to package is incompatible.
1493
1494#### `format` {#format}
1495
1496Adds 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`.
1497
1498This needs to be turned off or fixed for errors similar to:
1499
1500```
1501/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]
1502 printf(help_message);
1503 ^
1504cc1plus: some warnings being treated as errors
1505```
1506
1507#### `stackprotector` {#stackprotector}
1508
1509Adds 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).
1510
1511This needs to be turned off or fixed for errors similar to:
1512
1513```
1514bin/blib.a(bios_console.o): In function `bios_handle_cup':
1515/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'
1516```
1517
1518#### `fortify` {#fortify}
1519
1520Adds 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.
1521
1522Additionally, some warnings are enabled which might trigger build failures if compiler warnings are treated as errors in the package build. In this case, set `env.NIX_CFLAGS_COMPILE` to `-Wno-error=warning-type`.
1523
1524This needs to be turned off or fixed for errors similar to:
1525
1526```
1527malloc.c:404:15: error: return type is an incomplete type
1528malloc.c:410:19: error: storage size of 'ms' isn't known
1529
1530strdup.h:22:1: error: expected identifier or '(' before '__extension__'
1531
1532strsep.c:65:23: error: register name not specified for 'delim'
1533
1534installwatch.c:3751:5: error: conflicting types for '__open_2'
1535
1536fcntl2.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
1537```
1538
1539#### `pic` {#pic}
1540
1541Adds the `-fPIC` compiler options. This options adds support for position independent code in shared libraries and thus making ASLR possible.
1542
1543Most 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.
1544
1545This needs to be turned off or fixed for assembler errors similar to:
1546
1547```
1548ccbLfRgg.s: Assembler messages:
1549ccbLfRgg.s:33: Error: missing or invalid displacement expression `private_key_len@GOTOFF'
1550```
1551
1552#### `strictoverflow` {#strictoverflow}
1553
1554Signed 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.
1555
1556This flag should not trigger any build or runtime errors.
1557
1558#### `relro` {#relro}
1559
1560Adds 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.
1561
1562This 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.
1563
1564#### `bindnow` {#bindnow}
1565
1566Adds the `-z now` 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.
1567
1568This 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:
1569
1570```
1571intel_drv.so: undefined symbol: vgaHWFreeHWRec
1572```
1573
1574### Hardening flags disabled by default {#sec-hardening-flags-disabled-by-default}
1575
1576The following flags are disabled by default and should be enabled with `hardeningEnable` for packages that take untrusted input like network services.
1577
1578#### `pie` {#pie}
1579
1580This flag is disabled by default for normal `glibc` based NixOS package builds, but enabled by default for `musl` based package builds.
1581
1582Adds 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.
1583
1584Static libraries need to be compiled with `-fPIE` so that executables can link them in with the `-pie` linker option.
1585If the libraries lack `-fPIE`, you will get the error `recompile with -fPIE`.
1586
1587[^footnote-stdenv-ignored-build-platform]: 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 *specified* as interfaces, not concrete implementation.
1588[^footnote-stdenv-native-dependencies-in-path]: Currently, this means for native builds all dependencies are put on the `PATH`. 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 `depsBuild*` and `nativeBuildInputs` would be added to the `PATH`.
1589[^footnote-stdenv-propagated-dependencies]: Nix itself already takes a package’s transitive dependencies into account, but this propagation ensures nixpkgs-specific infrastructure like [setup hooks](#ssec-setup-hooks) also are run as if it were a propagated dependency.
1590[^footnote-stdenv-find-inputs-location]: The `findInputs` function, currently residing in `pkgs/stdenv/generic/setup.sh`, implements the propagation logic.
1591[^footnote-stdenv-sys-lib-search-path]: It clears the `sys_lib_*search_path` variables in the Libtool script to prevent Libtool from using libraries in `/usr/lib` and such.
1592[^footnote-stdenv-build-time-guessing-impurity]: Eventually these will be passed building natively as well, to improve determinism: build-time guessing, as is done today, is a risk of impurity.
1593[^footnote-stdenv-per-platform-wrapper]: 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.