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depot/third_party/nixpkgs/doc/stdenv/platform-notes.chapter.md

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Platform Notes

Darwin (macOS)

The Darwin stdenv differs from most other ones in Nixpkgs in a few key ways. These differences reflect the default assumptions for building software on that platform. In many cases, you can ignore these differences because the software you are packaging is already written with them in mind. When you do that, write your derivation as normal. You don’t have to include any Darwin-specific special cases. The easiest way to know whether your derivation requires special handling for Darwin is to write it as if it doesn’t and see if it works. If it does, you’re done; skip the rest of this.

  • Darwin uses Clang by default instead of GCC. Packages that refer to $CC or cc should just work in most cases. Some packages may hardcode gcc or g++. You can usually fix that by setting makeFlags = [ "CC=cc" "CXX=C++" ]. If that does not work, you will have to patch the build scripts yourself to use the correct compiler for Darwin.
  • Darwin needs an SDK to build software. The SDK provides a default set of frameworks and libraries to build software, most of which are specific to Darwin. There are multiple versions of the SDK packages in Nixpkgs, but one is included by default in the stdenv. Usually, you don’t have to change or pick a different SDK. When in doubt, use the default.
  • The SDK used by your build can be found using the DEVELOPER_DIR environment variable. There are also versions of this variable available when cross-compiling depending on the SDK’s role. The SDKROOT variable is also set with the path to the SDK’s libraries and frameworks. SDKROOT is always a sub-folder of DEVELOPER_DIR.
  • Darwin includes a platform-specific tool called xcrun to help builds locate binaries they need. A version of xcrun is part of the stdenv on Darwin. If your package invokes xcrun via an absolute path (such as /usr/bin/xcrun), you will need to patch the build scripts to use xcrun instead.

To reiterate: you usually don’t have to worry about this stuff. Start with writing your derivation as if everything is already set up for you (because in most cases it already is). If you run into issues or failures, continue reading below for how to deal with the most common issues you may encounter.

Darwin Issue Troubleshooting

Package requires a non-default SDK or fails to build due to missing frameworks or symbols

In some cases, you may have to use a non-default SDK. This can happen when a package requires APIs that are not present in the default SDK. For example, Metal Performance Shaders were added in macOS 12. If the default SDK is 11.3, then a package that requires Metal Performance Shaders will fail to build due to missing frameworks and symbols.

To use a non-default SDK, add it to your derivation’s buildInputs. It is not necessary to override the SDK in the stdenv nor is it necessary to override the SDK used by your dependencies. If your derivation needs a non-default SDK at build time (e.g., for a depsBuildBuild compiler), see the cross-compilation documentation for which input you should use.

When determining whether to use a non-default SDK, consider the following:

  • Try building your derivation with the default SDK. If it works, you’re done.
  • If the package specifies a specific version, use that. See below for how to map Xcode version to SDK version.
  • If the package’s documentation indicates it supports optional features on newer SDKs, consider using the SDK that enables those features. If you’re not sure, use the default SDK.

Note: It is possible to have multiple, different SDK versions in your inputs. When that happens, the one with the highest version is always used.

stdenv.mkDerivation {
  name = "libfoo-1.2.3";
  # ...
  buildInputs = [ apple-sdk_14 ];
}

What is a “deployment target” (or minimum version)?

The “deployment target” refers to the minimum version of macOS that is expected to run an application. In most cases, the default is fine, and you don’t have to do anything else. If you’re not sure, don’t do anything, and that will probably be fine.

Some packages require setting a non-default deployment target (or minimum version) to gain access to certain APIs. You do that using the darwinMinVersionHook, which takes the deployment target version as a parameter. There are primarily two ways to determine the deployment target.

  • The upstream documentation will specify a deployment target or minimum version. Use that.
  • The build will fail because an API requires a certain version. Use that.
  • In all other cases, you probably don’t need to specify a minimum version. The default is usually good enough.
stdenv.mkDerivation {
  name = "libfoo-1.2.3"; # Upstream specifies the minimum supported version as 12.5.
  buildInputs = [ (darwinMinVersionHook "12.5") ];
}

Note: It is possible to have multiple, different instances of darwinMinVerisonHook in your inputs. When that happens, the one with the highest version is always used.

Picking an SDK version

The following is a list of Xcode versions, the SDK version in Nixpkgs, and the attribute to use to add it. Check your package’s documentation (platform support or installation instructions) to find which Xcode or SDK version to use. Generally, only the last SDK release for a major version is packaged (each x in 10.x until 10.15 is considered a major version).

Xcode version SDK version Nixpkgs attribute
Varies by platform 10.12.2 (x86_64-darwin)
11.3 (aarch64-darwin)		 apple-sdk
8.0–8.3.3 10.12.2 apple-sdk_10_12
9.0–9.4.1 10.13.2 apple-sdk_10_13
10.0–10.3 10.14.6 apple-sdk_10_14
11.0–11.7 10.15.6 apple-sdk_10_15
12.0–12.5.1 11.3 apple-sdk_11
13.0–13.4.1 12.3 apple-sdk_12
14.0–14.3.1 13.3 apple-sdk_13
15.0–15.4 14.4 apple-sdk_14
16.0 15.0 apple-sdk_15

Darwin Default SDK versions

The current default versions of the deployment target (minimum version) and SDK are indicated by Darwin-specific attributes on the platform. Because of the ways that minimum version and SDK can be changed that are not visible to Nix, they should be treated as lower bounds. If you need to parameterize over a specific version, create a function that takes the version as a parameter instead of relying on these attributes.

  • darwinMinVersion defaults to 10.12 on x86_64-darwin and 11.0 on aarch64-darwin. It sets the default deployment target.
  • darwinSdkVersion defaults to 10.12 on x86-64-darwin and 11.0 on aarch64-darwin. Only the major version determines the SDK version, resulting in the 10.12.2 and 11.3 SDKs being used on these platforms respectively.

xcrun cannot find a binary

xcrun searches PATH and the SDK’s toolchain for binaries to run. If it cannot find a required binary, it will fail. When that happens, add the package for that binary to your derivation’s nativeBuildInputs (or nativeCheckInputs if the failure is happening when running tests).

stdenv.mkDerivation {
  name = "libfoo-1.2.3";
  # ...
  nativeBuildInputs = [ bison ];
  buildCommand = ''
    xcrun bison foo.y # produces foo.tab.c
    # ...
  '';
}

Package requires xcodebuild

The xcbuild package provides an xcodebuild command for packages that really depend on Xcode. This replacement is not 100% compatible and may run into some issues, but it is able to build many packages. To use xcodebuild, add xcbuildHook to your package’s nativeBuildInputs. It will provide a buildPhase for your derivation. You can use xcbuildFlags to specify flags to xcodebuild such as the required schema. If a schema has spaces in its name, you must set __structuredAttrs to true. See MoltenVK for an example of setting up xcbuild.

stdenv.mkDerivation {
  name = "libfoo-1.2.3";
  xcbuildFlags = [
    "-configuration"
    "Release"
    "-project"
    "libfoo-project.xcodeproj"
    "-scheme"
    "libfoo Package (macOS only)"
  ];
  __structuredAttrs = true;
}
Fixing absolute paths to xcodebuild, xcrun, and PlistBuddy

Many build systems hardcode the absolute paths to xcodebuild, xcrun, and PlistBuddy as /usr/bin/xcodebuild, /usr/bin/xcrun, and /usr/libexec/PlistBuddy respectively. These paths will need to be replaced with relative paths and the xcbuild package if xcodebuild or PListBuddy are used.

stdenv.mkDerivation {
  name = "libfoo-1.2.3";
  postPatch = ''
    substituteInPlace Makefile \
      --replace-fail '/usr/bin/xcodebuild' 'xcodebuild' \
      --replace-fail '/usr/bin/xcrun' 'xcrun' \
      --replace-fail '/usr/bin/PListBuddy' 'PListBuddy'
  '';
}

How to use libiconv on Darwin

The libiconv package is included in the SDK by default along with libresolv and libsbuf. You do not need to do anything to use these packages. They are available automatically. If your derivation needs the iconv binary, add the libiconv package to your nativeBuildInputs (or nativeCheckInputs for tests).

Library install name issues

Libraries on Darwin are usually linked with absolute paths. This is determined by something called an “install name”, which is resolved at link time. Sometimes packages will not set this correctly, causing binaries linking to it not to find their libraries at runtime. This can be fixed by adding extra linker flags or by using install_name_tool to set it in fixupPhase.

Setting the install name via linker flags
stdenv.mkDerivation {
  name = "libfoo-1.2.3";
  # ...
  makeFlags = lib.optional stdenv.hostPlatform.isDarwin "LDFLAGS=-Wl,-install_name,$(out)/lib/libfoo.dylib";
}
Setting the install name using install_name_tool
stdenv.mkDerivation {
  name = "libfoo-1.2.3";
  # ...
  postFixup = ''
    # `-id <install_name>` takes the install name. The last parameter is the path to the library.
    ${stdenv.cc.targetPrefix}install_name_tool -id "$out/lib/libfoo.dylib" "$out/lib/libfoo.dylib"
  '';
}

Even if libraries are linked using absolute paths and resolved via their install name correctly, tests in checkPhase can sometimes fail to run binaries because they are linked against libraries that have not yet been installed. This can usually be solved by running the tests after the installPhase or by using DYLD_LIBRARY_PATH (see {manpage}dyld(1) for more on setting DYLD_LIBRARY_PATH).

Setting the install name using fixDarwinDylibNames hook

If your package has numerous dylibs needing fixed, while it is preferable to fix the issue in the package’s build, you can update them all by adding the fixDarwinDylibNames hook to your nativeBuildInputs. This hook will scan your package’s outputs for dylibs and correct their install names. Note that if any binaries in your outputs linked those dylibs, you may need to use install_name_tool to replace references to them with the correct paths.

Propagating an SDK (advanced, compilers-only)

The SDK is a package, and it can be propagated. darwinMinVersionHook with a version specified can also be propagated. However, most packages should not do this. The exception is compilers. When you propagate an SDK, it becomes part of your derivation’s public API, and changing the SDK or removing it can be a breaking change. That is why propagating it is only recommended for compilers.

When authoring a compiler derivation, propagate the SDK only for the ways you expect users to use your compiler. Depending on your expected use cases, you may have to do one or all of these.

  • Put it in depsTargetTargetPropagated when your compiler is expected to be added to nativeBuildInputs. That will ensure the SDK is effectively part of the target derivation’s buildInputs.
  • If your compiler uses a hook, put it in the hook’s depsTargetTargetPropagated instead. The effect should be the same as the above.
  • If your package uses the builder pattern, update your builder to add the SDK to the derivation’s buildInputs.

If you’re not sure whether to propagate an SDK, don’t. If your package is a compiler or language, and you’re not sure, ask @NixOS/darwin-maintainers for help deciding.

Dealing with darwin.apple_sdk.frameworks

You may see references to darwin.apple_sdk.frameworks. This is the legacy SDK pattern, and it is being phased out. All packages in darwin.apple_sdk, darwin.apple_sdk_11_0, and darwin.apple_sdk_12_3 are stubs that do nothing. If your derivation references them, you can delete them. The default SDK should be enough to build your package.

Note: the new SDK pattern uses the name apple-sdk to better align with Nixpkgs naming conventions. The legacy SDK pattern uses apple_sdk. You always know you are using the old SDK pattern if the name is apple_sdk.

Some derivations may depend on the location of frameworks in those old packages. To update your derivation to find them in the new SDK, use $SDKROOT instead in preConfigure. For example, if you substitute ${darwin.apple_sdk.frameworks.OpenGL}/Library/Frameworks/OpenGL.framework in postPatch, replace it with $SDKROOT/System/Library/Frameworks/OpenGL.framework in preConfigure.

Note that if your derivation is changing a system path (such as /System/Library/Frameworks/OpenGL.framework), you may be able to remove the path. Compilers and binutils targeting Darwin look for system paths in the SDK sysroot. Some of them (such as Zig or bindgen for Rust) depend on it.

Updating legacy SDK overrides

The legacy SDK provided two ways of overriding the default SDK. These are both being phased out along with the legacy SDKs. They have been updated to set up the new SDK for you, but you should replace them with doing that directly.

  • pkgs.darwin.apple_sdk_11_0.callPackage - this pattern was used to provide frameworks from the 11.0 SDK. It now adds the apple-sdk_11 package to your derivation’s build inputs.
  • overrideSDK - this stdenv adapter would try to replace the frameworks used by your derivation and its transitive dependencies. It now adds the apple-sdk_11 package for 11.0 or the apple-sdk_12 package for 12.3. If darwinMinVersion is specified, it will add darwinMinVersionHook with the specified minimum version. No other SDK versions are supported.

Darwin Cross-Compilation

Darwin supports cross-compilation between Darwin platforms. Cross-compilation from Linux is not currently supported but may be supported in the future. To cross-compile to Darwin, you can set crossSystem or use one of the Darwin systems in pkgsCross. The darwinMinVersionHook and the SDKs support cross-compilation. If you need to specify a different SDK version for a depsBuildBuild compiler, add it to your nativeBuildInputs.

stdenv.mkDerivation {
  name = "libfoo-1.2.3";
  # ...
  depsBuildBuild = [ buildPackages.stdenv.cc ];
  nativeBuildInputs = [ apple-sdk_12 ];
  buildInputs = [ apple-sdk_13 ];
  depsTargetTargetPropagated = [ apple-sdk_14 ];
}
# The build-build `clang` will use the 12.3 SDK while the package build itself will use the 13.3 SDK.
# Derivations that add this package as an input will have the 14.4 SDK propagated to them.

The different target SDK and hooks are mangled based on role:

  • DEVELOPER_DIR_FOR_BUILD and MACOSX_DEPLOYMENT_TARGET_FOR_BUILD for the build platform;
  • DEVELOPER_DIR and MACOSX_DEPLOYMENT_TARGET for the host platform; and
  • DEVELOPER_DIR_FOR_TARGET and MACOSX_DEPLOYMENT_TARGET_FOR_TARGET for the build platform.

In static compilation situations, it is possible for the build and host platform to be the same platform but have different SDKs with the same version (one dynamic and one static). cc-wrapper and bintools-wrapper take care of handling this distinction.