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# Platform Notes {#chap-platform-notes}
## Darwin (macOS) {#sec-darwin}
Some common issues when packaging software for Darwin:
- The Darwin `stdenv` uses clang instead of gcc. When referring to the compiler `$CC` or `cc` will work in both cases. Some builds hardcode gcc/g++ in their build scripts, that can usually be fixed with using something like `makeFlags = [ "CC=cc" ];` or by patching the build scripts.
```nix
stdenv.mkDerivation {
name = "libfoo-1.2.3";
# ...
buildPhase = ''
$CC -o hello hello.c
'';
}
```
- On Darwin, libraries are linked using absolute paths, libraries are resolved by their `install_name` at link time. Sometimes packages wont set this correctly causing the library lookups to fail at runtime. This can be fixed by adding extra linker flags or by running `install_name_tool -id` during the `fixupPhase`.
```nix
stdenv.mkDerivation {
name = "libfoo-1.2.3";
# ...
makeFlags = lib.optional stdenv.isDarwin "LDFLAGS=-Wl,-install_name,$(out)/lib/libfoo.dylib";
}
```
- Even if the libraries are linked using absolute paths and resolved via their `install_name` correctly, tests can sometimes fail to run binaries. This happens because the `checkPhase` runs before the libraries are installed.
This can usually be solved by running the tests after the `installPhase` or alternatively by using `DYLD_LIBRARY_PATH`. More information about this variable can be found in the *dyld(1)* manpage.
```
dyld: Library not loaded: /nix/store/7hnmbscpayxzxrixrgxvvlifzlxdsdir-jq-1.5-lib/lib/libjq.1.dylib
Referenced from: /private/tmp/nix-build-jq-1.5.drv-0/jq-1.5/tests/../jq
Reason: image not found
./tests/jqtest: line 5: 75779 Abort trap: 6
```
```nix
stdenv.mkDerivation {
name = "libfoo-1.2.3";
# ...
doInstallCheck = true;
installCheckTarget = "check";
}
```
- Some packages assume xcode is available and use `xcrun` to resolve build tools like `clang`, etc. This causes errors like `xcode-select: error: no developer tools were found at '/Applications/Xcode.app'` while the build doesnt actually depend on xcode.
```nix
stdenv.mkDerivation {
name = "libfoo-1.2.3";
# ...
prePatch = ''
substituteInPlace Makefile \
--replace '/usr/bin/xcrun clang' clang
'';
}
```
The package `xcbuild` can be used to build projects that really depend on Xcode. However, this replacement is not 100% compatible with Xcode and can occasionally cause issues.
- x86_64-darwin uses the 10.12 SDK by default, but some software is not compatible with that version of the SDK. In that case,
the 11.0 SDK used by aarch64-darwin is available for use on x86_64-darwin. To use it, reference `apple_sdk_11_0` instead of
`apple_sdk` in your derivation and use `pkgs.darwin.apple_sdk_11_0.callPackage` instead of `pkgs.callPackage`. On Linux, this will
have the same effect as `pkgs.callPackage`, so you can use `pkgs.darwin.apple_sdk_11_0.callPackage` regardless of platform.