# Platform Notes

## Darwin (macOS)

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.

            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 won't 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`.

            stdenv.mkDerivation {
              name = "libfoo-1.2.3";
              # ...
              makeFlags = stdenv.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

            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 doesn't actually depend on xcode.

            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.