1 [build constraints]: https://golang.org/pkg/go/build/#hdr-Build_Constraints 2 [select]: https://docs.bazel.build/versions/master/be/functions.html#select 3 [config_setting]: https://docs.bazel.build/versions/master/be/general.html#config_setting 4 [Gazelle]: https://github.com/bazelbuild/bazel-gazelle 5 6 7## Platform-specific dependencies 8 9When cross-compiling, you may have some platform-specific sources and 10dependencies. Source files from all platforms can be mixed freely in a single 11`srcs` list. Source files are filtered using [build constraints] (filename 12suffixes and `+build` tags) before being passed to the compiler. 13 14Platform-specific dependencies are another story. For example, if you are 15building a binary for Linux, and it has dependency that should only be built 16when targeting Windows, you will need to filter it out using Bazel [select] 17expressions: 18 19``` bzl 20go_binary( 21 name = "cmd", 22 srcs = [ 23 "foo_linux.go", 24 "foo_windows.go", 25 ], 26 deps = [ 27 # platform agnostic dependencies 28 "//bar", 29 ] + select({ 30 # OS-specific dependencies 31 "@io_bazel_rules_go//go/platform:linux": [ 32 "//baz_linux", 33 ], 34 "@io_bazel_rules_go//go/platform:windows": [ 35 "//quux_windows", 36 ], 37 "//conditions:default": [], 38 }), 39) 40``` 41 42`select` accepts a dictionary argument. The keys are labels that reference [config_setting] rules. 43The values are lists of labels. Exactly one of these 44lists will be selected, depending on the target configuration. rules_go has 45pre-declared `config_setting` rules for each OS, architecture, and 46OS-architecture pair. For a full list, run this command: 47 48``` bash 49$ bazel query 'kind(config_setting, @io_bazel_rules_go//go/platform:all)' 50``` 51 52[Gazelle] will generate dependencies in this format automatically. 53 54