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[build constraints]: https://golang.org/pkg/go/build/#hdr-Build_Constraints
[select]: https://docs.bazel.build/versions/master/be/functions.html#select
[config_setting]: https://docs.bazel.build/versions/master/be/general.html#config_setting
[Gazelle]: https://github.com/bazelbuild/bazel-gazelle
## Platform-specific dependencies
When cross-compiling, you may have some platform-specific sources and
dependencies. Source files from all platforms can be mixed freely in a single
`srcs` list. Source files are filtered using [build constraints] (filename
suffixes and `+build` tags) before being passed to the compiler.
Platform-specific dependencies are another story. For example, if you are
building a binary for Linux, and it has dependency that should only be built
when targeting Windows, you will need to filter it out using Bazel [select]
expressions:
``` bzl
go_binary(
name = "cmd",
srcs = [
"foo_linux.go",
"foo_windows.go",
],
deps = [
# platform agnostic dependencies
"//bar",
] + select({
# OS-specific dependencies
"@io_bazel_rules_go//go/platform:linux": [
"//baz_linux",
],
"@io_bazel_rules_go//go/platform:windows": [
"//quux_windows",
],
"//conditions:default": [],
}),
)
```
`select` accepts a dictionary argument. The keys are labels that reference [config_setting] rules.
The values are lists of labels. Exactly one of these
lists will be selected, depending on the target configuration. rules_go has
pre-declared `config_setting` rules for each OS, architecture, and
OS-architecture pair. For a full list, run this command:
``` bash
$ bazel query 'kind(config_setting, @io_bazel_rules_go//go/platform:all)'
```
[Gazelle] will generate dependencies in this format automatically.
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