Index: docs/gn_check.md |
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+GN has several different ways to check dependencies. Many of them are checked by the `gn check` command. Running checks involve opening and scanning all source files so this isn't run every time a build is updated. To run check on an existing build: |
+``` |
+gn check out/mybuild |
+``` |
+ |
+To run the check as part of the "gen" command to update the build (this is what the bots do): |
+``` |
+gn gen out/mybuild --check |
+``` |
+ |
+# Concepts |
+ |
+## Visibility |
+ |
+Targets can control which other targets may depend on them by specifying `visibility`. Visibility is always checked when running any GN command (not just `gn check`. |
+ |
+By default, targets are "public" meaning any target can depend on them. If you supply a list, visibility will be listed to those targets (possibly including wildcards): |
+ |
+``` |
+visibility = [ |
+ ":*", # All targets in this file. |
+ "//content/*", # All targets in content and any subdirectory thereof. |
+ "//tools:doom_melon", # This specific target. |
+] |
+``` |
+ |
+See `gn help visibility` for more details and examples. |
+ |
+## Public header files |
+ |
+Targets can control which headers may be included by dependent targets so as to define a public API. If your target specifies only `sources`, then all headers listed there are public and can be included by all dependents. |
+ |
+If your target defines a `public` variable, only the files listed in that list will be public. Files in `sources` but not `public` (they can be in both or only one) may not be included by dependent targets. |
+ |
+``` |
+source_set("foo") { |
+ public = [ |
+ "foo.h", |
+ "foo_config.h", |
+ ] |
+ sources = [ |
+ "foo.cc", |
+ "foo.h", |
+ "bar.cc", |
+ "bar.h", |
+ ] |
+} |
+``` |
+ |
+## Public dependencies |
+ |
+In order to include files from your target, that target must be listed in your target's dependencies. By default, transitively depending on a target doesn't give your files this privilege. |
+ |
+If a target exposes a dependency as part of its public API, then it can list that dependency as a `public_deps`: |
+``` |
+source_set("foo") { |
+ sources = [ ... ] |
+ public_deps = [ |
+ "//base", |
+ ] |
+ deps = [ |
+ "//tools/doom_melon", |
+ ] |
+} |
+``` |
+Targets that depend on `foo` can include files from `base` but not from `doom_melon`. To include public headers from `doom\_melon, a target would need to depend directly on it. |
+ |
+Public dependencies work transitively, so listing a target as a public dependency also exposes that target's public dependencies. Along with the ability to include headers, public dependencies forward the `public_configs` which allow settings like defines and include directories to apply to dependents. |
+ |
+# Putting it all together |
+ |
+In order to include a header from target Y in a file that is part of target X: |
+ |
+ * X must be in Y's `visibility` list (or B must have no `visibility` defined). |
+ * The header must be in Y's `public` headers (or Y must have no `public` variable defined). |
+ * X must depend directly on Y, or there must be a path from X to Y following only public dependencies. |
+ |
+### What gets checked |
+ |
+Chrome currently doesn't come close to passing a `gn check` pass. You can check specific targets or subtrees for issues: |
+``` |
+ gn check out/mybuild //base |
+ |
+ gn check out/mybuild "//mojo/*" |
+``` |