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# GN Check
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
[TOC]
## 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/*"
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