gn/docs/quick_start.md

# GN Quick Start guide

[TOC]

## Running GN

You just run `gn` from the command line. For large projects, GN is versioned
and distributed with the source checkout.

  * For Chromium and Chromium-based projects, there is a script in
    `depot_tools`, which is presumably in your PATH, with this name. The script
    will find the binary in the source tree containing the current directory and
    run it.

  * For Fuchsia in-tree development, run `fx gn ...` which will find the right
    GN binary and run it with the given arguments.

  * For other projects, see your project's documentation.

## Setting up a build

Unlike some other build systems, with GN you set up your own build directories
with the settings you want. This lets you maintain as many different builds in
parallel as you need.

Once you set up a build directory, the Ninja files will be automatically
regenerated if they're out of date when you build in that directory so you
should not have to re-run GN.

To make a build directory:

```
gn gen out/my_build
```

## Passing build arguments

Set build arguments on your build directory by running:

```
gn args out/my_build
```

This will bring up an editor. Type build args into that file like this:

```
is_component_build = true
is_debug = false
```

The available variables will depend on your build (this example is from
Chromium). You can see the list of available arguments and their default values
by typing

```
gn args --list out/my_build
```

on the command line. Note that you have to specify the build directory
for this command because the available arguments can change according
to the build.

Chrome developers can also read the [Chrome-specific build
configuration](http://www.chromium.org/developers/gn-build-configuration)
instructions for more information.

## Cross-compiling to a target OS or architecture

Run `gn args out/Default` (substituting your build directory as needed) and
add one or more of the following lines for common cross-compiling options.

```
target_os = "chromeos"
target_os = "android"

target_cpu = "arm"
target_cpu = "x86"
target_cpu = "x64"
```

See [GN cross compiles](cross_compiles.md) for more info.

## Step-by-step

### Adding a build file

Go to the directory `examples/simple_build`. This is the root of a minimal GN
repository.

In that directory there is a `tutorial` directory. There is already a
`tutorial.cc` file that's not hooked up to the build. Create a new `BUILD.gn`
file in that directory for our new target.

```
executable("tutorial") {
  sources = [
    "tutorial.cc",
  ]
}
```

Now we just need to tell the build about this new target. Open the `BUILD.gn`
file in the parent (`simple_build`) directory. GN starts by loading this root
file, and then loads all dependencies ourward from here, so we just need to add
a reference to our new target from this file.

You could add our new target as a dependency from one of the existing targets in
the `simple_build/BUILD.gn` file, but it usually doesn't make a lot of sense to
have an executable as a depdency of another executable (they can't be linked).
So let's make a "tools" group. In GN, a "group" is just a collection of
dependencies that's not complied or linked:

```
group("tools") {
  deps = [
    # This will expand to the name "//tutorial:tutorial" which is the full name
    # of our new target. Run "gn help labels" for more.
    "//tutorial",
  ]
}
```

### Testing your addition

From the command line in the `simple_build` directory:

```
gn gen out
ninja -C out tutorial
out/tutorial
```

You should see "Hello, world." output to the console.

Side note: GN encourages target names for static libraries that aren't globally
unique. To build one of these, you can pass the label with its path (but no leading
"//") to ninja:

```
ninja -C out some/path/to/target:my_target
```

### Declaring dependencies

Let's look at the targets defined in
[examples/simple_build/BUILD.gn](../examples/simple_build/BUILD.gn). There is a
static library that defines one function, `GetStaticText()`:

```
static_library("hello_static") {
  sources = [
    "hello_static.cc",
    "hello_static.h",
  ]
}
```

There is also a shared library that defines one function `GetSharedText()`:

```
shared_library("hello_shared") {
  sources = [
    "hello_shared.cc",
    "hello_shared.h",
  ]

  defines = [ "HELLO_SHARED_IMPLEMENTATION" ]
}
```

This also illustrates how to set preprocessor defines for a target. To set more
than one or to assign values, use this form:

```
defines = [
  "HELLO_SHARED_IMPLEMENTATION",
  "ENABLE_DOOM_MELON=0",
]
```

Now let's look at the executable that depends on these two libraries:

```
executable("hello") {
  sources = [
    "hello.cc",
  ]

  deps = [
    ":hello_shared",
    ":hello_static",
  ]
}
```

This executable includes one source file and depends on the previous
two libraries. Labels starting with a colon refer to a target with that name in
the current BUILD.gn file.

### Test the binary

From the command line in the `simple_build` directory:

```
ninja -C out hello
out/hello
```

Note that you **didn't** need to re-run GN. GN will automatically rebuild
the ninja files when any build file has changed. You know this happens
when ninja prints `[1/1] Regenerating ninja files` at the beginning of
execution.

### Putting settings in a config

Users of a library often need compiler flags, defines, and include directories
applied to them. To do this, put all such settings into a "config" which is a
named collection of settings (but not sources or dependencies):

```
config("my_lib_config") {
  defines = [ "ENABLE_DOOM_MELON" ]
  include_dirs = [ "//third_party/something" ]
}
```

To apply a config's settings to a target, add it to the `configs` list:

```
static_library("hello_shared") {
  ...
  # Note "+=" here is usually required, see "default configs" below.
  configs += [
    ":my_lib_config",
  ]
}
```

A config can be applied to all targets that depend on the current one by putting
its label in the `public_configs` list:

```
static_library("hello_shared") {
  ...
  public_configs = [
    ":my_lib_config",
  ]
}
```

The `public_configs` also applies to the current target, so there's no need to
list a config in both places.

### Default configs

The build configuration will set up some settings that apply to every target by
default. These will normally be set as a default list of configs. You can see
this using the "print" command which is useful for debugging:

```
executable("hello") {
  print(configs)
}
```

Running GN will print something like:

```
$ gn gen out
["//build:compiler_defaults", "//build:executable_ldconfig"]
Done. Made 5 targets from 5 files in 9ms
```

Targets can modify this list to change their defaults. For example, the build
setup might turn off exceptions by default by adding a `no_exceptions` config,
but a target might re-enable them by replacing it with a different one:

```
executable("hello") {
  ...
  configs -= [ "//build:no_exceptions" ]  # Remove global default.
  configs += [ "//build:exceptions" ]  # Replace with a different one.
}
```

Our print command from above could also be expressed using string interpolation.
This is a way to convert values to strings. It uses the symbol "$" to refer to a
variable:

```
print("The configs for the target $target_name are $configs")
```

## Add a new build argument

You declare which arguments you accept and specify default values via
`declare_args`.

```
declare_args() {
  enable_teleporter = true
  enable_doom_melon = false
}
```

See `gn help buildargs` for an overview of how this works.
See `gn help declare_args` for specifics on declaring them.

It is an error to declare a given argument more than once in a given scope, so
care should be used in scoping and naming arguments.

## Don't know what's going on?

You can run GN in verbose mode to see lots of messages about what it's
doing. Use `-v` for this.

### The "desc" command

You can run `gn desc <build_dir> <targetname>` to get information about
a given target:

```
gn desc out/Default //foo/bar:say_hello
```

will print out lots of exciting information. You can also print just one
section. Lets say you wanted to know where your `TWO_PEOPLE` define
came from on the `say_hello` target:

```
> gn desc out/Default //foo/bar:say_hello defines --blame
...lots of other stuff omitted...
  From //foo/bar:hello_config
       (Added by //foo/bar/BUILD.gn:12)
    TWO_PEOPLE
```

Another particularly interesting variation:

```
gn desc out/Default //base:base_i18n deps --tree
```

See `gn help desc` for more.